Difference between revisions of "RFC8724"

Latest revision as of 21:37, 5 May 2021

Internet Engineering Task Force (IETF) A. Minaburo Request for Comments: 8724 Acklio Category: Standards Track L. Toutain ISSN: 2070-1721 IMT Atlantique

                                                            C. Gomez
                                Universitat Politecnica de Catalunya
                                                          D. Barthel
                                                         Orange Labs
                                                          JC. Zuniga
                                                              SIGFOX
                                                          April 2020

SCHC: Generic Framework for Static Context Header Compression and

                         Fragmentation

Abstract

This document defines the Static Context Header Compression and fragmentation (SCHC) framework, which provides both a header compression mechanism and an optional fragmentation mechanism. SCHC has been designed with Low-Power Wide Area Networks (LPWANs) in mind.

SCHC compression is based on a common static context stored both in the LPWAN device and in the network infrastructure side. This document defines a generic header compression mechanism and its application to compress IPv6/UDP headers.

This document also specifies an optional fragmentation and reassembly mechanism. It can be used to support the IPv6 MTU requirement over the LPWAN technologies. Fragmentation is needed for IPv6 datagrams that, after SCHC compression or when such compression was not possible, still exceed the Layer 2 maximum payload size.

The SCHC header compression and fragmentation mechanisms are independent of the specific LPWAN technology over which they are used. This document defines generic functionalities and offers flexibility with regard to parameter settings and mechanism choices. This document standardizes the exchange over the LPWAN between two SCHC entities. Settings and choices specific to a technology or a product are expected to be grouped into profiles, which are specified in other documents. Data models for the context and profiles are out of scope.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8724.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Introduction 2. Requirements Notation 3. LPWAN Architecture 4. Terminology 5. SCHC Overview

 5.1.  SCHC Packet Format
 5.2.  Functional Mapping

6. RuleID 7. Compression/Decompression

 7.1.  SCHC C/D Rules
 7.2.  Packet Processing
 7.3.  Matching Operators
 7.4.  Compression/Decompression Actions (CDA)
   7.4.1.  Processing Fixed-Length Fields
   7.4.2.  Processing Variable-Length Fields
   7.4.3.  Not-Sent CDA
   7.4.4.  Value-Sent CDA
   7.4.5.  Mapping-Sent CDA
   7.4.6.  LSB CDA
   7.4.7.  DevIID, AppIID CDA
   7.4.8.  Compute-*

8. Fragmentation/Reassembly

 8.1.  Overview
 8.2.  SCHC F/R Protocol Elements
   8.2.1.  Messages
   8.2.2.  Tiles, Windows, Bitmaps, Timers, Counters
   8.2.3.  Integrity Checking
   8.2.4.  Header Fields
 8.3.  SCHC F/R Message Formats
   8.3.1.  SCHC Fragment Format
   8.3.2.  SCHC ACK Format
   8.3.3.  SCHC ACK REQ Format
   8.3.4.  SCHC Sender-Abort Format
   8.3.5.  SCHC Receiver-Abort Format
 8.4.  SCHC F/R Modes
   8.4.1.  No-ACK Mode
   8.4.2.  ACK-Always Mode
   8.4.3.  ACK-on-Error Mode

9. Padding Management 10. SCHC Compression for IPv6 and UDP Headers

 10.1.  IPv6 Version Field
 10.2.  IPv6 Traffic Class Field
 10.3.  Flow Label Field
 10.4.  Payload Length Field
 10.5.  Next Header Field
 10.6.  Hop Limit Field
 10.7.  IPv6 Addresses Fields
   10.7.1.  IPv6 Source and Destination Prefixes
   10.7.2.  IPv6 Source and Destination IID
 10.8.  IPv6 Extension Headers
 10.9.  UDP Source and Destination Ports
 10.10. UDP Length Field
 10.11. UDP Checksum Field

11. IANA Considerations 12. Security Considerations

 12.1.  Security Considerations for SCHC Compression/Decompression
   12.1.1.  Forged SCHC Packet
   12.1.2.  Compressed Packet Size as a Side Channel to Guess a
           Secret Token
   12.1.3.  Decompressed Packet Different from the Original Packet
 12.2.  Security Considerations for SCHC Fragmentation/Reassembly
   12.2.1.  Buffer Reservation Attack
   12.2.2.  Corrupt Fragment Attack
   12.2.3.  Fragmentation as a Way to Bypass Network Inspection
   12.2.4.  Privacy Issues Associated with SCHC Header Fields

13. References

 13.1.  Normative References
 13.2.  Informative References

Appendix A. Compression Examples Appendix B. Fragmentation Examples Appendix C. Fragmentation State Machines Appendix D. SCHC Parameters Appendix E. Supporting Multiple Window Sizes for Fragmentation Appendix F. ACK-Always and ACK-on-Error on Quasi-Bidirectional

       Links

Acknowledgements Authors' Addresses

Introduction

This document defines the Static Context Header Compression and fragmentation (SCHC) framework, which provides both a header compression mechanism and an optional fragmentation mechanism. SCHC has been designed with Low-Power Wide Area Networks (LPWANs) in mind.

LPWAN technologies impose some strict limitations on traffic. For instance, devices sleep most of the time and may only receive data during short periods of time after transmission, in order to preserve battery. LPWAN technologies are also characterized by a greatly reduced data unit and/or payload size (see RFC8376).

Header compression is needed for efficient Internet connectivity to a node within an LPWAN. The following properties of LPWANs can be exploited to get an efficient header compression:

The network topology is star-oriented, which means that all

  packets between the same source-destination pair follow the same
  path.  For the needs of this document, the architecture can simply
  be described as Devices (Dev) exchanging information with LPWAN
  Application Servers (Apps) through a Network Gateway (NGW).

Because devices embed built-in applications, the traffic flows to

  be compressed are known in advance.  Indeed, new applications are
  less frequently installed in an LPWAN device than they are in a
  general-purpose computer or smartphone.

SCHC compression uses a Context (a set of Rules) in which information about header fields is stored. This Context is static: the values of the header fields and the actions to do compression/decompression do not change over time. This avoids the need for complex resynchronization mechanisms. Indeed, a return path may be more restricted/expensive, or may sometimes be completely unavailable RFC8376. A compression protocol that relies on feedback is not compatible with the characteristics of such LPWANs.

In most cases, a small Rule identifier is enough to represent the full IPv6/UDP headers. The SCHC header compression mechanism is independent of the specific LPWAN technology over which it is used.

Furthermore, some LPWAN technologies do not provide a fragmentation functionality; to support the IPv6 MTU requirement of 1280 bytes RFC8200, they require a fragmentation protocol at the adaptation layer below IPv6. Accordingly, this document defines an optional fragmentation/reassembly mechanism to help LPWAN technologies support the IPv6 MTU requirement.

This document defines generic functionality and offers flexibility with regard to parameter settings and mechanism choices. Technology- specific settings are expected to be grouped into Profiles specified in other documents.

Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 RFC2119 RFC8174 when, and only when, they appear in all capitals, as shown here.

LPWAN Architecture

LPWAN architectures are similar among them, but each LPWAN technology names architecture elements differently. In this document, we use terminology from RFC8376, which identifies the following entities in a typical LPWAN (see Figure 1):

Devices (Dev) are the end-devices or hosts (e.g., sensors,

  actuators, etc.).  There can be a very high density of devices per
  Radio Gateway.

The Radio Gateway (RGW) is the endpoint of the constrained link.

The Network Gateway (NGW) is the interconnection node between the

  Radio Gateway and the Internet.

The Application Server (App) is the endpoint of the application-

  level protocol on the Internet side.

()   ()   ()       |
 ()  () () ()     / \       +---------+

() () () () () () / \======| ^ | +-----------+

()  ()   ()     |           | <--|--> |             |Application|

() () () () / \==========| v |=============| Server |

 ()  ()  ()   /   \         +---------+             +-----------+
Dev            RGWs             NGW                      App

Figure 1: LPWAN Architecture (Simplified from That Shown in RFC 8376)

Terminology

This section defines terminology and abbreviations used in this document. It extends the terminology of RFC8376.

The SCHC acronym is pronounced like "sheek" in English (or "chic" in French). Therefore, this document writes "a SCHC Packet" instead of "an SCHC Packet".

App: LPWAN Application Server, as defined by RFC8376. It runs

        an application sending/receiving packets to/from the Dev.

AppIID: Application Interface Identifier. The IID that identifies

        the App interface.

Compression Residue: The bits that remain to be sent (beyond the

        RuleID itself) after applying the SCHC compression.

Context: A set of Rules used to compress/decompress headers, or to

        fragment/reassemble a packet.

Dev: Device, as defined by RFC8376.

DevIID: Device Interface Identifier. The IID that identifies the

        Dev interface.

Downlink: From the App to the Dev.

IID: Interface Identifier. See the IPv6 addressing architecture

        RFC7136.

L2: Layer 2. The immediate lower layer that SCHC interfaces

        with, for example an underlying LPWAN technology.  It does
        not necessarily correspond to the OSI model definition of
        Layer 2.

L2 Word: This is the minimum subdivision of payload data that the L2

        will carry.  In most L2 technologies, the L2 Word is an
        octet.  In bit-oriented radio technologies, the L2 Word
        might be a single bit.  The L2 Word size is assumed to be
        constant over time for each device.

Padding: Extra bits that may be appended by SCHC to a data unit that

        it passes down to L2 for transmission.  SCHC itself operates
        on bits, not bytes, and does not have any alignment
        prerequisite.  See Section 9.

Profile: SCHC offers variations in the way it is operated, with a

        number of parameters listed in Appendix D.  A Profile
        indicates a particular setting of all these parameters.
        Both ends of a SCHC communication must be provisioned with
        the same Profile information and with the same set of Rules
        before the communication starts, so that there is no
        ambiguity in how they expect to communicate.

Rule: Part of the Context that describes how a packet is

        compressed/decompressed or fragmented/reassembled.

RuleID: Rule Identifier. An identifier for a Rule.

SCHC: Static Context Header Compression and fragmentation (SCHC),

        a generic framework.

SCHC C/D: SCHC Compressor/Decompressor, or SCHC Compression/

        Decompression.  The SCHC entity or mechanism used on both
        sides, at the Dev and at the network, to achieve
        compression/decompression of headers.

SCHC F/R: SCHC Fragmenter/Reassembler or SCHC Fragmentation/

        Reassembly.  The SCHC entity or mechanism used on both
        sides, at the Dev and at the network, to achieve
        fragmentation/reassembly of SCHC Packets.

SCHC Packet: A packet (e.g., an IPv6 packet) whose header has been

        compressed as per the header compression mechanism defined
        in this document.  If the header compression process is
        unable to actually compress the packet header, the packet
        with the uncompressed header is still called a SCHC Packet
        (in this case, a RuleID is used to indicate that the packet
        header has not been compressed).  See Section 7 for more
        details.

Uplink: From the Dev to the App.

Additional terminology for the optional SCHC F/R is found in Section 8.2.

Additional terminology for SCHC C/D is found in Section 7.1.

SCHC Overview

SCHC can be characterized as an adaptation layer between an upper layer (for example, IPv6) and an underlying layer (for example, an LPWAN technology). SCHC comprises two sublayers (i.e., the Compression sublayer and the Fragmentation sublayer), as shown in Figure 2.

            +----------------+
            |      IPv6      |
         +- +----------------+
         |  |   Compression  |
   SCHC <   +----------------+
         |  |  Fragmentation |
         +- +----------------+
            |LPWAN technology|
            +----------------+

 Figure 2: Example of Protocol Stack Comprising IPv6, SCHC, and an
                          LPWAN Technology

Before an upper layer packet (e.g., an IPv6 packet) is transmitted to the underlying layer, header compression is first attempted. The resulting packet is called a "SCHC Packet", whether or not any compression is performed. If needed by the underlying layer, the optional SCHC fragmentation MAY be applied to the SCHC Packet. The inverse operations take place at the receiver. This process is illustrated in Figure 3.

A packet (e.g., an IPv6 packet)

        |                                           ^
        v                                           |

+------------------+ +--------------------+ | SCHC Compression | | SCHC Decompression | +------------------+ +--------------------+

        |                                           ^
        |   If no fragmentation (*)                 |
        +-------------- SCHC Packet  -------------->|
        |                                           |
        v                                           |

+--------------------+ +-----------------+ | SCHC Fragmentation | | SCHC Reassembly | +--------------------+ +-----------------+

     |     ^                                     |     ^
     |     |                                     |     |
     |     +---------- SCHC ACK (+) -------------+     |
     |                                                 |
     +-------------- SCHC Fragments -------------------+

       Sender                                    Receiver

the decision not to use SCHC fragmentation is left to each Profile

+: optional, depends on Fragmentation mode

      Figure 3: SCHC Operations at the Sender and the Receiver

SCHC Packet Format

The SCHC Packet is composed of the Compressed Header followed by the payload from the original packet (see Figure 4). The Compressed Header itself is composed of the RuleID and a Compression Residue, which is the output of compressing the packet header with the Rule identified by that RuleID (see Section 7). The Compression Residue may be empty. Both the RuleID and the Compression Residue potentially have a variable size, and are not necessarily a multiple of bytes in size.

|------- Compressed Header -------| +---------------------------------+--------------------+ | RuleID | Compression Residue | Payload | +---------------------------------+--------------------+

                       Figure 4: SCHC Packet

Functional Mapping

Figure 5 maps the functional elements of Figure 3 onto the LPWAN architecture elements of Figure 1.

       Dev                                               App

+----------------+ +----+ +----+ +----+ | App1 App2 App3 | |App1| |App2| |App3| | | | | | | | | | UDP | |UDP | |UDP | |UDP | | IPv6 | |IPv6| |IPv6| |IPv6| | | | | | | | | |SCHC C/D and F/R| | | | | | | +--------+-------+ +----+ +----+ +----+

        |  +---+     +---+    +----+    +----+    .      .      .

           +---+     +---+    |F/R |    |C/D |
                              +----+    +----+

                  Figure 5: Architectural Mapping

SCHC C/D and SCHC F/R are located on both sides of the LPWAN transmission, hereafter called the "Dev side" and the "Network Infrastructure side".

The operation in the Uplink direction is as follows. The Device application uses IPv6 or IPv6/UDP protocols. Before sending the packets, the Dev compresses their headers using SCHC C/D; if the SCHC Packet resulting from the compression needs to be fragmented by SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC Fragments are sent to an LPWAN Radio Gateway (RGW), which forwards them to a Network Gateway (NGW). The NGW sends the data to a SCHC F/R for reassembly (if needed) and then to the SCHC C/D for decompression. After decompression, the packet can be sent over the Internet to one or several Apps.

The SCHC F/R and SCHC C/D on the Network Infrastructure side can be part of the NGW or located in the Internet as long as a tunnel is established between them and the NGW. For some LPWAN technologies, it may be suitable to locate the SCHC F/R functionality nearer the NGW, in order to better deal with time constraints of such technologies.

The SCHC C/Ds on both sides MUST share the same set of Rules. So MUST the SCHC F/Rs on both sides.

The operation in the Downlink direction is similar to that in the Uplink direction, only reversing the order in which the architecture elements are traversed.

RuleID

RuleIDs identify the Rules used for compression/decompression or for fragmentation/reassembly.

The scope of the RuleID of a compression/decompression Rule is the link between the SCHC C/D in a given Dev and the corresponding SCHC C/D in the Network Infrastructure side. The scope of the RuleID of a fragmentation/reassembly Rule is the link between the SCHC F/R in a given Dev and the corresponding SCHC F/R in the Network Infrastructure side. If such a link is bidirectional, the scope includes both directions.

The RuleIDs are therefore specific to the Context related to one Dev. Hence, multiple Dev instances, which refer to different Contexts, MAY reuse the same RuleID for different Rules. On the Network Infrastructure side, in order to identify the correct Rule to be applied to Uplink traffic, the SCHC C/D or SCHC F/R needs to associate the RuleID with the Dev identifier. Similarly, for Downlink traffic, the SCHC C/D or SCHC F/R on the Network Infrastructure side first needs to identify the destination Dev before looking for the appropriate Rule (and associated RuleID) in the Context of that Dev.

Inside their scopes, Rules for compression/decompression and Rules for fragmentation/reassembly share the same RuleID space.

The size of the RuleIDs is not specified in this document, as it is implementation-specific and can vary according to the LPWAN technology and the number of Rules, among other things. It is defined in Profiles.

The RuleIDs are used:

For SCHC C/D, to identify the Rule that is used to compress a

  packet header.

  -  At least one RuleID MUST be allocated to tagging packets for
     which SCHC compression was not possible (i.e., no matching
     compression Rule was found).

In SCHC F/R, to identify the specific mode and settings of

  fragmentation/reassembly for one direction of data traffic (Uplink
  or Downlink).

  -  When SCHC F/R is used for both communication directions, at
     least two RuleID values are needed for fragmentation/
     reassembly: one per direction of data traffic.  This is because
     fragmentation/reassembly may entail control messages flowing in
     the reverse direction compared to data traffic.

Compression/Decompression

Compression with SCHC is based on using a set of Rules, which constitutes the Context of SCHC C/D, to compress or decompress headers. SCHC avoids Context synchronization traffic, which consumes considerable bandwidth in other header compression mechanisms such as RObust Header Compression (RoHC) RFC5795. Since the content of packets is highly predictable in LPWANs, static Contexts can be stored beforehand. The Contexts MUST be stored at both ends, and they can be learned by a provisioning protocol, by out-of-band means, or by pre-provisioning. The way the Contexts are provisioned is out of the scope of this document.

SCHC C/D Rules

The main idea of the SCHC compression scheme is to transmit the RuleID to the other end instead of sending known field values. This RuleID identifies a Rule that matches the original packet values. Hence, when a value is known by both ends, it is only necessary to send the corresponding RuleID over the LPWAN. The manner by which Rules are generated is out of the scope of this document. The Rules MAY be changed at run-time, but the mechanism is out of scope of this document.

The SCHC C/D Context is a set of Rules. See Figure 6 for a high- level, abstract representation of the Context. The formal specification of the representation of the Rules is outside the scope of this document.

Each Rule itself contains a list of Field Descriptors composed of a Field Identifier (FID), a Field Length (FL), a Field Position (FP), a Direction Indicator (DI), a Target Value (TV), a Matching Operator (MO), and a Compression/Decompression Action (CDA).

 /-----------------------------------------------------------------\
 |                         Rule N                                  |
/-----------------------------------------------------------------\|
|                       Rule i                                    ||

/-----------------------------------------------------------------\|| | (FID) Rule 1 ||| |+-------+--+--+--+------------+-----------------+---------------+||| ||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||| |+-------+--+--+--+------------+-----------------+---------------+||| ||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||| |+-------+--+--+--+------------+-----------------+---------------+||| ||... |..|..|..| ... | ... | ... |||| |+-------+--+--+--+------------+-----------------+---------------+||/ ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||| |+-------+--+--+--+------------+-----------------+---------------+|/ | | \-----------------------------------------------------------------/

                    Figure 6: A SCHC C/D Context

A Rule does not describe how the compressor parses a packet header to find and identify each field (e.g., the IPv6 Source Address, the UDP Destination Port, or a CoAP URI path option). It is assumed that there is a protocol parser alongside SCHC that is able to identify all the fields encountered in the headers to be compressed, and to label them with a Field ID. Rules only describe the compression/ decompression behavior for each header field, after it has been identified.

In a Rule, the Field Descriptors are listed in the order in which the fields appear in the packet header. The Field Descriptors describe the header fields with the following entries:

Field Identifier (FID) designates a protocol and field (e.g., UDP

  Destination Port), unambiguously among all protocols that a SCHC
  compressor processes.  In the presence of protocol nesting, the
  Field ID also identifies the nesting.

Field Length (FL) represents the length of the original field. It

  can be either a fixed value (in bits) if the length is known when
  the Rule is created or a type if the length is variable.  The
  length of a header field is defined by its own protocol
  specification (e.g., IPv6 or UDP).  If the length is variable, the
  type defines the process to compute the length and its unit (bits,
  bytes...).

Field Position (FP): most often, a field only occurs once in a

  packet header.  However, some fields may occur multiple times.  An
  example is the uri-path of CoAP.  FP indicates which occurrence
  this Field Descriptor applies to.  The default value is 1.  The
  value 1 designates the first occurrence.  The value 0 is special.
  It means "don't care", see Section 7.2.

A Direction Indicator (DI) indicates the packet direction(s) this

  Field Descriptor applies to.  It allows for asymmetric processing,
  using the same Rule.  Three values are possible:

  Up:  this Field Descriptor is only applicable to packets traveling
     Uplink.

  Dw:  this Field Descriptor is only applicable to packets traveling
     Downlink.

  Bi:  this Field Descriptor is applicable to packets traveling
     Uplink or Downlink.

Target Value (TV) is the value used to match against the packet

  header field.  The Target Value can be a scalar value of any type
  (integer, strings, etc.) or a more complex structure (array, list,
  etc.).  The types and representations are out of scope for this
  document.

Matching Operator (MO) is the operator used to match the field

  value and the Target Value.  The Matching Operator may require
  some parameters.  The set of MOs defined in this document can be
  found in Section 7.3.

Compression/Decompression Action (CDA) describes the pair of

  actions that are performed at the compressor to compress a header
  field and at the decompressor to recover the original value of the
  header field.  Some CDAs might use parameter values for their
  operation.  The set of CDAs defined in this document can be found
  in Section 7.4.

Packet Processing

The compression/decompression process follows several phases:

Compression Rule selection: the general idea is to browse the Rule

  set to find a Rule that has a matching Field Descriptor (given the
  DI and FP) for all and only those header fields that appear in the
  packet being compressed.  The detailed algorithm is the following:

  *  The first step is to check the FIDs.  If any header field of
     the packet being examined cannot be matched with a Field
     Descriptor with the correct FID, the Rule MUST be disregarded.
     If any Field Descriptor in the Rule has a FID that cannot be
     matched to one of the header fields of the packet being
     examined, the Rule MUST be disregarded.

  *  The next step is to match the Field Descriptors by their
     direction, using the DI.  If any field of the packet header
     cannot be matched with a Field Descriptor with the correct FID
     and DI, the Rule MUST be disregarded.

  *  Then, the Field Descriptors are further selected according to
     FP.  If any field of the packet header cannot be matched with a
     Field Descriptor with the correct FID, DI and FP, the Rule MUST
     be disregarded.

     The value 0 for FP means "don't care", i.e., the comparison of
     this Field Descriptor's FP with the position of the field of
     the packet header being compressed returns True, whatever that
     position.  FP=0 can be useful to build compression Rules for
     protocol headers in which some fields order is irrelevant.  An
     example could be uri-queries in CoAP.  Care needs to be
     exercised when writing Rules containing FP=0 values.  Indeed,
     it may result in decompressed packets having fields ordered
     differently compared to the original packet.

  *  Once each header field has been associated with a Field
     Descriptor with matching FID, DI, and FP, each packet field's
     value is then compared to the corresponding TV stored in the
     Rule for that specific field, using the MO.  If every field in
     the packet header satisfies the corresponding MOs of a Rule
     (i.e., all MO results are True), that Rule is valid for use to
     compress the header.  Otherwise, the Rule MUST be disregarded.

     This specification does not prevent multiple Rules from
     matching the above steps and, therefore, being valid for use.
     Which Rule to use among multiple valid Rules is left to the
     implementation.  As long as the same Rule set is installed at
     both ends, this degree of freedom does not constitute an
     interoperability issue.

  *  If no valid compression Rule is found, then the packet MUST be
     sent uncompressed using the RuleID dedicated to this purpose
     (see Section 6).  The entire packet header is the Compression
     Residue (see Figure 4).  Sending an uncompressed header is
     likely to require SCHC F/R.

Compression: if a valid Rule is found, each field of the header is

  compressed according to the CDAs of the Rule.  The fields are
  compressed in the order that the Field Descriptors appear in the
  Rule.  The compression of each field results in a residue, which
  may be empty.  The Compression Residue for the packet header is
  the concatenation of the non-empty residues for each field of the
  header, in the order the Field Descriptors appear in the Rule.
  The order in which the Field Descriptors appear in the Rule is
  therefore semantically important.

   |------------------- Compression Residue -------------------|
   +-----------------+-----------------+-----+-----------------+
   | field 1 residue | field 2 residue | ... | field N residue |
   +-----------------+-----------------+-----+-----------------+

              Figure 7: Compression Residue Structure

Sending: The RuleID is sent to the other end jointly with the

  Compression Residue (which could be empty) or the uncompressed
  header, and directly followed by the payload (see Figure 4).  The
  way the RuleID is sent will be specified in the Profile and is out
  of the scope of the present document.  For example, it could be
  included in an L2 header or sent as part of the L2 payload.

Decompression: when decompressing, on the Network Infrastructure

  side, the SCHC C/D needs to find the correct Rule based on the L2
  address of the Dev.  On the Dev side, only the RuleID is needed to
  identify the correct Rule since the Dev typically only holds Rules
  that apply to itself.

  This Rule describes the compressed header format.  From this, the
  decompressor determines the order of the residues, the fixed-size
  or variable-size nature of each residue (see Section 7.4.2), and
  the size of the fixed-size residues.

  Therefore, from the received compressed header, it can retrieve
  all the residue values and associate them to the corresponding
  header fields.

  For each field in the header, the receiver applies the CDA action
  associated with that field in order to reconstruct the original
  header field value.  The CDA application order can be different
  from the order in which the fields are listed in the Rule.  In
  particular, Compute-* MUST be applied after the application of the
  CDAs of all the fields it computes on.

Matching Operators

MOs are functions used at the compression side of SCHC C/D. They are not typed and can be applied to integer, string or any other data type. The result of the operation can either be True or False. The following MOs are defined:

equal: The match result is True if the field value in the packet

  matches the TV.

ignore: No matching is attempted between the field value in the

  packet and the TV in the Rule.  The result is always True.

MSB(x): A match is obtained if the most significant (leftmost) x

  bits of the packet header field value are equal to the TV in the
  Rule.  The x parameter of the MSB MO indicates how many bits are
  involved in the comparison.  If the FL is described as variable,
  the x parameter must be a multiple of the FL unit.  For example, x
  must be multiple of 8 if the unit of the variable length is bytes.

match-mapping: With match-mapping, TV is a list of values. Each

  value of the list is identified by an index.  Compression is
  achieved by sending the index instead of the original header field
  value.  This operator matches if the header field value is equal
  to one of the values in the target list.

Compression/Decompression Actions (CDA)

The CDA specifies the actions taken during the compression of header fields and the inverse action taken by the decompressor to restore the original value. The CDAs defined by this document are described in detail in Section 7.4.3 to Section 7.4.8. They are summarized in Table 1.

 +--------------+------------------------+-----------------------+
 | Action       | Compression            | Decompression         |
 +==============+========================+=======================+
 | not-sent     | elided                 | use TV stored in Rule |
 +--------------+------------------------+-----------------------+
 | value-sent   | send                   | use received value    |
 +--------------+------------------------+-----------------------+
 | mapping-sent | send index             | retrieve value from   |
 |              |                        | TV list               |
 +--------------+------------------------+-----------------------+
 | LSB          | send least significant | concatenate TV and    |
 |              | bits (LSB)             | received value        |
 +--------------+------------------------+-----------------------+
 | compute-*    | elided                 | recompute at          |
 |              |                        | decompressor          |
 +--------------+------------------------+-----------------------+
 | DevIID       | elided                 | build IID from L2 Dev |
 |              |                        | addr                  |
 +--------------+------------------------+-----------------------+
 | AppIID       | elided                 | build IID from L2 App |
 |              |                        | addr                  |
 +--------------+------------------------+-----------------------+

           Table 1: Compression and Decompression Actions

The first column shows the action's name. The second and third columns show the compression and decompression behaviors for each action.

Processing Fixed-Length Fields

If the field is identified in the Field Descriptor as being of fixed length, then applying the CDA to compress this field results in a fixed amount of bits. The residue for that field is simply the bits resulting from applying the CDA to the field. This value may be empty (e.g., not-sent CDA), in which case the field residue is absent from the Compression Residue.

|- field residue -| +-----------------+ | value | +-----------------+

            Figure 8: Fixed-Size Field Residue Structure

Processing Variable-Length Fields

If the field is identified in the Field Descriptor as being of variable length, then applying the CDA to compress this field may result in a value of fixed size (e.g., not-sent or mapping-sent) or of variable size (e.g., value-sent or LSB). In the latter case, the residue for that field is the bits that result from applying the CDA to the field, preceded with the size of the value. The most significant bit of the size is stored to the left (leftmost bit of the residue field).

|--- field residue ---| +-------+-------------+ | size | value | +-------+-------------+

          Figure 9: Variable-Size Field Residue Structure

The size (using the unit defined in the FL) is encoded on 4, 12, or 28 bits as follows:

If the size is between 0 and 14, it is encoded as a 4-bit unsigned

  integer.

Sizes between 15 and 254 are encoded as 0b1111 followed by the

  8-bit unsigned integer.

Larger sizes are encoded as 0xfff followed by the 16-bit unsigned

  integer.

If the field is identified in the Field Descriptor as being of variable length and this field is not present in the packet header being compressed, size 0 MUST be sent to denote its absence.

Not-Sent CDA

The not-sent action can be used when the field value is specified in a Rule and, therefore, known by both the Compressor and the Decompressor. This action SHOULD be used with the "equal" MO. If MO is "ignore", there is a risk of having a decompressed field value that is different from the original field that was compressed.

The compressor does not send any residue for a field on which not- sent compression is applied.

The decompressor restores the field value with the TV stored in the matched Rule identified by the received RuleID.

Value-Sent CDA

The value-sent action can be used when the field value is not known by both the Compressor and the Decompressor. The field is sent in its entirety, using the same bit order as in the original packet header.

If this action is performed on a variable-length field, the size of the residue value (using the units defined in FL) MUST be sent as described in Section 7.4.2.

This action is generally used with the "ignore" MO.

Mapping-Sent CDA

The mapping-sent action is used to send an index (the index into the TV list of values) instead of the original value. This action is used together with the "match-mapping" MO.

On the compressor side, the match-mapping MO searches the TV for a match with the header field value. The mapping-sent CDA then sends the corresponding index as the field residue. The most significant bit of the index is stored to the left (leftmost bit of the residue field).

On the decompressor side, the CDA uses the received index to restore the field value by looking up the list in the TV.

The number of bits sent is the minimal size for coding all the possible indices.

The first element in the list MUST be represented by index value 0, and successive elements in the list MUST have indices incremented by 1.

LSB CDA

The LSB action is used together with the "MSB(x)" MO to avoid sending the most significant part of the packet field if that part is already known by the receiving end.

The compressor sends the LSBs as the field residue value. The number of bits sent is the original header field length minus the length specified in the MSB(x) MO. The bits appear in the residue in the same bit order as in the original packet header.

The decompressor concatenates the x most significant bits of the TV and the received residue value.

If this action is performed on a variable-length field, the size of the residue value (using the units defined in FL) MUST be sent as described in Section 7.4.2.

DevIID, AppIID CDA

These actions are used to process the DevIID and AppIID of the IPv6 addresses, respectively. AppIID CDA is less common since most current LPWAN technologies frames contain a single L2 address, which is the Dev's address.

The DevIID value MAY be computed from the Dev ID present in the L2 header, or from some other stable identifier. The computation is specific to each Profile and MAY depend on the Dev ID size.

In the Downlink direction, at the compressor, the DevIID CDA may be used to generate the L2 addresses on the LPWAN, based on the packet's Destination Address.

Compute-*

Some fields can be elided at the compressor and recomputed locally at the decompressor.

Because the field is uniquely identified by its FID (e.g., IPv6 length), the relevant protocol specification unambiguously defines the algorithm for such computation.

An example of a field that knows how to recompute itself is IPv6 length.

Fragmentation/Reassembly

Overview

In LPWAN technologies, the L2 MTU typically ranges from tens to hundreds of bytes. Some of these technologies do not have an internal fragmentation/reassembly mechanism.

The optional SCHC F/R functionality enables such LPWAN technologies to comply with the IPv6 MTU requirement of 1280 bytes RFC8200. It is OPTIONAL to implement per this specification, but Profiles may specify that it is REQUIRED.

This specification includes several SCHC F/R modes, which allow for a range of reliability options such as optional SCHC Fragment retransmission. More modes may be defined in the future.

The same SCHC F/R mode MUST be used for all SCHC Fragments of a given SCHC Packet. This document does not specify which mode(s) must be implemented and used over a specific LPWAN technology. That information will be given in Profiles.

SCHC allows transmitting non-fragmented SCHC Packet concurrently with fragmented SCHC Packets. In addition, SCHC F/R provides protocol elements that allow transmitting several fragmented SCHC Packets concurrently, i.e., interleaving the transmission of fragments from different fragmented SCHC Packets. A Profile MAY restrict the latter behavior.

The L2 Word size (see Section 4) determines the encoding of some messages. SCHC F/R usually generates SCHC Fragments and SCHC ACKs that are multiples of L2 Words.

SCHC F/R Protocol Elements

This subsection describes the different elements that are used to enable the SCHC F/R functionality defined in this document. These elements include the SCHC F/R messages, tiles, windows, bitmaps, counters, timers, and header fields.

The elements are described here in a generic manner. Their application to each SCHC F/R mode is found in Section 8.4.

Messages

SCHC F/R defines the following messages:

SCHC Fragment: A message that carries part of a SCHC Packet from the

  sender to the receiver.

SCHC ACK: An acknowledgement for fragmentation, by the receiver to

  the sender.  This message is used to indicate whether or not the
  reception of pieces of, or the whole of, the fragmented SCHC
  Packet was successful.

SCHC ACK REQ: A request by the sender for a SCHC ACK from the

  receiver.

SCHC Sender-Abort: A message by the sender telling the receiver that

  it has aborted the transmission of a fragmented SCHC Packet.

SCHC Receiver-Abort: A message by the receiver to tell the sender to

  abort the transmission of a fragmented SCHC Packet.

The format of these messages is provided in Section 8.3.

Tiles, Windows, Bitmaps, Timers, Counters

Tiles

The SCHC Packet is fragmented into pieces, hereafter called "tiles". The tiles MUST be non-empty and pairwise disjoint. Their union MUST be equal to the SCHC Packet.

See Figure 10 for an example.

                               SCHC Packet
       +----+--+-----+---+----+-+---+-----+...-----+----+---+------+

       +----+--+-----+---+----+-+---+-----+...-----+----+---+------+

             Figure 10: SCHC Packet Fragmented in Tiles

Modes (see Section 8.4) MAY place additional constraints on tile sizes.

Each SCHC Fragment message carries at least one tile in its Payload, if the Payload field is present.

Windows

Some SCHC F/R modes may handle successive tiles in groups, called windows.

If windows are used:

all the windows of a SCHC Packet, except the last one, MUST

  contain the same number of tiles.  This number is WINDOW_SIZE.

WINDOW_SIZE MUST be specified in a Profile.

the windows are numbered.

their numbers MUST increment by 1 from 0 upward, from the start of

  the SCHC Packet to its end.

the last window MUST contain WINDOW_SIZE tiles or less.

tiles are numbered within each window.

the tile indices MUST decrement by 1 from WINDOW_SIZE - 1

  downward, looking from the start of the SCHC Packet toward its
  end.

therefore, each tile of a SCHC Packet is uniquely identified by a

  window number and a tile index within this window.

See Figure 11 for an example.

       +---------------------------------------------...-----------+
       |                       SCHC Packet                         |
       +---------------------------------------------...-----------+

Tile# | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3| Window# |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-|

 Figure 11: SCHC Packet Fragmented in Tiles Grouped in 29 Windows,
                        with WINDOW_SIZE = 5

Appendix E discusses the benefits of selecting one among multiple window sizes depending on the size of the SCHC Packet to be fragmented.

When windows are used:

Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to

  the sender in a SCHC ACK message.

A Bitmap corresponds to exactly one Window.

Bitmaps

Each bit in the Bitmap for a window corresponds to a tile in the window. Therefore, each Bitmap has WINDOW_SIZE bits. The bit at the leftmost position corresponds to the tile numbered WINDOW_SIZE - 1. Consecutive bits, going right, correspond to sequentially decreasing tile indices. In Bitmaps for windows that are not the last one of a SCHC Packet, the bit at the rightmost position corresponds to the tile numbered 0. In the Bitmap for the last window, the bit at the rightmost position corresponds either to the tile numbered 0 or to a tile that is sent/received as "the last one of the SCHC Packet" without explicitly stating its number (see Section 8.3.1.2).

At the receiver:

a bit set to 1 in the Bitmap indicates that a tile associated with

  that bit position has been correctly received for that window.

a bit set to 0 in the Bitmap indicates that there has been no tile

  correctly received, associated with that bit position, for that
  window.  Possible reasons include that the tile was not sent at
  all, not received, or received with errors.

Timers and Counters

Some SCHC F/R modes can use the following timers and counters:

Inactivity Timer: a SCHC Fragment receiver uses this timer to abort

  waiting for a SCHC F/R message.

Retransmission Timer: a SCHC Fragment sender uses this timer to

  abort waiting for an expected SCHC ACK.

Attempts: this counter counts the requests for SCHC ACKs, up to

  MAX_ACK_REQUESTS.

Integrity Checking

The integrity of the fragmentation-reassembly process of a SCHC Packet MUST be checked at the receive end. A Profile MUST specify how integrity checking is performed.

It is RECOMMENDED that integrity checking be performed by computing a Reassembly Check Sequence (RCS) based on the SCHC Packet at the sender side and transmitting it to the receiver for comparison with the RCS locally computed after reassembly.

The RCS supports UDP checksum elision by SCHC C/D (see Section 10.11).

The CRC32 polynomial 0xEDB88320 (i.e., the reversed polynomial representation, which is used in the Ethernet standard [ETHERNET]) is RECOMMENDED as the default algorithm for computing the RCS.

The RCS MUST be computed on the full SCHC Packet concatenated with the padding bits, if any, of the SCHC Fragment carrying the last tile. The rationale is that the SCHC reassembler has no way of knowing the boundary between the last tile and the padding bits. Indeed, this requires decompressing the SCHC Packet, which is out of the scope of the SCHC reassembler.

The concatenation of the complete SCHC Packet and any padding bits, if present, of the last SCHC Fragment does not generally constitute an integer number of bytes. CRC libraries are usually byte oriented. It is RECOMMENDED that the concatenation of the complete SCHC Packet and any last fragment padding bits be zero-extended to the next byte boundary and that the RCS be computed on that byte array.

Header Fields

The SCHC F/R messages contain the following fields (see the formats in Section 8.3):

RuleID: this field is present in all the SCHC F/R messages. The

  Rule identifies:

  *  that a SCHC F/R message is being carried, as opposed to an
     unfragmented SCHC Packet,

  *  which SCHC F/R mode is used,

  *  in case this mode uses windows, what the value of WINDOW_SIZE
     is, and

  *  what other optional fields are present and what the field sizes
     are.

  The Rule tells apart a non-fragmented SCHC Packet from SCHC
  Fragments.  It will also tell apart SCHC Fragments of fragmented
  SCHC Packets that use different SCHC F/R modes or different
  parameters.  Therefore, interleaved transmission of these is
  possible.

  All SCHC F/R messages pertaining to the same SCHC Packet MUST bear
  the same RuleID.

Datagram Tag (DTag): This field allows differentiating SCHC F/R

  messages belonging to different SCHC Packets that may be using the
  same RuleID simultaneously.  Hence, it allows interleaving
  fragments of a new SCHC Packet with fragments of a previous SCHC
  Packet under the same RuleID.

  The size of the DTag field (called "T", in bits) is defined by
  each Profile for each RuleID.  When T is 0, the DTag field does
  not appear in the SCHC F/R messages and the DTag value is defined
  as 0.

  When T is 0, there can be no more than one fragmented SCHC Packet
  in transit for each fragmentation RuleID.

  If T is not 0, DTag:

  *  MUST be set to the same value for all the SCHC F/R messages
     related to the same fragmented SCHC Packet, and

  *  MUST be set to different values for SCHC F/R messages related
     to different SCHC Packets that are being fragmented under the
     same RuleID and whose transmission may overlap.

W: The W field is optional. It is only present if windows are used.

  Its presence and size (called "M", in bits) is defined by each
  SCHC F/R mode and each Profile for each RuleID.

  This field carries information pertaining to the window a SCHC F/R
  message relates to.  If present, W MUST carry the same value for
  all the SCHC F/R messages related to the same window.  Depending
  on the mode and Profile, W may carry the full window number, or
  just the LSB or any other partial representation of the window
  number.

Fragment Compressed Number (FCN): The FCN field is present in the

  SCHC Fragment Header.  Its size (called "N", in bits) is defined
  by each Profile for each RuleID.

  This field conveys information about the progress in the sequence
  of tiles being transmitted by SCHC Fragment messages.  For
  example, it can contain a partial, efficient representation of a
  larger-sized tile index.  The description of the exact use of the
  FCN field is left to each SCHC F/R mode.  However, two values are
  reserved for special purposes.  They help control the SCHC F/R
  process:

  *  The FCN value with all the bits equal to 1 (called "All-1")
     signals that the very last tile of a SCHC Packet has been
     transmitted.  By extension, if windows are used, the last
     window of a packet is called the "All-1" window.

  *  If windows are used, the FCN value with all the bits equal to 0
     (called "All-0") signals the last tile of a window that is not
     the last one of the SCHC packet.  By extension, such a window
     is called an "All-0 window".

Reassembly Check Sequence (RCS): This field only appears in the

  All-1 SCHC Fragments.  Its size (called "U", in bits) is defined
  by each Profile for each RuleID.

  See Section 8.2.3 for the RCS default size, default polynomial and
  details on RCS computation.

C (integrity Check): C is a 1-bit field. This field is used in the

  SCHC ACK message to report on the reassembled SCHC Packet
  integrity check (see Section 8.2.3).

  A value of 1 tells that the integrity check was performed and is
  successful.  A value of 0 tells that the integrity check was not
  performed or that it was a failure.

Compressed Bitmap: The Compressed Bitmap is used together with

  windows and Bitmaps (see Section 8.2.2.3).  Its presence and size
  is defined for each SCHC F/R mode for each RuleID.

  This field appears in the SCHC ACK message to report on the
  receiver Bitmap (see Section 8.3.2.1).

SCHC F/R Message Formats

This section defines the SCHC Fragment formats, the SCHC ACK format, the SCHC ACK REQ format and the SCHC Abort formats.

SCHC Fragment Format

A SCHC Fragment conforms to the general format shown in Figure 12. It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The SCHC Fragment Payload carries one or several tile(s).

+-----------------+-----------------------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ | Fragment Header | Fragment Payload | padding (as needed) +-----------------+-----------------------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~

              Figure 12: SCHC Fragment General Format

Regular SCHC Fragment

The Regular SCHC Fragment format is shown in Figure 13. Regular SCHC Fragments are generally used to carry tiles that are not the last one of a SCHC Packet. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile.

|-- SCHC Fragment Header ----|

        |-- T --|-M-|-- N --|

+-- ... -+- ... -+---+- ... -+--------...-------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC) | RuleID | DTag | W | FCN | Fragment Payload | padding (as needed) +-- ... -+- ... -+---+- ... -+--------...-------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)

    Figure 13: Detailed Header Format for Regular SCHC Fragments

The FCN field MUST NOT contain all bits set to 1.

Profiles MUST ensure that a SCHC Fragment with FCN equal to 0 (called an "All-0 SCHC Fragment") is distinguishable by size, even in the presence of padding, from a SCHC ACK REQ message (see Section 8.3.3) with the same RuleID value and with the same T, M, and N values. This condition is met if the Payload is at least the size of an L2 Word. This condition is also met if the SCHC Fragment Header is a multiple of L2 Words.

All-1 SCHC Fragment

The All-1 SCHC Fragment format is shown in Figure 14. The sender uses the All-1 SCHC Fragment format for the message that completes the emission of a fragmented SCHC Packet. The DTag field, the W field, the RCS field and the Payload are OPTIONAL, their presence is specified by each mode and Profile. At least one of RCS field or Fragment Payload MUST be present. The FCN field is all ones.

|------- SCHC Fragment Header -------|

        |-- T --|-M-|-- N --|-- U --|

                      (FCN)

   Figure 14: Detailed Header Format for the All-1 SCHC Fragment

Profiles MUST ensure that an All-1 SCHC Fragment message is distinguishable by size, even in the presence of padding, from a SCHC Sender-Abort message (see Section 8.3.4) with the same RuleID value and with the same T, M, and N values. This condition is met if the RCS is present and is at least the size of an L2 Word or if the Payload is present and is at least the size an L2 Word. This condition is also met if the SCHC Sender-Abort Header is a multiple of L2 Words.

SCHC ACK Format

The SCHC ACK message is shown in Figure 15. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile. The Compressed Bitmap field MUST be present in SCHC F/R modes that use windows and MUST NOT be present in other modes.

|--- SCHC ACK Header ----|

        |-- T --|-M-| 1 |

+-- ... -+- ... -+---+---+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) | RuleID | DTag | W |C=1| padding as needed (success) +-- ... -+- ... -+---+---+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk)

             Figure 15: Format of the SCHC ACK Message

The SCHC ACK Header contains a C bit (see Section 8.2.4).

If the C bit is set to 1 (integrity check successful), no Bitmap is carried.

If the C bit is set to 0 (integrity check not performed or failed) and if windows are used, a Compressed Bitmap for the window referred to by the W field is transmitted as specified in Section 8.3.2.1.

Bitmap Compression

For transmission, the Compressed Bitmap in the SCHC ACK message is defined by the following algorithm (see Figure 16 for a follow-along example):

Build a temporary SCHC ACK message that contains the Header

  followed by the original Bitmap (see Section 8.2.2.3 for a
  description of Bitmaps).

Position scissors at the end of the Bitmap, after its last bit.

While the bit on the left of the scissors is 1 and belongs to the

  Bitmap, keep moving left, then stop.

Then, while the scissors are not on an L2 Word boundary of the

  SCHC ACK message and there is a Bitmap bit on the right of the
  scissors, keep moving right, then stop.

At this point, cut and drop off any bits to the right of the

  scissors.

When one or more bits have effectively been dropped off as a result of the above algorithm, the SCHC ACK message is a multiple of L2 Words; no padding bits will be appended.

Because the SCHC Fragment sender knows the size of the original Bitmap, it can reconstruct the original Bitmap from the Compressed Bitmap received in the SCHC ACK message.

Figure 16 shows an example where L2 Words are actually bytes and where the original Bitmap contains 17 bits, the last 15 of which are all set to 1.

|--- SCHC ACK Header ----|-------- Bitmap --------|

        |-- T --|-M-| 1 |

+-- ... -+- ... -+---+---+---------------------------------+ | RuleID | DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1| +-- ... -+- ... -+---+---+---------------------------------+

      next L2 Word boundary ->|

        Figure 16: SCHC ACK Header Plus Uncompressed Bitmap

Figure 17 shows that the last 14 bits are not sent.

|--- SCHC ACK Header ----|CpBmp|

        |-- T --|-M-| 1 |

+-- ... -+- ... -+---+---+-----+ | RuleID | DTag | W |C=0|1 0 1| +-- ... -+- ... -+---+---+-----+

      next L2 Word boundary ->|

    Figure 17: Resulting SCHC ACK Message with Compressed Bitmap

Figure 18 shows an example of a SCHC ACK with tile indices ranging from 6 down to 0, where the Bitmap indicates that the second and the fourth tile of the window have not been correctly received.

|--- SCHC ACK Header ----|--- Bitmap --|

        |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)

+--------+-------+---+---+-------------+ | RuleID | DTag | W |C=0|1 0 1 0 1 1 1| uncompressed Bitmap +--------+-------+---+---+-------------+

  next L2 Word boundary ->|<-- L2 Word --->|

+--------+-------+---+---+-------------+Admin (talk) 21:37, 5 May 2021 (UTC)+ | RuleID | DTag | W |C=0|1 0 1 0 1 1 1|pad.| transmitted SCHC ACK +--------+-------+---+---+-------------+Admin (talk) 21:37, 5 May 2021 (UTC)+

  next L2 Word boundary ->|<-- L2 Word --->|

      Figure 18: Example of a SCHC ACK Message, Missing Tiles

Figure 19 shows an example of a SCHC ACK with tile indices ranging from 6 down to 0, where integrity check has not been performed or has failed and the Bitmap indicates that there is no missing tile in that window.

|--- SCHC ACK Header ----|--- Bitmap --|

        |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)

+--------+-------+---+---+-------------+ | RuleID | DTag | W |C=0|1 1 1 1 1 1 1| with uncompressed Bitmap +--------+-------+---+---+-------------+

  next L2 Word boundary ->|

+-- ... -+- ... -+---+---+-+ | RuleID | DTag | W |C=0|1| transmitted SCHC ACK +-- ... -+- ... -+---+---+-+

  next L2 Word boundary ->|

     Figure 19: Example of a SCHC ACK Message, No Missing Tile

SCHC ACK REQ Format

The SCHC ACK REQ is used by a sender to request a SCHC ACK from the receiver. Its format is shown in Figure 20. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile. The FCN field is all zero.

|--- SCHC ACK REQ Header ----|

        |-- T --|-M-|-- N --|

+-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ | RuleID | DTag | W | 0..0 | padding (as needed) (no payload) +-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~

                   Figure 20: SCHC ACK REQ Format

SCHC Sender-Abort Format

When a SCHC Fragment sender needs to abort an ongoing fragmented SCHC Packet transmission, it sends a SCHC Sender-Abort message to the SCHC Fragment receiver.

The SCHC Sender-Abort format is shown in Figure 21. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile. The FCN field is all ones.

|--- Sender-Abort Header ----|

        |-- T --|-M-|-- N --|

+-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ | RuleID | DTag | W | 11..1 | padding (as needed) +-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~

                Figure 21: SCHC Sender-Abort Format

If the W field is present:

the fragment sender MUST set it to all ones. Other values are

  RESERVED.

the fragment receiver MUST check its value. If the value is

  different from all ones, the message MUST be ignored.

The SCHC Sender-Abort MUST NOT be acknowledged.

SCHC Receiver-Abort Format

When a SCHC Fragment receiver needs to abort an ongoing fragmented SCHC Packet transmission, it transmits a SCHC Receiver-Abort message to the SCHC Fragment sender.

The SCHC Receiver-Abort format is shown in Figure 22. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile.

|-- Receiver-Abort Header ---|

          |--- T ---|-M-| 1 |

+--- ... --+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+ | RuleID | DTag | W |C=1| 1..1| 1..1 | +--- ... --+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+

          next L2 Word boundary ->|<-- L2 Word -->|

               Figure 22: SCHC Receiver-Abort Format

If the W field is present:

the fragment receiver MUST set it to all ones. Other values are

  RESERVED.

if the value is different from all ones, the fragment sender MUST

  ignore the message.

The SCHC Receiver-Abort has the same header as a SCHC ACK message. The bits that follow the SCHC Receiver-Abort Header MUST be as follows:

if the Header does not end at an L2 Word boundary, append bits set

  to 1 as needed to reach the next L2 Word boundary.

append exactly one more L2 Word with bits all set to ones.

Such a bit pattern never occurs in a legitimate SCHC ACK. This is how the fragment sender recognizes a SCHC Receiver-Abort.

The SCHC Receiver-Abort MUST NOT be acknowledged.

SCHC F/R Modes

This specification includes several SCHC F/R modes that:

allow for a range of reliability options, such as optional SCHC

  Fragment retransmission.

support various LPWAN characteristics, such as links with variable

  MTU or unidirectional links.

More modes may be defined in the future.

Appendix B provides examples of fragmentation sessions based on the modes described hereafter.

Appendix C provides examples of Finite State Machines implementing the SCHC F/R modes described hereafter.

No-ACK Mode

The No-ACK mode has been designed under the assumption that data unit out-of-sequence delivery does not occur between the entity performing fragmentation and the entity performing reassembly. This mode supports L2 technologies that have a variable MTU.

In No-ACK mode, there is no communication from the fragment receiver to the fragment sender. The sender transmits all the SCHC Fragments without expecting any acknowledgement. Therefore, No-ACK does not require bidirectional links: unidirectional links are just fine.

In No-ACK mode, only the All-1 SCHC Fragment is padded as needed. The other SCHC Fragments are intrinsically aligned to L2 Words.

The tile sizes are not required to be uniform. Windows are not used. The Retransmission Timer is not used. The Attempts counter is not used.

Each Profile MUST specify which RuleID value(s) corresponds to SCHC F/R messages operating in this mode.

The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort MAY be sent. SCHC Receiver-Abort MUST NOT be sent.

The value of N (size of the FCN field) is RECOMMENDED to be 1.

Each Profile, for each RuleID value, MUST define:

the size of the DTag field,

the size and algorithm for the RCS field, and

the expiration time of the Inactivity Timer.

Each Profile, for each RuleID value, MAY define

a value of N different from the recommended one, and

the meaning of values sent in the FCN field, for values different

  from the All-1 value.

For each active pair of RuleID and DTag values, the receiver MUST maintain an Inactivity Timer. If the receiver is under-resourced to do this, it MUST silently drop the related messages.

Sender Behavior

At the beginning of the fragmentation of a new SCHC Packet, the fragment sender MUST select a RuleID and DTag value pair for this SCHC Packet.

Each SCHC Fragment MUST contain exactly one tile in its Payload. The tile MUST be at least the size of an L2 Word. The sender MUST transmit the SCHC Fragments messages in the order that the tiles appear in the SCHC Packet. Except for the last tile of a SCHC Packet, each tile MUST be of a size that complements the SCHC Fragment Header so that the SCHC Fragment is a multiple of L2 Words without the need for padding bits. Except for the last one, the SCHC Fragments MUST use the Regular SCHC Fragment format specified in Section 8.3.1.1. The SCHC Fragment that carries the last tile MUST be an All-1 SCHC Fragment, described in Section 8.3.1.2.

The sender MAY transmit a SCHC Sender-Abort.

Figure 39 shows an example of a corresponding state machine.

Receiver Behavior

Upon receiving each Regular SCHC Fragment:

the receiver MUST reset the Inactivity Timer.

the receiver assembles the payloads of the SCHC Fragments.

On receiving an All-1 SCHC Fragment:

the receiver MUST append the All-1 SCHC Fragment Payload and the

  padding bits to the previously received SCHC Fragment Payloads for
  this SCHC Packet.

the receiver MUST perform the integrity check.

if integrity checking fails, the receiver MUST drop the

  reassembled SCHC Packet.

the reassembly operation concludes.

On expiration of the Inactivity Timer, the receiver MUST drop the SCHC Packet being reassembled.

On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC Packet being reassembled.

Figure 40 shows an example of a corresponding state machine.

ACK-Always Mode

The ACK-Always mode has been designed under the following assumptions:

Data unit out-of-sequence delivery does not occur between the

  entity performing fragmentation and the entity performing
  reassembly,

The L2 MTU value does not change while the fragments of a SCHC

  Packet are being transmitted, and

There is a feedback path from the reassembler to the fragmenter.

  See Appendix F for a discussion on using ACK-Always mode on quasi-
  bidirectional links.

In ACK-Always mode, windows are used. An acknowledgement, positive or negative, is transmitted by the fragment receiver to the fragment sender at the end of the transmission of each window of SCHC Fragments.

The tiles are not required to be of uniform size. In ACK-Always mode, only the All-1 SCHC Fragment is padded as needed. The other SCHC Fragments are intrinsically aligned to L2 Words.

Briefly, the algorithm is as follows: after a first blind transmission of all the tiles of a window, the fragment sender iterates retransmitting the tiles that are reported missing until the fragment receiver reports that all the tiles belonging to the window have been correctly received or until too many attempts were made. The fragment sender only advances to the next window of tiles when it has ascertained that all the tiles belonging to the current window have been fully and correctly received. This results in a per-window lock-step behavior between the sender and the receiver.

Each Profile MUST specify which RuleID value(s) correspond to SCHC F/ R messages operating in this mode.

The W field MUST be present and its size M MUST be 1 bit.

Each Profile, for each RuleID value, MUST define:

the value of N,

the value of WINDOW_SIZE, which MUST be strictly less than 2^N,

the size and algorithm for the RCS field,

the value of T,

the value of MAX_ACK_REQUESTS,

the expiration time of the Retransmission Timer, and

the expiration time of the Inactivity Timer.

For each active pair of RuleID and DTag values, the sender MUST maintain:

one Attempts counter

one Retransmission Timer

For each active pair of RuleID and DTag values, the receiver MUST maintain

one Inactivity Timer, and

one Attempts counter.

Sender Behavior

At the beginning of the fragmentation of a new SCHC Packet, the fragment sender MUST select a RuleID and DTag value pair for this SCHC Packet.

Each SCHC Fragment MUST contain exactly one tile in its Payload. All tiles with the index 0, as well as the last tile, MUST be at least the size of an L2 Word.

In all SCHC Fragment messages, the W field MUST be filled with the LSB of the window number that the sender is currently processing.

For a SCHC Fragment that carries a tile other than the last one of the SCHC Packet:

the Fragment MUST be of the Regular type specified in

  Section 8.3.1.1.

the FCN field MUST contain the tile index.

each tile MUST be of a size that complements the SCHC Fragment

  Header so that the SCHC Fragment is a multiple of L2 Words without
  the need for padding bits.

The SCHC Fragment that carries the last tile MUST be an All-1 SCHC Fragment, described in Section 8.3.1.2.

The fragment sender MUST start by transmitting the window numbered 0.

All message receptions being discussed in the rest of this section are to be understood as "matching the RuleID and DTag pair being processed", even if not spelled out, for brevity.

The sender starts by a "blind transmission" phase, in which it MUST transmit all the tiles composing the window, in decreasing tile index order.

Then, it enters a "retransmission phase" in which it MUST initialize an Attempts counter to 0, it MUST start a Retransmission Timer and it MUST await a SCHC ACK.

Then, upon receiving a SCHC ACK:

  -  if the SCHC ACK indicates that some tiles are missing at the
     receiver, then the sender MUST transmit all the tiles that have
     been reported missing, it MUST increment Attempts, it MUST
     reset the Retransmission Timer, and MUST await the next SCHC
     ACK.

  -  if the current window is not the last one and the SCHC ACK
     indicates that all tiles were correctly received, the sender
     MUST stop the Retransmission Timer, it MUST advance to the next
     fragmentation window, and it MUST start a blind transmission
     phase as described above.

  -  if the current window is the last one and the SCHC ACK
     indicates that more tiles were received than the sender sent,
     the fragment sender MUST send a SCHC Sender-Abort, and it MAY
     exit with an error condition.

  -  if the current window is the last one and the SCHC ACK
     indicates that all tiles were correctly received, yet the
     integrity check was a failure, the fragment sender MUST send a
     SCHC Sender-Abort, and it MAY exit with an error condition.

  -  if the current window is the last one and the SCHC ACK
     indicates that integrity checking was successful, the sender
     exits successfully.

on Retransmission Timer expiration:

  -  if Attempts is strictly less that MAX_ACK_REQUESTS, the
     fragment sender MUST send a SCHC ACK REQ and MUST increment the
     Attempts counter.

  -  otherwise, the fragment sender MUST send a SCHC Sender-Abort,
     and it MAY exit with an error condition.

At any time:

on receiving a SCHC Receiver-Abort, the fragment sender MAY exit

  with an error condition.

on receiving a SCHC ACK that bears a W value different from the W

  value that it currently uses, the fragment sender MUST silently
  discard and ignore that SCHC ACK.

Figure 41 shows an example of a corresponding state machine.

Receiver Behavior

On receiving a SCHC Fragment with a RuleID and DTag pair not being processed at that time:

the receiver SHOULD check if the DTag value has not recently been

  used for that RuleID value, thereby ensuring that the received
  SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
  transmission.  The initial value of the Inactivity Timer is the
  RECOMMENDED lifetime for the DTag value at the receiver.  If the
  SCHC Fragment is determined to be such a remnant, the receiver MAY
  silently ignore it and discard it.

the receiver MUST start a process to assemble a new SCHC Packet

  with that RuleID and DTag value pair.

the receiver MUST start an Inactivity Timer for that RuleID and

  DTag pair.  It MUST initialize an Attempts counter to 0 for that
  RuleID and DTag pair.  It MUST initialize a window counter to 0.
  If the receiver is under-resourced to do this, it MUST respond to
  the sender with a SCHC Receiver-Abort.

In the rest of this section, "local W bit" means the least significant bit of the window counter of the receiver.

On reception of any SCHC F/R message for the RuleID and DTag pair being processed, the receiver MUST reset the Inactivity Timer pertaining to that RuleID and DTag pair.

All message receptions being discussed in the rest of this section are to be understood as "matching the RuleID and DTag pair being processed", even if not spelled out, for brevity.

The receiver MUST first initialize an empty Bitmap for the first window then enter an "acceptance phase", in which:

on receiving a SCHC Fragment or a SCHC ACK REQ, either one having

  the W bit different from the local W bit, the receiver MUST
  silently ignore and discard that message.

on receiving a SCHC ACK REQ with the W bit equal to the local W

  bit, the receiver MUST send a SCHC ACK for this window.

on receiving a SCHC Fragment with the W bit equal to the local W

  bit, the receiver MUST assemble the received tile based on the
  window counter and on the FCN field in the SCHC Fragment, and it
  MUST update the Bitmap.

  -  if the SCHC Fragment received is an All-0 SCHC Fragment, the
     current window is determined to be a not-last window, the
     receiver MUST send a SCHC ACK for this window and it MUST enter
     the "retransmission phase" for this window.

  -  if the SCHC Fragment received is an All-1 SCHC Fragment, the
     current window is determined to be the last window, the padding
     bits of the All-1 SCHC Fragment MUST be assembled after the
     received tile, the receiver MUST perform the integrity check
     and it MUST send a SCHC ACK for this window.  Then:

     o  If the integrity check indicates that the full SCHC Packet
        has been correctly reassembled, the receiver MUST enter the
        "clean-up phase" for this window.

     o  If the integrity check indicates that the full SCHC Packet
        has not been correctly reassembled, the receiver enters the
        "retransmission phase" for this window.

In the "retransmission phase":

if the window is a not-last window:

  -  on receiving a SCHC Fragment that is not All-0 or All-1 and
     that has a W bit different from the local W bit, the receiver
     MUST increment its window counter and allocate a fresh Bitmap,
     it MUST assemble the tile received and update the Bitmap, and
     it MUST enter the "acceptance phase" for that new window.

  -  on receiving a SCHC ACK REQ with a W bit different from the
     local W bit, the receiver MUST increment its window counter and
     allocate a fresh Bitmap, it MUST send a SCHC ACK for that new
     window, and it MUST enter the "acceptance phase" for that new
     window.

  -  on receiving a SCHC All-0 Fragment with a W bit different from
     the local W bit, the receiver MUST increment its window counter
     and allocate a fresh Bitmap, it MUST assemble the tile received
     and update the Bitmap, it MUST send a SCHC ACK for that new
     window, and it MUST stay in the "retransmission phase" for that
     new window.

  -  on receiving a SCHC All-1 Fragment with a W bit different from
     the local W bit, the receiver MUST increment its window counter
     and allocate a fresh Bitmap; it MUST assemble the tile
     received, including the padding bits; it MUST update the Bitmap
     and perform the integrity check; it MUST send a SCHC ACK for
     the new window, which is determined to be the last window.
     Then:

     o  If the integrity check indicates that the full SCHC Packet
        has been correctly reassembled, the receiver MUST enter the
        "clean-up phase" for that new window.

     o  If the integrity check indicates that the full SCHC Packet
        has not been correctly reassembled, the receiver enters the
        "retransmission phase" for that new window.

  -  on receiving a SCHC Fragment with a W bit equal to the local W
     bit:

     o  if the SCHC Fragment received is an All-1 SCHC Fragment, the
        receiver MUST silently ignore it and discard it.

     o  otherwise, the receiver MUST assemble the tile received and
        update the Bitmap.  If the Bitmap becomes fully populated
        with 1's or if the SCHC Fragment is an All-0, the receiver
        MUST send a SCHC ACK for this window.

  -  on receiving a SCHC ACK REQ with the W bit equal to the local W
     bit, the receiver MUST send a SCHC ACK for this window.

if the window is the last window:

  -  on receiving a SCHC Fragment or a SCHC ACK REQ, either one
     having a W bit different from the local W bit, the receiver
     MUST silently ignore and discard that message.

  -  on receiving a SCHC ACK REQ with the W bit equal to the local W
     bit, the receiver MUST send a SCHC ACK for this window.

  -  on receiving a SCHC Fragment with a W bit equal to the local W
     bit:

     o  if the SCHC Fragment received is an All-0 SCHC Fragment, the
        receiver MUST silently ignore it and discard it.

     o  otherwise, the receiver MUST update the Bitmap, and it MUST
        assemble the tile received.  If the SCHC Fragment received
        is an All-1 SCHC Fragment, the receiver MUST assemble the
        padding bits of the All-1 SCHC Fragment after the received
        tile, it MUST perform the integrity check and:

        +  if the integrity check indicates that the full SCHC
           Packet has been correctly reassembled, the receiver MUST
           send a SCHC ACK and it enters the "clean-up phase".

        +  if the integrity check indicates that the full SCHC
           Packet has not been correctly reassembled:

           *  if the SCHC Fragment received was an All-1 SCHC
              Fragment, the receiver MUST send a SCHC ACK for this
              window.

In the "clean-up phase":

On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, either one

  having the W bit equal to the local W bit, the receiver MUST send
  a SCHC ACK.

Any other SCHC Fragment received MUST be silently ignored and

  discarded.

At any time, on sending a SCHC ACK, the receiver MUST increment the Attempts counter.

At any time, on incrementing its window counter, the receiver MUST reset the Attempts counter.

At any time, on expiration of the Inactivity Timer, on receiving a SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the receiver MUST send a SCHC Receiver-Abort, and it MAY exit the receive process for that SCHC Packet.

Figure 42 shows an example of a corresponding state machine.

ACK-on-Error Mode

The ACK-on-Error mode supports L2 technologies that have variable MTU and out-of-order delivery. It requires an L2 that provides a feedback path from the reassembler to the fragmenter. See Appendix F for a discussion on using ACK-on-Error mode on quasi-bidirectional links.

In ACK-on-Error mode, windows are used.

All tiles except the last one and the penultimate one MUST be of equal size, hereafter called "regular". The size of the last tile MUST be smaller than or equal to the regular tile size. Regarding the penultimate tile, a Profile MUST pick one of the following two options:

The penultimate tile size MUST be the regular tile size, or

the penultimate tile size MUST be either the regular tile size or

  the regular tile size minus one L2 Word.

A SCHC Fragment message carries one or several contiguous tiles, which may span multiple windows. A SCHC ACK reports on the reception of exactly one window of tiles.

See Figure 23 for an example.

       +---------------------------------------------...-----------+
       |                       SCHC Packet                         |
       +---------------------------------------------...-----------+

Tile# | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3| Window# |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-|

SCHC Fragment msg |-----------|

   Figure 23: SCHC Packet Fragmented in Tiles, ACK-on-Error Mode

The W field is wide enough that it unambiguously represents an absolute window number. The fragment receiver sends SCHC ACKs to the fragment sender about windows for which tiles are missing. No SCHC ACK is sent by the fragment receiver for windows that it knows have been fully received.

The fragment sender retransmits SCHC Fragments for tiles that are reported missing. It can advance to next windows even before it has ascertained that all tiles belonging to previous windows have been correctly received, and it can still later retransmit SCHC Fragments with tiles belonging to previous windows. Therefore, the sender and the receiver may operate in a decoupled fashion. The fragmented SCHC Packet transmission concludes when:

integrity checking shows that the fragmented SCHC Packet has been

  correctly reassembled at the receive end, and this information has
  been conveyed back to the sender, or

too many retransmission attempts were made, or

the receiver determines that the transmission of this fragmented

  SCHC Packet has been inactive for too long.

Each Profile MUST specify which RuleID value(s) corresponds to SCHC F/R messages operating in this mode.

The W field MUST be present in the SCHC F/R messages.

Each Profile, for each RuleID value, MUST define:

the tile size (a tile does not need to be multiple of an L2 Word,

  but it MUST be at least the size of an L2 Word),

the value of M,

the value of N,

the value of WINDOW_SIZE, which MUST be strictly less than 2^N,

the size and algorithm for the RCS field,

the value of T,

the value of MAX_ACK_REQUESTS,

the expiration time of the Retransmission Timer,

the expiration time of the Inactivity Timer,

if the last tile is carried in a Regular SCHC Fragment or an All-1

  SCHC Fragment (see Section 8.4.3.1), and

if the penultimate tile MAY be one L2 Word smaller than the

  regular tile size.  In this case, the regular tile size MUST be at
  least twice the L2 Word size.

For each active pair of RuleID and DTag values, the sender MUST maintain:

one Attempts counter, and

one Retransmission Timer.

For each active pair of RuleID and DTag values, the receiver MUST maintain:

one Inactivity Timer, and

one Attempts counter.

Sender Behavior

At the beginning of the fragmentation of a new SCHC Packet:

the fragment sender MUST select a RuleID and DTag value pair for

  this SCHC Packet.  A Rule MUST NOT be selected if the values of M
  and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot
  be fragmented in (2^M) * WINDOW_SIZE tiles or less.

the fragment sender MUST initialize the Attempts counter to 0 for

  that RuleID and DTag value pair.

A Regular SCHC Fragment message carries in its payload one or more tiles. If more than one tile is carried in one Regular SCHC Fragment:

the selected tiles MUST be contiguous in the original SCHC Packet,

and

they MUST be placed in the SCHC Fragment Payload adjacent to one

  another, in the order they appear in the SCHC Packet, from the
  start of the SCHC Packet toward its end.

Tiles that are not the last one MUST be sent in Regular SCHC Fragments specified in Section 8.3.1.1. The FCN field MUST contain the tile index of the first tile sent in that SCHC Fragment.

In a Regular SCHC Fragment message, the sender MUST fill the W field with the window number of the first tile sent in that SCHC Fragment.

A Profile MUST define if the last tile of a SCHC Packet is sent:

in a Regular SCHC Fragment, alone or as part of a multi-tiles

  Payload,

alone in an All-1 SCHC Fragment, or

with any of the above two methods.

In an All-1 SCHC Fragment message, the sender MUST fill the W field with the window number of the last tile of the SCHC Packet.

The fragment sender MUST send SCHC Fragments such that, all together, they contain all the tiles of the fragmented SCHC Packet.

The fragment sender MUST send at least one All-1 SCHC Fragment.

In doing the two items above, the sender MUST ascertain that the receiver will not receive the last tile through both a Regular SCHC Fragment and an All-1 SCHC Fragment.

The fragment sender MUST listen for SCHC ACK messages after having sent:

an All-1 SCHC Fragment, or

a SCHC ACK REQ.

A Profile MAY specify other times at which the fragment sender MUST listen for SCHC ACK messages. For example, this could be after sending a complete window of tiles.

Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC ACK REQ:

it MUST increment the Attempts counter, and

it MUST reset the Retransmission Timer.

On Retransmission Timer expiration:

if the Attempts counter is strictly less than MAX_ACK_REQUESTS,

  the fragment sender MUST send either the All-1 SCHC Fragment or a
  SCHC ACK REQ with the W field corresponding to the last window,

otherwise, the fragment sender MUST send a SCHC Sender-Abort, and

  it MAY exit with an error condition.

All message receptions being discussed in the rest of this section are to be understood as "matching the RuleID and DTag pair being processed", even if not spelled out, for brevity.

On receiving a SCHC ACK:

if the W field in the SCHC ACK corresponds to the last window of

  the SCHC Packet:

  -  if the C bit is set, the sender MAY exit successfully.

  -  otherwise:

     o  if the Profile mandates that the last tile be sent in an
        All-1 SCHC Fragment:

        +  if the SCHC ACK shows no missing tile at the receiver,
           the sender:

           *  MUST send a SCHC Sender-Abort, and

           *  MAY exit with an error condition.

        +  otherwise:

           *  the fragment sender MUST send SCHC Fragment messages
              containing all the tiles that are reported missing in
              the SCHC ACK.

           *  if the last of these SCHC Fragment messages is not an
              All-1 SCHC Fragment, then the fragment sender MUST in
              addition send after it a SCHC ACK REQ with the W field
              corresponding to the last window.

           *  in doing the two items above, the sender MUST
              ascertain that the receiver will not receive the last
              tile through both a Regular SCHC Fragment and an All-1
              SCHC Fragment.

     o  otherwise:

        +  if the SCHC ACK shows no missing tile at the receiver,
           the sender MUST send the All-1 SCHC Fragment

        +  otherwise:

           *  the fragment sender MUST send SCHC Fragment messages
              containing all the tiles that are reported missing in
              the SCHC ACK.

           *  the fragment sender MUST then send either the All-1
              SCHC Fragment or a SCHC ACK REQ with the W field
              corresponding to the last window.

otherwise, the fragment sender:

  -  MUST send SCHC Fragment messages containing the tiles that are
     reported missing in the SCHC ACK.

  -  then, it MAY send a SCHC ACK REQ with the W field corresponding
     to the last window.

See Figure 43 for one among several possible examples of a Finite State Machine implementing a sender behavior obeying this specification.

Receiver Behavior

On receiving a SCHC Fragment with a RuleID and DTag pair not being processed at that time:

the receiver SHOULD check if the DTag value has not recently been

  used for that RuleID value, thereby ensuring that the received
  SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
  transmission.  The initial value of the Inactivity Timer is the
  RECOMMENDED lifetime for the DTag value at the receiver.  If the
  SCHC Fragment is determined to be such a remnant, the receiver MAY
  silently ignore it and discard it.

the receiver MUST start a process to assemble a new SCHC Packet

  with that RuleID and DTag value pair.  The receiver MUST start an
  Inactivity Timer for that RuleID and DTag value pair.  It MUST
  initialize an Attempts counter to 0 for that RuleID and DTag value
  pair.  If the receiver is under-resourced to do this, it MUST
  respond to the sender with a SCHC Receiver-Abort.

On reception of any SCHC F/R message for the RuleID and DTag pair being processed, the receiver MUST reset the Inactivity Timer pertaining to that RuleID and DTag pair.

All message receptions being discussed in the rest of this section are to be understood as "matching the RuleID and DTag pair being processed", even if not spelled out, for brevity.

On receiving a SCHC Fragment message, the receiver determines what tiles were received, based on the payload length and on the W and FCN fields of the SCHC Fragment.

if the FCN is All-1, if a Payload is present, the full SCHC

  Fragment Payload MUST be assembled including the padding bits.
  This is because the size of the last tile is not known by the
  receiver; therefore, padding bits are indistinguishable from the
  tile data bits, at this stage.  They will be removed by the SCHC
  C/D sublayer.  If the size of the SCHC Fragment Payload exceeds or
  equals the size of one regular tile plus the size of an L2 Word,
  this SHOULD raise an error flag.

otherwise, tiles MUST be assembled based on the a priori known

  tile size.

  -  If allowed by the Profile, the end of the payload MAY contain
     the last tile, which may be shorter.  Padding bits are
     indistinguishable from the tile data bits, at this stage.

  -  The payload may contain the penultimate tile that, if allowed
     by the Profile, MAY be exactly one L2 Word shorter than the
     regular tile size.

  -  Otherwise, padding bits MUST be discarded.  This is possible
     because:

     o  the size of the tiles is known a priori,

     o  tiles are larger than an L2 Word, and

     o  padding bits are always strictly less than an L2 Word.

On receiving a SCHC ACK REQ or an All-1 SCHC Fragment:

if the receiver knows of any windows with missing tiles for the

  packet being reassembled, it MUST return a SCHC ACK for the
  lowest-numbered such window:

otherwise:

  -  if it has received at least one tile, it MUST return a SCHC ACK
     for the highest-numbered window it currently has tiles for,

  -  otherwise, it MUST return a SCHC ACK for window numbered 0.

A Profile MAY specify other times and circumstances at which a receiver sends a SCHC ACK, and which window the SCHC ACK reports about in these circumstances.

Upon sending a SCHC ACK, the receiver MUST increase the Attempts counter.

After receiving an All-1 SCHC Fragment, a receiver MUST check the integrity of the reassembled SCHC Packet at least every time it prepares for sending a SCHC ACK for the last window.

Upon receiving a SCHC Sender-Abort, the receiver MAY exit with an error condition.

Upon expiration of the Inactivity Timer, the receiver MUST send a SCHC Receiver-Abort, and it MAY exit with an error condition.

On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST send a SCHC Receiver-Abort, and it MAY exit with an error condition.

Reassembly of the SCHC Packet concludes when:

a Sender-Abort has been received, or

the Inactivity Timer has expired, or

the Attempts counter has exceeded MAX_ACK_REQUESTS, or

at least an All-1 SCHC Fragment has been received and integrity

  checking of the reassembled SCHC Packet is successful.

See Figure 44 for one among several possible examples of a Finite State Machine implementing a receiver behavior obeying this specification. The example provided is meant to match the sender Finite State Machine of Figure 43.

Padding Management

SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does not have any alignment prerequisite. The size of SCHC Packets can be any number of bits.

If the L2 constrains the payload to align to coarser boundaries (for example, bytes), the SCHC messages MUST be padded. When padding occurs, the number of appended bits MUST be strictly less than the L2 Word size.

If a SCHC Packet is sent unfragmented (see Figure 24), it is padded as needed for transmission.

If a SCHC Packet needs to be fragmented for transmission, it is not padded in itself. Only the SCHC F/R messages are padded as needed for transmission. Some SCHC F/R messages are intrinsically aligned to L2 Words.

A packet (e.g., an IPv6 packet)

        |                                           ^ (padding bits
        v                                           |       dropped)

+------------------+ +--------------------+ | SCHC Compression | | SCHC Decompression | +------------------+ +--------------------+

        |                                           ^
        |   If no fragmentation,                    |
        +---- SCHC Packet + padding as needed ----->|
        |                                           | (integrity
        v                                           |  checked)

+--------------------+ +-----------------+ | SCHC Fragmentation | | SCHC Reassembly | +--------------------+ +-----------------+

    |       ^                                   |       ^
    |       |                                   |       |
    |       +--- SCHC ACK + padding as needed --+       |
    |                                                   |
    +------- SCHC Fragments + padding as needed---------+

       Sender                                    Receiver

      Figure 24: SCHC Operations, Including Padding as Needed

Each Profile MUST specify the size of the L2 Word. The L2 Word might actually be a single bit, in which case no padding will take place at all.

A Profile MUST define the value of the padding bits if the L2 Word is wider than a single bit. The RECOMMENDED value is 0.

10. SCHC Compression for IPv6 and UDP Headers

This section lists the IPv6 and UDP header fields and describes how they can be compressed. An example of a set of Rules for UDP/IPv6 header compression is provided in Appendix A.

10.1. IPv6 Version Field

The IPv6 version field is labeled by the protocol parser as being the "version" field of the IPv6 protocol. Therefore, it only exists for IPv6 packets. In the Rule, TV is set to 6, MO to "ignore" and CDA to "not-sent".

10.2. IPv6 Traffic Class Field

If the Diffserv field does not vary and is known by both sides, the Field Descriptor in the Rule SHOULD contain a TV with this well-known value, an "equal" MO, and a "not-sent" CDA.

Otherwise (e.g., ECN bits are to be transmitted), two possibilities can be considered depending on the variability of the value:

One possibility is to not compress the field and send the original

  value.  In the Rule, TV is not set to any particular value, MO is
  set to "ignore", and CDA is set to "value-sent".

If some upper bits in the field are constant and known, a better

  option is to only send the LSBs.  In the Rule, TV is set to a
  value with the stable known upper part, MO is set to MSB(x), and
  CDA to LSB.

  ECN functionality depends on both bits of the ECN field, which are
  the 2 LSBs of this field; hence, sending only a single LSB of this
  field is NOT RECOMMENDED.

10.3. Flow Label Field

If the flow label is not set, i.e., its value is zero, the Field Descriptor in the Rule SHOULD contain a TV set to zero, an "equal" MO, and a "not-sent" CDA.

If the flow label is set to a pseudorandom value according to RFC6437, in the Rule, TV is not set to any particular value, MO is set to "ignore", and CDA is set to "value-sent".

If the flow label is set according to some prior agreement, i.e., by a flow state establishment method as allowed by RFC6437, the Field Descriptor in the Rule SHOULD contain a TV with this agreed-upon value, an "equal" MO, and a "not-sent" CDA.

10.4. Payload Length Field

This field can be elided for the transmission on the LPWAN. The SCHC C/D recomputes the original payload length value. In the Field Descriptor, TV is not set, MO is set to "ignore", and CDA is "compute-*".

10.5. Next Header Field

If the Next Header field does not vary and is known by both sides, the Field Descriptor in the Rule SHOULD contain a TV with this Next Header value, the MO SHOULD be "equal", and the CDA SHOULD be "not- sent".

Otherwise, TV is not set in the Field Descriptor, MO is set to "ignore", and CDA is set to "value-sent". Alternatively, a matching- list MAY also be used.

10.6. Hop Limit Field

The field behavior for this field is different for Uplink and Downlink. In Uplink, since there is no IP forwarding between the Dev and the SCHC C/D, the value is relatively constant. On the other hand, the Downlink value depends on Internet routing and can change more frequently. The DI can be used to distinguish both directions:

in an Up Field Descriptor, elide the field: the TV is set to the

  known constant value, the MO is set to "equal" and the CDA is set
  to "not-sent".

in a Dw Field Descriptor, the Hop Limit is elided for transmission

  and forced to 1 at the receiver, by setting TV to 1, MO to
  "ignore" and CDA to "not-sent".  This prevents any further
  forwarding.

10.7. IPv6 Addresses Fields

As in 6LoWPAN RFC4944, IPv6 addresses are split into two 64-bit- long fields; one for the prefix and one for the Interface Identifier (IID). These fields SHOULD be compressed. To allow for a single Rule being used for both directions, these values are identified by their role (Dev or App) and not by their position in the header (source or destination).

10.7.1. IPv6 Source and Destination Prefixes

Both ends MUST be configured with the appropriate prefixes. For a specific flow, the source and destination prefixes can be unique and stored in the Context. In that case, the TV for the source and destination prefixes contain the values, the MO is set to "equal" and the CDA is set to "not-sent".

If the Rule is intended to compress packets with different prefix values, match-mapping SHOULD be used. The different prefixes are listed in the TV, the MO is set to "match-mapping" and the CDA is set to "mapping-sent". See Figure 26.

Otherwise, the TV is not set, the MO is set to "ignore", and the CDA is set to "value-sent".

10.7.2. IPv6 Source and Destination IID

If the Dev or App IID are based on an L2 address, then the IID can be reconstructed with information coming from the L2 header. In that case, the TV is not set, the MO is set to "ignore" and the CDA is set to "DevIID" or "AppIID". On LPWAN technologies where the frames carry a single identifier (corresponding to the Dev), AppIID cannot be used.

As described in RFC8065, it may be undesirable to build the Dev IPv6 IID out of the Dev address. Another static value is used instead. In that case, the TV contains the static value, the MO operator is set to "equal" and the CDA is set to "not-sent".

If several IIDs are possible, then the TV contains the list of possible IIDs, the MO is set to "match-mapping" and the CDA is set to "mapping-sent".

It may also happen that the IID variability only expresses itself on a few bytes. In that case, the TV is set to the stable part of the IID, the MO is set to "MSB" and the CDA is set to "LSB".

Finally, the IID can be sent in its entirety on the L2. In that case, the TV is not set, the MO is set to "ignore", and the CDA is set to "value-sent".

10.8. IPv6 Extension Headers

This document does not provide recommendations on how to compress IPv6 extension headers.

10.9. UDP Source and Destination Ports

To allow for a single Rule being used for both directions, the UDP port values are identified by their role (Dev or App) and not by their position in the header (source or destination). The SCHC C/D MUST be aware of the traffic direction (Uplink, Downlink) to select the appropriate field. The following Rules apply for Dev and App port numbers.

If both ends know the port number, it can be elided. The TV contains the port number, the MO is set to "equal", and the CDA is set to "not-sent".

If the port variation is on few bits, the TV contains the stable part of the port number, the MO is set to "MSB", and the CDA is set to "LSB".

If some well-known values are used, the TV can contain the list of these values, the MO is set to "match-mapping", and the CDA is set to "mapping-sent".

Otherwise, the port numbers are sent over the L2. The TV is not set, the MO is set to "ignore" and the CDA is set to "value-sent".

10.10. UDP Length Field

The parser MUST NOT label this field unless the UDP Length value matches the Payload Length value from the IPv6 header. The TV is not set, the MO is set to "ignore", and the CDA is set to "compute-*".

10.11. UDP Checksum Field

The UDP checksum operation is mandatory with IPv6 for most packets, but there are exceptions RFC8200.

For instance, protocols that use UDP as a tunnel encapsulation may enable zero-checksum mode for a specific port (or set of ports) for sending and/or receiving. RFC8200 requires any node implementing zero-checksum mode to follow the requirements specified in "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums" RFC6936.

6LoWPAN Header Compression RFC6282 also specifies that a UDP checksum can be elided by the compressor and recomputed by the decompressor when an upper layer guarantees the integrity of the UDP payload and pseudo-header. A specific example of this is when a message integrity check protects the compressed message between the compressor that elides the UDP checksum and the decompressor that computes it, with a strength that is identical or better to the UDP checksum.

Similarly, a SCHC compressor MAY elide the UDP checksum when another layer guarantees at least equal integrity protection for the UDP payload and the pseudo-header. In this case, the TV is not set, the MO is set to "ignore", and the CDA is set to "compute-*".

In particular, when SCHC fragmentation is used, a fragmentation RCS of 2 bytes or more provides equal or better protection than the UDP checksum; in that case, if the compressor is collocated with the fragmentation point and the decompressor is collocated with the packet reassembly point, and if the SCHC Packet is fragmented even when it would fit unfragmented in the L2 MTU, then the compressor MAY verify and then elide the UDP checksum. Whether and when the UDP Checksum is elided is to be specified in the Profile.

Since the compression happens before the fragmentation, implementers should understand the risks when dealing with unprotected data below the transport layer and take special care when manipulating that data.

In other cases, the checksum SHOULD be explicitly sent. The TV is not set, the MO is set to "ignore" and the CDA is set to "value- sent".

11. IANA Considerations

This document has no IANA actions.

12. Security Considerations

As explained in Section 5, SCHC is expected to be implemented on top of LPWAN technologies, which are expected to implement security measures.

In this section, we analyze the potential security threats that could be introduced into an LPWAN by adding the SCHC functionalities.

12.1. Security Considerations for SCHC Compression/Decompression

12.1.1. Forged SCHC Packet

Let's assume that an attacker is able to send a forged SCHC Packet to a SCHC decompressor.

Let's first consider the case where the RuleID contained in that forged SCHC Packet does not correspond to a Rule allocated in the Rule table. An implementation should detect that the RuleID is invalid and should silently drop the offending SCHC Packet.

Let's now consider that the RuleID corresponds to a Rule in the table. With the CDAs defined in this document, the reconstructed packet is, at most, a constant number of bits bigger than the SCHC Packet that was received. This assumes that the compute-* decompression actions produce a bounded number of bits, irrespective of the incoming SCHC Packet. This property is true for IPv6 Length, UDP Length, and UDP Checksum, for which the compute-* CDA is recommended by this document.

As a consequence, SCHC decompression does not amplify attacks, beyond adding a bounded number of bits to the SCHC Packet received. This bound is determined by the Rule stored in the receiving device.

As a general safety measure, a SCHC decompressor should never reconstruct a packet larger than MAX_PACKET_SIZE (defined in a Profile, with 1500 bytes as generic default).

12.1.2. Compressed Packet Size as a Side Channel to Guess a Secret

     Token

Some packet compression methods are known to be susceptible to attacks, such as BREACH and CRIME. The attack involves injecting arbitrary data into the packet and observing the resulting compressed packet size. The observed size potentially reflects correlation between the arbitrary data and some content that was meant to remain secret, such as a security token, thereby allowing the attacker to get at the secret.

By contrast, SCHC compression takes place header field by header field, with the SCHC Packet being a mere concatenation of the compression residues of each of the individual field. Any correlation between header fields does not result in a change in the SCHC Packet size compressed under the same Rule.

If SCHC C/D is used to compress packets that include a secret information field, such as a token, the Rule set should be designed so that the size of the compression residue for the field to remain secret is the same irrespective of the value of the secret information. This is achieved by, e.g., sending this field in extenso with the "ignore" MO and the "value-sent" CDA. This recommendation is disputable if it is ascertained that the Rule set itself will remain secret.

12.1.3. Decompressed Packet Different from the Original Packet

As explained in Section 7.2, using FPs with value 0 in Field Descriptors in a Rule may result in header fields appearing in the decompressed packet in an order different from that in the original packet. Likewise, as stated in Section 7.4.3, using an "ignore" MO together with a "not-sent" CDA will result in the header field taking the TV value, which is likely to be different from the original value.

Depending on the protocol, the order of header fields in the packet may or may not be functionally significant.

Furthermore, if the packet is protected by a checksum or a similar integrity protection mechanism, and if the checksum is transmitted instead of being recomputed as part of the decompression, these situations may result in the packet being considered corrupt and dropped.

12.2. Security Considerations for SCHC Fragmentation/Reassembly

12.2.1. Buffer Reservation Attack

Let's assume that an attacker is able to send a forged SCHC Fragment to a SCHC reassembler.

A node can perform a buffer reservation attack: the receiver will reserve buffer space for the SCHC Packet. If the implementation has only one buffer, other incoming fragmented SCHC Packets will be dropped while the reassembly buffer is occupied during the reassembly timeout. Once that timeout expires, the attacker can repeat the same procedure, and iterate, thus, creating a denial-of-service attack. An implementation may have multiple reassembly buffers. The cost to mount this attack is linear with the number of buffers at the target node. Better, the cost for an attacker can be increased if individual fragments of multiple SCHC Packets can be stored in the reassembly buffer. The finer grained the reassembly buffer (down to the smallest tile size), the higher the cost of the attack. If buffer overload does occur, a smart receiver could selectively discard SCHC Packets being reassembled based on the sender behavior, which may help identify which SCHC Fragments have been sent by the attacker. Another mild countermeasure is for the target to abort the fragmentation/reassembly session as early as it detects a non- identical SCHC Fragment duplicate, anticipating for an eventual corrupt SCHC Packet, so as to save the sender the hassle of sending the rest of the fragments for this SCHC Packet.

12.2.2. Corrupt Fragment Attack

Let's assume that an attacker is able to send a forged SCHC Fragment to a SCHC reassembler. The malicious node is additionally assumed to be able to hear an incoming communication destined to the target node.

It can then send a forged SCHC Fragment that looks like it belongs to a SCHC Packet already being reassembled at the target node. This can cause the SCHC Packet to be considered corrupt and to be dropped by the receiver. The amplification happens here by a single spoofed SCHC Fragment rendering a full sequence of legitimate SCHC Fragments useless. If the target uses ACK-Always or ACK-on-Error mode, such a malicious node can also interfere with the acknowledgement and repetition algorithm of SCHC F/R. A single spoofed ACK, with all Bitmap bits set to 0, will trigger the repetition of WINDOW_SIZE tiles. This protocol loop amplification depletes the energy source of the target node and consumes the channel bandwidth. Similarly, a spoofed ACK REQ will trigger the sending of a SCHC ACK, which may be much larger than the ACK REQ if WINDOW_SIZE is large. These consequences should be borne in mind when defining profiles for SCHC over specific LPWAN technologies.

12.2.3. Fragmentation as a Way to Bypass Network Inspection

Fragmentation is known for potentially allowing one to force through a Network Inspection device (e.g., firewall) packets that would be rejected if unfragmented. This involves sending overlapping fragments to rewrite fields whose initial value led the Network Inspection device to allow the flow to go through.

SCHC F/R is expected to be used over one LPWAN link, where no Network Inspection device is expected to sit. As described in Section 5.2, even if the SCHC F/R on the Network Infrastructure side is located in the Internet, a tunnel is to be established between it and the NGW.

12.2.4. Privacy Issues Associated with SCHC Header Fields

SCHC F/R allocates a DTag value to fragments belonging to the same SCHC Packet. Concerns were raised that, if DTag is a wide counter that is incremented in a predictable fashion for each new fragmented SCHC Packet, it might lead to a privacy issue, such as enabling tracking of a device across LPWANs.

However, SCHC F/R is expected to be used over exactly one LPWAN link. As described in Section 5.2, even if the SCHC F/R on the Network Infrastructure side is located in the Internet, a tunnel is to be established between it and the NGW. Therefore, assuming the tunnel provides confidentiality, neither the DTag field nor any other SCHC- introduced field is visible over the Internet.

13. References

13.1. Normative References

RFC2119 Bradner, S., "Key words for use in RFCs to Indicate

          Requirement Levels", BCP 14, RFC 2119,
          DOI 10.17487/RFC2119, March 1997,
          <https://www.rfc-editor.org/info/rfc2119>.

RFC6936 Fairhurst, G. and M. Westerlund, "Applicability Statement

          for the Use of IPv6 UDP Datagrams with Zero Checksums",
          RFC 6936, DOI 10.17487/RFC6936, April 2013,
          <https://www.rfc-editor.org/info/rfc6936>.

RFC8174 Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC

          2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
          May 2017, <https://www.rfc-editor.org/info/rfc8174>.

RFC8200 Deering, S. and R. Hinden, "Internet Protocol, Version 6

          (IPv6) Specification", STD 86, RFC 8200,
          DOI 10.17487/RFC8200, July 2017,
          <https://www.rfc-editor.org/info/rfc8200>.

RFC8376 Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)

          Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
          <https://www.rfc-editor.org/info/rfc8376>.

13.2. Informative References

[ETHERNET] IEEE, "IEEE Standard for Ethernet",

          DOI 10.1109/IEEESTD.2012.6419735, IEEE
          Standard 802.3-2012, December 2012,
          <https://ieeexplore.ieee.org/document/6419735>.

RFC4944 Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,

          "Transmission of IPv6 Packets over IEEE 802.15.4
          Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
          <https://www.rfc-editor.org/info/rfc4944>.

RFC5795 Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust

          Header Compression (ROHC) Framework", RFC 5795,
          DOI 10.17487/RFC5795, March 2010,
          <https://www.rfc-editor.org/info/rfc5795>.

RFC6282 Hui, J., Ed. and P. Thubert, "Compression Format for IPv6

          Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
          DOI 10.17487/RFC6282, September 2011,
          <https://www.rfc-editor.org/info/rfc6282>.

RFC6437 Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,

          "IPv6 Flow Label Specification", RFC 6437,
          DOI 10.17487/RFC6437, November 2011,
          <https://www.rfc-editor.org/info/rfc6437>.

RFC7136 Carpenter, B. and S. Jiang, "Significance of IPv6

          Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
          February 2014, <https://www.rfc-editor.org/info/rfc7136>.

RFC8065 Thaler, D., "Privacy Considerations for IPv6 Adaptation-

          Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
          February 2017, <https://www.rfc-editor.org/info/rfc8065>.

Appendix A. Compression Examples

This section gives some scenarios of the compression mechanism for IPv6/UDP. The goal is to illustrate the behavior of SCHC.

The mechanisms defined in this document can be applied to a Dev that embeds some applications running over CoAP. In this example, three flows are considered. The first flow is for the device management based on CoAP using Link Local IPv6 addresses and UDP ports 123 and 124 for Dev and App, respectively. The second flow is a CoAP server for measurements done by the Dev (using ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to beta::1/64. The last flow is for legacy applications using different ports numbers, the destination IPv6 address prefix is gamma::1/64.

Figure 25 presents the protocol stack. IPv6 and UDP are represented with dotted lines since these protocols are compressed on the radio link.

Management   Data

+----------+---------+---------+ | CoAP | CoAP | legacy | +----||----+---||----+---||----+ . UDP . UDP | UDP |

        Dev or NGW

          Figure 25: Simplified Protocol Stack for LP-WAN

Rule 0

 Special RuleID used to tag an uncompressed UDP/IPV6 packet.

Rule 1

+----------------+--+--+--+---------+--------+------------++------+
|       FID      |FL|FP|DI|    TV   |   MO   |     CDA    || Sent |
|                |  |  |  |         |        |            ||[bits]|
+----------------+--+--+--+---------+---------------------++------+
|IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent   ||      |
|IPv6 Diffserv   |8 |1 |Bi|0        | equal  | not-sent   ||      |
|IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
|IPv6 Length     |16|1 |Bi|         | ignore | compute-*  ||      |
|IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
|IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
|IPv6 DevPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
|IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
|IPv6 AppPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
|IPv6 AppIID     |64|1 |Bi|::1      | equal  | not-sent   ||      |
+================+==+==+==+=========+========+============++======+
|UDP DevPort     |16|1 |Bi|123      | equal  | not-sent   ||      |
|UDP AppPort     |16|1 |Bi|124      | equal  | not-sent   ||      |
|UDP Length      |16|1 |Bi|         | ignore | compute-*  ||      |
|UDP checksum    |16|1 |Bi|         | ignore | compute-*  ||      |
+================+==+==+==+=========+========+============++======+

            Figure 26: Context Rules - Rule 0 and Rule 1

Rule 2
+----------------+--+--+--+---------+--------+------------++------+
|       FID      |FL|FP|DI|    TV   |   MO   |     CDA    || Sent |
|                |  |  |  |         |        |            ||[bits]|
+----------------+--+--+--+---------+--------+------------++------+
|IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent   ||      |
|IPv6 Diffserv   |8 |1 |Bi|0        | equal  | not-sent   ||      |
|IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
|IPv6 Length     |16|1 |Bi|         | ignore | compute-*  ||      |
|IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
|IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
|IPv6 DevPrefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||   1  |
|                |  |  |  |fe80::/64] mapping|            ||      |
|IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
|IPv6 AppPrefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||   2  |
|                |  |  |  |alpha/64,| mapping|            ||      |
|                |  |  |  |fe80::64]|        |            ||      |
|IPv6 AppIID     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
+================+==+==+==+=========+========+============++======+
|UDP DevPort     |16|1 |Bi|5683     | equal  | not-sent   ||      |
|UDP AppPort     |16|1 |Bi|5683     | equal  | not-sent   ||      |
|UDP Length      |16|1 |Bi|         | ignore | compute-*  ||      |
|UDP checksum    |16|1 |Bi|         | ignore | compute-*  ||      |
+================+==+==+==+=========+========+============++======+

                 Figure 27: Context Rules - Rule 2

Rule 3
+----------------+--+--+--+---------+--------+------------++------+
|       FID      |FL|FP|DI|    TV   |   MO   |     CDA    || Sent |
|                |  |  |  |         |        |            ||[bits]|
+----------------+--+--+--+---------+--------+------------++------+
|IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent   ||      |
|IPv6 Diffserv   |8 |1 |Bi|0        | equal  | not-sent   ||      |
|IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
|IPv6 Length     |16|1 |Bi|         | ignore | compute-*  ||      |
|IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
|IPv6 Hop Limit  |8 |1 |Up|255      | ignore | not-sent   ||      |
|IPv6 Hop Limit  |8 |1 |Dw|         | ignore | value-sent ||   8  |
|IPv6 DevPrefix  |64|1 |Bi|alpha/64 | equal  | not-sent   ||      |
|IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
|IPv6 AppPrefix  |64|1 |Bi|gamma/64 | equal  | not-sent   ||      |
|IPv6 AppIID     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
+================+==+==+==+=========+========+============++======+
|UDP DevPort     |16|1 |Bi|8720     | MSB(12)| LSB        ||   4  |
|UDP AppPort     |16|1 |Bi|8720     | MSB(12)| LSB        ||   4  |
|UDP Length      |16|1 |Bi|         | ignore | compute-*  ||      |
|UDP checksum    |16|1 |Bi|         | ignore | compute-*  ||      |
+================+==+==+==+=========+========+============++======+

                 Figure 28: Context Rules - Rule 3

Figures 26 to 28 describe an example of a Rule set.

In this example, 0 was chosen as the special RuleID that tags packets that cannot be compressed with any compression Rule.

All the fields described in Rules 1-3 are present in the IPv6 and UDP headers. The DevIID value is inferred from the L2 header.

Rules 2-3 use global addresses. The way the Dev learns the prefix is not in the scope of the document.

Rule 3 compresses each port number to 4 bits.

Appendix B. Fragmentation Examples

This section provides examples for the various fragment reliability modes specified in this document. In the drawings, Bitmaps are shown in their uncompressed form.

Figure 29 illustrates the transmission in No-ACK mode of a SCHC Packet that needs 11 SCHC Fragments. FCN is 1 bit wide.

       Sender               Receiver
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-------FCN=0-------->|
         |-----FCN=1 + RCS --->| Integrity check: success
       (End)

             Figure 29: No-ACK Mode, 11 SCHC Fragments

In the following examples, N (the size of the FCN field) is 3 bits. The All-1 FCN value is therefore 7.

Figure 30 illustrates the transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, WINDOW_SIZE=7 and no lost SCHC Fragment.

       Sender               Receiver
         |-----W=0, FCN=6----->|
         |-----W=0, FCN=5----->|
         |-----W=0, FCN=4----->|
         |-----W=0, FCN=3----->|
         |-----W=0, FCN=2----->|
         |-----W=0, FCN=1----->|
         |-----W=0, FCN=0----->|
     (no ACK)
         |-----W=1, FCN=6----->|
         |-----W=1, FCN=5----->|
         |-----W=1, FCN=4----->|
         |--W=1, FCN=7 + RCS-->| Integrity check: success
         |<-- ACK, W=1, C=1 ---| C=1
       (End)

     Figure 30: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
                  Fragment, No Lost SCHC Fragment

Figure 31 illustrates the transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, WINDOW_SIZE=7, and three lost SCHC Fragments.

       Sender               Receiver
         |-----W=0, FCN=6----->|
         |-----W=0, FCN=5----->|
         |-----W=0, FCN=4--X-->|
         |-----W=0, FCN=3----->|
         |-----W=0, FCN=2--X-->|
         |-----W=0, FCN=1----->|
         |-----W=0, FCN=0----->|        6543210
         |<-- ACK, W=0, C=0 ---| Bitmap:1101011
         |-----W=0, FCN=4----->|
         |-----W=0, FCN=2----->|
     (no ACK)
         |-----W=1, FCN=6----->|
         |-----W=1, FCN=5----->|
         |-----W=1, FCN=4--X-->|
         |- W=1, FCN=7 + RCS ->| Integrity check: failure
         |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
         |-----W=1, FCN=4----->| Integrity check: success
         |<-- ACK, W=1, C=1 ---| C=1
       (End)

     Figure 31: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
                   Fragment, Lost SCHC Fragments

Figure 32 shows an example of a transmission in ACK-on-Error mode of a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2, and three lost SCHC Fragments.

  Sender               Receiver
   |-----W=0, FCN=27----->| 4 tiles sent
   |-----W=0, FCN=23----->| 4 tiles sent
   |-----W=0, FCN=19----->| 4 tiles sent
   |-----W=0, FCN=15--X-->| 4 tiles sent (not received)
   |-----W=0, FCN=11----->| 4 tiles sent
   |-----W=0, FCN=7 ----->| 4 tiles sent
   |-----W=0, FCN=3 ----->| 4 tiles sent
   |-----W=1, FCN=27----->| 4 tiles sent
   |-----W=1, FCN=23----->| 4 tiles sent
   |-----W=1, FCN=19----->| 4 tiles sent
   |-----W=1, FCN=15----->| 4 tiles sent
   |-----W=1, FCN=11----->| 4 tiles sent
   |-----W=1, FCN=7 ----->| 4 tiles sent
   |-----W=1, FCN=3 --X-->| 4 tiles sent (not received)
   |-----W=2, FCN=27----->| 4 tiles sent
   |-----W=2, FCN=23----->| 4 tiles sent

   |-----W=2, FCN=16----->| 1 tile sent

s |-----W=2, FCN=15----->| 1 tile sent m |-----W=2, FCN=14----->| 1 tile sent a |-----W=2, FCN=13--X-->| 1 tile sent (not received) l |-----W=2, FCN=12----->| 1 tile sent l |---W=2, FCN=31 + RCS->| Integrity check: failure e |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111 r |-----W=0, FCN=15----->| 1 tile sent

   |-----W=0, FCN=14----->| 1 tile sent

L |-----W=0, FCN=13----->| 1 tile sent 2 |-----W=0, FCN=12----->| 1 tile sent

   |<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000

   |-----W=1, FCN=0 ----->| 1 tile sent

| |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001 | |-----W=2, FCN=13----->| Integrity check: success V |<--- ACK, W=2, C=1 ---| C=1

 (End)

             Figure 32: ACK-on-Error Mode, Variable MTU

In this example, the L2 MTU becomes reduced just before sending the "W=2, FCN=19" fragment, leaving space for only one tile in each forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are carried by a total of 25 SCHC Fragments, the last nine being of smaller size.

Note: other sequences of events (e.g., regarding when ACKs are sent by the Receiver) are also allowed by this specification. Profiles may restrict this flexibility.

Figure 33 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with N=3, WINDOW_SIZE=7, and no loss.

       Sender               Receiver
         |-----W=0, FCN=6----->|
         |-----W=0, FCN=5----->|
         |-----W=0, FCN=4----->|
         |-----W=0, FCN=3----->|
         |-----W=0, FCN=2----->|
         |-----W=0, FCN=1----->|
         |-----W=0, FCN=0----->|
         |<-- ACK, W=0, C=0 ---| Bitmap:1111111
         |-----W=1, FCN=6----->|
         |-----W=1, FCN=5----->|
         |-----W=1, FCN=4----->|
         |--W=1, FCN=7 + RCS-->| Integrity check: success
         |<-- ACK, W=1, C=1 ---| C=1
       (End)

 Figure 33: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
                              No Loss

Figure 34 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3, WINDOW_SIZE=7 and three lost SCHC Fragments.

       Sender               Receiver
         |-----W=0, FCN=6----->|
         |-----W=0, FCN=5----->|
         |-----W=0, FCN=4--X-->|
         |-----W=0, FCN=3----->|
         |-----W=0, FCN=2--X-->|
         |-----W=0, FCN=1----->|
         |-----W=0, FCN=0----->|        6543210
         |<-- ACK, W=0, C=0 ---| Bitmap:1101011
         |-----W=0, FCN=4----->|
         |-----W=0, FCN=2----->|
         |<-- ACK, W=0, C=0 ---| Bitmap:1111111
         |-----W=1, FCN=6----->|
         |-----W=1, FCN=5----->|
         |-----W=1, FCN=4--X-->|
         |--W=1, FCN=7 + RCS-->| Integrity check: failure
         |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
         |-----W=1, FCN=4----->| Integrity check: success
         |<-- ACK, W=1, C=1 ---| C=1
       (End)

 Figure 34: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
                     Three Lost SCHC Fragments

Figure 35 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in six tiles, with one tile per SCHC Fragment, N=3, WINDOW_SIZE=7, three lost SCHC Fragments, and only one retry needed to recover each lost SCHC Fragment.

         Sender                Receiver
            |-----W=0, FCN=6----->|
            |-----W=0, FCN=5----->|
            |-----W=0, FCN=4--X-->|
            |-----W=0, FCN=3--X-->|
            |-----W=0, FCN=2--X-->|
            |--W=0, FCN=7 + RCS-->| Integrity check: failure
            |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
            |-----W=0, FCN=4----->| Integrity check: failure
            |-----W=0, FCN=3----->| Integrity check: failure
            |-----W=0, FCN=2----->| Integrity check: success
            |<-- ACK, W=0, C=1 ---| C=1
          (End)

      Figure 35: ACK-Always Mode, Six Tiles, One Tile per SCHC
                Fragment, Three Lost SCHC Fragments

Figure 36 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in six tiles, with one tile per SCHC Fragment, N=3, WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK lost.

         Sender                Receiver
            |-----W=0, FCN=6----->|
            |-----W=0, FCN=5----->|
            |-----W=0, FCN=4--X-->|
            |-----W=0, FCN=3--X-->|
            |-----W=0, FCN=2--X-->|
            |--W=0, FCN=7 + RCS-->| Integrity check: failure
            |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
            |-----W=0, FCN=4----->| Integrity check: failure
            |-----W=0, FCN=3----->| Integrity check: failure
            |-----W=0, FCN=2----->| Integrity check: success
            |<-X-ACK, W=0, C=1 ---| C=1
   timeout  |                     |
            |--- W=0, ACK REQ --->| ACK REQ
            |<-- ACK, W=0, C=1 ---| C=1
          (End)

      Figure 36: ACK-Always Mode, Six Tiles, One Tile per SCHC
                      Fragment, SCHC ACK Loss

Figure 37 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in six tiles, with N=3, WINDOW_SIZE=7, with three lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.

          Sender                Receiver
            |-----W=0, FCN=6----->|
            |-----W=0, FCN=5----->|
            |-----W=0, FCN=4--X-->|
            |-----W=0, FCN=3--X-->|
            |-----W=0, FCN=2--X-->|
            |--W=0, FCN=7 + RCS-->| Integrity check: failure
            |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
            |-----W=0, FCN=4----->| Integrity check: failure
            |-----W=0, FCN=3----->| Integrity check: failure
            |-----W=0, FCN=2--X-->|
     timeout|                     |
            |--- W=0, ACK REQ --->| ACK REQ
            |<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
            |-----W=0, FCN=2----->| Integrity check: success
            |<-- ACK, W=0, C=1 ---| C=1
          (End)

     Figure 37: ACK-Always Mode, Six Tiles, Retransmitted SCHC
                        Fragment Lost Again

Figure 38 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5, WINDOW_SIZE=24, and two lost SCHC Fragments.

     Sender               Receiver
       |-----W=0, FCN=23----->|
       |-----W=0, FCN=22----->|
       |-----W=0, FCN=21--X-->|
       |-----W=0, FCN=20----->|
       |-----W=0, FCN=19----->|
       |-----W=0, FCN=18----->|
       |-----W=0, FCN=17----->|
       |-----W=0, FCN=16----->|
       |-----W=0, FCN=15----->|
       |-----W=0, FCN=14----->|
       |-----W=0, FCN=13----->|
       |-----W=0, FCN=12----->|
       |-----W=0, FCN=11----->|
       |-----W=0, FCN=10--X-->|
       |-----W=0, FCN=9 ----->|
       |-----W=0, FCN=8 ----->|
       |-----W=0, FCN=7 ----->|
       |-----W=0, FCN=6 ----->|
       |-----W=0, FCN=5 ----->|
       |-----W=0, FCN=4 ----->|
       |-----W=0, FCN=3 ----->|
       |-----W=0, FCN=2 ----->|
       |-----W=0, FCN=1 ----->|
       |-----W=0, FCN=0 ----->|
       |                      |
       |<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111
       |-----W=0, FCN=21----->|
       |-----W=0, FCN=10----->|
       |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
       |-----W=1, FCN=23----->|
       |-----W=1, FCN=22----->|
       |-----W=1, FCN=21----->|
       |--W=1, FCN=31 + RCS-->| Integrity check: success
       |<--- ACK, W=1, C=1 ---| C=1
     (End)

 Figure 38: ACK-Always Mode, 28 Tiles, One Tile per SCHC Fragment,
                        Lost SCHC Fragments

Appendix C. Fragmentation State Machines

The fragmentation state machines of the sender and the receiver, one for each of the different reliability modes, are described in the following figures:

            +===========+

          +===+=======+
              |  last fragment
              |  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
              |  FCN = 1
              v  send fragment+RCS
          +============+
          |    END     |
          +============+

        Figure 39: Sender State Machine for the No-ACK Mode

                     +------+ Not All-1
          +==========+=+    | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) 21:37, 5 May 2021 (UTC)
          |            + <--+ set Inactivity Timer
          |  RCV Frag  +-------+
          +=+===+======+       |All-1 &
  All-1 &   |   |              |RCS correct
RCS wrong   |   |Inactivity    |
            |   |Timer Exp.    |
            v   |              |
 +==========++  |              v
 |   Error   |<-+     +========+==+
 +===========+        |    END    |
                      +===========+

       Figure 40: Receiver State Machine for the No-ACK Mode

             +=======+
             | INIT  |       FCN!=0 & more frags
             |       |       21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
             +======++  +--+ send Window + frag(FCN)
                W=0 |   |  | FCN-
 Clear lcl_bm       |   |  v set lcl_bm
      FCN=max value |  ++==+========+
                    +> |            |

                +----+     Wait      |            |Frag   |

not expected wnd | | Bitmap | +=======+ 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ +--->+ ++Retrans_Timer Exp |

   discard frag      +==+=+===+=+==+=+| 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~ |
                        | |   | ^  ^  |reSend(empty)All-* |
                        | |   | |  |  |Set Retrans_Timer  |
                        | |   | |  +--+Attempt++          |
 C_bit==1 &             | |   | +-------------------------+

Recv_window==window & | | | all missing frags sent

            no more frag| |   |   21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) 21:37, 5 May 2021 (UTC)| |   |   Set Retrans_Timer
      Stop Retrans_Timer| |   |
+=============+         | |   |
|     END     +<--------+ |   |
+=============+           |   | Attempt > MAX_ACK_REQUESTS
           All-1 Window & |   | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk)
              C_bit ==0 & |   v Send Abort
         lcl_bm==recv_bm  | +=+===========+
             21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~ +>|    ERROR    |
               Send Abort   +=============+

      Figure 41: Sender State Machine for the ACK-Always Mode

Not All- & w=expected +---+   +---+w = Not expected
21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ |   |   |   |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
Set lcl_bm(FCN)       |   v   v   |discard
                     ++===+===+===+=+

                           +==========+<---------------+

      --->* ABORT

      In any state
         on receiving a SCHC ACK REQ
            Send a SCHC ACK for the current window

     Figure 42: Receiver State Machine for the ACK-Always Mode

                 +=======+
                 |       |
                 | INIT  |
                 |       |       FCN!=0 & more frags
                 +======++       21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
    Frag RuleID trigger |   +--+ Send cur_W + frag(FCN);
    21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) 21:37, 5 May 2021 (UTC) |   |  | FCN--;
 cur_W=0; FCN=max_value;|   |  | set [cur_W, cur_Bmp]
   clear [cur_W, Bmp_n];|   |  v
         clear rcv_Bmp  |  ++==+==========+       **BACK_TO_SEND
                        +->+              |   cur_W==rcv_W &
     **BACK_TO_SEND        |     SEND     |   [cur_W,Bmp_n]==rcv_Bmp

                cur_W==rcv_W&|   |
      [cur_W,Bmp_n]==rcv_Bmp&|   | Attempts > MAX_ACK_REQUESTS
 No more Frags & RCS flag==OK|   | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
           21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk)|   | send Abort
+=========+stop Retrans_Timer|   |  +===========+
|   END   +<-----------------+   +->+   ERROR   |
+=========+                         +===========+

     Figure 43: Sender State Machine for the ACK-on-Error Mode

This is an example only. It is not normative. The specification in Section 8.4.3.1 allows for sequences of operations different from the one shown here.

                +=======+        New frag RuleID received
                |       |        21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk)
                | INIT  +-------+cur_W=0;clear([cur_W,Bmp_n]);
                +=======+       |sync=0
                                |
   Not All* & rcv_W==cur_W+---+ | +--+
     21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC) |   | | | (E)
     set[cur_W,Bmp_n(FCN)]|   v v v  |
                         ++===+=+=+==+=+
  +----------------------+             +--+ All-0&Full[cur_W,Bmp_n]
  |           ABORT *<---+  Rcv Window |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
  |  +-------------------+             +<-+ cur_W++;set Inact_timer;
  |  |                +->+=+=+=+=+=+===+    clear [cur_W,Bmp_n]
  |  | All-0 empty(Wn)|    | | | ^ ^
  |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) 21:37, 5 May 2021 (UTC) +----+ | | | |rcv_W==cur_W & sync==0;
  |  | sendACK([Wn,Bmp_n])   | | | |& Full([cur_W,Bmp_n])
  |  |                       | | | |& All* || last_miss_frag
  |  |                       | | | |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
  |  |    All* & rcv_W==cur_W|(C)| |sendACK([cur_W,Bmp_n]);
  |  |              & sync==0| | | |cur_W++; clear([cur_W,Bmp_n])
  |  |&no_full([cur_W,Bmp_n])| |(E)|
  |  |      21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ | | | |              +========+
  |  | sendACK([cur_W,Bmp_n])| | | |              | Error/ |
  |  |                       | | | |   +----+     | Abort  |
  |  |                       v v | |   |    |     +===+====+
  |  |                   +===+=+=+=+===+=+ (D)        ^
  |  |                +--+    Wait x     |  |         |
  |  | All-0 empty(Wn)+->| Missing Frags |<-+         |
  |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) 21:37, 5 May 2021 (UTC)    +=============+=+            |
  |  | sendACK([Wn,Bmp_n])             +--------------+
  |  |                                       *ABORT
  v  v
 (A)(B)
                                  (D) All* || last_miss_frag
   (C) All* & sync>0                  & rcv_W!=cur_W & sync>0
       21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~                   & Full([rcv_W,Bmp_n])
       Wn=oldest[not full(W)];        21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
       sendACK([Wn,Bmp_n])            Wn=oldest[not full(W)];
                                      sendACK([Wn,Bmp_n]);sync--

                            ABORT-->* Uplink Only &
                                      Inact_Timer expires
   (E) Not All* & rcv_W!=cur_W        || Attempts > MAX_ACK_REQUESTS
       21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)           21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
       sync++; cur_W=rcv_W;           send Abort
       set[cur_W,Bmp_n(FCN)]

 (A)(B)
  |  |
  |  | All-1 & rcv_W==cur_W & RCS!=OK        All-0 empty(Wn)
  |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk)     +-+  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
  |  | sendACK([cur_W,Bmp_n],C=0)       | v  sendACK([Wn,Bmp_n])
  |  |                      +===========+=++
  |  +--------------------->+   Wait End   +-+
  |                         +=====+=+====+=+ | All-1
  |     rcv_W==cur_W & RCS==OK    | |    ^   | & rcv_W==cur_W
  |     21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~    | |    +---+ & RCS!=OK
  |  sendACK([cur_W,Bmp_n],C=1)   | |          21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) 21:37, 5 May 2021 (UTC)
  |                               | | sendACK([cur_W,Bmp_n],C=0);
  |                               | |          Attempts++
  |All-1 & Full([cur_W,Bmp_n])    | |
  |& RCS==OK & sync==0            | +-->* ABORT
  |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)Admin (talk) 21:37, 5 May 2021 (UTC)            v
  |sendACK([cur_W,Bmp_n],C=1)   +=+=========+
  +---------------------------->+    END    |
                                +===========+

    Figure 44: Receiver State Machine for the ACK-on-Error Mode

Appendix D. SCHC Parameters

This section lists the information that needs to be provided in the LPWAN technology-specific documents.

Most common uses cases, deployment scenarios.

Mapping of the SCHC architectural elements onto the LPWAN

  architecture.

Assessment of LPWAN integrity checking.

Various potential channel conditions for the technology and the

  corresponding recommended use of SCHC C/D and SCHC F/R.

This section lists the parameters that need to be defined in the Profile.

RuleID numbering scheme, fixed-size or variable-size RuleIDs,

  number of Rules, the way the RuleID is transmitted.

maximum packet size that should ever be reconstructed by SCHC

  decompression (MAX_PACKET_SIZE).  See Section 12.

Padding: size of the L2 Word (for most LPWAN technologies, this

  would be a byte; for some technologies, a bit).

Decision to use SCHC fragmentation mechanism or not. If yes, the

  document must describe:

  -  reliability mode(s) used, in which cases (e.g., based on link
     channel condition).

  -  RuleID values assigned to each mode in use.

  -  presence and number of bits for DTag (T) for each RuleID value,
     lifetime of DTag at the receiver.

  -  support for interleaved packet transmission, to what extent.

  -  WINDOW_SIZE, for modes that use windows.

  -  number of bits for W (M) for each RuleID value, for modes that
     use windows.

  -  number of bits for FCN (N) for each RuleID value, meaning of
     the FCN values.

  -  what makes an All-0 SCHC Fragment and a SCHC ACK REQ
     distinguishable (see Section 8.3.1.1).

  -  what makes an All-1 SCHC Fragment and a SCHC Sender-Abort
     distinguishable (see Section 8.3.1.2).

  -  for RuleIDs that use ACK-on-Error mode: when the last tile of a
     SCHC Packet is to be sent in a Regular SCHC Fragment, alone in
     an All-1 SCHC Fragment or with any of these two methods.

  -  for RuleIDs that use ACK-on-Error mode: if the penultimate tile
     of a SCHC Packet is of the regular size only or if it can also
     be one L2 Word shorter.

  -  for RuleIDs that use ACK-on-Error mode: times at which the
     sender must listen for SCHC ACKs.

  -  size of RCS and algorithm for its computation, for each RuleID,
     if different from the default CRC32.  Byte fill-up with zeroes
     or other mechanism, to be specified.  Support for UDP checksum
     elision.

  -  Retransmission Timer duration for each RuleID value, if
     applicable to the SCHC F/R mode.

  -  Inactivity Timer duration for each RuleID value, if applicable
     to the SCHC F/R mode.

  -  MAX_ACK_REQUESTS value for each RuleID value, if applicable to
     the SCHC F/R mode.

if L2 Word is wider than a bit and SCHC fragmentation is used,

  value of the padding bits (0 or 1).

A Profile may define a delay to be added after each SCHC message transmission for compliance with local regulations or other constraints imposed by the applications.

In some LPWAN technologies, as part of energy-saving techniques,

  Downlink transmission is only possible immediately after an Uplink
  transmission.  In order to avoid potentially high delay in the
  Downlink transmission of a fragmented SCHC Packet, the SCHC
  Fragment receiver may perform an Uplink transmission as soon as
  possible after reception of a SCHC Fragment that is not the last
  one.  Such Uplink transmission may be triggered by the L2 (e.g.,
  an L2 ACK sent in response to a SCHC Fragment encapsulated in a L2
  PDU that requires an L2 ACK) or it may be triggered from an upper
  layer.  See Appendix F.

the following parameters need to be addressed in documents other

  than this one but not necessarily in the LPWAN technology-specific
  documents:

  -  The way the Contexts are provisioned.

  -  The way the Rules are generated.

Appendix E. Supporting Multiple Window Sizes for Fragmentation

For ACK-Always or ACK-on-Error, implementers may opt to support a single window size or multiple window sizes. The latter, when feasible, may provide performance optimizations. For example, a large WINDOW_SIZE should be used for packets that need to be split into a large number of tiles. However, when the number of tiles required to carry a packet is low, a smaller WINDOW_SIZE and, thus, a shorter Bitmap, may be sufficient to provide reception status on all tiles. If multiple window sizes are supported, the RuleID signals what WINDOW_SIZE is in use for a specific packet transmission.

Appendix F. ACK-Always and ACK-on-Error on Quasi-Bidirectional Links

The ACK-Always and ACK-on-Error modes of SCHC F/R are bidirectional protocols: they require a feedback path from the reassembler to the fragmenter.

Some LPWAN technologies provide quasi-bidirectional connectivity, whereby a Downlink transmission from the Network Infrastructure can only take place right after an Uplink transmission by the Dev.

When using SCHC F/R to send fragmented SCHC Packets Downlink over these quasi-bidirectional links, the following situation may arise: if an Uplink SCHC ACK is lost, the SCHC ACK REQ message by the sender could be stuck indefinitely in the Downlink queue at the Network Infrastructure, waiting for a transmission opportunity.

There are many ways by which this deadlock can be avoided. The Dev application might be sending recurring Uplink messages such as keep- alive, or the Dev application stack might be sending other recurring Uplink messages as part of its operation. However, these are out of the control of this generic SCHC specification.

In order to cope with quasi-bidirectional links, a SCHC-over-foo specification may want to amend the SCHC F/R specification to add a timer-based retransmission of the SCHC ACK. Below is an example of the suggested behavior for ACK-Always mode. Because it is an example, RFC2119 language is deliberately not used here.

For Downlink transmission of a fragmented SCHC Packet in ACK-Always mode, the SCHC Fragment receiver may support timer-based SCHC ACK retransmission. In this mechanism, the SCHC Fragment receiver initializes and starts a timer (the UplinkACK Timer) after the transmission of a SCHC ACK, except when the SCHC ACK is sent in response to the last SCHC Fragment of a packet (All-1 fragment). In the latter case, the SCHC Fragment receiver does not start a timer after transmission of the SCHC ACK.

If, after transmission of a SCHC ACK that is not an All-1 fragment, and before expiration of the corresponding UplinkACK timer, the SCHC Fragment receiver receives a SCHC Fragment that belongs to the current window (e.g., a missing SCHC Fragment from the current window) or to the next window, the UplinkACK timer for the SCHC ACK is stopped. However, if the UplinkACK timer expires, the SCHC ACK is resent and the UplinkACK timer is reinitialized and restarted.

The default initial value for the UplinkACK Timer, as well as the maximum number of retries for a specific SCHC ACK, denoted MAX_ACK_REQUESTS, is to be defined in a Profile. The initial value of the UplinkACK timer is expected to be greater than that of the Retransmission timer, in order to make sure that a (buffered) SCHC Fragment to be retransmitted finds an opportunity for that transmission. One exception to this recommendation is the special case of the All-1 SCHC Fragment transmission.

When the SCHC Fragment sender transmits the All-1 SCHC Fragment, it starts its Retransmission Timer with a large timeout value (e.g., several times that of the initial UplinkACK Timer). If a SCHC ACK is received before expiration of this timer, the SCHC Fragment sender retransmits any lost SCHC Fragments as reported by the SCHC ACK, or if the SCHC ACK confirms successful reception of all SCHC Fragments of the last window, the transmission of the fragmented SCHC Packet is considered complete. If the timer expires, and no SCHC ACK has been received since the start of the timer, the SCHC Fragment sender assumes that the All-1 SCHC Fragment has been successfully received (and possibly, the last SCHC ACK has been lost: this mechanism assumes that the Retransmission Timer for the All-1 SCHC Fragment is long enough to allow several SCHC ACK retries if the All-1 SCHC Fragment has not been received by the SCHC Fragment receiver, and it also assumes that it is unlikely that several ACKs become all lost).

Acknowledgements

Thanks to (in alphabetical order) Sergio Aguilar Romero, David Black, Carsten Bormann, Deborah Brungard, Brian Carpenter, Philippe Clavier, Alissa Cooper, Roman Danyliw, Daniel Ducuara Beltran, Diego Dujovne, Eduardo Ingles Sanchez, Rahul Jadhav, Benjamin Kaduk, Arunprabhu Kandasamy, Suresh Krishnan, Mirja Kuehlewind, Barry Leiba, Sergio Lopez Bernal, Antoni Markovski, Alexey Melnikov, Georgios Papadopoulos, Alexander Pelov, Charles Perkins, Edgar Ramos, Alvaro Retana, Adam Roach, Shoichi Sakane, Joseph Salowey, Pascal Thubert, and Eric Vyncke for useful design considerations, reviews and comments.

Carles Gomez has been funded in part by the Spanish Government (Ministerio de Educacion, Cultura y Deporte) through the Jose Castillejo grant CAS15/00336 and by the ERDF and the Spanish Government through project TEC2016-79988-P. Part of his contribution to this work has been carried out during his stay as a visiting scholar at the Computer Laboratory of the University of Cambridge.

Authors' Addresses

Ana Minaburo Acklio 1137A avenue des Champs Blancs 35510 Cesson-Sevigne Cedex France

Email: [email protected]

Laurent Toutain IMT Atlantique 2 rue de la Chataigneraie CS 17607 35576 Cesson-Sevigne Cedex France

Email: [email protected]

Carles Gomez Universitat Politecnica de Catalunya C/Esteve Terradas, 7 08860 Castelldefels Spain

Email: [email protected]

Dominique Barthel Orange Labs 28 chemin du Vieux Chene 38243 Meylan France

Email: [email protected]

Juan Carlos Zuniga SIGFOX 425 rue Jean Rostand 31670 Labege France

Email: [email protected]

@@ Line 1: / Line 1: @@
 Internet Engineering Task Force (IETF)                      A. Minaburo
@@ Line 7: / Line 5: @@
 Category: Standards Track                                    L. Toutain
 ISSN: 2070-1721                                          IMT Atlantique
-                                                                C. Gomez
+                                                            C. Gomez
-                                    Universitat Politecnica de Catalunya
+                                Universitat Politecnica de Catalunya
-                                                              D. Barthel
+                                                          D. Barthel
-                                                            Orange Labs
+                                                          Orange Labs
-                                                              JC. Zuniga
+                                                          JC. Zuniga
-                                                                  SIGFOX
+                                                              SIGFOX
-                                                              April 2020
+                                                          April 2020
+SCHC: Generic Framework for Static Context Header Compression and
+                          Fragmentation
-  SCHC: Generic Framework for Static Context Header Compression and
+'''Abstract'''
-                            Fragmentation
-Abstract
+This document defines the Static Context Header Compression and
+fragmentation (SCHC) framework, which provides both a header
+compression mechanism and an optional fragmentation mechanism.  SCHC
+has been designed with Low-Power Wide Area Networks (LPWANs) in mind.
-  This document defines the Static Context Header Compression and
+SCHC compression is based on a common static context stored both in
-  fragmentation (SCHC) framework, which provides both a header
+the LPWAN device and in the network infrastructure side.  This
-  compression mechanism and an optional fragmentation mechanism.  SCHC
+document defines a generic header compression mechanism and its
-  has been designed with Low-Power Wide Area Networks (LPWANs) in mind.
+application to compress IPv6/UDP headers.
-  SCHC compression is based on a common static context stored both in
+This document also specifies an optional fragmentation and reassembly
-  the LPWAN device and in the network infrastructure side.  This
+mechanism.  It can be used to support the IPv6 MTU requirement over
-  document defines a generic header compression mechanism and its
+the LPWAN technologies.  Fragmentation is needed for IPv6 datagrams
-  application to compress IPv6/UDP headers.
+that, after SCHC compression or when such compression was not
+possible, still exceed the Layer 2 maximum payload size.
-  This document also specifies an optional fragmentation and reassembly
+The SCHC header compression and fragmentation mechanisms are
-  mechanism.  It can be used to support the IPv6 MTU requirement over
+independent of the specific LPWAN technology over which they are
-  the LPWAN technologies.  Fragmentation is needed for IPv6 datagrams
+used.  This document defines generic functionalities and offers
-  that, after SCHC compression or when such compression was not
+flexibility with regard to parameter settings and mechanism choices.
-  possible, still exceed the Layer 2 maximum payload size.
+This document standardizes the exchange over the LPWAN between two
+SCHC entities.  Settings and choices specific to a technology or a
+product are expected to be grouped into profiles, which are specified
+in other documents.  Data models for the context and profiles are out
+of scope.
-  The SCHC header compression and fragmentation mechanisms are
+'''Status of This Memo'''
-  independent of the specific LPWAN technology over which they are
-  used.  This document defines generic functionalities and offers
-  flexibility with regard to parameter settings and mechanism choices.
-  This document standardizes the exchange over the LPWAN between two
-  SCHC entities.  Settings and choices specific to a technology or a
-  product are expected to be grouped into profiles, which are specified
-  in other documents.  Data models for the context and profiles are out
-  of scope.
-Status of This Memo
+This is an Internet Standards Track document.
-  This is an Internet Standards Track document.
+This document is a product of the Internet Engineering Task Force
+(IETF).  It represents the consensus of the IETF community.  It has
+received public review and has been approved for publication by the
+Internet Engineering Steering Group (IESG).  Further information on
+Internet Standards is available in Section 2 of [[RFC7841|RFC 7841]].
-  This document is a product of the Internet Engineering Task Force
+Information about the current status of this document, any errata,
-  (IETF).  It represents the consensus of the IETF community.  It has
+and how to provide feedback on it may be obtained at
-  received public review and has been approved for publication by the
+https://www.rfc-editor.org/info/rfc8724.
-  Internet Engineering Steering Group (IESG).  Further information on
-  Internet Standards is available in Section 2 of RFC 7841.
-  Information about the current status of this document, any errata,
+'''Copyright Notice'''
-  and how to provide feedback on it may be obtained at
-  https://www.rfc-editor.org/info/rfc8724.
-Copyright Notice
+Copyright (c) 2020 IETF Trust and the persons identified as the
+document authors.  All rights reserved.
-  Copyright (c) 2020 IETF Trust and the persons identified as the
+This document is subject to [[BCP78|BCP 78]] and the IETF Trust's Legal
-  document authors.  All rights reserved.
+Provisions Relating to IETF Documents
+(https://trustee.ietf.org/license-info) in effect on the date of
+publication of this document.  Please review these documents
+carefully, as they describe your rights and restrictions with respect
+to this document.  Code Components extracted from this document must
+include Simplified BSD License text as described in Section 4.e of
+the Trust Legal Provisions and are provided without warranty as
+described in the Simplified BSD License.
-  This document is subject to BCP 78 and the IETF Trust's Legal
+.  Introduction
-  Provisions Relating to IETF Documents
+.  Requirements Notation
-  (https://trustee.ietf.org/license-info) in effect on the date of
+.  LPWAN Architecture
-  publication of this document.  Please review these documents
+.  Terminology
-  carefully, as they describe your rights and restrictions with respect
+.  SCHC Overview
-  to this document.  Code Components extracted from this document must
+.1.  SCHC Packet Format
-  include Simplified BSD License text as described in Section 4.e of
+.2.  Functional Mapping
-  the Trust Legal Provisions and are provided without warranty as
+.  RuleID
-  described in the Simplified BSD License.
+.  Compression/Decompression
+.1.  SCHC C/D Rules
+.2.  Packet Processing
+.3.  Matching Operators
+.4.  Compression/Decompression Actions (CDA)
+.4.1.  Processing Fixed-Length Fields
+.4.2.  Processing Variable-Length Fields
+.4.3.  Not-Sent CDA
+.4.4.  Value-Sent CDA
+.4.5.  Mapping-Sent CDA
+.4.6.  LSB CDA
+.4.7.  DevIID, AppIID CDA
+.4.8.  Compute-*
+.  Fragmentation/Reassembly
+.1.  Overview
+.2.  SCHC F/R Protocol Elements
+.2.1.  Messages
+.2.2.  Tiles, Windows, Bitmaps, Timers, Counters
+.2.3.  Integrity Checking
+.2.4.  Header Fields
+.3.  SCHC F/R Message Formats
+.3.1.  SCHC Fragment Format
+.3.2.  SCHC ACK Format
+.3.3.  SCHC ACK REQ Format
+.3.4.  SCHC Sender-Abort Format
+.3.5.  SCHC Receiver-Abort Format
+.4.  SCHC F/R Modes
+.4.1.  No-ACK Mode
+.4.2.  ACK-Always Mode
+.4.3.  ACK-on-Error Mode
+.  Padding Management
+. SCHC Compression for IPv6 and UDP Headers
+.1.  IPv6 Version Field
+.2.  IPv6 Traffic Class Field
+.3.  Flow Label Field
+.4.  Payload Length Field
+.5.  Next Header Field
+.6.  Hop Limit Field
+.7.  IPv6 Addresses Fields
+.7.1.  IPv6 Source and Destination Prefixes
+.7.2.  IPv6 Source and Destination IID
+.8.  IPv6 Extension Headers
+.9.  UDP Source and Destination Ports
+.10. UDP Length Field
+.11. UDP Checksum Field
+. IANA Considerations
+. Security Considerations
+.1.  Security Considerations for SCHC Compression/Decompression
+.1.1.  Forged SCHC Packet
+.1.2.  Compressed Packet Size as a Side Channel to Guess a
+            Secret Token
+.1.3.  Decompressed Packet Different from the Original Packet
+.2.  Security Considerations for SCHC Fragmentation/Reassembly
+.2.1.  Buffer Reservation Attack
+.2.2.  Corrupt Fragment Attack
+.2.3.  Fragmentation as a Way to Bypass Network Inspection
+.2.4.  Privacy Issues Associated with SCHC Header Fields
+. References
+.1.  Normative References
+.2.  Informative References
+Appendix A.  Compression Examples
+Appendix B.  Fragmentation Examples
+Appendix C.  Fragmentation State Machines
+Appendix D.  SCHC Parameters
+Appendix E.  Supporting Multiple Window Sizes for Fragmentation
+Appendix F.  ACK-Always and ACK-on-Error on Quasi-Bidirectional
+        Links
+Acknowledgements
+Authors' Addresses
-Table of Contents
+== Introduction ==
-.  Introduction
+This document defines the Static Context Header Compression and
-.  Requirements Notation
+fragmentation (SCHC) framework, which provides both a header
-.  LPWAN Architecture
+compression mechanism and an optional fragmentation mechanism.  SCHC
-.  Terminology
+has been designed with Low-Power Wide Area Networks (LPWANs) in mind.
-.  SCHC Overview
-.1.  SCHC Packet Format
-.2.  Functional Mapping
-.  RuleID
-.  Compression/Decompression
-.1.  SCHC C/D Rules
-.2.  Packet Processing
-.3.  Matching Operators
-.4.  Compression/Decompression Actions (CDA)
-.4.1.  Processing Fixed-Length Fields
-.4.2.  Processing Variable-Length Fields
-.4.3.  Not-Sent CDA
-.4.4.  Value-Sent CDA
-.4.5.  Mapping-Sent CDA
-.4.6.  LSB CDA
-.4.7.  DevIID, AppIID CDA
-.4.8.  Compute-*
-.  Fragmentation/Reassembly
-.1.  Overview
-.2.  SCHC F/R Protocol Elements
-.2.1.  Messages
-.2.2.  Tiles, Windows, Bitmaps, Timers, Counters
-.2.3.  Integrity Checking
-.2.4.  Header Fields
-.3.  SCHC F/R Message Formats
-.3.1.  SCHC Fragment Format
-.3.2.  SCHC ACK Format
-.3.3.  SCHC ACK REQ Format
-.3.4.  SCHC Sender-Abort Format
-.3.5.  SCHC Receiver-Abort Format
-.4.  SCHC F/R Modes
-.4.1.  No-ACK Mode
-.4.2.  ACK-Always Mode
-.4.3.  ACK-on-Error Mode
-.  Padding Management
-. SCHC Compression for IPv6 and UDP Headers
-.1.  IPv6 Version Field
-.2.  IPv6 Traffic Class Field
-.3.  Flow Label Field
-.4.  Payload Length Field
-.5.  Next Header Field
-.6.  Hop Limit Field
-.7.  IPv6 Addresses Fields
-.7.1.  IPv6 Source and Destination Prefixes
-.7.2.  IPv6 Source and Destination IID
-.8.  IPv6 Extension Headers
-.9.  UDP Source and Destination Ports
-.10. UDP Length Field
-.11. UDP Checksum Field
-. IANA Considerations
-. Security Considerations
-.1.  Security Considerations for SCHC Compression/Decompression
-.1.1.  Forged SCHC Packet
-.1.2.  Compressed Packet Size as a Side Channel to Guess a
-              Secret Token
-.1.3.  Decompressed Packet Different from the Original Packet
-.2.  Security Considerations for SCHC Fragmentation/Reassembly
-.2.1.  Buffer Reservation Attack
-.2.2.  Corrupt Fragment Attack
-.2.3.  Fragmentation as a Way to Bypass Network Inspection
-.2.4.  Privacy Issues Associated with SCHC Header Fields
-. References
-.1.  Normative References
-.2.  Informative References
-  Appendix A.  Compression Examples
-  Appendix B.  Fragmentation Examples
-  Appendix C.  Fragmentation State Machines
-  Appendix D.  SCHC Parameters
-  Appendix E.  Supporting Multiple Window Sizes for Fragmentation
-  Appendix F.  ACK-Always and ACK-on-Error on Quasi-Bidirectional
-          Links
-  Acknowledgements
-  Authors' Addresses
-.  Introduction
+LPWAN technologies impose some strict limitations on traffic.  For
+instance, devices sleep most of the time and may only receive data
+during short periods of time after transmission, in order to preserve
+battery.  LPWAN technologies are also characterized by a greatly
+reduced data unit and/or payload size (see [[RFC8376]]).
-  This document defines the Static Context Header Compression and
+Header compression is needed for efficient Internet connectivity to a
-  fragmentation (SCHC) framework, which provides both a header
+node within an LPWAN.  The following properties of LPWANs can be
-  compression mechanism and an optional fragmentation mechanism.  SCHC
+exploited to get an efficient header compression:
-  has been designed with Low-Power Wide Area Networks (LPWANs) in mind.
-  LPWAN technologies impose some strict limitations on traffic.  For
+*  The network topology is star-oriented, which means that all
-   instance, devices sleep most of the time and may only receive data
+   packets between the same source-destination pair follow the same
-   during short periods of time after transmission, in order to preserve
+   path.  For the needs of this document, the architecture can simply
-   battery.  LPWAN technologies are also characterized by a greatly
+   be described as Devices (Dev) exchanging information with LPWAN
-   reduced data unit and/or payload size (see [RFC8376]).
+   Application Servers (Apps) through a Network Gateway (NGW).
-  Header compression is needed for efficient Internet connectivity to a
+*  Because devices embed built-in applications, the traffic flows to
-   node within an LPWAN.  The following properties of LPWANs can be
+   be compressed are known in advance.  Indeed, new applications are
-   exploited to get an efficient header compression:
+   less frequently installed in an LPWAN device than they are in a
+  general-purpose computer or smartphone.
-  *  The network topology is star-oriented, which means that all
+SCHC compression uses a Context (a set of Rules) in which information
-      packets between the same source-destination pair follow the same
+about header fields is stored.  This Context is static: the values of
-      path.  For the needs of this document, the architecture can simply
+the header fields and the actions to do compression/decompression do
-      be described as Devices (Dev) exchanging information with LPWAN
+not change over time.  This avoids the need for complex
-      Application Servers (Apps) through a Network Gateway (NGW).
+resynchronization mechanisms.  Indeed, a return path may be more
+restricted/expensive, or may sometimes be completely unavailable
+[[RFC8376]].  A compression protocol that relies on feedback is not
+compatible with the characteristics of such LPWANs.
-  *  Because devices embed built-in applications, the traffic flows to
+In most cases, a small Rule identifier is enough to represent the
-      be compressed are known in advance.  Indeed, new applications are
+full IPv6/UDP headers.  The SCHC header compression mechanism is
-      less frequently installed in an LPWAN device than they are in a
+independent of the specific LPWAN technology over which it is used.
-      general-purpose computer or smartphone.
-  SCHC compression uses a Context (a set of Rules) in which information
+Furthermore, some LPWAN technologies do not provide a fragmentation
-  about header fields is stored.  This Context is static: the values of
+functionality; to support the IPv6 MTU requirement of 1280 bytes
-  the header fields and the actions to do compression/decompression do
+[[RFC8200]], they require a fragmentation protocol at the adaptation
-  not change over time.  This avoids the need for complex
+layer below IPv6.  Accordingly, this document defines an optional
-  resynchronization mechanisms.  Indeed, a return path may be more
+fragmentation/reassembly mechanism to help LPWAN technologies support
-  restricted/expensive, or may sometimes be completely unavailable
+the IPv6 MTU requirement.
-  [RFC8376].  A compression protocol that relies on feedback is not
-  compatible with the characteristics of such LPWANs.
-  In most cases, a small Rule identifier is enough to represent the
+This document defines generic functionality and offers flexibility
-  full IPv6/UDP headers.  The SCHC header compression mechanism is
+with regard to parameter settings and mechanism choices.  Technology-
-  independent of the specific LPWAN technology over which it is used.
+specific settings are expected to be grouped into Profiles specified
+in other documents.
-  Furthermore, some LPWAN technologies do not provide a fragmentation
+== Requirements Notation ==
-  functionality; to support the IPv6 MTU requirement of 1280 bytes
-  [RFC8200], they require a fragmentation protocol at the adaptation
-  layer below IPv6.  Accordingly, this document defines an optional
-  fragmentation/reassembly mechanism to help LPWAN technologies support
-  the IPv6 MTU requirement.
-  This document defines generic functionality and offers flexibility
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
-  with regard to parameter settings and mechanism choices.  Technology-
+"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
-  specific settings are expected to be grouped into Profiles specified
+"OPTIONAL" in this document are to be interpreted as described in
-  in other documents.
+[[BCP14|BCP 14]] [[RFC2119]] [[RFC8174]] when, and only when, they appear in all
+capitals, as shown here.
-.  Requirements Notation
+== LPWAN Architecture ==
-  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+LPWAN architectures are similar among them, but each LPWAN technology
-  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
+names architecture elements differently.  In this document, we use
-  "OPTIONAL" in this document are to be interpreted as described in
+terminology from [[RFC8376]], which identifies the following entities
-  BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
+in a typical LPWAN (see Figure 1):
-  capitals, as shown here.
-.  LPWAN Architecture
+*  Devices (Dev) are the end-devices or hosts (e.g., sensors,
+  actuators, etc.).  There can be a very high density of devices per
+  Radio Gateway.
-  LPWAN architectures are similar among them, but each LPWAN technology
+*  The Radio Gateway (RGW) is the endpoint of the constrained link.
-  names architecture elements differently.  In this document, we use
-  terminology from [RFC8376], which identifies the following entities
-  in a typical LPWAN (see Figure 1):
-  *  Devices (Dev) are the end-devices or hosts (e.g., sensors,
+*  The Network Gateway (NGW) is the interconnection node between the
-      actuators, etc.).  There can be a very high density of devices per
+  Radio Gateway and the Internet.
-      Radio Gateway.
-  *  The Radio Gateway (RGW) is the endpoint of the constrained link.
+*  The Application Server (App) is the endpoint of the application-
+  level protocol on the Internet side.
-   *  The Network Gateway (NGW) is the interconnection node between the
+ ()  ()  ()      |
-      Radio Gateway and the Internet.
+  ()  () () ()    / \      +---------+
+() () () () () () /  \======|    ^    |            +-----------+
+ ()  ()  ()    |          | <--|--> |            |Application|
+()  ()  ()  ()  / \==========|   v    |=============|  Server  |
+  ()  ()  ()   /  \        +---------+            +-----------+
+ Dev            RGWs            NGW                      App
-  *  The Application Server (App) is the endpoint of the application-
+Figure 1: LPWAN Architecture (Simplified from That Shown in [[RFC8376|RFC 8376]])
-      level protocol on the Internet side.
-    ()  ()  ()      |
+== Terminology ==
-    ()  () () ()    / \      +---------+
-  () () () () () () /  \======|    ^    |            +-----------+
-    ()  ()  ()    |          | <--|--> |            |Application|
-  ()  ()  ()  ()  / \==========|    v    |=============|  Server  |
-    ()  ()  ()  /  \        +---------+            +-----------+
-    Dev            RGWs            NGW                      App
-  Figure 1: LPWAN Architecture (Simplified from That Shown in RFC 8376)
+This section defines terminology and abbreviations used in this
+document.  It extends the terminology of [[RFC8376]].
-.  Terminology
+The SCHC acronym is pronounced like "sheek" in English (or "chic" in
+French).  Therefore, this document writes "a SCHC Packet" instead of
+"an SCHC Packet".
-  This section defines terminology and abbreviations used in this
+App:    LPWAN Application Server, as defined by [[RFC8376]].  It runs
-  document.  It extends the terminology of [RFC8376].
+        an application sending/receiving packets to/from the Dev.
-  The SCHC acronym is pronounced like "sheek" in English (or "chic" in
+AppIID:  Application Interface Identifier.  The IID that identifies
-  French).  Therefore, this document writes "a SCHC Packet" instead of
+        the App interface.
-  "an SCHC Packet".
-  App:     LPWAN Application Server, as defined by [RFC8376].  It runs
+Compression Residue:  The bits that remain to be sent (beyond the
-            an application sending/receiving packets to/from the Dev.
+        RuleID itself) after applying the SCHC compression.
-  AppIID:  Application Interface Identifier.  The IID that identifies
+Context:  A set of Rules used to compress/decompress headers, or to
-            the App interface.
+        fragment/reassemble a packet.
-  Compression Residue:  The bits that remain to be sent (beyond the
+Dev:     Device, as defined by [[RFC8376]].
-            RuleID itself) after applying the SCHC compression.
-  Context:  A set of Rules used to compress/decompress headers, or to
+DevIID:  Device Interface Identifier.  The IID that identifies the
-            fragment/reassemble a packet.
+        Dev interface.
-  Dev:    Device, as defined by [RFC8376].
+Downlink:  From the App to the Dev.
-  DevIID:  Device Interface Identifier.  The IID that identifies the
+IID:     Interface Identifier.  See the IPv6 addressing architecture
-            Dev interface.
+        [[RFC7136]].
-  Downlink:  From the App to the Dev.
+L2:     Layer 2.  The immediate lower layer that SCHC interfaces
+        with, for example an underlying LPWAN technology.  It does
+        not necessarily correspond to the OSI model definition of
+        Layer 2.
-  IID:     Interface Identifier.  See the IPv6 addressing architecture
+L2 Word:  This is the minimum subdivision of payload data that the L2
-            [RFC7136].
+        will carry.  In most L2 technologies, the L2 Word is an
+        octet.  In bit-oriented radio technologies, the L2 Word
+        might be a single bit.  The L2 Word size is assumed to be
+        constant over time for each device.
-  L2:     Layer 2.  The immediate lower layer that SCHC interfaces
+Padding:  Extra bits that may be appended by SCHC to a data unit that
-            with, for example an underlying LPWAN technology.  It does
+        it passes down to L2 for transmission.  SCHC itself operates
-            not necessarily correspond to the OSI model definition of
+        on bits, not bytes, and does not have any alignment
-            Layer 2.
+        prerequisite.  See Section 9.
-  L2 Word:  This is the minimum subdivision of payload data that the L2
+Profile:  SCHC offers variations in the way it is operated, with a
-            will carry.  In most L2 technologies, the L2 Word is an
+        number of parameters listed in Appendix D.  A Profile
-            octet.  In bit-oriented radio technologies, the L2 Word
+        indicates a particular setting of all these parameters.
-            might be a single bit.  The L2 Word size is assumed to be
+        Both ends of a SCHC communication must be provisioned with
-            constant over time for each device.
+        the same Profile information and with the same set of Rules
+        before the communication starts, so that there is no
+        ambiguity in how they expect to communicate.
-   Padding:  Extra bits that may be appended by SCHC to a data unit that
+Rule:   Part of the Context that describes how a packet is
-            it passes down to L2 for transmission.  SCHC itself operates
+        compressed/decompressed or fragmented/reassembled.
-            on bits, not bytes, and does not have any alignment
-            prerequisite.  See Section 9.
-  Profile:  SCHC offers variations in the way it is operated, with a
+RuleID:  Rule Identifier.  An identifier for a Rule.
-            number of parameters listed in Appendix D.  A Profile
-            indicates a particular setting of all these parameters.
-            Both ends of a SCHC communication must be provisioned with
-            the same Profile information and with the same set of Rules
-            before the communication starts, so that there is no
-            ambiguity in how they expect to communicate.
-  Rule:    Part of the Context that describes how a packet is
+SCHC:    Static Context Header Compression and fragmentation (SCHC),
-            compressed/decompressed or fragmented/reassembled.
+        a generic framework.
-  RuleID:  Rule Identifier.  An identifier for a Rule.
+SCHC C/D:  SCHC Compressor/Decompressor, or SCHC Compression/
+        Decompression.  The SCHC entity or mechanism used on both
+        sides, at the Dev and at the network, to achieve
+        compression/decompression of headers.
-  SCHC:   Static Context Header Compression and fragmentation (SCHC),
+SCHC F/R:  SCHC Fragmenter/Reassembler or SCHC Fragmentation/
-            a generic framework.
+        Reassembly.  The SCHC entity or mechanism used on both
+        sides, at the Dev and at the network, to achieve
+        fragmentation/reassembly of SCHC Packets.
-  SCHC C/D:  SCHC Compressor/Decompressor, or SCHC Compression/
+SCHC Packet:  A packet (e.g., an IPv6 packet) whose header has been
-            Decompression.  The SCHC entity or mechanism used on both
+        compressed as per the header compression mechanism defined
-            sides, at the Dev and at the network, to achieve
+        in this document.  If the header compression process is
-            compression/decompression of headers.
+        unable to actually compress the packet header, the packet
+        with the uncompressed header is still called a SCHC Packet
+        (in this case, a RuleID is used to indicate that the packet
+        header has not been compressed).  See Section 7 for more
+        details.
-  SCHC F/R:  SCHC Fragmenter/Reassembler or SCHC Fragmentation/
+Uplink:  From the Dev to the App.
-            Reassembly.  The SCHC entity or mechanism used on both
-            sides, at the Dev and at the network, to achieve
-            fragmentation/reassembly of SCHC Packets.
-  SCHC Packet:  A packet (e.g., an IPv6 packet) whose header has been
+Additional terminology for the optional SCHC F/R is found in
-            compressed as per the header compression mechanism defined
+Section 8.2.
-            in this document.  If the header compression process is
-            unable to actually compress the packet header, the packet
-            with the uncompressed header is still called a SCHC Packet
-            (in this case, a RuleID is used to indicate that the packet
-            header has not been compressed).  See Section 7 for more
-            details.
-  Uplink:  From the Dev to the App.
+Additional terminology for SCHC C/D is found in Section 7.1.
-  Additional terminology for the optional SCHC F/R is found in
+== SCHC Overview ==
-  Section 8.2.
-  Additional terminology for SCHC C/D is found in Section 7.1.
+SCHC can be characterized as an adaptation layer between an upper
+layer (for example, IPv6) and an underlying layer (for example, an
+LPWAN technology).  SCHC comprises two sublayers (i.e., the
+Compression sublayer and the Fragmentation sublayer), as shown in
+Figure 2.
-.  SCHC Overview
+            +----------------+
+            |      IPv6      |
+          +- +----------------+
+          |  |  Compression  |
+    SCHC <  +----------------+
+          |  |  Fragmentation |
+          +- +----------------+
+            |LPWAN technology|
+            +----------------+
-  SCHC can be characterized as an adaptation layer between an upper
+  Figure 2: Example of Protocol Stack Comprising IPv6, SCHC, and an
-  layer (for example, IPv6) and an underlying layer (for example, an
+                          LPWAN Technology
-  LPWAN technology).  SCHC comprises two sublayers (i.e., the
-  Compression sublayer and the Fragmentation sublayer), as shown in
-  Figure 2.
-                +----------------+
+Before an upper layer packet (e.g., an IPv6 packet) is transmitted to
-                |      IPv6     |
+the underlying layer, header compression is first attempted.  The
-            +- +----------------+
+resulting packet is called a "SCHC Packet", whether or not any
-            |  |  Compression  |
+compression is performed.  If needed by the underlying layer, the
-      SCHC <  +----------------+
+optional SCHC fragmentation MAY be applied to the SCHC Packet.  The
-            |  |  Fragmentation |
+inverse operations take place at the receiver.  This process is
-            +- +----------------+
+illustrated in Figure 3.
-                |LPWAN technology|
-                +----------------+
-    Figure 2: Example of Protocol Stack Comprising IPv6, SCHC, and an
+A packet (e.g., an IPv6 packet)
-                              LPWAN Technology
+        |                                          ^
+        v                                          |
++------------------+                      +--------------------+
+| SCHC Compression |                      | SCHC Decompression |
++------------------+                      +--------------------+
+        |                                          ^
+        |  If no fragmentation (*)                |
+        +-------------- SCHC Packet  -------------->|
+        |                                          |
+        v                                          |
++--------------------+                      +-----------------+
+| SCHC Fragmentation |                      | SCHC Reassembly |
++--------------------+                      +-----------------+
+      |    ^                                    |    ^
+      |    |                                    |    |
+      |    +---------- SCHC ACK (+) -------------+    |
+      |                                                |
+      +-------------- SCHC Fragments -------------------+
-  Before an upper layer packet (e.g., an IPv6 packet) is transmitted to
+        Sender                                    Receiver
-  the underlying layer, header compression is first attempted.  The
-  resulting packet is called a "SCHC Packet", whether or not any
-  compression is performed.  If needed by the underlying layer, the
-  optional SCHC fragmentation MAY be applied to the SCHC Packet.  The
-  inverse operations take place at the receiver.  This process is
-  illustrated in Figure 3.
-  A packet (e.g., an IPv6 packet)
+*: the decision not to use SCHC fragmentation is left to each Profile
-            |                                          ^
++: optional, depends on Fragmentation mode
-            v                                          |
-  +------------------+                      +--------------------+
-  | SCHC Compression |                      | SCHC Decompression |
-  +------------------+                      +--------------------+
-            |                                          ^
-            |  If no fragmentation (*)                |
-            +-------------- SCHC Packet  -------------->|
-            |                                          |
-            v                                          |
-  +--------------------+                      +-----------------+
-  | SCHC Fragmentation |                      | SCHC Reassembly |
-  +--------------------+                      +-----------------+
-        |    ^                                    |    ^
-        |    |                                    |    |
-        |    +---------- SCHC ACK (+) -------------+    |
-        |                                                |
-        +-------------- SCHC Fragments -------------------+
-          Sender                                   Receiver
+      Figure 3: SCHC Operations at the Sender and the Receiver
-  *: the decision not to use SCHC fragmentation is left to each Profile
+=== SCHC Packet Format ===
-  +: optional, depends on Fragmentation mode
-          Figure 3: SCHC Operations at the Sender and the Receiver
+The SCHC Packet is composed of the Compressed Header followed by the
+payload from the original packet (see Figure 4).  The Compressed
+Header itself is composed of the RuleID and a Compression Residue,
+which is the output of compressing the packet header with the Rule
+identified by that RuleID (see Section 7).  The Compression Residue
+may be empty.  Both the RuleID and the Compression Residue
+potentially have a variable size, and are not necessarily a multiple
+of bytes in size.
-.1.  SCHC Packet Format
+|------- Compressed Header -------|
++---------------------------------+--------------------+
+|  RuleID  |  Compression Residue |      Payload      |
++---------------------------------+--------------------+
-  The SCHC Packet is composed of the Compressed Header followed by the
+                        Figure 4: SCHC Packet
-  payload from the original packet (see Figure 4).  The Compressed
-  Header itself is composed of the RuleID and a Compression Residue,
-  which is the output of compressing the packet header with the Rule
-  identified by that RuleID (see Section 7).  The Compression Residue
-  may be empty.  Both the RuleID and the Compression Residue
-  potentially have a variable size, and are not necessarily a multiple
-  of bytes in size.
-  |------- Compressed Header -------|
+=== Functional Mapping ===
-  +---------------------------------+--------------------+
-  |  RuleID  |  Compression Residue |      Payload      |
-  +---------------------------------+--------------------+
-                          Figure 4: SCHC Packet
+Figure 5 maps the functional elements of Figure 3 onto the LPWAN
+architecture elements of Figure 1.
-.2.  Functional Mapping
+        Dev                                              App
++----------------+                              +----+ +----+ +----+
+| App1 App2 App3 |                              |App1| |App2| |App3|
+|                |                              |    | |    | |    |
+|      UDP      |                              |UDP | |UDP | |UDP |
+|      IPv6      |                              |IPv6| |IPv6| |IPv6|
+|                |                              |    | |    | |    |
+|SCHC C/D and F/R|                              |    | |    | |    |
++--------+-------+                              +----+ +----+ +----+
+        |  +---+    +---+    +----+    +----+    .     .      .
-   Figure 5 maps the functional elements of Figure 3 onto the LPWAN
+            +---+    +---+   |F/R |    |C/D |
-   architecture elements of Figure 1.
+                              +----+   +----+
-          Dev                                              App
+                  Figure 5: Architectural Mapping
-  +----------------+                              +----+ +----+ +----+
-  | App1 App2 App3 |                              |App1| |App2| |App3|
-  |                |                              |    | |    | |    |
-  |      UDP      |                              |UDP | |UDP | |UDP |
-  |      IPv6      |                              |IPv6| |IPv6| |IPv6|
-  |                |                              |    | |    | |    |
-  |SCHC C/D and F/R|                              |    | |    | |    |
-  +--------+-------+                              +----+ +----+ +----+
-            |  +---+    +---+    +----+    +----+    .      .      .
-            +~ |RGW| === |NGW| == |SCHC| == |SCHC|..... Internet ....
-              +---+    +---+    |F/R |    |C/D |
-                                  +----+    +----+
-                      Figure 5: Architectural Mapping
+SCHC C/D and SCHC F/R are located on both sides of the LPWAN
+transmission, hereafter called the "Dev side" and the "Network
+Infrastructure side".
-  SCHC C/D and SCHC F/R are located on both sides of the LPWAN
+The operation in the Uplink direction is as follows.  The Device
-  transmission, hereafter called the "Dev side" and the "Network
+application uses IPv6 or IPv6/UDP protocols.  Before sending the
-  Infrastructure side".
+packets, the Dev compresses their headers using SCHC C/D; if the SCHC
+Packet resulting from the compression needs to be fragmented by SCHC,
+SCHC F/R is performed (see Section 8).  The resulting SCHC Fragments
+are sent to an LPWAN Radio Gateway (RGW), which forwards them to a
+Network Gateway (NGW).  The NGW sends the data to a SCHC F/R for
+reassembly (if needed) and then to the SCHC C/D for decompression.
+After decompression, the packet can be sent over the Internet to one
+or several Apps.
-  The operation in the Uplink direction is as follows.  The Device
+The SCHC F/R and SCHC C/D on the Network Infrastructure side can be
-  application uses IPv6 or IPv6/UDP protocols.  Before sending the
+part of the NGW or located in the Internet as long as a tunnel is
-  packets, the Dev compresses their headers using SCHC C/D; if the SCHC
+established between them and the NGW.  For some LPWAN technologies,
-  Packet resulting from the compression needs to be fragmented by SCHC,
+it may be suitable to locate the SCHC F/R functionality nearer the
-  SCHC F/R is performed (see Section 8).  The resulting SCHC Fragments
+NGW, in order to better deal with time constraints of such
-  are sent to an LPWAN Radio Gateway (RGW), which forwards them to a
+technologies.
-  Network Gateway (NGW).  The NGW sends the data to a SCHC F/R for
-  reassembly (if needed) and then to the SCHC C/D for decompression.
-  After decompression, the packet can be sent over the Internet to one
-  or several Apps.
-  The SCHC F/R and SCHC C/D on the Network Infrastructure side can be
+The SCHC C/Ds on both sides MUST share the same set of Rules.  So
-  part of the NGW or located in the Internet as long as a tunnel is
+MUST the SCHC F/Rs on both sides.
-  established between them and the NGW.  For some LPWAN technologies,
-  it may be suitable to locate the SCHC F/R functionality nearer the
-  NGW, in order to better deal with time constraints of such
-  technologies.
-  The SCHC C/Ds on both sides MUST share the same set of Rules.  So
+The operation in the Downlink direction is similar to that in the
-  MUST the SCHC F/Rs on both sides.
+Uplink direction, only reversing the order in which the architecture
+elements are traversed.
-  The operation in the Downlink direction is similar to that in the
+== RuleID ==
-  Uplink direction, only reversing the order in which the architecture
-  elements are traversed.
-.  RuleID
+RuleIDs identify the Rules used for compression/decompression or for
+fragmentation/reassembly.
-  RuleIDs identify the Rules used for compression/decompression or for
+The scope of the RuleID of a compression/decompression Rule is the
-  fragmentation/reassembly.
+link between the SCHC C/D in a given Dev and the corresponding SCHC
+C/D in the Network Infrastructure side.  The scope of the RuleID of a
+fragmentation/reassembly Rule is the link between the SCHC F/R in a
+given Dev and the corresponding SCHC F/R in the Network
+Infrastructure side.  If such a link is bidirectional, the scope
+includes both directions.
-  The scope of the RuleID of a compression/decompression Rule is the
+The RuleIDs are therefore specific to the Context related to one Dev.
-  link between the SCHC C/D in a given Dev and the corresponding SCHC
+Hence, multiple Dev instances, which refer to different Contexts, MAY
-  C/D in the Network Infrastructure side.  The scope of the RuleID of a
+reuse the same RuleID for different Rules.  On the Network
-  fragmentation/reassembly Rule is the link between the SCHC F/R in a
+Infrastructure side, in order to identify the correct Rule to be
-  given Dev and the corresponding SCHC F/R in the Network
+applied to Uplink traffic, the SCHC C/D or SCHC F/R needs to
-  Infrastructure side.  If such a link is bidirectional, the scope
+associate the RuleID with the Dev identifier.  Similarly, for
-  includes both directions.
+Downlink traffic, the SCHC C/D or SCHC F/R on the Network
+Infrastructure side first needs to identify the destination Dev
+before looking for the appropriate Rule (and associated RuleID) in
+the Context of that Dev.
-  The RuleIDs are therefore specific to the Context related to one Dev.
+Inside their scopes, Rules for compression/decompression and Rules
-  Hence, multiple Dev instances, which refer to different Contexts, MAY
+for fragmentation/reassembly share the same RuleID space.
-  reuse the same RuleID for different Rules.  On the Network
-  Infrastructure side, in order to identify the correct Rule to be
-  applied to Uplink traffic, the SCHC C/D or SCHC F/R needs to
-  associate the RuleID with the Dev identifier.  Similarly, for
-  Downlink traffic, the SCHC C/D or SCHC F/R on the Network
-  Infrastructure side first needs to identify the destination Dev
-  before looking for the appropriate Rule (and associated RuleID) in
-  the Context of that Dev.
-  Inside their scopes, Rules for compression/decompression and Rules
+The size of the RuleIDs is not specified in this document, as it is
-  for fragmentation/reassembly share the same RuleID space.
+implementation-specific and can vary according to the LPWAN
+technology and the number of Rules, among other things.  It is
+defined in Profiles.
-  The size of the RuleIDs is not specified in this document, as it is
+The RuleIDs are used:
-  implementation-specific and can vary according to the LPWAN
-  technology and the number of Rules, among other things.  It is
-  defined in Profiles.
-   The RuleIDs are used:
+*  For SCHC C/D, to identify the Rule that is used to compress a
+   packet header.
-   *  For SCHC C/D, to identify the Rule that is used to compress a
+   -  At least one RuleID MUST be allocated to tagging packets for
-       packet header.
+      which SCHC compression was not possible (i.e., no matching
+       compression Rule was found).
-      -  At least one RuleID MUST be allocated to tagging packets for
+*  In SCHC F/R, to identify the specific mode and settings of
-        which SCHC compression was not possible (i.e., no matching
+  fragmentation/reassembly for one direction of data traffic (Uplink
-        compression Rule was found).
+  or Downlink).
-   *  In SCHC F/R, to identify the specific mode and settings of
+   -  When SCHC F/R is used for both communication directions, at
-       fragmentation/reassembly for one direction of data traffic (Uplink
+       least two RuleID values are needed for fragmentation/
-       or Downlink).
+      reassembly: one per direction of data traffic.  This is because
+       fragmentation/reassembly may entail control messages flowing in
+      the reverse direction compared to data traffic.
-      -  When SCHC F/R is used for both communication directions, at
+== Compression/Decompression ==
-        least two RuleID values are needed for fragmentation/
-        reassembly: one per direction of data traffic.  This is because
-        fragmentation/reassembly may entail control messages flowing in
-        the reverse direction compared to data traffic.
-.  Compression/Decompression
+Compression with SCHC is based on using a set of Rules, which
+constitutes the Context of SCHC C/D, to compress or decompress
-  Compression with SCHC is based on using a set of Rules, which
+headers.  SCHC avoids Context synchronization traffic, which consumes
-  constitutes the Context of SCHC C/D, to compress or decompress
+considerable bandwidth in other header compression mechanisms such as
-  headers.  SCHC avoids Context synchronization traffic, which consumes
+RObust Header Compression (RoHC) [[RFC5795]].  Since the content of
-  considerable bandwidth in other header compression mechanisms such as
+packets is highly predictable in LPWANs, static Contexts can be
-  RObust Header Compression (RoHC) [RFC5795].  Since the content of
+stored beforehand.  The Contexts MUST be stored at both ends, and
-  packets is highly predictable in LPWANs, static Contexts can be
+they can be learned by a provisioning protocol, by out-of-band means,
-  stored beforehand.  The Contexts MUST be stored at both ends, and
+or by pre-provisioning.  The way the Contexts are provisioned is out
-  they can be learned by a provisioning protocol, by out-of-band means,
+of the scope of this document.
-  or by pre-provisioning.  The way the Contexts are provisioned is out
-  of the scope of this document.
-.1.  SCHC C/D Rules
+=== SCHC C/D Rules ===
-  The main idea of the SCHC compression scheme is to transmit the
+The main idea of the SCHC compression scheme is to transmit the
-  RuleID to the other end instead of sending known field values.  This
+RuleID to the other end instead of sending known field values.  This
-  RuleID identifies a Rule that matches the original packet values.
+RuleID identifies a Rule that matches the original packet values.
-  Hence, when a value is known by both ends, it is only necessary to
+Hence, when a value is known by both ends, it is only necessary to
-  send the corresponding RuleID over the LPWAN.  The manner by which
+send the corresponding RuleID over the LPWAN.  The manner by which
-  Rules are generated is out of the scope of this document.  The Rules
+Rules are generated is out of the scope of this document.  The Rules
-  MAY be changed at run-time, but the mechanism is out of scope of this
+MAY be changed at run-time, but the mechanism is out of scope of this
-  document.
+document.
-  The SCHC C/D Context is a set of Rules.  See Figure 6 for a high-
+The SCHC C/D Context is a set of Rules.  See Figure 6 for a high-
-  level, abstract representation of the Context.  The formal
+level, abstract representation of the Context.  The formal
-  specification of the representation of the Rules is outside the scope
+specification of the representation of the Rules is outside the scope
-  of this document.
+of this document.
-  Each Rule itself contains a list of Field Descriptors composed of a
+Each Rule itself contains a list of Field Descriptors composed of a
-  Field Identifier (FID), a Field Length (FL), a Field Position (FP), a
+Field Identifier (FID), a Field Length (FL), a Field Position (FP), a
-  Direction Indicator (DI), a Target Value (TV), a Matching Operator
+Direction Indicator (DI), a Target Value (TV), a Matching Operator
-  (MO), and a Compression/Decompression Action (CDA).
+(MO), and a Compression/Decompression Action (CDA).
-    /-----------------------------------------------------------------\
+  /-----------------------------------------------------------------\
-    |                        Rule N                                  |
+  |                        Rule N                                  |
-    /-----------------------------------------------------------------\|
+ /-----------------------------------------------------------------\|
-    |                      Rule i                                    ||
+ |                      Rule i                                    ||
-  /-----------------------------------------------------------------\||
+/-----------------------------------------------------------------\||
-  |  (FID)            Rule 1                                        |||
+|  (FID)            Rule 1                                        |||
-  |+-------+--+--+--+------------+-----------------+---------------+|||
+|+-------+--+--+--+------------+-----------------+---------------+|||
-  ||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
+||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
-  |+-------+--+--+--+------------+-----------------+---------------+|||
+|+-------+--+--+--+------------+-----------------+---------------+|||
-  ||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
+||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
-  |+-------+--+--+--+------------+-----------------+---------------+|||
+|+-------+--+--+--+------------+-----------------+---------------+|||
-  ||...    |..|..|..|  ...      | ...            | ...          ||||
+||...    |..|..|..|  ...      | ...            | ...          ||||
-  |+-------+--+--+--+------------+-----------------+---------------+||/
+|+-------+--+--+--+------------+-----------------+---------------+||/
-  ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
+||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
-  |+-------+--+--+--+------------+-----------------+---------------+|/
+|+-------+--+--+--+------------+-----------------+---------------+|/
-  |                                                                |
+|                                                                |
-  \-----------------------------------------------------------------/
+\-----------------------------------------------------------------/
-                        Figure 6: A SCHC C/D Context
+                    Figure 6: A SCHC C/D Context
-  A Rule does not describe how the compressor parses a packet header to
+A Rule does not describe how the compressor parses a packet header to
-  find and identify each field (e.g., the IPv6 Source Address, the UDP
+find and identify each field (e.g., the IPv6 Source Address, the UDP
-  Destination Port, or a CoAP URI path option).  It is assumed that
+Destination Port, or a CoAP URI path option).  It is assumed that
-  there is a protocol parser alongside SCHC that is able to identify
+there is a protocol parser alongside SCHC that is able to identify
-  all the fields encountered in the headers to be compressed, and to
+all the fields encountered in the headers to be compressed, and to
-  label them with a Field ID.  Rules only describe the compression/
+label them with a Field ID.  Rules only describe the compression/
-  decompression behavior for each header field, after it has been
+decompression behavior for each header field, after it has been
-  identified.
+identified.
-  In a Rule, the Field Descriptors are listed in the order in which the
+In a Rule, the Field Descriptors are listed in the order in which the
-  fields appear in the packet header.  The Field Descriptors describe
+fields appear in the packet header.  The Field Descriptors describe
-  the header fields with the following entries:
+the header fields with the following entries:
-  *  Field Identifier (FID) designates a protocol and field (e.g., UDP
+*  Field Identifier (FID) designates a protocol and field (e.g., UDP
-      Destination Port), unambiguously among all protocols that a SCHC
+  Destination Port), unambiguously among all protocols that a SCHC
-      compressor processes.  In the presence of protocol nesting, the
+  compressor processes.  In the presence of protocol nesting, the
-      Field ID also identifies the nesting.
+  Field ID also identifies the nesting.
-  *  Field Length (FL) represents the length of the original field.  It
+*  Field Length (FL) represents the length of the original field.  It
-      can be either a fixed value (in bits) if the length is known when
+  can be either a fixed value (in bits) if the length is known when
-      the Rule is created or a type if the length is variable.  The
+  the Rule is created or a type if the length is variable.  The
-      length of a header field is defined by its own protocol
+  length of a header field is defined by its own protocol
-      specification (e.g., IPv6 or UDP).  If the length is variable, the
+  specification (e.g., IPv6 or UDP).  If the length is variable, the
-      type defines the process to compute the length and its unit (bits,
+  type defines the process to compute the length and its unit (bits,
-      bytes...).
+  bytes...).
-  *  Field Position (FP): most often, a field only occurs once in a
+*  Field Position (FP): most often, a field only occurs once in a
-      packet header.  However, some fields may occur multiple times.  An
+  packet header.  However, some fields may occur multiple times.  An
-      example is the uri-path of CoAP.  FP indicates which occurrence
+  example is the uri-path of CoAP.  FP indicates which occurrence
-      this Field Descriptor applies to.  The default value is 1.  The
+  this Field Descriptor applies to.  The default value is 1.  The
-      value 1 designates the first occurrence.  The value 0 is special.
+  value 1 designates the first occurrence.  The value 0 is special.
-      It means "don't care", see Section 7.2.
+  It means "don't care", see Section 7.2.
-  *  A Direction Indicator (DI) indicates the packet direction(s) this
+*  A Direction Indicator (DI) indicates the packet direction(s) this
-      Field Descriptor applies to.  It allows for asymmetric processing,
+  Field Descriptor applies to.  It allows for asymmetric processing,
-      using the same Rule.  Three values are possible:
+  using the same Rule.  Three values are possible:
-      Up:  this Field Descriptor is only applicable to packets traveling
+  Up:  this Field Descriptor is only applicable to packets traveling
-        Uplink.
+      Uplink.
-      Dw:  this Field Descriptor is only applicable to packets traveling
+  Dw:  this Field Descriptor is only applicable to packets traveling
-        Downlink.
+      Downlink.
-      Bi:  this Field Descriptor is applicable to packets traveling
+  Bi:  this Field Descriptor is applicable to packets traveling
-        Uplink or Downlink.
+      Uplink or Downlink.
-  *  Target Value (TV) is the value used to match against the packet
+*  Target Value (TV) is the value used to match against the packet
-      header field.  The Target Value can be a scalar value of any type
+  header field.  The Target Value can be a scalar value of any type
-      (integer, strings, etc.) or a more complex structure (array, list,
+  (integer, strings, etc.) or a more complex structure (array, list,
-      etc.).  The types and representations are out of scope for this
+  etc.).  The types and representations are out of scope for this
-      document.
+  document.
-  *  Matching Operator (MO) is the operator used to match the field
+*  Matching Operator (MO) is the operator used to match the field
-      value and the Target Value.  The Matching Operator may require
+  value and the Target Value.  The Matching Operator may require
-      some parameters.  The set of MOs defined in this document can be
+  some parameters.  The set of MOs defined in this document can be
-      found in Section 7.3.
+  found in Section 7.3.
-  *  Compression/Decompression Action (CDA) describes the pair of
+*  Compression/Decompression Action (CDA) describes the pair of
-      actions that are performed at the compressor to compress a header
+  actions that are performed at the compressor to compress a header
-      field and at the decompressor to recover the original value of the
+  field and at the decompressor to recover the original value of the
-      header field.  Some CDAs might use parameter values for their
+  header field.  Some CDAs might use parameter values for their
-      operation.  The set of CDAs defined in this document can be found
+  operation.  The set of CDAs defined in this document can be found
-      in Section 7.4.
+  in Section 7.4.
-.2.  Packet Processing
+=== Packet Processing ===
-  The compression/decompression process follows several phases:
+The compression/decompression process follows several phases:
-  Compression Rule selection:  the general idea is to browse the Rule
+Compression Rule selection:  the general idea is to browse the Rule
-      set to find a Rule that has a matching Field Descriptor (given the
+  set to find a Rule that has a matching Field Descriptor (given the
-      DI and FP) for all and only those header fields that appear in the
+  DI and FP) for all and only those header fields that appear in the
-      packet being compressed.  The detailed algorithm is the following:
+  packet being compressed.  The detailed algorithm is the following:
-      *  The first step is to check the FIDs.  If any header field of
+  *  The first step is to check the FIDs.  If any header field of
-        the packet being examined cannot be matched with a Field
+      the packet being examined cannot be matched with a Field
-        Descriptor with the correct FID, the Rule MUST be disregarded.
+      Descriptor with the correct FID, the Rule MUST be disregarded.
-        If any Field Descriptor in the Rule has a FID that cannot be
+      If any Field Descriptor in the Rule has a FID that cannot be
-        matched to one of the header fields of the packet being
+      matched to one of the header fields of the packet being
-        examined, the Rule MUST be disregarded.
+      examined, the Rule MUST be disregarded.
-      *  The next step is to match the Field Descriptors by their
+  *  The next step is to match the Field Descriptors by their
-        direction, using the DI.  If any field of the packet header
+      direction, using the DI.  If any field of the packet header
-        cannot be matched with a Field Descriptor with the correct FID
+      cannot be matched with a Field Descriptor with the correct FID
-        and DI, the Rule MUST be disregarded.
+      and DI, the Rule MUST be disregarded.
-      *  Then, the Field Descriptors are further selected according to
+  *  Then, the Field Descriptors are further selected according to
-        FP.  If any field of the packet header cannot be matched with a
+      FP.  If any field of the packet header cannot be matched with a
-        Field Descriptor with the correct FID, DI and FP, the Rule MUST
+      Field Descriptor with the correct FID, DI and FP, the Rule MUST
-        be disregarded.
+      be disregarded.
-        The value 0 for FP means "don't care", i.e., the comparison of
+      The value 0 for FP means "don't care", i.e., the comparison of
-        this Field Descriptor's FP with the position of the field of
+      this Field Descriptor's FP with the position of the field of
-        the packet header being compressed returns True, whatever that
+      the packet header being compressed returns True, whatever that
-        position.  FP=0 can be useful to build compression Rules for
+      position.  FP=0 can be useful to build compression Rules for
-        protocol headers in which some fields order is irrelevant.  An
+      protocol headers in which some fields order is irrelevant.  An
-        example could be uri-queries in CoAP.  Care needs to be
+      example could be uri-queries in CoAP.  Care needs to be
-        exercised when writing Rules containing FP=0 values.  Indeed,
+      exercised when writing Rules containing FP=0 values.  Indeed,
-        it may result in decompressed packets having fields ordered
+      it may result in decompressed packets having fields ordered
-        differently compared to the original packet.
+      differently compared to the original packet.
-      *  Once each header field has been associated with a Field
+  *  Once each header field has been associated with a Field
-        Descriptor with matching FID, DI, and FP, each packet field's
+      Descriptor with matching FID, DI, and FP, each packet field's
-        value is then compared to the corresponding TV stored in the
+      value is then compared to the corresponding TV stored in the
-        Rule for that specific field, using the MO.  If every field in
+      Rule for that specific field, using the MO.  If every field in
-        the packet header satisfies the corresponding MOs of a Rule
+      the packet header satisfies the corresponding MOs of a Rule
-        (i.e., all MO results are True), that Rule is valid for use to
+      (i.e., all MO results are True), that Rule is valid for use to
-        compress the header.  Otherwise, the Rule MUST be disregarded.
+      compress the header.  Otherwise, the Rule MUST be disregarded.
-        This specification does not prevent multiple Rules from
+      This specification does not prevent multiple Rules from
-        matching the above steps and, therefore, being valid for use.
+      matching the above steps and, therefore, being valid for use.
-        Which Rule to use among multiple valid Rules is left to the
+      Which Rule to use among multiple valid Rules is left to the
-        implementation.  As long as the same Rule set is installed at
+      implementation.  As long as the same Rule set is installed at
-        both ends, this degree of freedom does not constitute an
+      both ends, this degree of freedom does not constitute an
-        interoperability issue.
+      interoperability issue.
-      *  If no valid compression Rule is found, then the packet MUST be
+  *  If no valid compression Rule is found, then the packet MUST be
-        sent uncompressed using the RuleID dedicated to this purpose
+      sent uncompressed using the RuleID dedicated to this purpose
-        (see Section 6).  The entire packet header is the Compression
+      (see Section 6).  The entire packet header is the Compression
-        Residue (see Figure 4).  Sending an uncompressed header is
+      Residue (see Figure 4).  Sending an uncompressed header is
-        likely to require SCHC F/R.
+      likely to require SCHC F/R.
-  Compression:  if a valid Rule is found, each field of the header is
+Compression:  if a valid Rule is found, each field of the header is
-      compressed according to the CDAs of the Rule.  The fields are
+  compressed according to the CDAs of the Rule.  The fields are
-      compressed in the order that the Field Descriptors appear in the
+  compressed in the order that the Field Descriptors appear in the
-      Rule.  The compression of each field results in a residue, which
+  Rule.  The compression of each field results in a residue, which
-      may be empty.  The Compression Residue for the packet header is
+  may be empty.  The Compression Residue for the packet header is
-      the concatenation of the non-empty residues for each field of the
+  the concatenation of the non-empty residues for each field of the
-      header, in the order the Field Descriptors appear in the Rule.
+  header, in the order the Field Descriptors appear in the Rule.
-      The order in which the Field Descriptors appear in the Rule is
+  The order in which the Field Descriptors appear in the Rule is
-      therefore semantically important.
+  therefore semantically important.
-      |------------------- Compression Residue -------------------|
+    |------------------- Compression Residue -------------------|
-      +-----------------+-----------------+-----+-----------------+
+    +-----------------+-----------------+-----+-----------------+
-      | field 1 residue | field 2 residue | ... | field N residue |
+    | field 1 residue | field 2 residue | ... | field N residue |
-      +-----------------+-----------------+-----+-----------------+
+    +-----------------+-----------------+-----+-----------------+
-                  Figure 7: Compression Residue Structure
+              Figure 7: Compression Residue Structure
-  Sending:  The RuleID is sent to the other end jointly with the
+Sending:  The RuleID is sent to the other end jointly with the
-      Compression Residue (which could be empty) or the uncompressed
+  Compression Residue (which could be empty) or the uncompressed
-      header, and directly followed by the payload (see Figure 4).  The
+  header, and directly followed by the payload (see Figure 4).  The
-      way the RuleID is sent will be specified in the Profile and is out
+  way the RuleID is sent will be specified in the Profile and is out
-      of the scope of the present document.  For example, it could be
+  of the scope of the present document.  For example, it could be
-      included in an L2 header or sent as part of the L2 payload.
+  included in an L2 header or sent as part of the L2 payload.
-  Decompression:  when decompressing, on the Network Infrastructure
+Decompression:  when decompressing, on the Network Infrastructure
-      side, the SCHC C/D needs to find the correct Rule based on the L2
+  side, the SCHC C/D needs to find the correct Rule based on the L2
-      address of the Dev.  On the Dev side, only the RuleID is needed to
+  address of the Dev.  On the Dev side, only the RuleID is needed to
-      identify the correct Rule since the Dev typically only holds Rules
+  identify the correct Rule since the Dev typically only holds Rules
-      that apply to itself.
+  that apply to itself.
-      This Rule describes the compressed header format.  From this, the
+  This Rule describes the compressed header format.  From this, the
-      decompressor determines the order of the residues, the fixed-size
+  decompressor determines the order of the residues, the fixed-size
-      or variable-size nature of each residue (see Section 7.4.2), and
+  or variable-size nature of each residue (see Section 7.4.2), and
-      the size of the fixed-size residues.
+  the size of the fixed-size residues.
-      Therefore, from the received compressed header, it can retrieve
+  Therefore, from the received compressed header, it can retrieve
-      all the residue values and associate them to the corresponding
+  all the residue values and associate them to the corresponding
-      header fields.
+  header fields.
-      For each field in the header, the receiver applies the CDA action
+  For each field in the header, the receiver applies the CDA action
-      associated with that field in order to reconstruct the original
+  associated with that field in order to reconstruct the original
-      header field value.  The CDA application order can be different
+  header field value.  The CDA application order can be different
-      from the order in which the fields are listed in the Rule.  In
+  from the order in which the fields are listed in the Rule.  In
-      particular, Compute-* MUST be applied after the application of the
+  particular, Compute-* MUST be applied after the application of the
-      CDAs of all the fields it computes on.
+  CDAs of all the fields it computes on.
-.3.  Matching Operators
+=== Matching Operators ===
-  MOs are functions used at the compression side of SCHC C/D.  They are
+MOs are functions used at the compression side of SCHC C/D.  They are
-  not typed and can be applied to integer, string or any other data
+not typed and can be applied to integer, string or any other data
-  type.  The result of the operation can either be True or False.  The
+type.  The result of the operation can either be True or False.  The
-  following MOs are defined:
+following MOs are defined:
-  equal:  The match result is True if the field value in the packet
+equal:  The match result is True if the field value in the packet
-      matches the TV.
+  matches the TV.
-  ignore:  No matching is attempted between the field value in the
+ignore:  No matching is attempted between the field value in the
-      packet and the TV in the Rule.  The result is always True.
+  packet and the TV in the Rule.  The result is always True.
-  MSB(x):  A match is obtained if the most significant (leftmost) x
+MSB(x):  A match is obtained if the most significant (leftmost) x
-      bits of the packet header field value are equal to the TV in the
+  bits of the packet header field value are equal to the TV in the
-      Rule.  The x parameter of the MSB MO indicates how many bits are
+  Rule.  The x parameter of the MSB MO indicates how many bits are
-      involved in the comparison.  If the FL is described as variable,
+  involved in the comparison.  If the FL is described as variable,
-      the x parameter must be a multiple of the FL unit.  For example, x
+  the x parameter must be a multiple of the FL unit.  For example, x
-      must be multiple of 8 if the unit of the variable length is bytes.
+  must be multiple of 8 if the unit of the variable length is bytes.
-  match-mapping:  With match-mapping, TV is a list of values.  Each
+match-mapping:  With match-mapping, TV is a list of values.  Each
-      value of the list is identified by an index.  Compression is
+  value of the list is identified by an index.  Compression is
-      achieved by sending the index instead of the original header field
+  achieved by sending the index instead of the original header field
-      value.  This operator matches if the header field value is equal
+  value.  This operator matches if the header field value is equal
-      to one of the values in the target list.
+  to one of the values in the target list.
-.4.  Compression/Decompression Actions (CDA)
+=== Compression/Decompression Actions (CDA) ===
-  The CDA specifies the actions taken during the compression of header
+The CDA specifies the actions taken during the compression of header
-  fields and the inverse action taken by the decompressor to restore
+fields and the inverse action taken by the decompressor to restore
-  the original value.  The CDAs defined by this document are described
+the original value.  The CDAs defined by this document are described
-  in detail in Section 7.4.3 to Section 7.4.8.  They are summarized in
+in detail in Section 7.4.3 to Section 7.4.8.  They are summarized in
-  Table 1.
+Table 1.
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-    | Action      | Compression            | Decompression        |
+  | Action      | Compression            | Decompression        |
-    +==============+========================+=======================+
+  +==============+========================+=======================+
-    | not-sent    | elided                | use TV stored in Rule |
+  | not-sent    | elided                | use TV stored in Rule |
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-    | value-sent  | send                  | use received value    |
+  | value-sent  | send                  | use received value    |
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-    | mapping-sent | send index            | retrieve value from  |
+  | mapping-sent | send index            | retrieve value from  |
-    |              |                        | TV list              |
+  |              |                        | TV list              |
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-    | LSB          | send least significant | concatenate TV and    |
+  | LSB          | send least significant | concatenate TV and    |
-    |              | bits (LSB)            | received value        |
+  |              | bits (LSB)            | received value        |
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-    | compute-*    | elided                | recompute at          |
+  | compute-*    | elided                | recompute at          |
-    |              |                        | decompressor          |
+  |              |                        | decompressor          |
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-    | DevIID      | elided                | build IID from L2 Dev |
+  | DevIID      | elided                | build IID from L2 Dev |
-    |              |                        | addr                  |
+  |              |                        | addr                  |
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-    | AppIID      | elided                | build IID from L2 App |
+  | AppIID      | elided                | build IID from L2 App |
-    |              |                        | addr                  |
+  |              |                        | addr                  |
-    +--------------+------------------------+-----------------------+
+  +--------------+------------------------+-----------------------+
-              Table 1: Compression and Decompression Actions
+            Table 1: Compression and Decompression Actions
-  The first column shows the action's name.  The second and third
+The first column shows the action's name.  The second and third
-  columns show the compression and decompression behaviors for each
+columns show the compression and decompression behaviors for each
-  action.
+action.
-.4.1.  Processing Fixed-Length Fields
+==== Processing Fixed-Length Fields ====
-  If the field is identified in the Field Descriptor as being of fixed
+If the field is identified in the Field Descriptor as being of fixed
-  length, then applying the CDA to compress this field results in a
+length, then applying the CDA to compress this field results in a
-  fixed amount of bits.  The residue for that field is simply the bits
+fixed amount of bits.  The residue for that field is simply the bits
-  resulting from applying the CDA to the field.  This value may be
+resulting from applying the CDA to the field.  This value may be
-  empty (e.g., not-sent CDA), in which case the field residue is absent
+empty (e.g., not-sent CDA), in which case the field residue is absent
-  from the Compression Residue.
+from the Compression Residue.
-  |- field residue -|
+|- field residue -|
-  +-----------------+
++-----------------+
-  |      value      |
+|      value      |
-  +-----------------+
++-----------------+
-                Figure 8: Fixed-Size Field Residue Structure
+            Figure 8: Fixed-Size Field Residue Structure
-.4.2.  Processing Variable-Length Fields
+==== Processing Variable-Length Fields ====
-  If the field is identified in the Field Descriptor as being of
+If the field is identified in the Field Descriptor as being of
-  variable length, then applying the CDA to compress this field may
+variable length, then applying the CDA to compress this field may
-  result in a value of fixed size (e.g., not-sent or mapping-sent) or
+result in a value of fixed size (e.g., not-sent or mapping-sent) or
-  of variable size (e.g., value-sent or LSB).  In the latter case, the
+of variable size (e.g., value-sent or LSB).  In the latter case, the
-  residue for that field is the bits that result from applying the CDA
+residue for that field is the bits that result from applying the CDA
-  to the field, preceded with the size of the value.  The most
+to the field, preceded with the size of the value.  The most
-  significant bit of the size is stored to the left (leftmost bit of
+significant bit of the size is stored to the left (leftmost bit of
-  the residue field).
+the residue field).
-  |--- field residue ---|
+|--- field residue ---|
-  +-------+-------------+
++-------+-------------+
-  |  size |    value    |
+|  size |    value    |
-  +-------+-------------+
++-------+-------------+
-              Figure 9: Variable-Size Field Residue Structure
+          Figure 9: Variable-Size Field Residue Structure
-  The size (using the unit defined in the FL) is encoded on 4, 12, or
+The size (using the unit defined in the FL) is encoded on 4, 12, or
 bits as follows:
-  *  If the size is between 0 and 14, it is encoded as a 4-bit unsigned
+*  If the size is between 0 and 14, it is encoded as a 4-bit unsigned
-      integer.
+  integer.
-  *  Sizes between 15 and 254 are encoded as 0b1111 followed by the
+*  Sizes between 15 and 254 are encoded as 0b1111 followed by the
 -bit unsigned integer.
-  *  Larger sizes are encoded as 0xfff followed by the 16-bit unsigned
+*  Larger sizes are encoded as 0xfff followed by the 16-bit unsigned
-      integer.
+  integer.
-  If the field is identified in the Field Descriptor as being of
+If the field is identified in the Field Descriptor as being of
-  variable length and this field is not present in the packet header
+variable length and this field is not present in the packet header
-  being compressed, size 0 MUST be sent to denote its absence.
+being compressed, size 0 MUST be sent to denote its absence.
-.4.3.  Not-Sent CDA
+==== Not-Sent CDA ====
-  The not-sent action can be used when the field value is specified in
+The not-sent action can be used when the field value is specified in
-  a Rule and, therefore, known by both the Compressor and the
+a Rule and, therefore, known by both the Compressor and the
-  Decompressor.  This action SHOULD be used with the "equal" MO.  If MO
+Decompressor.  This action SHOULD be used with the "equal" MO.  If MO
-  is "ignore", there is a risk of having a decompressed field value
+is "ignore", there is a risk of having a decompressed field value
-  that is different from the original field that was compressed.
+that is different from the original field that was compressed.
-  The compressor does not send any residue for a field on which not-
+The compressor does not send any residue for a field on which not-
-  sent compression is applied.
+sent compression is applied.
-  The decompressor restores the field value with the TV stored in the
+The decompressor restores the field value with the TV stored in the
-  matched Rule identified by the received RuleID.
+matched Rule identified by the received RuleID.
-.4.4.  Value-Sent CDA
+==== Value-Sent CDA ====
-  The value-sent action can be used when the field value is not known
+The value-sent action can be used when the field value is not known
-  by both the Compressor and the Decompressor.  The field is sent in
+by both the Compressor and the Decompressor.  The field is sent in
-  its entirety, using the same bit order as in the original packet
+its entirety, using the same bit order as in the original packet
-  header.
+header.
-  If this action is performed on a variable-length field, the size of
+If this action is performed on a variable-length field, the size of
-  the residue value (using the units defined in FL) MUST be sent as
+the residue value (using the units defined in FL) MUST be sent as
-  described in Section 7.4.2.
+described in Section 7.4.2.
-  This action is generally used with the "ignore" MO.
+This action is generally used with the "ignore" MO.
-.4.5.  Mapping-Sent CDA
+==== Mapping-Sent CDA ====
-  The mapping-sent action is used to send an index (the index into the
+The mapping-sent action is used to send an index (the index into the
-  TV list of values) instead of the original value.  This action is
+TV list of values) instead of the original value.  This action is
-  used together with the "match-mapping" MO.
+used together with the "match-mapping" MO.
-  On the compressor side, the match-mapping MO searches the TV for a
+On the compressor side, the match-mapping MO searches the TV for a
-  match with the header field value.  The mapping-sent CDA then sends
+match with the header field value.  The mapping-sent CDA then sends
-  the corresponding index as the field residue.  The most significant
+the corresponding index as the field residue.  The most significant
-  bit of the index is stored to the left (leftmost bit of the residue
+bit of the index is stored to the left (leftmost bit of the residue
-  field).
+field).
-  On the decompressor side, the CDA uses the received index to restore
+On the decompressor side, the CDA uses the received index to restore
-  the field value by looking up the list in the TV.
+the field value by looking up the list in the TV.
-  The number of bits sent is the minimal size for coding all the
+The number of bits sent is the minimal size for coding all the
-  possible indices.
+possible indices.
-  The first element in the list MUST be represented by index value 0,
+The first element in the list MUST be represented by index value 0,
-  and successive elements in the list MUST have indices incremented by
+and successive elements in the list MUST have indices incremented by
 .
-.4.6.  LSB CDA
+==== LSB CDA ====
-  The LSB action is used together with the "MSB(x)" MO to avoid sending
+The LSB action is used together with the "MSB(x)" MO to avoid sending
-  the most significant part of the packet field if that part is already
+the most significant part of the packet field if that part is already
-  known by the receiving end.
+known by the receiving end.
-  The compressor sends the LSBs as the field residue value.  The number
+The compressor sends the LSBs as the field residue value.  The number
-  of bits sent is the original header field length minus the length
+of bits sent is the original header field length minus the length
-  specified in the MSB(x) MO.  The bits appear in the residue in the
+specified in the MSB(x) MO.  The bits appear in the residue in the
-  same bit order as in the original packet header.
+same bit order as in the original packet header.
-  The decompressor concatenates the x most significant bits of the TV
+The decompressor concatenates the x most significant bits of the TV
-  and the received residue value.
+and the received residue value.
-  If this action is performed on a variable-length field, the size of
+If this action is performed on a variable-length field, the size of
-  the residue value (using the units defined in FL) MUST be sent as
+the residue value (using the units defined in FL) MUST be sent as
-  described in Section 7.4.2.
+described in Section 7.4.2.
-.4.7.  DevIID, AppIID CDA
+==== DevIID, AppIID CDA ====
-  These actions are used to process the DevIID and AppIID of the IPv6
+These actions are used to process the DevIID and AppIID of the IPv6
-  addresses, respectively.  AppIID CDA is less common since most
+addresses, respectively.  AppIID CDA is less common since most
-  current LPWAN technologies frames contain a single L2 address, which
+current LPWAN technologies frames contain a single L2 address, which
-  is the Dev's address.
+is the Dev's address.
-  The DevIID value MAY be computed from the Dev ID present in the L2
+The DevIID value MAY be computed from the Dev ID present in the L2
-  header, or from some other stable identifier.  The computation is
+header, or from some other stable identifier.  The computation is
-  specific to each Profile and MAY depend on the Dev ID size.
+specific to each Profile and MAY depend on the Dev ID size.
-  In the Downlink direction, at the compressor, the DevIID CDA may be
+In the Downlink direction, at the compressor, the DevIID CDA may be
-  used to generate the L2 addresses on the LPWAN, based on the packet's
+used to generate the L2 addresses on the LPWAN, based on the packet's
-  Destination Address.
+Destination Address.
-.4.8.  Compute-*
+==== Compute-* ====
-  Some fields can be elided at the compressor and recomputed locally at
+Some fields can be elided at the compressor and recomputed locally at
-  the decompressor.
+the decompressor.
-  Because the field is uniquely identified by its FID (e.g., IPv6
+Because the field is uniquely identified by its FID (e.g., IPv6
-  length), the relevant protocol specification unambiguously defines
+length), the relevant protocol specification unambiguously defines
-  the algorithm for such computation.
+the algorithm for such computation.
-  An example of a field that knows how to recompute itself is IPv6
+An example of a field that knows how to recompute itself is IPv6
-  length.
+length.
-.  Fragmentation/Reassembly
+== Fragmentation/Reassembly ==
-.1.  Overview
+=== Overview ===
-  In LPWAN technologies, the L2 MTU typically ranges from tens to
+In LPWAN technologies, the L2 MTU typically ranges from tens to
-  hundreds of bytes.  Some of these technologies do not have an
+hundreds of bytes.  Some of these technologies do not have an
-  internal fragmentation/reassembly mechanism.
+internal fragmentation/reassembly mechanism.
-  The optional SCHC F/R functionality enables such LPWAN technologies
+The optional SCHC F/R functionality enables such LPWAN technologies
-  to comply with the IPv6 MTU requirement of 1280 bytes [RFC8200].  It
+to comply with the IPv6 MTU requirement of 1280 bytes [[RFC8200]].  It
-  is OPTIONAL to implement per this specification, but Profiles may
+is OPTIONAL to implement per this specification, but Profiles may
-  specify that it is REQUIRED.
+specify that it is REQUIRED.
-  This specification includes several SCHC F/R modes, which allow for a
+This specification includes several SCHC F/R modes, which allow for a
-  range of reliability options such as optional SCHC Fragment
+range of reliability options such as optional SCHC Fragment
-  retransmission.  More modes may be defined in the future.
+retransmission.  More modes may be defined in the future.
-  The same SCHC F/R mode MUST be used for all SCHC Fragments of a given
+The same SCHC F/R mode MUST be used for all SCHC Fragments of a given
-  SCHC Packet.  This document does not specify which mode(s) must be
+SCHC Packet.  This document does not specify which mode(s) must be
-  implemented and used over a specific LPWAN technology.  That
+implemented and used over a specific LPWAN technology.  That
-  information will be given in Profiles.
+information will be given in Profiles.
-  SCHC allows transmitting non-fragmented SCHC Packet concurrently with
+SCHC allows transmitting non-fragmented SCHC Packet concurrently with
-  fragmented SCHC Packets.  In addition, SCHC F/R provides protocol
+fragmented SCHC Packets.  In addition, SCHC F/R provides protocol
-  elements that allow transmitting several fragmented SCHC Packets
+elements that allow transmitting several fragmented SCHC Packets
-  concurrently, i.e., interleaving the transmission of fragments from
+concurrently, i.e., interleaving the transmission of fragments from
-  different fragmented SCHC Packets.  A Profile MAY restrict the latter
+different fragmented SCHC Packets.  A Profile MAY restrict the latter
-  behavior.
+behavior.
-  The L2 Word size (see Section 4) determines the encoding of some
+The L2 Word size (see Section 4) determines the encoding of some
-  messages.  SCHC F/R usually generates SCHC Fragments and SCHC ACKs
+messages.  SCHC F/R usually generates SCHC Fragments and SCHC ACKs
-  that are multiples of L2 Words.
+that are multiples of L2 Words.
-.2.  SCHC F/R Protocol Elements
+=== SCHC F/R Protocol Elements ===
-  This subsection describes the different elements that are used to
+This subsection describes the different elements that are used to
-  enable the SCHC F/R functionality defined in this document.  These
+enable the SCHC F/R functionality defined in this document.  These
-  elements include the SCHC F/R messages, tiles, windows, bitmaps,
+elements include the SCHC F/R messages, tiles, windows, bitmaps,
-  counters, timers, and header fields.
+counters, timers, and header fields.
-  The elements are described here in a generic manner.  Their
+The elements are described here in a generic manner.  Their
-  application to each SCHC F/R mode is found in Section 8.4.
+application to each SCHC F/R mode is found in Section 8.4.
-.2.1.  Messages
+==== Messages ====
-  SCHC F/R defines the following messages:
+SCHC F/R defines the following messages:
-  SCHC Fragment:  A message that carries part of a SCHC Packet from the
+SCHC Fragment:  A message that carries part of a SCHC Packet from the
-      sender to the receiver.
+  sender to the receiver.
-  SCHC ACK:  An acknowledgement for fragmentation, by the receiver to
+SCHC ACK:  An acknowledgement for fragmentation, by the receiver to
-      the sender.  This message is used to indicate whether or not the
+  the sender.  This message is used to indicate whether or not the
-      reception of pieces of, or the whole of, the fragmented SCHC
+  reception of pieces of, or the whole of, the fragmented SCHC
-      Packet was successful.
+  Packet was successful.
-  SCHC ACK REQ:  A request by the sender for a SCHC ACK from the
+SCHC ACK REQ:  A request by the sender for a SCHC ACK from the
-      receiver.
+  receiver.
-  SCHC Sender-Abort:  A message by the sender telling the receiver that
+SCHC Sender-Abort:  A message by the sender telling the receiver that
-      it has aborted the transmission of a fragmented SCHC Packet.
+  it has aborted the transmission of a fragmented SCHC Packet.
-  SCHC Receiver-Abort:  A message by the receiver to tell the sender to
+SCHC Receiver-Abort:  A message by the receiver to tell the sender to
-      abort the transmission of a fragmented SCHC Packet.
+  abort the transmission of a fragmented SCHC Packet.
-  The format of these messages is provided in Section 8.3.
+The format of these messages is provided in Section 8.3.
-.2.2.  Tiles, Windows, Bitmaps, Timers, Counters
+==== Tiles, Windows, Bitmaps, Timers, Counters ====
-.2.2.1.  Tiles
+===== Tiles =====
-  The SCHC Packet is fragmented into pieces, hereafter called "tiles".
+The SCHC Packet is fragmented into pieces, hereafter called "tiles".
-  The tiles MUST be non-empty and pairwise disjoint.  Their union MUST
+The tiles MUST be non-empty and pairwise disjoint.  Their union MUST
-  be equal to the SCHC Packet.
+be equal to the SCHC Packet.
-  See Figure 10 for an example.
+See Figure 10 for an example.
-                                  SCHC Packet
+                                SCHC Packet
-          +----+--+-----+---+----+-+---+-----+...-----+----+---+------+
+        +----+--+-----+---+----+-+---+-----+...-----+----+---+------+
-  Tiles  |    |  |    |  |    | |  |    |        |    |  |      |
+Tiles  |    |  |    |  |    | |  |    |        |    |  |      |
-          +----+--+-----+---+----+-+---+-----+...-----+----+---+------+
+        +----+--+-----+---+----+-+---+-----+...-----+----+---+------+
-                Figure 10: SCHC Packet Fragmented in Tiles
+              Figure 10: SCHC Packet Fragmented in Tiles
-  Modes (see Section 8.4) MAY place additional constraints on tile
+Modes (see Section 8.4) MAY place additional constraints on tile
-  sizes.
+sizes.
-  Each SCHC Fragment message carries at least one tile in its Payload,
+Each SCHC Fragment message carries at least one tile in its Payload,
-  if the Payload field is present.
+if the Payload field is present.
-.2.2.2.  Windows
+===== Windows =====
-  Some SCHC F/R modes may handle successive tiles in groups, called
+Some SCHC F/R modes may handle successive tiles in groups, called
-  windows.
+windows.
-  If windows are used:
+If windows are used:
-  *  all the windows of a SCHC Packet, except the last one, MUST
+*  all the windows of a SCHC Packet, except the last one, MUST
-      contain the same number of tiles.  This number is WINDOW_SIZE.
+  contain the same number of tiles.  This number is WINDOW_SIZE.
-  *  WINDOW_SIZE MUST be specified in a Profile.
+*  WINDOW_SIZE MUST be specified in a Profile.
-  *  the windows are numbered.
+*  the windows are numbered.
-  *  their numbers MUST increment by 1 from 0 upward, from the start of
+*  their numbers MUST increment by 1 from 0 upward, from the start of
-      the SCHC Packet to its end.
+  the SCHC Packet to its end.
-  *  the last window MUST contain WINDOW_SIZE tiles or less.
+*  the last window MUST contain WINDOW_SIZE tiles or less.
-  *  tiles are numbered within each window.
+*  tiles are numbered within each window.
-  *  the tile indices MUST decrement by 1 from WINDOW_SIZE - 1
+*  the tile indices MUST decrement by 1 from WINDOW_SIZE - 1
-      downward, looking from the start of the SCHC Packet toward its
+  downward, looking from the start of the SCHC Packet toward its
-      end.
+  end.
-  *  therefore, each tile of a SCHC Packet is uniquely identified by a
+*  therefore, each tile of a SCHC Packet is uniquely identified by a
-      window number and a tile index within this window.
+  window number and a tile index within this window.
-  See Figure 11 for an example.
+See Figure 11 for an example.
-          +---------------------------------------------...-----------+
+        +---------------------------------------------...-----------+
-          |                      SCHC Packet                        |
+        |                      SCHC Packet                        |
-          +---------------------------------------------...-----------+
+        +---------------------------------------------...-----------+
-  Tile#  | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 |    | 0 | 4 |3|
+Tile#  | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 |    | 0 | 4 |3|
-  Window# |-------- 0 --------|-------- 1 --------|- 2  ... 27 -|- 28-|
+Window# |-------- 0 --------|-------- 1 --------|- 2  ... 27 -|- 28-|
-    Figure 11: SCHC Packet Fragmented in Tiles Grouped in 29 Windows,
+  Figure 11: SCHC Packet Fragmented in Tiles Grouped in 29 Windows,
-                            with WINDOW_SIZE = 5
+                        with WINDOW_SIZE = 5
-  Appendix E discusses the benefits of selecting one among multiple
+Appendix E discusses the benefits of selecting one among multiple
-  window sizes depending on the size of the SCHC Packet to be
+window sizes depending on the size of the SCHC Packet to be
-  fragmented.
+fragmented.
-  When windows are used:
+When windows are used:
-  *  Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to
+*  Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to
-      the sender in a SCHC ACK message.
+  the sender in a SCHC ACK message.
-  *  A Bitmap corresponds to exactly one Window.
+*  A Bitmap corresponds to exactly one Window.
-.2.2.3.  Bitmaps
+===== Bitmaps =====
-  Each bit in the Bitmap for a window corresponds to a tile in the
+Each bit in the Bitmap for a window corresponds to a tile in the
-  window.  Therefore, each Bitmap has WINDOW_SIZE bits.  The bit at the
+window.  Therefore, each Bitmap has WINDOW_SIZE bits.  The bit at the
-  leftmost position corresponds to the tile numbered WINDOW_SIZE - 1.
+leftmost position corresponds to the tile numbered WINDOW_SIZE - 1.
-  Consecutive bits, going right, correspond to sequentially decreasing
+Consecutive bits, going right, correspond to sequentially decreasing
-  tile indices.  In Bitmaps for windows that are not the last one of a
+tile indices.  In Bitmaps for windows that are not the last one of a
-  SCHC Packet, the bit at the rightmost position corresponds to the
+SCHC Packet, the bit at the rightmost position corresponds to the
-  tile numbered 0.  In the Bitmap for the last window, the bit at the
+tile numbered 0.  In the Bitmap for the last window, the bit at the
-  rightmost position corresponds either to the tile numbered 0 or to a
+rightmost position corresponds either to the tile numbered 0 or to a
-  tile that is sent/received as "the last one of the SCHC Packet"
+tile that is sent/received as "the last one of the SCHC Packet"
-  without explicitly stating its number (see Section 8.3.1.2).
+without explicitly stating its number (see Section 8.3.1.2).
-  At the receiver:
+At the receiver:
-  *  a bit set to 1 in the Bitmap indicates that a tile associated with
+*  a bit set to 1 in the Bitmap indicates that a tile associated with
-      that bit position has been correctly received for that window.
+  that bit position has been correctly received for that window.
-  *  a bit set to 0 in the Bitmap indicates that there has been no tile
+*  a bit set to 0 in the Bitmap indicates that there has been no tile
-      correctly received, associated with that bit position, for that
+  correctly received, associated with that bit position, for that
-      window.  Possible reasons include that the tile was not sent at
+  window.  Possible reasons include that the tile was not sent at
-      all, not received, or received with errors.
+  all, not received, or received with errors.
-.2.2.4.  Timers and Counters
+===== Timers and Counters =====
-  Some SCHC F/R modes can use the following timers and counters:
+Some SCHC F/R modes can use the following timers and counters:
-  Inactivity Timer:  a SCHC Fragment receiver uses this timer to abort
+Inactivity Timer:  a SCHC Fragment receiver uses this timer to abort
-      waiting for a SCHC F/R message.
+  waiting for a SCHC F/R message.
-  Retransmission Timer:  a SCHC Fragment sender uses this timer to
+Retransmission Timer:  a SCHC Fragment sender uses this timer to
-      abort waiting for an expected SCHC ACK.
+  abort waiting for an expected SCHC ACK.
-  Attempts:  this counter counts the requests for SCHC ACKs, up to
+Attempts:  this counter counts the requests for SCHC ACKs, up to
-      MAX_ACK_REQUESTS.
+  MAX_ACK_REQUESTS.
-.2.3.  Integrity Checking
+==== Integrity Checking ====
-  The integrity of the fragmentation-reassembly process of a SCHC
+The integrity of the fragmentation-reassembly process of a SCHC
-  Packet MUST be checked at the receive end.  A Profile MUST specify
+Packet MUST be checked at the receive end.  A Profile MUST specify
-  how integrity checking is performed.
+how integrity checking is performed.
-  It is RECOMMENDED that integrity checking be performed by computing a
+It is RECOMMENDED that integrity checking be performed by computing a
-  Reassembly Check Sequence (RCS) based on the SCHC Packet at the
+Reassembly Check Sequence (RCS) based on the SCHC Packet at the
-  sender side and transmitting it to the receiver for comparison with
+sender side and transmitting it to the receiver for comparison with
-  the RCS locally computed after reassembly.
+the RCS locally computed after reassembly.
-  The RCS supports UDP checksum elision by SCHC C/D (see
+The RCS supports UDP checksum elision by SCHC C/D (see
-  Section 10.11).
+Section 10.11).
-  The CRC32 polynomial 0xEDB88320 (i.e., the reversed polynomial
+The CRC32 polynomial 0xEDB88320 (i.e., the reversed polynomial
-  representation, which is used in the Ethernet standard [ETHERNET]) is
+representation, which is used in the Ethernet standard [ETHERNET]) is
-  RECOMMENDED as the default algorithm for computing the RCS.
+RECOMMENDED as the default algorithm for computing the RCS.
-  The RCS MUST be computed on the full SCHC Packet concatenated with
+The RCS MUST be computed on the full SCHC Packet concatenated with
-  the padding bits, if any, of the SCHC Fragment carrying the last
+the padding bits, if any, of the SCHC Fragment carrying the last
-  tile.  The rationale is that the SCHC reassembler has no way of
+tile.  The rationale is that the SCHC reassembler has no way of
-  knowing the boundary between the last tile and the padding bits.
+knowing the boundary between the last tile and the padding bits.
-  Indeed, this requires decompressing the SCHC Packet, which is out of
+Indeed, this requires decompressing the SCHC Packet, which is out of
-  the scope of the SCHC reassembler.
+the scope of the SCHC reassembler.
-  The concatenation of the complete SCHC Packet and any padding bits,
+The concatenation of the complete SCHC Packet and any padding bits,
-  if present, of the last SCHC Fragment does not generally constitute
+if present, of the last SCHC Fragment does not generally constitute
-  an integer number of bytes.  CRC libraries are usually byte oriented.
+an integer number of bytes.  CRC libraries are usually byte oriented.
-  It is RECOMMENDED that the concatenation of the complete SCHC Packet
+It is RECOMMENDED that the concatenation of the complete SCHC Packet
-  and any last fragment padding bits be zero-extended to the next byte
+and any last fragment padding bits be zero-extended to the next byte
-  boundary and that the RCS be computed on that byte array.
+boundary and that the RCS be computed on that byte array.
-.2.4.  Header Fields
+==== Header Fields ====
-  The SCHC F/R messages contain the following fields (see the formats
+The SCHC F/R messages contain the following fields (see the formats
-  in Section 8.3):
+in Section 8.3):
-  RuleID:  this field is present in all the SCHC F/R messages.  The
+RuleID:  this field is present in all the SCHC F/R messages.  The
-      Rule identifies:
+  Rule identifies:
-      *  that a SCHC F/R message is being carried, as opposed to an
+  *  that a SCHC F/R message is being carried, as opposed to an
-        unfragmented SCHC Packet,
+      unfragmented SCHC Packet,
-      *  which SCHC F/R mode is used,
+  *  which SCHC F/R mode is used,
-      *  in case this mode uses windows, what the value of WINDOW_SIZE
+  *  in case this mode uses windows, what the value of WINDOW_SIZE
-        is, and
+      is, and
-      *  what other optional fields are present and what the field sizes
+  *  what other optional fields are present and what the field sizes
-        are.
+      are.
-      The Rule tells apart a non-fragmented SCHC Packet from SCHC
+  The Rule tells apart a non-fragmented SCHC Packet from SCHC
-      Fragments.  It will also tell apart SCHC Fragments of fragmented
+  Fragments.  It will also tell apart SCHC Fragments of fragmented
-      SCHC Packets that use different SCHC F/R modes or different
+  SCHC Packets that use different SCHC F/R modes or different
-      parameters.  Therefore, interleaved transmission of these is
+  parameters.  Therefore, interleaved transmission of these is
-      possible.
+  possible.
-      All SCHC F/R messages pertaining to the same SCHC Packet MUST bear
+  All SCHC F/R messages pertaining to the same SCHC Packet MUST bear
-      the same RuleID.
+  the same RuleID.
-  Datagram Tag (DTag):  This field allows differentiating SCHC F/R
+Datagram Tag (DTag):  This field allows differentiating SCHC F/R
-      messages belonging to different SCHC Packets that may be using the
+  messages belonging to different SCHC Packets that may be using the
-      same RuleID simultaneously.  Hence, it allows interleaving
+  same RuleID simultaneously.  Hence, it allows interleaving
-      fragments of a new SCHC Packet with fragments of a previous SCHC
+  fragments of a new SCHC Packet with fragments of a previous SCHC
-      Packet under the same RuleID.
+  Packet under the same RuleID.
-      The size of the DTag field (called "T", in bits) is defined by
+  The size of the DTag field (called "T", in bits) is defined by
-      each Profile for each RuleID.  When T is 0, the DTag field does
+  each Profile for each RuleID.  When T is 0, the DTag field does
-      not appear in the SCHC F/R messages and the DTag value is defined
+  not appear in the SCHC F/R messages and the DTag value is defined
-      as 0.
+  as 0.
-      When T is 0, there can be no more than one fragmented SCHC Packet
+  When T is 0, there can be no more than one fragmented SCHC Packet
-      in transit for each fragmentation RuleID.
+  in transit for each fragmentation RuleID.
-      If T is not 0, DTag:
+  If T is not 0, DTag:
-      *  MUST be set to the same value for all the SCHC F/R messages
+  *  MUST be set to the same value for all the SCHC F/R messages
-        related to the same fragmented SCHC Packet, and
+      related to the same fragmented SCHC Packet, and
-      *  MUST be set to different values for SCHC F/R messages related
+  *  MUST be set to different values for SCHC F/R messages related
-        to different SCHC Packets that are being fragmented under the
+      to different SCHC Packets that are being fragmented under the
-        same RuleID and whose transmission may overlap.
+      same RuleID and whose transmission may overlap.
-  W:  The W field is optional.  It is only present if windows are used.
+W:  The W field is optional.  It is only present if windows are used.
-      Its presence and size (called "M", in bits) is defined by each
+  Its presence and size (called "M", in bits) is defined by each
-      SCHC F/R mode and each Profile for each RuleID.
+  SCHC F/R mode and each Profile for each RuleID.
-      This field carries information pertaining to the window a SCHC F/R
+  This field carries information pertaining to the window a SCHC F/R
-      message relates to.  If present, W MUST carry the same value for
+  message relates to.  If present, W MUST carry the same value for
-      all the SCHC F/R messages related to the same window.  Depending
+  all the SCHC F/R messages related to the same window.  Depending
-      on the mode and Profile, W may carry the full window number, or
+  on the mode and Profile, W may carry the full window number, or
-      just the LSB or any other partial representation of the window
+  just the LSB or any other partial representation of the window
-      number.
+  number.
-  Fragment Compressed Number (FCN):  The FCN field is present in the
+Fragment Compressed Number (FCN):  The FCN field is present in the
-      SCHC Fragment Header.  Its size (called "N", in bits) is defined
+  SCHC Fragment Header.  Its size (called "N", in bits) is defined
-      by each Profile for each RuleID.
+  by each Profile for each RuleID.
-      This field conveys information about the progress in the sequence
+  This field conveys information about the progress in the sequence
-      of tiles being transmitted by SCHC Fragment messages.  For
+  of tiles being transmitted by SCHC Fragment messages.  For
-      example, it can contain a partial, efficient representation of a
+  example, it can contain a partial, efficient representation of a
-      larger-sized tile index.  The description of the exact use of the
+  larger-sized tile index.  The description of the exact use of the
-      FCN field is left to each SCHC F/R mode.  However, two values are
+  FCN field is left to each SCHC F/R mode.  However, two values are
-      reserved for special purposes.  They help control the SCHC F/R
+  reserved for special purposes.  They help control the SCHC F/R
-      process:
+  process:
-      *  The FCN value with all the bits equal to 1 (called "All-1")
+  *  The FCN value with all the bits equal to 1 (called "All-1")
-        signals that the very last tile of a SCHC Packet has been
+      signals that the very last tile of a SCHC Packet has been
-        transmitted.  By extension, if windows are used, the last
+      transmitted.  By extension, if windows are used, the last
-        window of a packet is called the "All-1" window.
+      window of a packet is called the "All-1" window.
-      *  If windows are used, the FCN value with all the bits equal to 0
+  *  If windows are used, the FCN value with all the bits equal to 0
-        (called "All-0") signals the last tile of a window that is not
+      (called "All-0") signals the last tile of a window that is not
-        the last one of the SCHC packet.  By extension, such a window
+      the last one of the SCHC packet.  By extension, such a window
-        is called an "All-0 window".
+      is called an "All-0 window".
-  Reassembly Check Sequence (RCS):  This field only appears in the
+Reassembly Check Sequence (RCS):  This field only appears in the
-      All-1 SCHC Fragments.  Its size (called "U", in bits) is defined
+  All-1 SCHC Fragments.  Its size (called "U", in bits) is defined
-      by each Profile for each RuleID.
+  by each Profile for each RuleID.
-      See Section 8.2.3 for the RCS default size, default polynomial and
+  See Section 8.2.3 for the RCS default size, default polynomial and
-      details on RCS computation.
+  details on RCS computation.
-  C (integrity Check):  C is a 1-bit field.  This field is used in the
+C (integrity Check):  C is a 1-bit field.  This field is used in the
-      SCHC ACK message to report on the reassembled SCHC Packet
+  SCHC ACK message to report on the reassembled SCHC Packet
-      integrity check (see Section 8.2.3).
+  integrity check (see Section 8.2.3).
-      A value of 1 tells that the integrity check was performed and is
+  A value of 1 tells that the integrity check was performed and is
-      successful.  A value of 0 tells that the integrity check was not
+  successful.  A value of 0 tells that the integrity check was not
-      performed or that it was a failure.
+  performed or that it was a failure.
-  Compressed Bitmap:  The Compressed Bitmap is used together with
+Compressed Bitmap:  The Compressed Bitmap is used together with
-      windows and Bitmaps (see Section 8.2.2.3).  Its presence and size
+  windows and Bitmaps (see Section 8.2.2.3).  Its presence and size
-      is defined for each SCHC F/R mode for each RuleID.
+  is defined for each SCHC F/R mode for each RuleID.
-      This field appears in the SCHC ACK message to report on the
+  This field appears in the SCHC ACK message to report on the
-      receiver Bitmap (see Section 8.3.2.1).
+  receiver Bitmap (see Section 8.3.2.1).
-.3.  SCHC F/R Message Formats
+=== SCHC F/R Message Formats ===
-  This section defines the SCHC Fragment formats, the SCHC ACK format,
+This section defines the SCHC Fragment formats, the SCHC ACK format,
-  the SCHC ACK REQ format and the SCHC Abort formats.
+the SCHC ACK REQ format and the SCHC Abort formats.
-.3.1.  SCHC Fragment Format
+==== SCHC Fragment Format ====
-  A SCHC Fragment conforms to the general format shown in Figure 12.
+A SCHC Fragment conforms to the general format shown in Figure 12.
-  It comprises a SCHC Fragment Header and a SCHC Fragment Payload.  The
+It comprises a SCHC Fragment Header and a SCHC Fragment Payload.  The
-  SCHC Fragment Payload carries one or several tile(s).
+SCHC Fragment Payload carries one or several tile(s).
-  +-----------------+-----------------------+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
++-----------------+-----------------------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-  | Fragment Header | Fragment Payload      | padding (as needed)
+| Fragment Header | Fragment Payload      | padding (as needed)
-  +-----------------+-----------------------+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
++-----------------+-----------------------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-                  Figure 12: SCHC Fragment General Format
+              Figure 12: SCHC Fragment General Format
-.3.1.1.  Regular SCHC Fragment
+===== Regular SCHC Fragment =====
-  The Regular SCHC Fragment format is shown in Figure 13.  Regular SCHC
+The Regular SCHC Fragment format is shown in Figure 13.  Regular SCHC
-  Fragments are generally used to carry tiles that are not the last one
+Fragments are generally used to carry tiles that are not the last one
-  of a SCHC Packet.  The DTag field and the W field are OPTIONAL, their
+of a SCHC Packet.  The DTag field and the W field are OPTIONAL, their
-  presence is specified by each mode and Profile.
+presence is specified by each mode and Profile.
-  |-- SCHC Fragment Header ----|
+|-- SCHC Fragment Header ----|
-            |-- T --|-M-|-- N --|
+        |-- T --|-M-|-- N --|
-  +-- ... -+- ... -+---+- ... -+--------...-------+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
++-- ... -+- ... -+---+- ... -+--------...-------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-  | RuleID | DTag  | W |  FCN  | Fragment Payload | padding (as needed)
+| RuleID | DTag  | W |  FCN  | Fragment Payload | padding (as needed)
-  +-- ... -+- ... -+---+- ... -+--------...-------+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
++-- ... -+- ... -+---+- ... -+--------...-------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-        Figure 13: Detailed Header Format for Regular SCHC Fragments
+    Figure 13: Detailed Header Format for Regular SCHC Fragments
-  The FCN field MUST NOT contain all bits set to 1.
+The FCN field MUST NOT contain all bits set to 1.
-  Profiles MUST ensure that a SCHC Fragment with FCN equal to 0 (called
+Profiles MUST ensure that a SCHC Fragment with FCN equal to 0 (called
-  an "All-0 SCHC Fragment") is distinguishable by size, even in the
+an "All-0 SCHC Fragment") is distinguishable by size, even in the
-  presence of padding, from a SCHC ACK REQ message (see Section 8.3.3)
+presence of padding, from a SCHC ACK REQ message (see Section 8.3.3)
-  with the same RuleID value and with the same T, M, and N values.
+with the same RuleID value and with the same T, M, and N values.
-  This condition is met if the Payload is at least the size of an L2
+This condition is met if the Payload is at least the size of an L2
-  Word.  This condition is also met if the SCHC Fragment Header is a
+Word.  This condition is also met if the SCHC Fragment Header is a
-  multiple of L2 Words.
+multiple of L2 Words.
-.3.1.2.  All-1 SCHC Fragment
+===== All-1 SCHC Fragment =====
-  The All-1 SCHC Fragment format is shown in Figure 14.  The sender
+The All-1 SCHC Fragment format is shown in Figure 14.  The sender
-  uses the All-1 SCHC Fragment format for the message that completes
+uses the All-1 SCHC Fragment format for the message that completes
-  the emission of a fragmented SCHC Packet.  The DTag field, the W
+the emission of a fragmented SCHC Packet.  The DTag field, the W
-  field, the RCS field and the Payload are OPTIONAL, their presence is
+field, the RCS field and the Payload are OPTIONAL, their presence is
-  specified by each mode and Profile.  At least one of RCS field or
+specified by each mode and Profile.  At least one of RCS field or
-  Fragment Payload MUST be present.  The FCN field is all ones.
+Fragment Payload MUST be present.  The FCN field is all ones.
-  |------- SCHC Fragment Header -------|
+|------- SCHC Fragment Header -------|
-            |-- T --|-M-|-- N --|-- U --|
+        |-- T --|-M-|-- N --|-- U --|
-  +-- ... -+- ... -+---+- ... -+- ... -+-----...-----+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
++-- ... -+- ... -+---+- ... -+- ... -+-----...-----+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-  | RuleID | DTag  | W | 11..1 |  RCS  | FragPayload | pad. (as needed)
+| RuleID | DTag  | W | 11..1 |  RCS  | FragPayload | pad. (as needed)
-  +-- ... -+- ... -+---+- ... -+- ... -+-----...-----+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
++-- ... -+- ... -+---+- ... -+- ... -+-----...-----+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-                          (FCN)
+                      (FCN)
-      Figure 14: Detailed Header Format for the All-1 SCHC Fragment
+    Figure 14: Detailed Header Format for the All-1 SCHC Fragment
-  Profiles MUST ensure that an All-1 SCHC Fragment message is
+Profiles MUST ensure that an All-1 SCHC Fragment message is
-  distinguishable by size, even in the presence of padding, from a SCHC
+distinguishable by size, even in the presence of padding, from a SCHC
-  Sender-Abort message (see Section 8.3.4) with the same RuleID value
+Sender-Abort message (see Section 8.3.4) with the same RuleID value
-  and with the same T, M, and N values.  This condition is met if the
+and with the same T, M, and N values.  This condition is met if the
-  RCS is present and is at least the size of an L2 Word or if the
+RCS is present and is at least the size of an L2 Word or if the
-  Payload is present and is at least the size an L2 Word.  This
+Payload is present and is at least the size an L2 Word.  This
-  condition is also met if the SCHC Sender-Abort Header is a multiple
+condition is also met if the SCHC Sender-Abort Header is a multiple
-  of L2 Words.
+of L2 Words.
-.3.2.  SCHC ACK Format
+==== SCHC ACK Format ====
-  The SCHC ACK message is shown in Figure 15.  The DTag field and the W
+The SCHC ACK message is shown in Figure 15.  The DTag field and the W
-  field are OPTIONAL, their presence is specified by each mode and
+field are OPTIONAL, their presence is specified by each mode and
-  Profile.  The Compressed Bitmap field MUST be present in SCHC F/R
+Profile.  The Compressed Bitmap field MUST be present in SCHC F/R
-  modes that use windows and MUST NOT be present in other modes.
+modes that use windows and MUST NOT be present in other modes.
-  |--- SCHC ACK Header ----|
+|--- SCHC ACK Header ----|
-            |-- T --|-M-| 1 |
+        |-- T --|-M-| 1 |
-  +-- ... -+- ... -+---+---+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
++-- ... -+- ... -+---+---+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
-  | RuleID |  DTag | W |C=1| padding as needed                (success)
+| RuleID |  DTag | W |C=1| padding as needed                (success)
-  +-- ... -+- ... -+---+---+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
++-- ... -+- ... -+---+---+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
-  +-- ... -+- ... -+---+---+------ ... ------+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
++-- ... -+- ... -+---+---+------ ... ------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-  | RuleID |  DTag | W |C=0|Compressed Bitmap| pad. as needed (failure)
+| RuleID |  DTag | W |C=0|Compressed Bitmap| pad. as needed (failure)
-  +-- ... -+- ... -+---+---+------ ... ------+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
++-- ... -+- ... -+---+---+------ ... ------+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-                Figure 15: Format of the SCHC ACK Message
+              Figure 15: Format of the SCHC ACK Message
-  The SCHC ACK Header contains a C bit (see Section 8.2.4).
+The SCHC ACK Header contains a C bit (see Section 8.2.4).
-  If the C bit is set to 1 (integrity check successful), no Bitmap is
+If the C bit is set to 1 (integrity check successful), no Bitmap is
-  carried.
+carried.
-  If the C bit is set to 0 (integrity check not performed or failed)
+If the C bit is set to 0 (integrity check not performed or failed)
-  and if windows are used, a Compressed Bitmap for the window referred
+and if windows are used, a Compressed Bitmap for the window referred
-  to by the W field is transmitted as specified in Section 8.3.2.1.
+to by the W field is transmitted as specified in Section 8.3.2.1.
-.3.2.1.  Bitmap Compression
+===== Bitmap Compression =====
-  For transmission, the Compressed Bitmap in the SCHC ACK message is
+For transmission, the Compressed Bitmap in the SCHC ACK message is
-  defined by the following algorithm (see Figure 16 for a follow-along
+defined by the following algorithm (see Figure 16 for a follow-along
-  example):
+example):
-  *  Build a temporary SCHC ACK message that contains the Header
+*  Build a temporary SCHC ACK message that contains the Header
-      followed by the original Bitmap (see Section 8.2.2.3 for a
+  followed by the original Bitmap (see Section 8.2.2.3 for a
-      description of Bitmaps).
+  description of Bitmaps).
-  *  Position scissors at the end of the Bitmap, after its last bit.
+*  Position scissors at the end of the Bitmap, after its last bit.
-  *  While the bit on the left of the scissors is 1 and belongs to the
+*  While the bit on the left of the scissors is 1 and belongs to the
-      Bitmap, keep moving left, then stop.
+  Bitmap, keep moving left, then stop.
-  *  Then, while the scissors are not on an L2 Word boundary of the
+*  Then, while the scissors are not on an L2 Word boundary of the
-      SCHC ACK message and there is a Bitmap bit on the right of the
+  SCHC ACK message and there is a Bitmap bit on the right of the
-      scissors, keep moving right, then stop.
+  scissors, keep moving right, then stop.
-  *  At this point, cut and drop off any bits to the right of the
+*  At this point, cut and drop off any bits to the right of the
-      scissors.
+  scissors.
-  When one or more bits have effectively been dropped off as a result
+When one or more bits have effectively been dropped off as a result
-  of the above algorithm, the SCHC ACK message is a multiple of L2
+of the above algorithm, the SCHC ACK message is a multiple of L2
-  Words; no padding bits will be appended.
+Words; no padding bits will be appended.
-  Because the SCHC Fragment sender knows the size of the original
+Because the SCHC Fragment sender knows the size of the original
-  Bitmap, it can reconstruct the original Bitmap from the Compressed
+Bitmap, it can reconstruct the original Bitmap from the Compressed
-  Bitmap received in the SCHC ACK message.
+Bitmap received in the SCHC ACK message.
-  Figure 16 shows an example where L2 Words are actually bytes and
+Figure 16 shows an example where L2 Words are actually bytes and
-  where the original Bitmap contains 17 bits, the last 15 of which are
+where the original Bitmap contains 17 bits, the last 15 of which are
-  all set to 1.
+all set to 1.
-  |--- SCHC ACK Header ----|--------      Bitmap    --------|
+|--- SCHC ACK Header ----|--------      Bitmap    --------|
-            |-- T --|-M-| 1 |
+        |-- T --|-M-| 1 |
-  +-- ... -+- ... -+---+---+---------------------------------+
++-- ... -+- ... -+---+---+---------------------------------+
-  | RuleID |  DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+| RuleID |  DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
-  +-- ... -+- ... -+---+---+---------------------------------+
++-- ... -+- ... -+---+---+---------------------------------+
-          next L2 Word boundary ->|
+      next L2 Word boundary ->|
-            Figure 16: SCHC ACK Header Plus Uncompressed Bitmap
+        Figure 16: SCHC ACK Header Plus Uncompressed Bitmap
-  Figure 17 shows that the last 14 bits are not sent.
+Figure 17 shows that the last 14 bits are not sent.
-  |--- SCHC ACK Header ----|CpBmp|
+|--- SCHC ACK Header ----|CpBmp|
-            |-- T --|-M-| 1 |
+        |-- T --|-M-| 1 |
-  +-- ... -+- ... -+---+---+-----+
++-- ... -+- ... -+---+---+-----+
-  | RuleID |  DTag | W |C=0|1 0 1|
+| RuleID |  DTag | W |C=0|1 0 1|
-  +-- ... -+- ... -+---+---+-----+
++-- ... -+- ... -+---+---+-----+
-          next L2 Word boundary ->|
+      next L2 Word boundary ->|
-        Figure 17: Resulting SCHC ACK Message with Compressed Bitmap
+    Figure 17: Resulting SCHC ACK Message with Compressed Bitmap
-  Figure 18 shows an example of a SCHC ACK with tile indices ranging
+Figure 18 shows an example of a SCHC ACK with tile indices ranging
-  from 6 down to 0, where the Bitmap indicates that the second and the
+from 6 down to 0, where the Bitmap indicates that the second and the
-  fourth tile of the window have not been correctly received.
+fourth tile of the window have not been correctly received.
-  |--- SCHC ACK Header ----|--- Bitmap --|
+|--- SCHC ACK Header ----|--- Bitmap --|
-            |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+        |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
-  +--------+-------+---+---+-------------+
++--------+-------+---+---+-------------+
-  | RuleID |  DTag | W |C=0|1 0 1 0 1 1 1|    uncompressed Bitmap
+| RuleID |  DTag | W |C=0|1 0 1 0 1 1 1|    uncompressed Bitmap
-  +--------+-------+---+---+-------------+
++--------+-------+---+---+-------------+
-      next L2 Word boundary ->|<-- L2 Word --->|
+  next L2 Word boundary ->|<-- L2 Word --->|
-  +--------+-------+---+---+-------------+[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)+
++--------+-------+---+---+-------------+[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)+
-  | RuleID |  DTag | W |C=0|1 0 1 0 1 1 1|pad.| transmitted SCHC ACK
+| RuleID |  DTag | W |C=0|1 0 1 0 1 1 1|pad.| transmitted SCHC ACK
-  +--------+-------+---+---+-------------+[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)+
++--------+-------+---+---+-------------+[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)+
-      next L2 Word boundary ->|<-- L2 Word --->|
+  next L2 Word boundary ->|<-- L2 Word --->|
-          Figure 18: Example of a SCHC ACK Message, Missing Tiles
+      Figure 18: Example of a SCHC ACK Message, Missing Tiles
-  Figure 19 shows an example of a SCHC ACK with tile indices ranging
+Figure 19 shows an example of a SCHC ACK with tile indices ranging
-  from 6 down to 0, where integrity check has not been performed or has
+from 6 down to 0, where integrity check has not been performed or has
-  failed and the Bitmap indicates that there is no missing tile in that
+failed and the Bitmap indicates that there is no missing tile in that
-  window.
+window.
-  |--- SCHC ACK Header ----|--- Bitmap --|
+|--- SCHC ACK Header ----|--- Bitmap --|
-            |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+        |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
-  +--------+-------+---+---+-------------+
++--------+-------+---+---+-------------+
-  | RuleID |  DTag | W |C=0|1 1 1 1 1 1 1|  with uncompressed Bitmap
+| RuleID |  DTag | W |C=0|1 1 1 1 1 1 1|  with uncompressed Bitmap
-  +--------+-------+---+---+-------------+
++--------+-------+---+---+-------------+
-      next L2 Word boundary ->|
+  next L2 Word boundary ->|
-  +-- ... -+- ... -+---+---+-+
++-- ... -+- ... -+---+---+-+
-  | RuleID |  DTag | W |C=0|1|                  transmitted SCHC ACK
+| RuleID |  DTag | W |C=0|1|                  transmitted SCHC ACK
-  +-- ... -+- ... -+---+---+-+
++-- ... -+- ... -+---+---+-+
-      next L2 Word boundary ->|
+  next L2 Word boundary ->|
-        Figure 19: Example of a SCHC ACK Message, No Missing Tile
+      Figure 19: Example of a SCHC ACK Message, No Missing Tile
-.3.3.  SCHC ACK REQ Format
+==== SCHC ACK REQ Format ====
-  The SCHC ACK REQ is used by a sender to request a SCHC ACK from the
+The SCHC ACK REQ is used by a sender to request a SCHC ACK from the
-  receiver.  Its format is shown in Figure 20.  The DTag field and the
+receiver.  Its format is shown in Figure 20.  The DTag field and the
-  W field are OPTIONAL, their presence is specified by each mode and
+W field are OPTIONAL, their presence is specified by each mode and
-  Profile.  The FCN field is all zero.
+Profile.  The FCN field is all zero.
-  |--- SCHC ACK REQ Header ----|
+|--- SCHC ACK REQ Header ----|
-            |-- T --|-M-|-- N --|
+        |-- T --|-M-|-- N --|
-  +-- ... -+- ... -+---+- ... -+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
++-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-  | RuleID | DTag  | W |  0..0 | padding (as needed)      (no payload)
+| RuleID | DTag  | W |  0..0 | padding (as needed)      (no payload)
-  +-- ... -+- ... -+---+- ... -+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
++-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-                      Figure 20: SCHC ACK REQ Format
+                    Figure 20: SCHC ACK REQ Format
-.3.4.  SCHC Sender-Abort Format
+==== SCHC Sender-Abort Format ====
-  When a SCHC Fragment sender needs to abort an ongoing fragmented SCHC
+When a SCHC Fragment sender needs to abort an ongoing fragmented SCHC
-  Packet transmission, it sends a SCHC Sender-Abort message to the SCHC
+Packet transmission, it sends a SCHC Sender-Abort message to the SCHC
-  Fragment receiver.
+Fragment receiver.
-  The SCHC Sender-Abort format is shown in Figure 21.  The DTag field
+The SCHC Sender-Abort format is shown in Figure 21.  The DTag field
-  and the W field are OPTIONAL, their presence is specified by each
+and the W field are OPTIONAL, their presence is specified by each
-  mode and Profile.  The FCN field is all ones.
+mode and Profile.  The FCN field is all ones.
-  |--- Sender-Abort Header ----|
+|--- Sender-Abort Header ----|
-            |-- T --|-M-|-- N --|
+        |-- T --|-M-|-- N --|
-  +-- ... -+- ... -+---+- ... -+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
++-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-  | RuleID | DTag  | W | 11..1 | padding (as needed)
+| RuleID | DTag  | W | 11..1 | padding (as needed)
-  +-- ... -+- ... -+---+- ... -+21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
++-- ... -+- ... -+---+- ... -+21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-                    Figure 21: SCHC Sender-Abort Format
+                Figure 21: SCHC Sender-Abort Format
-  If the W field is present:
+If the W field is present:
-  *  the fragment sender MUST set it to all ones.  Other values are
+*  the fragment sender MUST set it to all ones.  Other values are
-      RESERVED.
+  RESERVED.
-  *  the fragment receiver MUST check its value.  If the value is
+*  the fragment receiver MUST check its value.  If the value is
-      different from all ones, the message MUST be ignored.
+  different from all ones, the message MUST be ignored.
-  The SCHC Sender-Abort MUST NOT be acknowledged.
+The SCHC Sender-Abort MUST NOT be acknowledged.
-.3.5.  SCHC Receiver-Abort Format
+==== SCHC Receiver-Abort Format ====
-  When a SCHC Fragment receiver needs to abort an ongoing fragmented
+When a SCHC Fragment receiver needs to abort an ongoing fragmented
-  SCHC Packet transmission, it transmits a SCHC Receiver-Abort message
+SCHC Packet transmission, it transmits a SCHC Receiver-Abort message
-  to the SCHC Fragment sender.
+to the SCHC Fragment sender.
-  The SCHC Receiver-Abort format is shown in Figure 22.  The DTag field
+The SCHC Receiver-Abort format is shown in Figure 22.  The DTag field
-  and the W field are OPTIONAL, their presence is specified by each
+and the W field are OPTIONAL, their presence is specified by each
-  mode and Profile.
+mode and Profile.
-  |-- Receiver-Abort Header ---|
+|-- Receiver-Abort Header ---|
-              |--- T ---|-M-| 1 |
+          |--- T ---|-M-| 1 |
-  +--- ... --+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
++--- ... --+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
-  |  RuleID  |  DTag  | W |C=1| 1..1|      1..1    |
+|  RuleID  |  DTag  | W |C=1| 1..1|      1..1    |
-  +--- ... --+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
++--- ... --+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
-              next L2 Word boundary ->|<-- L2 Word -->|
+          next L2 Word boundary ->|<-- L2 Word -->|
-                  Figure 22: SCHC Receiver-Abort Format
+                Figure 22: SCHC Receiver-Abort Format
-  If the W field is present:
+If the W field is present:
-  *  the fragment receiver MUST set it to all ones.  Other values are
+*  the fragment receiver MUST set it to all ones.  Other values are
-      RESERVED.
+  RESERVED.
-  *  if the value is different from all ones, the fragment sender MUST
+*  if the value is different from all ones, the fragment sender MUST
-      ignore the message.
+  ignore the message.
-  The SCHC Receiver-Abort has the same header as a SCHC ACK message.
+The SCHC Receiver-Abort has the same header as a SCHC ACK message.
-  The bits that follow the SCHC Receiver-Abort Header MUST be as
+The bits that follow the SCHC Receiver-Abort Header MUST be as
-  follows:
+follows:
-  *  if the Header does not end at an L2 Word boundary, append bits set
+*  if the Header does not end at an L2 Word boundary, append bits set
-      to 1 as needed to reach the next L2 Word boundary.
+  to 1 as needed to reach the next L2 Word boundary.
-  *  append exactly one more L2 Word with bits all set to ones.
+*  append exactly one more L2 Word with bits all set to ones.
-  Such a bit pattern never occurs in a legitimate SCHC ACK.  This is
+Such a bit pattern never occurs in a legitimate SCHC ACK.  This is
-  how the fragment sender recognizes a SCHC Receiver-Abort.
+how the fragment sender recognizes a SCHC Receiver-Abort.
-  The SCHC Receiver-Abort MUST NOT be acknowledged.
+The SCHC Receiver-Abort MUST NOT be acknowledged.
-.4.  SCHC F/R Modes
+=== SCHC F/R Modes ===
-  This specification includes several SCHC F/R modes that:
+This specification includes several SCHC F/R modes that:
-  *  allow for a range of reliability options, such as optional SCHC
+*  allow for a range of reliability options, such as optional SCHC
-      Fragment retransmission.
+  Fragment retransmission.
-  *  support various LPWAN characteristics, such as links with variable
+*  support various LPWAN characteristics, such as links with variable
-      MTU or unidirectional links.
+  MTU or unidirectional links.
-  More modes may be defined in the future.
+More modes may be defined in the future.
-  Appendix B provides examples of fragmentation sessions based on the
+Appendix B provides examples of fragmentation sessions based on the
-  modes described hereafter.
+modes described hereafter.
-  Appendix C provides examples of Finite State Machines implementing
+Appendix C provides examples of Finite State Machines implementing
-  the SCHC F/R modes described hereafter.
+the SCHC F/R modes described hereafter.
-.4.1.  No-ACK Mode
+==== No-ACK Mode ====
-  The No-ACK mode has been designed under the assumption that data unit
+The No-ACK mode has been designed under the assumption that data unit
-  out-of-sequence delivery does not occur between the entity performing
+out-of-sequence delivery does not occur between the entity performing
-  fragmentation and the entity performing reassembly.  This mode
+fragmentation and the entity performing reassembly.  This mode
-  supports L2 technologies that have a variable MTU.
+supports L2 technologies that have a variable MTU.
-  In No-ACK mode, there is no communication from the fragment receiver
+In No-ACK mode, there is no communication from the fragment receiver
-  to the fragment sender.  The sender transmits all the SCHC Fragments
+to the fragment sender.  The sender transmits all the SCHC Fragments
-  without expecting any acknowledgement.  Therefore, No-ACK does not
+without expecting any acknowledgement.  Therefore, No-ACK does not
-  require bidirectional links: unidirectional links are just fine.
+require bidirectional links: unidirectional links are just fine.
-  In No-ACK mode, only the All-1 SCHC Fragment is padded as needed.
+In No-ACK mode, only the All-1 SCHC Fragment is padded as needed.
-  The other SCHC Fragments are intrinsically aligned to L2 Words.
+The other SCHC Fragments are intrinsically aligned to L2 Words.
-  The tile sizes are not required to be uniform.  Windows are not used.
+The tile sizes are not required to be uniform.  Windows are not used.
-  The Retransmission Timer is not used.  The Attempts counter is not
+The Retransmission Timer is not used.  The Attempts counter is not
-  used.
+used.
-  Each Profile MUST specify which RuleID value(s) corresponds to SCHC
+Each Profile MUST specify which RuleID value(s) corresponds to SCHC
-  F/R messages operating in this mode.
+F/R messages operating in this mode.
-  The W field MUST NOT be present in the SCHC F/R messages.  SCHC ACK
+The W field MUST NOT be present in the SCHC F/R messages.  SCHC ACK
-  MUST NOT be sent.  SCHC ACK REQ MUST NOT be sent.  SCHC Sender-Abort
+MUST NOT be sent.  SCHC ACK REQ MUST NOT be sent.  SCHC Sender-Abort
-  MAY be sent.  SCHC Receiver-Abort MUST NOT be sent.
+MAY be sent.  SCHC Receiver-Abort MUST NOT be sent.
-  The value of N (size of the FCN field) is RECOMMENDED to be 1.
+The value of N (size of the FCN field) is RECOMMENDED to be 1.
-  Each Profile, for each RuleID value, MUST define:
+Each Profile, for each RuleID value, MUST define:
-  *  the size of the DTag field,
+*  the size of the DTag field,
-  *  the size and algorithm for the RCS field, and
+*  the size and algorithm for the RCS field, and
-  *  the expiration time of the Inactivity Timer.
+*  the expiration time of the Inactivity Timer.
-  Each Profile, for each RuleID value, MAY define
+Each Profile, for each RuleID value, MAY define
-  *  a value of N different from the recommended one, and
+*  a value of N different from the recommended one, and
-  *  the meaning of values sent in the FCN field, for values different
+*  the meaning of values sent in the FCN field, for values different
-      from the All-1 value.
+  from the All-1 value.
-  For each active pair of RuleID and DTag values, the receiver MUST
+For each active pair of RuleID and DTag values, the receiver MUST
-  maintain an Inactivity Timer.  If the receiver is under-resourced to
+maintain an Inactivity Timer.  If the receiver is under-resourced to
-  do this, it MUST silently drop the related messages.
+do this, it MUST silently drop the related messages.
-.4.1.1.  Sender Behavior
+===== Sender Behavior =====
-  At the beginning of the fragmentation of a new SCHC Packet, the
+At the beginning of the fragmentation of a new SCHC Packet, the
-  fragment sender MUST select a RuleID and DTag value pair for this
+fragment sender MUST select a RuleID and DTag value pair for this
-  SCHC Packet.
+SCHC Packet.
-  Each SCHC Fragment MUST contain exactly one tile in its Payload.  The
+Each SCHC Fragment MUST contain exactly one tile in its Payload.  The
-  tile MUST be at least the size of an L2 Word.  The sender MUST
+tile MUST be at least the size of an L2 Word.  The sender MUST
-  transmit the SCHC Fragments messages in the order that the tiles
+transmit the SCHC Fragments messages in the order that the tiles
-  appear in the SCHC Packet.  Except for the last tile of a SCHC
+appear in the SCHC Packet.  Except for the last tile of a SCHC
-  Packet, each tile MUST be of a size that complements the SCHC
+Packet, each tile MUST be of a size that complements the SCHC
-  Fragment Header so that the SCHC Fragment is a multiple of L2 Words
+Fragment Header so that the SCHC Fragment is a multiple of L2 Words
-  without the need for padding bits.  Except for the last one, the SCHC
+without the need for padding bits.  Except for the last one, the SCHC
-  Fragments MUST use the Regular SCHC Fragment format specified in
+Fragments MUST use the Regular SCHC Fragment format specified in
-  Section 8.3.1.1.  The SCHC Fragment that carries the last tile MUST
+Section 8.3.1.1.  The SCHC Fragment that carries the last tile MUST
-  be an All-1 SCHC Fragment, described in Section 8.3.1.2.
+be an All-1 SCHC Fragment, described in Section 8.3.1.2.
-  The sender MAY transmit a SCHC Sender-Abort.
+The sender MAY transmit a SCHC Sender-Abort.
-  Figure 39 shows an example of a corresponding state machine.
+Figure 39 shows an example of a corresponding state machine.
-.4.1.2.  Receiver Behavior
+===== Receiver Behavior =====
-  Upon receiving each Regular SCHC Fragment:
+Upon receiving each Regular SCHC Fragment:
-  *  the receiver MUST reset the Inactivity Timer.
+*  the receiver MUST reset the Inactivity Timer.
-  *  the receiver assembles the payloads of the SCHC Fragments.
+*  the receiver assembles the payloads of the SCHC Fragments.
-  On receiving an All-1 SCHC Fragment:
+On receiving an All-1 SCHC Fragment:
-  *  the receiver MUST append the All-1 SCHC Fragment Payload and the
+*  the receiver MUST append the All-1 SCHC Fragment Payload and the
-      padding bits to the previously received SCHC Fragment Payloads for
+  padding bits to the previously received SCHC Fragment Payloads for
-      this SCHC Packet.
+  this SCHC Packet.
-  *  the receiver MUST perform the integrity check.
+*  the receiver MUST perform the integrity check.
-  *  if integrity checking fails, the receiver MUST drop the
+*  if integrity checking fails, the receiver MUST drop the
-      reassembled SCHC Packet.
+  reassembled SCHC Packet.
-  *  the reassembly operation concludes.
+*  the reassembly operation concludes.
-  On expiration of the Inactivity Timer, the receiver MUST drop the
+On expiration of the Inactivity Timer, the receiver MUST drop the
-  SCHC Packet being reassembled.
+SCHC Packet being reassembled.
-  On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC
+On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC
-  Packet being reassembled.
+Packet being reassembled.
-  Figure 40 shows an example of a corresponding state machine.
+Figure 40 shows an example of a corresponding state machine.
-.4.2.  ACK-Always Mode
+==== ACK-Always Mode ====
-  The ACK-Always mode has been designed under the following
+The ACK-Always mode has been designed under the following
-  assumptions:
+assumptions:
-  *  Data unit out-of-sequence delivery does not occur between the
+*  Data unit out-of-sequence delivery does not occur between the
-      entity performing fragmentation and the entity performing
+  entity performing fragmentation and the entity performing
-      reassembly,
+  reassembly,
-  *  The L2 MTU value does not change while the fragments of a SCHC
+*  The L2 MTU value does not change while the fragments of a SCHC
-      Packet are being transmitted, and
+  Packet are being transmitted, and
-  *  There is a feedback path from the reassembler to the fragmenter.
+*  There is a feedback path from the reassembler to the fragmenter.
-      See Appendix F for a discussion on using ACK-Always mode on quasi-
+  See Appendix F for a discussion on using ACK-Always mode on quasi-
-      bidirectional links.
+  bidirectional links.
-  In ACK-Always mode, windows are used.  An acknowledgement, positive
+In ACK-Always mode, windows are used.  An acknowledgement, positive
-  or negative, is transmitted by the fragment receiver to the fragment
+or negative, is transmitted by the fragment receiver to the fragment
-  sender at the end of the transmission of each window of SCHC
+sender at the end of the transmission of each window of SCHC
-  Fragments.
+Fragments.
-  The tiles are not required to be of uniform size.  In ACK-Always
+The tiles are not required to be of uniform size.  In ACK-Always
-  mode, only the All-1 SCHC Fragment is padded as needed.  The other
+mode, only the All-1 SCHC Fragment is padded as needed.  The other
-  SCHC Fragments are intrinsically aligned to L2 Words.
+SCHC Fragments are intrinsically aligned to L2 Words.
-  Briefly, the algorithm is as follows: after a first blind
+Briefly, the algorithm is as follows: after a first blind
-  transmission of all the tiles of a window, the fragment sender
+transmission of all the tiles of a window, the fragment sender
-  iterates retransmitting the tiles that are reported missing until the
+iterates retransmitting the tiles that are reported missing until the
-  fragment receiver reports that all the tiles belonging to the window
+fragment receiver reports that all the tiles belonging to the window
-  have been correctly received or until too many attempts were made.
+have been correctly received or until too many attempts were made.
-  The fragment sender only advances to the next window of tiles when it
+The fragment sender only advances to the next window of tiles when it
-  has ascertained that all the tiles belonging to the current window
+has ascertained that all the tiles belonging to the current window
-  have been fully and correctly received.  This results in a per-window
+have been fully and correctly received.  This results in a per-window
-  lock-step behavior between the sender and the receiver.
+lock-step behavior between the sender and the receiver.
-  Each Profile MUST specify which RuleID value(s) correspond to SCHC F/
+Each Profile MUST specify which RuleID value(s) correspond to SCHC F/
-  R messages operating in this mode.
+R messages operating in this mode.
-  The W field MUST be present and its size M MUST be 1 bit.
+The W field MUST be present and its size M MUST be 1 bit.
-  Each Profile, for each RuleID value, MUST define:
+Each Profile, for each RuleID value, MUST define:
-  *  the value of N,
+*  the value of N,
-  *  the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
+*  the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
-  *  the size and algorithm for the RCS field,
+*  the size and algorithm for the RCS field,
-  *  the value of T,
+*  the value of T,
-  *  the value of MAX_ACK_REQUESTS,
+*  the value of MAX_ACK_REQUESTS,
-  *  the expiration time of the Retransmission Timer, and
+*  the expiration time of the Retransmission Timer, and
-  *  the expiration time of the Inactivity Timer.
+*  the expiration time of the Inactivity Timer.
-  For each active pair of RuleID and DTag values, the sender MUST
+For each active pair of RuleID and DTag values, the sender MUST
-  maintain:
+maintain:
-  *  one Attempts counter
+*  one Attempts counter
-  *  one Retransmission Timer
+*  one Retransmission Timer
-  For each active pair of RuleID and DTag values, the receiver MUST
+For each active pair of RuleID and DTag values, the receiver MUST
-  maintain
+maintain
-  *  one Inactivity Timer, and
+*  one Inactivity Timer, and
-  *  one Attempts counter.
+*  one Attempts counter.
-.4.2.1.  Sender Behavior
+===== Sender Behavior =====
-  At the beginning of the fragmentation of a new SCHC Packet, the
+At the beginning of the fragmentation of a new SCHC Packet, the
-  fragment sender MUST select a RuleID and DTag value pair for this
+fragment sender MUST select a RuleID and DTag value pair for this
-  SCHC Packet.
+SCHC Packet.
-  Each SCHC Fragment MUST contain exactly one tile in its Payload.  All
+Each SCHC Fragment MUST contain exactly one tile in its Payload.  All
-  tiles with the index 0, as well as the last tile, MUST be at least
+tiles with the index 0, as well as the last tile, MUST be at least
-  the size of an L2 Word.
+the size of an L2 Word.
-  In all SCHC Fragment messages, the W field MUST be filled with the
+In all SCHC Fragment messages, the W field MUST be filled with the
-  LSB of the window number that the sender is currently processing.
+LSB of the window number that the sender is currently processing.
-  For a SCHC Fragment that carries a tile other than the last one of
+For a SCHC Fragment that carries a tile other than the last one of
-  the SCHC Packet:
+the SCHC Packet:
-  *  the Fragment MUST be of the Regular type specified in
+*  the Fragment MUST be of the Regular type specified in
-      Section 8.3.1.1.
+  Section 8.3.1.1.
-  *  the FCN field MUST contain the tile index.
+*  the FCN field MUST contain the tile index.
-  *  each tile MUST be of a size that complements the SCHC Fragment
+*  each tile MUST be of a size that complements the SCHC Fragment
-      Header so that the SCHC Fragment is a multiple of L2 Words without
+  Header so that the SCHC Fragment is a multiple of L2 Words without
-      the need for padding bits.
+  the need for padding bits.
-  The SCHC Fragment that carries the last tile MUST be an All-1 SCHC
+The SCHC Fragment that carries the last tile MUST be an All-1 SCHC
-  Fragment, described in Section 8.3.1.2.
+Fragment, described in Section 8.3.1.2.
-  The fragment sender MUST start by transmitting the window numbered 0.
+The fragment sender MUST start by transmitting the window numbered 0.
-  All message receptions being discussed in the rest of this section
+All message receptions being discussed in the rest of this section
-  are to be understood as "matching the RuleID and DTag pair being
+are to be understood as "matching the RuleID and DTag pair being
-  processed", even if not spelled out, for brevity.
+processed", even if not spelled out, for brevity.
-  The sender starts by a "blind transmission" phase, in which it MUST
+The sender starts by a "blind transmission" phase, in which it MUST
-  transmit all the tiles composing the window, in decreasing tile index
+transmit all the tiles composing the window, in decreasing tile index
-  order.
+order.
-  Then, it enters a "retransmission phase" in which it MUST initialize
+Then, it enters a "retransmission phase" in which it MUST initialize
-  an Attempts counter to 0, it MUST start a Retransmission Timer and it
+an Attempts counter to 0, it MUST start a Retransmission Timer and it
-  MUST await a SCHC ACK.
+MUST await a SCHC ACK.
-  *  Then, upon receiving a SCHC ACK:
+*  Then, upon receiving a SCHC ACK:
-      -  if the SCHC ACK indicates that some tiles are missing at the
+  -  if the SCHC ACK indicates that some tiles are missing at the
-        receiver, then the sender MUST transmit all the tiles that have
+      receiver, then the sender MUST transmit all the tiles that have
-        been reported missing, it MUST increment Attempts, it MUST
+      been reported missing, it MUST increment Attempts, it MUST
-        reset the Retransmission Timer, and MUST await the next SCHC
+      reset the Retransmission Timer, and MUST await the next SCHC
-        ACK.
+      ACK.
-      -  if the current window is not the last one and the SCHC ACK
+  -  if the current window is not the last one and the SCHC ACK
-        indicates that all tiles were correctly received, the sender
+      indicates that all tiles were correctly received, the sender
-        MUST stop the Retransmission Timer, it MUST advance to the next
+      MUST stop the Retransmission Timer, it MUST advance to the next
-        fragmentation window, and it MUST start a blind transmission
+      fragmentation window, and it MUST start a blind transmission
-        phase as described above.
+      phase as described above.
-      -  if the current window is the last one and the SCHC ACK
+  -  if the current window is the last one and the SCHC ACK
-        indicates that more tiles were received than the sender sent,
+      indicates that more tiles were received than the sender sent,
-        the fragment sender MUST send a SCHC Sender-Abort, and it MAY
+      the fragment sender MUST send a SCHC Sender-Abort, and it MAY
-        exit with an error condition.
+      exit with an error condition.
-      -  if the current window is the last one and the SCHC ACK
+  -  if the current window is the last one and the SCHC ACK
-        indicates that all tiles were correctly received, yet the
+      indicates that all tiles were correctly received, yet the
-        integrity check was a failure, the fragment sender MUST send a
+      integrity check was a failure, the fragment sender MUST send a
-        SCHC Sender-Abort, and it MAY exit with an error condition.
+      SCHC Sender-Abort, and it MAY exit with an error condition.
-      -  if the current window is the last one and the SCHC ACK
+  -  if the current window is the last one and the SCHC ACK
-        indicates that integrity checking was successful, the sender
+      indicates that integrity checking was successful, the sender
-        exits successfully.
+      exits successfully.
-  *  on Retransmission Timer expiration:
+*  on Retransmission Timer expiration:
-      -  if Attempts is strictly less that MAX_ACK_REQUESTS, the
+  -  if Attempts is strictly less that MAX_ACK_REQUESTS, the
-        fragment sender MUST send a SCHC ACK REQ and MUST increment the
+      fragment sender MUST send a SCHC ACK REQ and MUST increment the
-        Attempts counter.
+      Attempts counter.
-      -  otherwise, the fragment sender MUST send a SCHC Sender-Abort,
+  -  otherwise, the fragment sender MUST send a SCHC Sender-Abort,
-        and it MAY exit with an error condition.
+      and it MAY exit with an error condition.
-  At any time:
+At any time:
-  *  on receiving a SCHC Receiver-Abort, the fragment sender MAY exit
+*  on receiving a SCHC Receiver-Abort, the fragment sender MAY exit
-      with an error condition.
+  with an error condition.
-  *  on receiving a SCHC ACK that bears a W value different from the W
+*  on receiving a SCHC ACK that bears a W value different from the W
-      value that it currently uses, the fragment sender MUST silently
+  value that it currently uses, the fragment sender MUST silently
-      discard and ignore that SCHC ACK.
+  discard and ignore that SCHC ACK.
-  Figure 41 shows an example of a corresponding state machine.
+Figure 41 shows an example of a corresponding state machine.
-.4.2.2.  Receiver Behavior
+===== Receiver Behavior =====
-  On receiving a SCHC Fragment with a RuleID and DTag pair not being
+On receiving a SCHC Fragment with a RuleID and DTag pair not being
-  processed at that time:
+processed at that time:
-  *  the receiver SHOULD check if the DTag value has not recently been
+*  the receiver SHOULD check if the DTag value has not recently been
-      used for that RuleID value, thereby ensuring that the received
+  used for that RuleID value, thereby ensuring that the received
-      SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
+  SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
-      transmission.  The initial value of the Inactivity Timer is the
+  transmission.  The initial value of the Inactivity Timer is the
-      RECOMMENDED lifetime for the DTag value at the receiver.  If the
+  RECOMMENDED lifetime for the DTag value at the receiver.  If the
-      SCHC Fragment is determined to be such a remnant, the receiver MAY
+  SCHC Fragment is determined to be such a remnant, the receiver MAY
-      silently ignore it and discard it.
+  silently ignore it and discard it.
-  *  the receiver MUST start a process to assemble a new SCHC Packet
+*  the receiver MUST start a process to assemble a new SCHC Packet
-      with that RuleID and DTag value pair.
+  with that RuleID and DTag value pair.
-  *  the receiver MUST start an Inactivity Timer for that RuleID and
+*  the receiver MUST start an Inactivity Timer for that RuleID and
-      DTag pair.  It MUST initialize an Attempts counter to 0 for that
+  DTag pair.  It MUST initialize an Attempts counter to 0 for that
-      RuleID and DTag pair.  It MUST initialize a window counter to 0.
+  RuleID and DTag pair.  It MUST initialize a window counter to 0.
-      If the receiver is under-resourced to do this, it MUST respond to
+  If the receiver is under-resourced to do this, it MUST respond to
-      the sender with a SCHC Receiver-Abort.
+  the sender with a SCHC Receiver-Abort.
-  In the rest of this section, "local W bit" means the least
+In the rest of this section, "local W bit" means the least
-  significant bit of the window counter of the receiver.
+significant bit of the window counter of the receiver.
-  On reception of any SCHC F/R message for the RuleID and DTag pair
+On reception of any SCHC F/R message for the RuleID and DTag pair
-  being processed, the receiver MUST reset the Inactivity Timer
+being processed, the receiver MUST reset the Inactivity Timer
-  pertaining to that RuleID and DTag pair.
+pertaining to that RuleID and DTag pair.
-  All message receptions being discussed in the rest of this section
+All message receptions being discussed in the rest of this section
-  are to be understood as "matching the RuleID and DTag pair being
+are to be understood as "matching the RuleID and DTag pair being
-  processed", even if not spelled out, for brevity.
+processed", even if not spelled out, for brevity.
-  The receiver MUST first initialize an empty Bitmap for the first
+The receiver MUST first initialize an empty Bitmap for the first
-  window then enter an "acceptance phase", in which:
+window then enter an "acceptance phase", in which:
-  *  on receiving a SCHC Fragment or a SCHC ACK REQ, either one having
+*  on receiving a SCHC Fragment or a SCHC ACK REQ, either one having
-      the W bit different from the local W bit, the receiver MUST
+  the W bit different from the local W bit, the receiver MUST
-      silently ignore and discard that message.
+  silently ignore and discard that message.
-  *  on receiving a SCHC ACK REQ with the W bit equal to the local W
+*  on receiving a SCHC ACK REQ with the W bit equal to the local W
-      bit, the receiver MUST send a SCHC ACK for this window.
+  bit, the receiver MUST send a SCHC ACK for this window.
-  *  on receiving a SCHC Fragment with the W bit equal to the local W
+*  on receiving a SCHC Fragment with the W bit equal to the local W
-      bit, the receiver MUST assemble the received tile based on the
+  bit, the receiver MUST assemble the received tile based on the
-      window counter and on the FCN field in the SCHC Fragment, and it
+  window counter and on the FCN field in the SCHC Fragment, and it
-      MUST update the Bitmap.
+  MUST update the Bitmap.
-      -  if the SCHC Fragment received is an All-0 SCHC Fragment, the
+  -  if the SCHC Fragment received is an All-0 SCHC Fragment, the
-        current window is determined to be a not-last window, the
+      current window is determined to be a not-last window, the
-        receiver MUST send a SCHC ACK for this window and it MUST enter
+      receiver MUST send a SCHC ACK for this window and it MUST enter
-        the "retransmission phase" for this window.
+      the "retransmission phase" for this window.
-      -  if the SCHC Fragment received is an All-1 SCHC Fragment, the
+  -  if the SCHC Fragment received is an All-1 SCHC Fragment, the
-        current window is determined to be the last window, the padding
+      current window is determined to be the last window, the padding
-        bits of the All-1 SCHC Fragment MUST be assembled after the
+      bits of the All-1 SCHC Fragment MUST be assembled after the
-        received tile, the receiver MUST perform the integrity check
+      received tile, the receiver MUST perform the integrity check
-        and it MUST send a SCHC ACK for this window.  Then:
+      and it MUST send a SCHC ACK for this window.  Then:
-        o  If the integrity check indicates that the full SCHC Packet
+      o  If the integrity check indicates that the full SCHC Packet
-            has been correctly reassembled, the receiver MUST enter the
+        has been correctly reassembled, the receiver MUST enter the
-            "clean-up phase" for this window.
+        "clean-up phase" for this window.
-        o  If the integrity check indicates that the full SCHC Packet
+      o  If the integrity check indicates that the full SCHC Packet
-            has not been correctly reassembled, the receiver enters the
+        has not been correctly reassembled, the receiver enters the
-            "retransmission phase" for this window.
+        "retransmission phase" for this window.
-  In the "retransmission phase":
+In the "retransmission phase":
-  *  if the window is a not-last window:
+*  if the window is a not-last window:
-      -  on receiving a SCHC Fragment that is not All-0 or All-1 and
+  -  on receiving a SCHC Fragment that is not All-0 or All-1 and
-        that has a W bit different from the local W bit, the receiver
+      that has a W bit different from the local W bit, the receiver
-        MUST increment its window counter and allocate a fresh Bitmap,
+      MUST increment its window counter and allocate a fresh Bitmap,
-        it MUST assemble the tile received and update the Bitmap, and
+      it MUST assemble the tile received and update the Bitmap, and
-        it MUST enter the "acceptance phase" for that new window.
+      it MUST enter the "acceptance phase" for that new window.
-      -  on receiving a SCHC ACK REQ with a W bit different from the
+  -  on receiving a SCHC ACK REQ with a W bit different from the
-        local W bit, the receiver MUST increment its window counter and
+      local W bit, the receiver MUST increment its window counter and
-        allocate a fresh Bitmap, it MUST send a SCHC ACK for that new
+      allocate a fresh Bitmap, it MUST send a SCHC ACK for that new
-        window, and it MUST enter the "acceptance phase" for that new
+      window, and it MUST enter the "acceptance phase" for that new
-        window.
+      window.
-      -  on receiving a SCHC All-0 Fragment with a W bit different from
+  -  on receiving a SCHC All-0 Fragment with a W bit different from
-        the local W bit, the receiver MUST increment its window counter
+      the local W bit, the receiver MUST increment its window counter
-        and allocate a fresh Bitmap, it MUST assemble the tile received
+      and allocate a fresh Bitmap, it MUST assemble the tile received
-        and update the Bitmap, it MUST send a SCHC ACK for that new
+      and update the Bitmap, it MUST send a SCHC ACK for that new
-        window, and it MUST stay in the "retransmission phase" for that
+      window, and it MUST stay in the "retransmission phase" for that
-        new window.
+      new window.
-      -  on receiving a SCHC All-1 Fragment with a W bit different from
+  -  on receiving a SCHC All-1 Fragment with a W bit different from
-        the local W bit, the receiver MUST increment its window counter
+      the local W bit, the receiver MUST increment its window counter
-        and allocate a fresh Bitmap; it MUST assemble the tile
+      and allocate a fresh Bitmap; it MUST assemble the tile
-        received, including the padding bits; it MUST update the Bitmap
+      received, including the padding bits; it MUST update the Bitmap
-        and perform the integrity check; it MUST send a SCHC ACK for
+      and perform the integrity check; it MUST send a SCHC ACK for
-        the new window, which is determined to be the last window.
+      the new window, which is determined to be the last window.
-        Then:
+      Then:
-        o  If the integrity check indicates that the full SCHC Packet
+      o  If the integrity check indicates that the full SCHC Packet
-            has been correctly reassembled, the receiver MUST enter the
+        has been correctly reassembled, the receiver MUST enter the
-            "clean-up phase" for that new window.
+        "clean-up phase" for that new window.
-        o  If the integrity check indicates that the full SCHC Packet
+      o  If the integrity check indicates that the full SCHC Packet
-            has not been correctly reassembled, the receiver enters the
+        has not been correctly reassembled, the receiver enters the
-            "retransmission phase" for that new window.
+        "retransmission phase" for that new window.
-      -  on receiving a SCHC Fragment with a W bit equal to the local W
+  -  on receiving a SCHC Fragment with a W bit equal to the local W
-        bit:
+      bit:
-        o  if the SCHC Fragment received is an All-1 SCHC Fragment, the
+      o  if the SCHC Fragment received is an All-1 SCHC Fragment, the
-            receiver MUST silently ignore it and discard it.
+        receiver MUST silently ignore it and discard it.
-        o  otherwise, the receiver MUST assemble the tile received and
+      o  otherwise, the receiver MUST assemble the tile received and
-            update the Bitmap.  If the Bitmap becomes fully populated
+        update the Bitmap.  If the Bitmap becomes fully populated
-            with 1's or if the SCHC Fragment is an All-0, the receiver
+        with 1's or if the SCHC Fragment is an All-0, the receiver
-            MUST send a SCHC ACK for this window.
+        MUST send a SCHC ACK for this window.
-      -  on receiving a SCHC ACK REQ with the W bit equal to the local W
+  -  on receiving a SCHC ACK REQ with the W bit equal to the local W
-        bit, the receiver MUST send a SCHC ACK for this window.
+      bit, the receiver MUST send a SCHC ACK for this window.
-  *  if the window is the last window:
+*  if the window is the last window:
-      -  on receiving a SCHC Fragment or a SCHC ACK REQ, either one
+  -  on receiving a SCHC Fragment or a SCHC ACK REQ, either one
-        having a W bit different from the local W bit, the receiver
+      having a W bit different from the local W bit, the receiver
-        MUST silently ignore and discard that message.
+      MUST silently ignore and discard that message.
-      -  on receiving a SCHC ACK REQ with the W bit equal to the local W
+  -  on receiving a SCHC ACK REQ with the W bit equal to the local W
-        bit, the receiver MUST send a SCHC ACK for this window.
+      bit, the receiver MUST send a SCHC ACK for this window.
-      -  on receiving a SCHC Fragment with a W bit equal to the local W
+  -  on receiving a SCHC Fragment with a W bit equal to the local W
-        bit:
+      bit:
-        o  if the SCHC Fragment received is an All-0 SCHC Fragment, the
+      o  if the SCHC Fragment received is an All-0 SCHC Fragment, the
-            receiver MUST silently ignore it and discard it.
+        receiver MUST silently ignore it and discard it.
-        o  otherwise, the receiver MUST update the Bitmap, and it MUST
+      o  otherwise, the receiver MUST update the Bitmap, and it MUST
-            assemble the tile received.  If the SCHC Fragment received
+        assemble the tile received.  If the SCHC Fragment received
-            is an All-1 SCHC Fragment, the receiver MUST assemble the
+        is an All-1 SCHC Fragment, the receiver MUST assemble the
-            padding bits of the All-1 SCHC Fragment after the received
+        padding bits of the All-1 SCHC Fragment after the received
-            tile, it MUST perform the integrity check and:
+        tile, it MUST perform the integrity check and:
-            +  if the integrity check indicates that the full SCHC
+        +  if the integrity check indicates that the full SCHC
-              Packet has been correctly reassembled, the receiver MUST
+            Packet has been correctly reassembled, the receiver MUST
-              send a SCHC ACK and it enters the "clean-up phase".
+            send a SCHC ACK and it enters the "clean-up phase".
-            +  if the integrity check indicates that the full SCHC
+        +  if the integrity check indicates that the full SCHC
-              Packet has not been correctly reassembled:
+            Packet has not been correctly reassembled:
-              *  if the SCHC Fragment received was an All-1 SCHC
+            *  if the SCHC Fragment received was an All-1 SCHC
-                  Fragment, the receiver MUST send a SCHC ACK for this
+              Fragment, the receiver MUST send a SCHC ACK for this
-                  window.
+              window.
-  In the "clean-up phase":
+In the "clean-up phase":
-  *  On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, either one
+*  On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, either one
-      having the W bit equal to the local W bit, the receiver MUST send
+  having the W bit equal to the local W bit, the receiver MUST send
-      a SCHC ACK.
+  a SCHC ACK.
-  *  Any other SCHC Fragment received MUST be silently ignored and
+*  Any other SCHC Fragment received MUST be silently ignored and
-      discarded.
+  discarded.
-  At any time, on sending a SCHC ACK, the receiver MUST increment the
+At any time, on sending a SCHC ACK, the receiver MUST increment the
-  Attempts counter.
+Attempts counter.
-  At any time, on incrementing its window counter, the receiver MUST
+At any time, on incrementing its window counter, the receiver MUST
-  reset the Attempts counter.
+reset the Attempts counter.
-  At any time, on expiration of the Inactivity Timer, on receiving a
+At any time, on expiration of the Inactivity Timer, on receiving a
-  SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the
+SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the
-  receiver MUST send a SCHC Receiver-Abort, and it MAY exit the receive
+receiver MUST send a SCHC Receiver-Abort, and it MAY exit the receive
-  process for that SCHC Packet.
+process for that SCHC Packet.
-  Figure 42 shows an example of a corresponding state machine.
+Figure 42 shows an example of a corresponding state machine.
-.4.3.  ACK-on-Error Mode
+==== ACK-on-Error Mode ====
-  The ACK-on-Error mode supports L2 technologies that have variable MTU
+The ACK-on-Error mode supports L2 technologies that have variable MTU
-  and out-of-order delivery.  It requires an L2 that provides a
+and out-of-order delivery.  It requires an L2 that provides a
-  feedback path from the reassembler to the fragmenter.  See Appendix F
+feedback path from the reassembler to the fragmenter.  See Appendix F
-  for a discussion on using ACK-on-Error mode on quasi-bidirectional
+for a discussion on using ACK-on-Error mode on quasi-bidirectional
-  links.
+links.
-  In ACK-on-Error mode, windows are used.
+In ACK-on-Error mode, windows are used.
-  All tiles except the last one and the penultimate one MUST be of
+All tiles except the last one and the penultimate one MUST be of
-  equal size, hereafter called "regular".  The size of the last tile
+equal size, hereafter called "regular".  The size of the last tile
-  MUST be smaller than or equal to the regular tile size.  Regarding
+MUST be smaller than or equal to the regular tile size.  Regarding
-  the penultimate tile, a Profile MUST pick one of the following two
+the penultimate tile, a Profile MUST pick one of the following two
-  options:
+options:
-  *  The penultimate tile size MUST be the regular tile size, or
+*  The penultimate tile size MUST be the regular tile size, or
-  *  the penultimate tile size MUST be either the regular tile size or
+*  the penultimate tile size MUST be either the regular tile size or
-      the regular tile size minus one L2 Word.
+  the regular tile size minus one L2 Word.
-  A SCHC Fragment message carries one or several contiguous tiles,
+A SCHC Fragment message carries one or several contiguous tiles,
-  which may span multiple windows.  A SCHC ACK reports on the reception
+which may span multiple windows.  A SCHC ACK reports on the reception
-  of exactly one window of tiles.
+of exactly one window of tiles.
-  See Figure 23 for an example.
+See Figure 23 for an example.
-          +---------------------------------------------...-----------+
+        +---------------------------------------------...-----------+
-          |                      SCHC Packet                        |
+        |                      SCHC Packet                        |
-          +---------------------------------------------...-----------+
+        +---------------------------------------------...-----------+
-  Tile#  | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 |    | 0 | 4 |3|
+Tile#  | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 |    | 0 | 4 |3|
-  Window# |-------- 0 --------|-------- 1 --------|- 2  ... 27 -|- 28-|
+Window# |-------- 0 --------|-------- 1 --------|- 2  ... 27 -|- 28-|
+SCHC Fragment msg  |-----------|
-  SCHC Fragment msg  |-----------|
+    Figure 23: SCHC Packet Fragmented in Tiles, ACK-on-Error Mode
-      Figure 23: SCHC Packet Fragmented in Tiles, ACK-on-Error Mode
+The W field is wide enough that it unambiguously represents an
+absolute window number.  The fragment receiver sends SCHC ACKs to the
+fragment sender about windows for which tiles are missing.  No SCHC
+ACK is sent by the fragment receiver for windows that it knows have
+been fully received.
-  The W field is wide enough that it unambiguously represents an
+The fragment sender retransmits SCHC Fragments for tiles that are
-  absolute window number.  The fragment receiver sends SCHC ACKs to the
+reported missing.  It can advance to next windows even before it has
-  fragment sender about windows for which tiles are missing.  No SCHC
+ascertained that all tiles belonging to previous windows have been
-  ACK is sent by the fragment receiver for windows that it knows have
+correctly received, and it can still later retransmit SCHC Fragments
-  been fully received.
+with tiles belonging to previous windows.  Therefore, the sender and
+the receiver may operate in a decoupled fashion.  The fragmented SCHC
+Packet transmission concludes when:
-  The fragment sender retransmits SCHC Fragments for tiles that are
+*  integrity checking shows that the fragmented SCHC Packet has been
-  reported missing.  It can advance to next windows even before it has
+   correctly reassembled at the receive end, and this information has
-  ascertained that all tiles belonging to previous windows have been
+   been conveyed back to the sender, or
-   correctly received, and it can still later retransmit SCHC Fragments
-   with tiles belonging to previous windows.  Therefore, the sender and
-  the receiver may operate in a decoupled fashion.  The fragmented SCHC
-  Packet transmission concludes when:
-  *  integrity checking shows that the fragmented SCHC Packet has been
+*  too many retransmission attempts were made, or
-      correctly reassembled at the receive end, and this information has
-      been conveyed back to the sender, or
-  *  too many retransmission attempts were made, or
+*  the receiver determines that the transmission of this fragmented
+  SCHC Packet has been inactive for too long.
-  *  the receiver determines that the transmission of this fragmented
+Each Profile MUST specify which RuleID value(s) corresponds to SCHC
-      SCHC Packet has been inactive for too long.
+F/R messages operating in this mode.
-  Each Profile MUST specify which RuleID value(s) corresponds to SCHC
+The W field MUST be present in the SCHC F/R messages.
-  F/R messages operating in this mode.
-  The W field MUST be present in the SCHC F/R messages.
+Each Profile, for each RuleID value, MUST define:
-   Each Profile, for each RuleID value, MUST define:
+*  the tile size (a tile does not need to be multiple of an L2 Word,
+   but it MUST be at least the size of an L2 Word),
-  *  the tile size (a tile does not need to be multiple of an L2 Word,
+*  the value of M,
-      but it MUST be at least the size of an L2 Word),
-  *  the value of M,
+*  the value of N,
-  *  the value of N,
+*  the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
-  *  the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
+*  the size and algorithm for the RCS field,
-  *  the size and algorithm for the RCS field,
+*  the value of T,
-  *  the value of T,
+*  the value of MAX_ACK_REQUESTS,
-  *  the value of MAX_ACK_REQUESTS,
+*  the expiration time of the Retransmission Timer,
-  *  the expiration time of the Retransmission Timer,
+*  the expiration time of the Inactivity Timer,
-  *  the expiration time of the Inactivity Timer,
+*  if the last tile is carried in a Regular SCHC Fragment or an All-1
+  SCHC Fragment (see Section 8.4.3.1), and
-  *  if the last tile is carried in a Regular SCHC Fragment or an All-1
+*  if the penultimate tile MAY be one L2 Word smaller than the
-      SCHC Fragment (see Section 8.4.3.1), and
+  regular tile size.  In this case, the regular tile size MUST be at
+  least twice the L2 Word size.
-  *  if the penultimate tile MAY be one L2 Word smaller than the
+For each active pair of RuleID and DTag values, the sender MUST
-      regular tile size.  In this case, the regular tile size MUST be at
+maintain:
-      least twice the L2 Word size.
-  For each active pair of RuleID and DTag values, the sender MUST
+*  one Attempts counter, and
-  maintain:
-  *  one Attempts counter, and
+*  one Retransmission Timer.
-  *  one Retransmission Timer.
+For each active pair of RuleID and DTag values, the receiver MUST
+maintain:
-  For each active pair of RuleID and DTag values, the receiver MUST
+*  one Inactivity Timer, and
-  maintain:
-  *  one Inactivity Timer, and
+*  one Attempts counter.
-  *  one Attempts counter.
+===== Sender Behavior =====
-.4.3.1.  Sender Behavior
+At the beginning of the fragmentation of a new SCHC Packet:
-   At the beginning of the fragmentation of a new SCHC Packet:
+*  the fragment sender MUST select a RuleID and DTag value pair for
+   this SCHC Packet.  A Rule MUST NOT be selected if the values of M
+  and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot
+  be fragmented in (2^M) * WINDOW_SIZE tiles or less.
-  *  the fragment sender MUST select a RuleID and DTag value pair for
+*  the fragment sender MUST initialize the Attempts counter to 0 for
-      this SCHC Packet.  A Rule MUST NOT be selected if the values of M
+  that RuleID and DTag value pair.
-      and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot
-      be fragmented in (2^M) * WINDOW_SIZE tiles or less.
-  *  the fragment sender MUST initialize the Attempts counter to 0 for
+A Regular SCHC Fragment message carries in its payload one or more
-      that RuleID and DTag value pair.
+tiles.  If more than one tile is carried in one Regular SCHC
+Fragment:
-  A Regular SCHC Fragment message carries in its payload one or more
+*  the selected tiles MUST be contiguous in the original SCHC Packet,
-  tiles.  If more than one tile is carried in one Regular SCHC
+   and
-   Fragment:
-  *  the selected tiles MUST be contiguous in the original SCHC Packet,
+*  they MUST be placed in the SCHC Fragment Payload adjacent to one
-      and
+  another, in the order they appear in the SCHC Packet, from the
+  start of the SCHC Packet toward its end.
-  *  they MUST be placed in the SCHC Fragment Payload adjacent to one
+Tiles that are not the last one MUST be sent in Regular SCHC
-      another, in the order they appear in the SCHC Packet, from the
+Fragments specified in Section 8.3.1.1.  The FCN field MUST contain
-      start of the SCHC Packet toward its end.
+the tile index of the first tile sent in that SCHC Fragment.
-  Tiles that are not the last one MUST be sent in Regular SCHC
+In a Regular SCHC Fragment message, the sender MUST fill the W field
-  Fragments specified in Section 8.3.1.1.  The FCN field MUST contain
+with the window number of the first tile sent in that SCHC Fragment.
-  the tile index of the first tile sent in that SCHC Fragment.
-  In a Regular SCHC Fragment message, the sender MUST fill the W field
+A Profile MUST define if the last tile of a SCHC Packet is sent:
-  with the window number of the first tile sent in that SCHC Fragment.
-  A Profile MUST define if the last tile of a SCHC Packet is sent:
+*  in a Regular SCHC Fragment, alone or as part of a multi-tiles
+  Payload,
-  *  in a Regular SCHC Fragment, alone or as part of a multi-tiles
+*  alone in an All-1 SCHC Fragment, or
-      Payload,
-  *  alone in an All-1 SCHC Fragment, or
+*  with any of the above two methods.
-  *  with any of the above two methods.
+In an All-1 SCHC Fragment message, the sender MUST fill the W field
+with the window number of the last tile of the SCHC Packet.
-  In an All-1 SCHC Fragment message, the sender MUST fill the W field
+The fragment sender MUST send SCHC Fragments such that, all together,
-  with the window number of the last tile of the SCHC Packet.
+they contain all the tiles of the fragmented SCHC Packet.
-  The fragment sender MUST send SCHC Fragments such that, all together,
+The fragment sender MUST send at least one All-1 SCHC Fragment.
-  they contain all the tiles of the fragmented SCHC Packet.
-  The fragment sender MUST send at least one All-1 SCHC Fragment.
+In doing the two items above, the sender MUST ascertain that the
+receiver will not receive the last tile through both a Regular SCHC
+Fragment and an All-1 SCHC Fragment.
-  In doing the two items above, the sender MUST ascertain that the
+The fragment sender MUST listen for SCHC ACK messages after having
-  receiver will not receive the last tile through both a Regular SCHC
+sent:
-  Fragment and an All-1 SCHC Fragment.
-  The fragment sender MUST listen for SCHC ACK messages after having
+*  an All-1 SCHC Fragment, or
-  sent:
-  *  an All-1 SCHC Fragment, or
+*  a SCHC ACK REQ.
-  *  a SCHC ACK REQ.
+A Profile MAY specify other times at which the fragment sender MUST
+listen for SCHC ACK messages.  For example, this could be after
+sending a complete window of tiles.
-  A Profile MAY specify other times at which the fragment sender MUST
+Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC
-  listen for SCHC ACK messages.  For example, this could be after
+ACK REQ:
-  sending a complete window of tiles.
-  Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC
+*  it MUST increment the Attempts counter, and
-  ACK REQ:
-  *  it MUST increment the Attempts counter, and
+*  it MUST reset the Retransmission Timer.
-  *  it MUST reset the Retransmission Timer.
+On Retransmission Timer expiration:
-   On Retransmission Timer expiration:
+*  if the Attempts counter is strictly less than MAX_ACK_REQUESTS,
+   the fragment sender MUST send either the All-1 SCHC Fragment or a
+  SCHC ACK REQ with the W field corresponding to the last window,
-  *  if the Attempts counter is strictly less than MAX_ACK_REQUESTS,
+*  otherwise, the fragment sender MUST send a SCHC Sender-Abort, and
-      the fragment sender MUST send either the All-1 SCHC Fragment or a
+  it MAY exit with an error condition.
-      SCHC ACK REQ with the W field corresponding to the last window,
-  *  otherwise, the fragment sender MUST send a SCHC Sender-Abort, and
+All message receptions being discussed in the rest of this section
-      it MAY exit with an error condition.
+are to be understood as "matching the RuleID and DTag pair being
+processed", even if not spelled out, for brevity.
-  All message receptions being discussed in the rest of this section
+On receiving a SCHC ACK:
-  are to be understood as "matching the RuleID and DTag pair being
-  processed", even if not spelled out, for brevity.
-  On receiving a SCHC ACK:
+*  if the W field in the SCHC ACK corresponds to the last window of
+  the SCHC Packet:
-  *  if the W field in the SCHC ACK corresponds to the last window of
-      the SCHC Packet:
-      -  if the C bit is set, the sender MAY exit successfully.
+  -  if the C bit is set, the sender MAY exit successfully.
-      -  otherwise:
+  -  otherwise:
-        o  if the Profile mandates that the last tile be sent in an
+      o  if the Profile mandates that the last tile be sent in an
-            All-1 SCHC Fragment:
+        All-1 SCHC Fragment:
-            +  if the SCHC ACK shows no missing tile at the receiver,
+        +  if the SCHC ACK shows no missing tile at the receiver,
-              the sender:
+            the sender:
-              *  MUST send a SCHC Sender-Abort, and
+            *  MUST send a SCHC Sender-Abort, and
-              *  MAY exit with an error condition.
+            *  MAY exit with an error condition.
-            +  otherwise:
+        +  otherwise:
-              *  the fragment sender MUST send SCHC Fragment messages
+            *  the fragment sender MUST send SCHC Fragment messages
-                  containing all the tiles that are reported missing in
+              containing all the tiles that are reported missing in
-                  the SCHC ACK.
+              the SCHC ACK.
-              *  if the last of these SCHC Fragment messages is not an
+            *  if the last of these SCHC Fragment messages is not an
-                  All-1 SCHC Fragment, then the fragment sender MUST in
+              All-1 SCHC Fragment, then the fragment sender MUST in
-                  addition send after it a SCHC ACK REQ with the W field
+              addition send after it a SCHC ACK REQ with the W field
-                  corresponding to the last window.
+              corresponding to the last window.
-              *  in doing the two items above, the sender MUST
+            *  in doing the two items above, the sender MUST
-                  ascertain that the receiver will not receive the last
+              ascertain that the receiver will not receive the last
-                  tile through both a Regular SCHC Fragment and an All-1
+              tile through both a Regular SCHC Fragment and an All-1
-                  SCHC Fragment.
+              SCHC Fragment.
-        o  otherwise:
+      o  otherwise:
-            +  if the SCHC ACK shows no missing tile at the receiver,
+        +  if the SCHC ACK shows no missing tile at the receiver,
-              the sender MUST send the All-1 SCHC Fragment
+            the sender MUST send the All-1 SCHC Fragment
-            +  otherwise:
+        +  otherwise:
-              *  the fragment sender MUST send SCHC Fragment messages
+            *  the fragment sender MUST send SCHC Fragment messages
-                  containing all the tiles that are reported missing in
+              containing all the tiles that are reported missing in
-                  the SCHC ACK.
+              the SCHC ACK.
-              *  the fragment sender MUST then send either the All-1
+            *  the fragment sender MUST then send either the All-1
-                  SCHC Fragment or a SCHC ACK REQ with the W field
+              SCHC Fragment or a SCHC ACK REQ with the W field
-                  corresponding to the last window.
+              corresponding to the last window.
-  *  otherwise, the fragment sender:
+*  otherwise, the fragment sender:
-      -  MUST send SCHC Fragment messages containing the tiles that are
+  -  MUST send SCHC Fragment messages containing the tiles that are
-        reported missing in the SCHC ACK.
+      reported missing in the SCHC ACK.
-      -  then, it MAY send a SCHC ACK REQ with the W field corresponding
+  -  then, it MAY send a SCHC ACK REQ with the W field corresponding
-        to the last window.
+      to the last window.
-  See Figure 43 for one among several possible examples of a Finite
+See Figure 43 for one among several possible examples of a Finite
-  State Machine implementing a sender behavior obeying this
+State Machine implementing a sender behavior obeying this
-  specification.
+specification.
-.4.3.2.  Receiver Behavior
+===== Receiver Behavior =====
-  On receiving a SCHC Fragment with a RuleID and DTag pair not being
+On receiving a SCHC Fragment with a RuleID and DTag pair not being
-  processed at that time:
+processed at that time:
-  *  the receiver SHOULD check if the DTag value has not recently been
+*  the receiver SHOULD check if the DTag value has not recently been
-      used for that RuleID value, thereby ensuring that the received
+  used for that RuleID value, thereby ensuring that the received
-      SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
+  SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
-      transmission.  The initial value of the Inactivity Timer is the
+  transmission.  The initial value of the Inactivity Timer is the
-      RECOMMENDED lifetime for the DTag value at the receiver.  If the
+  RECOMMENDED lifetime for the DTag value at the receiver.  If the
-      SCHC Fragment is determined to be such a remnant, the receiver MAY
+  SCHC Fragment is determined to be such a remnant, the receiver MAY
-      silently ignore it and discard it.
+  silently ignore it and discard it.
-  *  the receiver MUST start a process to assemble a new SCHC Packet
+*  the receiver MUST start a process to assemble a new SCHC Packet
-      with that RuleID and DTag value pair.  The receiver MUST start an
+  with that RuleID and DTag value pair.  The receiver MUST start an
-      Inactivity Timer for that RuleID and DTag value pair.  It MUST
+  Inactivity Timer for that RuleID and DTag value pair.  It MUST
-      initialize an Attempts counter to 0 for that RuleID and DTag value
+  initialize an Attempts counter to 0 for that RuleID and DTag value
-      pair.  If the receiver is under-resourced to do this, it MUST
+  pair.  If the receiver is under-resourced to do this, it MUST
-      respond to the sender with a SCHC Receiver-Abort.
+  respond to the sender with a SCHC Receiver-Abort.
-  On reception of any SCHC F/R message for the RuleID and DTag pair
+On reception of any SCHC F/R message for the RuleID and DTag pair
-  being processed, the receiver MUST reset the Inactivity Timer
+being processed, the receiver MUST reset the Inactivity Timer
-  pertaining to that RuleID and DTag pair.
+pertaining to that RuleID and DTag pair.
-  All message receptions being discussed in the rest of this section
+All message receptions being discussed in the rest of this section
-  are to be understood as "matching the RuleID and DTag pair being
+are to be understood as "matching the RuleID and DTag pair being
-  processed", even if not spelled out, for brevity.
+processed", even if not spelled out, for brevity.
-  On receiving a SCHC Fragment message, the receiver determines what
+On receiving a SCHC Fragment message, the receiver determines what
-  tiles were received, based on the payload length and on the W and FCN
+tiles were received, based on the payload length and on the W and FCN
-  fields of the SCHC Fragment.
+fields of the SCHC Fragment.
-  *  if the FCN is All-1, if a Payload is present, the full SCHC
+*  if the FCN is All-1, if a Payload is present, the full SCHC
-      Fragment Payload MUST be assembled including the padding bits.
+  Fragment Payload MUST be assembled including the padding bits.
-      This is because the size of the last tile is not known by the
+  This is because the size of the last tile is not known by the
-      receiver; therefore, padding bits are indistinguishable from the
+  receiver; therefore, padding bits are indistinguishable from the
-      tile data bits, at this stage.  They will be removed by the SCHC
+  tile data bits, at this stage.  They will be removed by the SCHC
-      C/D sublayer.  If the size of the SCHC Fragment Payload exceeds or
+  C/D sublayer.  If the size of the SCHC Fragment Payload exceeds or
-      equals the size of one regular tile plus the size of an L2 Word,
+  equals the size of one regular tile plus the size of an L2 Word,
-      this SHOULD raise an error flag.
+  this SHOULD raise an error flag.
-  *  otherwise, tiles MUST be assembled based on the a priori known
+*  otherwise, tiles MUST be assembled based on the a priori known
-      tile size.
+  tile size.
-      -  If allowed by the Profile, the end of the payload MAY contain
+  -  If allowed by the Profile, the end of the payload MAY contain
-        the last tile, which may be shorter.  Padding bits are
+      the last tile, which may be shorter.  Padding bits are
-        indistinguishable from the tile data bits, at this stage.
+      indistinguishable from the tile data bits, at this stage.
-      -  The payload may contain the penultimate tile that, if allowed
+  -  The payload may contain the penultimate tile that, if allowed
-        by the Profile, MAY be exactly one L2 Word shorter than the
+      by the Profile, MAY be exactly one L2 Word shorter than the
-        regular tile size.
+      regular tile size.
-      -  Otherwise, padding bits MUST be discarded.  This is possible
+  -  Otherwise, padding bits MUST be discarded.  This is possible
-        because:
+      because:
-        o  the size of the tiles is known a priori,
+      o  the size of the tiles is known a priori,
-        o  tiles are larger than an L2 Word, and
+      o  tiles are larger than an L2 Word, and
-        o  padding bits are always strictly less than an L2 Word.
+      o  padding bits are always strictly less than an L2 Word.
-  On receiving a SCHC ACK REQ or an All-1 SCHC Fragment:
+On receiving a SCHC ACK REQ or an All-1 SCHC Fragment:
-  *  if the receiver knows of any windows with missing tiles for the
+*  if the receiver knows of any windows with missing tiles for the
-      packet being reassembled, it MUST return a SCHC ACK for the
+  packet being reassembled, it MUST return a SCHC ACK for the
-      lowest-numbered such window:
+  lowest-numbered such window:
-  *  otherwise:
+*  otherwise:
-      -  if it has received at least one tile, it MUST return a SCHC ACK
+  -  if it has received at least one tile, it MUST return a SCHC ACK
-        for the highest-numbered window it currently has tiles for,
+      for the highest-numbered window it currently has tiles for,
-      -  otherwise, it MUST return a SCHC ACK for window numbered 0.
+  -  otherwise, it MUST return a SCHC ACK for window numbered 0.
-  A Profile MAY specify other times and circumstances at which a
+A Profile MAY specify other times and circumstances at which a
-  receiver sends a SCHC ACK, and which window the SCHC ACK reports
+receiver sends a SCHC ACK, and which window the SCHC ACK reports
-  about in these circumstances.
+about in these circumstances.
-  Upon sending a SCHC ACK, the receiver MUST increase the Attempts
+Upon sending a SCHC ACK, the receiver MUST increase the Attempts
-  counter.
+counter.
-  After receiving an All-1 SCHC Fragment, a receiver MUST check the
+After receiving an All-1 SCHC Fragment, a receiver MUST check the
-  integrity of the reassembled SCHC Packet at least every time it
+integrity of the reassembled SCHC Packet at least every time it
-  prepares for sending a SCHC ACK for the last window.
+prepares for sending a SCHC ACK for the last window.
-  Upon receiving a SCHC Sender-Abort, the receiver MAY exit with an
+Upon receiving a SCHC Sender-Abort, the receiver MAY exit with an
-  error condition.
+error condition.
-  Upon expiration of the Inactivity Timer, the receiver MUST send a
+Upon expiration of the Inactivity Timer, the receiver MUST send a
-  SCHC Receiver-Abort, and it MAY exit with an error condition.
+SCHC Receiver-Abort, and it MAY exit with an error condition.
-  On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST
+On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST
-  send a SCHC Receiver-Abort, and it MAY exit with an error condition.
+send a SCHC Receiver-Abort, and it MAY exit with an error condition.
-  Reassembly of the SCHC Packet concludes when:
+Reassembly of the SCHC Packet concludes when:
-  *  a Sender-Abort has been received, or
+*  a Sender-Abort has been received, or
-  *  the Inactivity Timer has expired, or
+*  the Inactivity Timer has expired, or
-  *  the Attempts counter has exceeded MAX_ACK_REQUESTS, or
+*  the Attempts counter has exceeded MAX_ACK_REQUESTS, or
-  *  at least an All-1 SCHC Fragment has been received and integrity
+*  at least an All-1 SCHC Fragment has been received and integrity
-      checking of the reassembled SCHC Packet is successful.
+  checking of the reassembled SCHC Packet is successful.
-  See Figure 44 for one among several possible examples of a Finite
+See Figure 44 for one among several possible examples of a Finite
-  State Machine implementing a receiver behavior obeying this
+State Machine implementing a receiver behavior obeying this
-  specification.  The example provided is meant to match the sender
+specification.  The example provided is meant to match the sender
-  Finite State Machine of Figure 43.
+Finite State Machine of Figure 43.
-.  Padding Management
+== Padding Management ==
-  SCHC C/D and SCHC F/R operate on bits, not bytes.  SCHC itself does
+SCHC C/D and SCHC F/R operate on bits, not bytes.  SCHC itself does
-  not have any alignment prerequisite.  The size of SCHC Packets can be
+not have any alignment prerequisite.  The size of SCHC Packets can be
-  any number of bits.
+any number of bits.
-  If the L2 constrains the payload to align to coarser boundaries (for
+If the L2 constrains the payload to align to coarser boundaries (for
-  example, bytes), the SCHC messages MUST be padded.  When padding
+example, bytes), the SCHC messages MUST be padded.  When padding
-  occurs, the number of appended bits MUST be strictly less than the L2
+occurs, the number of appended bits MUST be strictly less than the L2
-  Word size.
+Word size.
-  If a SCHC Packet is sent unfragmented (see Figure 24), it is padded
+If a SCHC Packet is sent unfragmented (see Figure 24), it is padded
-  as needed for transmission.
+as needed for transmission.
-  If a SCHC Packet needs to be fragmented for transmission, it is not
+If a SCHC Packet needs to be fragmented for transmission, it is not
-  padded in itself.  Only the SCHC F/R messages are padded as needed
+padded in itself.  Only the SCHC F/R messages are padded as needed
-  for transmission.  Some SCHC F/R messages are intrinsically aligned
+for transmission.  Some SCHC F/R messages are intrinsically aligned
-  to L2 Words.
+to L2 Words.
-  A packet (e.g., an IPv6 packet)
+A packet (e.g., an IPv6 packet)
-            |                                          ^ (padding bits
+        |                                          ^ (padding bits
-            v                                          |      dropped)
+        v                                          |      dropped)
-  +------------------+                      +--------------------+
++------------------+                      +--------------------+
-  | SCHC Compression |                      | SCHC Decompression |
+| SCHC Compression |                      | SCHC Decompression |
-  +------------------+                      +--------------------+
++------------------+                      +--------------------+
-            |                                          ^
+        |                                          ^
-            |  If no fragmentation,                    |
+        |  If no fragmentation,                    |
-            +---- SCHC Packet + padding as needed ----->|
+        +---- SCHC Packet + padding as needed ----->|
-            |                                          | (integrity
+        |                                          | (integrity
-            v                                          |  checked)
+        v                                          |  checked)
-  +--------------------+                      +-----------------+
++--------------------+                      +-----------------+
-  | SCHC Fragmentation |                      | SCHC Reassembly |
+| SCHC Fragmentation |                      | SCHC Reassembly |
-  +--------------------+                      +-----------------+
++--------------------+                      +-----------------+
-        |      ^                                  |      ^
+    |      ^                                  |      ^
-        |      |                                  |      |
+    |      |                                  |      |
-        |      +--- SCHC ACK + padding as needed --+      |
+    |      +--- SCHC ACK + padding as needed --+      |
-        |                                                  |
+    |                                                  |
-        +------- SCHC Fragments + padding as needed---------+
+    +------- SCHC Fragments + padding as needed---------+
-          Sender                                    Receiver
+        Sender                                    Receiver
-          Figure 24: SCHC Operations, Including Padding as Needed
+      Figure 24: SCHC Operations, Including Padding as Needed
-  Each Profile MUST specify the size of the L2 Word.  The L2 Word might
+Each Profile MUST specify the size of the L2 Word.  The L2 Word might
-  actually be a single bit, in which case no padding will take place at
+actually be a single bit, in which case no padding will take place at
-  all.
+all.
-  A Profile MUST define the value of the padding bits if the L2 Word is
+A Profile MUST define the value of the padding bits if the L2 Word is
-  wider than a single bit.  The RECOMMENDED value is 0.
+wider than a single bit.  The RECOMMENDED value is 0.
 .  SCHC Compression for IPv6 and UDP Headers
-  This section lists the IPv6 and UDP header fields and describes how
+This section lists the IPv6 and UDP header fields and describes how
-  they can be compressed.  An example of a set of Rules for UDP/IPv6
+they can be compressed.  An example of a set of Rules for UDP/IPv6
-  header compression is provided in Appendix A.
+header compression is provided in Appendix A.
 .1.  IPv6 Version Field
-  The IPv6 version field is labeled by the protocol parser as being the
+The IPv6 version field is labeled by the protocol parser as being the
-  "version" field of the IPv6 protocol.  Therefore, it only exists for
+"version" field of the IPv6 protocol.  Therefore, it only exists for
-  IPv6 packets.  In the Rule, TV is set to 6, MO to "ignore" and CDA to
+IPv6 packets.  In the Rule, TV is set to 6, MO to "ignore" and CDA to
-  "not-sent".
+"not-sent".
 .2.  IPv6 Traffic Class Field
-  If the Diffserv field does not vary and is known by both sides, the
+If the Diffserv field does not vary and is known by both sides, the
-  Field Descriptor in the Rule SHOULD contain a TV with this well-known
+Field Descriptor in the Rule SHOULD contain a TV with this well-known
-  value, an "equal" MO, and a "not-sent" CDA.
+value, an "equal" MO, and a "not-sent" CDA.
-  Otherwise (e.g., ECN bits are to be transmitted), two possibilities
+Otherwise (e.g., ECN bits are to be transmitted), two possibilities
-  can be considered depending on the variability of the value:
+can be considered depending on the variability of the value:
-  *  One possibility is to not compress the field and send the original
+*  One possibility is to not compress the field and send the original
-      value.  In the Rule, TV is not set to any particular value, MO is
+  value.  In the Rule, TV is not set to any particular value, MO is
-      set to "ignore", and CDA is set to "value-sent".
+  set to "ignore", and CDA is set to "value-sent".
-  *  If some upper bits in the field are constant and known, a better
+*  If some upper bits in the field are constant and known, a better
-      option is to only send the LSBs.  In the Rule, TV is set to a
+  option is to only send the LSBs.  In the Rule, TV is set to a
-      value with the stable known upper part, MO is set to MSB(x), and
+  value with the stable known upper part, MO is set to MSB(x), and
-      CDA to LSB.
+  CDA to LSB.
-      ECN functionality depends on both bits of the ECN field, which are
+  ECN functionality depends on both bits of the ECN field, which are
-      the 2 LSBs of this field; hence, sending only a single LSB of this
+  the 2 LSBs of this field; hence, sending only a single LSB of this
-      field is NOT RECOMMENDED.
+  field is NOT RECOMMENDED.
 .3.  Flow Label Field
-  If the flow label is not set, i.e., its value is zero, the Field
+If the flow label is not set, i.e., its value is zero, the Field
-  Descriptor in the Rule SHOULD contain a TV set to zero, an "equal"
+Descriptor in the Rule SHOULD contain a TV set to zero, an "equal"
-  MO, and a "not-sent" CDA.
+MO, and a "not-sent" CDA.
-  If the flow label is set to a pseudorandom value according to
+If the flow label is set to a pseudorandom value according to
-  [RFC6437], in the Rule, TV is not set to any particular value, MO is
+[[RFC6437]], in the Rule, TV is not set to any particular value, MO is
-  set to "ignore", and CDA is set to "value-sent".
+set to "ignore", and CDA is set to "value-sent".
-  If the flow label is set according to some prior agreement, i.e., by
+If the flow label is set according to some prior agreement, i.e., by
-  a flow state establishment method as allowed by [RFC6437], the Field
+a flow state establishment method as allowed by [[RFC6437]], the Field
-  Descriptor in the Rule SHOULD contain a TV with this agreed-upon
+Descriptor in the Rule SHOULD contain a TV with this agreed-upon
-  value, an "equal" MO, and a "not-sent" CDA.
+value, an "equal" MO, and a "not-sent" CDA.
 .4.  Payload Length Field
-  This field can be elided for the transmission on the LPWAN.  The SCHC
+This field can be elided for the transmission on the LPWAN.  The SCHC
-  C/D recomputes the original payload length value.  In the Field
+C/D recomputes the original payload length value.  In the Field
-  Descriptor, TV is not set, MO is set to "ignore", and CDA is
+Descriptor, TV is not set, MO is set to "ignore", and CDA is
-  "compute-*".
+"compute-*".
 .5.  Next Header Field
-  If the Next Header field does not vary and is known by both sides,
+If the Next Header field does not vary and is known by both sides,
-  the Field Descriptor in the Rule SHOULD contain a TV with this Next
+the Field Descriptor in the Rule SHOULD contain a TV with this Next
-  Header value, the MO SHOULD be "equal", and the CDA SHOULD be "not-
+Header value, the MO SHOULD be "equal", and the CDA SHOULD be "not-
-  sent".
+sent".
-  Otherwise, TV is not set in the Field Descriptor, MO is set to
+Otherwise, TV is not set in the Field Descriptor, MO is set to
-  "ignore", and CDA is set to "value-sent".  Alternatively, a matching-
+"ignore", and CDA is set to "value-sent".  Alternatively, a matching-
-  list MAY also be used.
+list MAY also be used.
 .6.  Hop Limit Field
-  The field behavior for this field is different for Uplink and
+The field behavior for this field is different for Uplink and
-  Downlink.  In Uplink, since there is no IP forwarding between the Dev
+Downlink.  In Uplink, since there is no IP forwarding between the Dev
-  and the SCHC C/D, the value is relatively constant.  On the other
+and the SCHC C/D, the value is relatively constant.  On the other
-  hand, the Downlink value depends on Internet routing and can change
+hand, the Downlink value depends on Internet routing and can change
-  more frequently.  The DI can be used to distinguish both directions:
+more frequently.  The DI can be used to distinguish both directions:
-  *  in an Up Field Descriptor, elide the field: the TV is set to the
+*  in an Up Field Descriptor, elide the field: the TV is set to the
-      known constant value, the MO is set to "equal" and the CDA is set
+  known constant value, the MO is set to "equal" and the CDA is set
-      to "not-sent".
+  to "not-sent".
-  *  in a Dw Field Descriptor, the Hop Limit is elided for transmission
+*  in a Dw Field Descriptor, the Hop Limit is elided for transmission
-      and forced to 1 at the receiver, by setting TV to 1, MO to
+  and forced to 1 at the receiver, by setting TV to 1, MO to
-      "ignore" and CDA to "not-sent".  This prevents any further
+  "ignore" and CDA to "not-sent".  This prevents any further
-      forwarding.
+  forwarding.
 .7.  IPv6 Addresses Fields
-  As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit-
+As in 6LoWPAN [[RFC4944]], IPv6 addresses are split into two 64-bit-
-  long fields; one for the prefix and one for the Interface Identifier
+long fields; one for the prefix and one for the Interface Identifier
-  (IID).  These fields SHOULD be compressed.  To allow for a single
+(IID).  These fields SHOULD be compressed.  To allow for a single
-  Rule being used for both directions, these values are identified by
+Rule being used for both directions, these values are identified by
-  their role (Dev or App) and not by their position in the header
+their role (Dev or App) and not by their position in the header
-  (source or destination).
+(source or destination).
 .7.1.  IPv6 Source and Destination Prefixes
-  Both ends MUST be configured with the appropriate prefixes.  For a
+Both ends MUST be configured with the appropriate prefixes.  For a
-  specific flow, the source and destination prefixes can be unique and
+specific flow, the source and destination prefixes can be unique and
-  stored in the Context.  In that case, the TV for the source and
+stored in the Context.  In that case, the TV for the source and
-  destination prefixes contain the values, the MO is set to "equal" and
+destination prefixes contain the values, the MO is set to "equal" and
-  the CDA is set to "not-sent".
+the CDA is set to "not-sent".
-  If the Rule is intended to compress packets with different prefix
+If the Rule is intended to compress packets with different prefix
-  values, match-mapping SHOULD be used.  The different prefixes are
+values, match-mapping SHOULD be used.  The different prefixes are
-  listed in the TV, the MO is set to "match-mapping" and the CDA is set
+listed in the TV, the MO is set to "match-mapping" and the CDA is set
-  to "mapping-sent".  See Figure 26.
+to "mapping-sent".  See Figure 26.
-  Otherwise, the TV is not set, the MO is set to "ignore", and the CDA
+Otherwise, the TV is not set, the MO is set to "ignore", and the CDA
-  is set to "value-sent".
+is set to "value-sent".
 .7.2.  IPv6 Source and Destination IID
-  If the Dev or App IID are based on an L2 address, then the IID can be
+If the Dev or App IID are based on an L2 address, then the IID can be
-  reconstructed with information coming from the L2 header.  In that
+reconstructed with information coming from the L2 header.  In that
-  case, the TV is not set, the MO is set to "ignore" and the CDA is set
+case, the TV is not set, the MO is set to "ignore" and the CDA is set
-  to "DevIID" or "AppIID".  On LPWAN technologies where the frames
+to "DevIID" or "AppIID".  On LPWAN technologies where the frames
-  carry a single identifier (corresponding to the Dev), AppIID cannot
+carry a single identifier (corresponding to the Dev), AppIID cannot
-  be used.
+be used.
-  As described in [RFC8065], it may be undesirable to build the Dev
+As described in [[RFC8065]], it may be undesirable to build the Dev
-  IPv6 IID out of the Dev address.  Another static value is used
+IPv6 IID out of the Dev address.  Another static value is used
-  instead.  In that case, the TV contains the static value, the MO
+instead.  In that case, the TV contains the static value, the MO
-  operator is set to "equal" and the CDA is set to "not-sent".
+operator is set to "equal" and the CDA is set to "not-sent".
-  If several IIDs are possible, then the TV contains the list of
+If several IIDs are possible, then the TV contains the list of
-  possible IIDs, the MO is set to "match-mapping" and the CDA is set to
+possible IIDs, the MO is set to "match-mapping" and the CDA is set to
-  "mapping-sent".
+"mapping-sent".
-  It may also happen that the IID variability only expresses itself on
+It may also happen that the IID variability only expresses itself on
-  a few bytes.  In that case, the TV is set to the stable part of the
+a few bytes.  In that case, the TV is set to the stable part of the
-  IID, the MO is set to "MSB" and the CDA is set to "LSB".
+IID, the MO is set to "MSB" and the CDA is set to "LSB".
-  Finally, the IID can be sent in its entirety on the L2.  In that
+Finally, the IID can be sent in its entirety on the L2.  In that
-  case, the TV is not set, the MO is set to "ignore", and the CDA is
+case, the TV is not set, the MO is set to "ignore", and the CDA is
-  set to "value-sent".
+set to "value-sent".
 .8.  IPv6 Extension Headers
-  This document does not provide recommendations on how to compress
+This document does not provide recommendations on how to compress
-  IPv6 extension headers.
+IPv6 extension headers.
 .9.  UDP Source and Destination Ports
-  To allow for a single Rule being used for both directions, the UDP
+To allow for a single Rule being used for both directions, the UDP
-  port values are identified by their role (Dev or App) and not by
+port values are identified by their role (Dev or App) and not by
-  their position in the header (source or destination).  The SCHC C/D
+their position in the header (source or destination).  The SCHC C/D
-  MUST be aware of the traffic direction (Uplink, Downlink) to select
+MUST be aware of the traffic direction (Uplink, Downlink) to select
-  the appropriate field.  The following Rules apply for Dev and App
+the appropriate field.  The following Rules apply for Dev and App
-  port numbers.
+port numbers.
-  If both ends know the port number, it can be elided.  The TV contains
+If both ends know the port number, it can be elided.  The TV contains
-  the port number, the MO is set to "equal", and the CDA is set to
+the port number, the MO is set to "equal", and the CDA is set to
-  "not-sent".
+"not-sent".
-  If the port variation is on few bits, the TV contains the stable part
+If the port variation is on few bits, the TV contains the stable part
-  of the port number, the MO is set to "MSB", and the CDA is set to
+of the port number, the MO is set to "MSB", and the CDA is set to
-  "LSB".
+"LSB".
-  If some well-known values are used, the TV can contain the list of
+If some well-known values are used, the TV can contain the list of
-  these values, the MO is set to "match-mapping", and the CDA is set to
+these values, the MO is set to "match-mapping", and the CDA is set to
-  "mapping-sent".
+"mapping-sent".
-  Otherwise, the port numbers are sent over the L2.  The TV is not set,
+Otherwise, the port numbers are sent over the L2.  The TV is not set,
-  the MO is set to "ignore" and the CDA is set to "value-sent".
+the MO is set to "ignore" and the CDA is set to "value-sent".
 .10.  UDP Length Field
-  The parser MUST NOT label this field unless the UDP Length value
+The parser MUST NOT label this field unless the UDP Length value
-  matches the Payload Length value from the IPv6 header.  The TV is not
+matches the Payload Length value from the IPv6 header.  The TV is not
-  set, the MO is set to "ignore", and the CDA is set to "compute-*".
+set, the MO is set to "ignore", and the CDA is set to "compute-*".
 .11.  UDP Checksum Field
-  The UDP checksum operation is mandatory with IPv6 for most packets,
+The UDP checksum operation is mandatory with IPv6 for most packets,
-  but there are exceptions [RFC8200].
+but there are exceptions [[RFC8200]].
-  For instance, protocols that use UDP as a tunnel encapsulation may
+For instance, protocols that use UDP as a tunnel encapsulation may
-  enable zero-checksum mode for a specific port (or set of ports) for
+enable zero-checksum mode for a specific port (or set of ports) for
-  sending and/or receiving.  [RFC8200] requires any node implementing
+sending and/or receiving.  [[RFC8200]] requires any node implementing
-  zero-checksum mode to follow the requirements specified in
+zero-checksum mode to follow the requirements specified in
-  "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero
+"Applicability Statement for the Use of IPv6 UDP Datagrams with Zero
-  Checksums" [RFC6936].
+Checksums" [[RFC6936]].
-LoWPAN Header Compression [RFC6282] also specifies that a UDP
+LoWPAN Header Compression [[RFC6282]] also specifies that a UDP
-  checksum can be elided by the compressor and recomputed by the
+checksum can be elided by the compressor and recomputed by the
-  decompressor when an upper layer guarantees the integrity of the UDP
+decompressor when an upper layer guarantees the integrity of the UDP
-  payload and pseudo-header.  A specific example of this is when a
+payload and pseudo-header.  A specific example of this is when a
-  message integrity check protects the compressed message between the
+message integrity check protects the compressed message between the
-  compressor that elides the UDP checksum and the decompressor that
+compressor that elides the UDP checksum and the decompressor that
-  computes it, with a strength that is identical or better to the UDP
+computes it, with a strength that is identical or better to the UDP
-  checksum.
+checksum.
-  Similarly, a SCHC compressor MAY elide the UDP checksum when another
+Similarly, a SCHC compressor MAY elide the UDP checksum when another
-  layer guarantees at least equal integrity protection for the UDP
+layer guarantees at least equal integrity protection for the UDP
-  payload and the pseudo-header.  In this case, the TV is not set, the
+payload and the pseudo-header.  In this case, the TV is not set, the
-  MO is set to "ignore", and the CDA is set to "compute-*".
+MO is set to "ignore", and the CDA is set to "compute-*".
-  In particular, when SCHC fragmentation is used, a fragmentation RCS
+In particular, when SCHC fragmentation is used, a fragmentation RCS
-  of 2 bytes or more provides equal or better protection than the UDP
+of 2 bytes or more provides equal or better protection than the UDP
-  checksum; in that case, if the compressor is collocated with the
+checksum; in that case, if the compressor is collocated with the
-  fragmentation point and the decompressor is collocated with the
+fragmentation point and the decompressor is collocated with the
-  packet reassembly point, and if the SCHC Packet is fragmented even
+packet reassembly point, and if the SCHC Packet is fragmented even
-  when it would fit unfragmented in the L2 MTU, then the compressor MAY
+when it would fit unfragmented in the L2 MTU, then the compressor MAY
-  verify and then elide the UDP checksum.  Whether and when the UDP
+verify and then elide the UDP checksum.  Whether and when the UDP
-  Checksum is elided is to be specified in the Profile.
+Checksum is elided is to be specified in the Profile.
-  Since the compression happens before the fragmentation, implementers
+Since the compression happens before the fragmentation, implementers
-  should understand the risks when dealing with unprotected data below
+should understand the risks when dealing with unprotected data below
-  the transport layer and take special care when manipulating that
+the transport layer and take special care when manipulating that
-  data.
+data.
-  In other cases, the checksum SHOULD be explicitly sent.  The TV is
+In other cases, the checksum SHOULD be explicitly sent.  The TV is
-  not set, the MO is set to "ignore" and the CDA is set to "value-
+not set, the MO is set to "ignore" and the CDA is set to "value-
-  sent".
+sent".
 .  IANA Considerations
-  This document has no IANA actions.
+This document has no IANA actions.
 .  Security Considerations
-  As explained in Section 5, SCHC is expected to be implemented on top
+As explained in Section 5, SCHC is expected to be implemented on top
-  of LPWAN technologies, which are expected to implement security
+of LPWAN technologies, which are expected to implement security
-  measures.
+measures.
-  In this section, we analyze the potential security threats that could
+In this section, we analyze the potential security threats that could
-  be introduced into an LPWAN by adding the SCHC functionalities.
+be introduced into an LPWAN by adding the SCHC functionalities.
 .1.  Security Considerations for SCHC Compression/Decompression
@@ Line 2,618: / Line 2,612: @@
 .1.1.  Forged SCHC Packet
-  Let's assume that an attacker is able to send a forged SCHC Packet to
+Let's assume that an attacker is able to send a forged SCHC Packet to
-  a SCHC decompressor.
+a SCHC decompressor.
-  Let's first consider the case where the RuleID contained in that
+Let's first consider the case where the RuleID contained in that
-  forged SCHC Packet does not correspond to a Rule allocated in the
+forged SCHC Packet does not correspond to a Rule allocated in the
-  Rule table.  An implementation should detect that the RuleID is
+Rule table.  An implementation should detect that the RuleID is
-  invalid and should silently drop the offending SCHC Packet.
+invalid and should silently drop the offending SCHC Packet.
-  Let's now consider that the RuleID corresponds to a Rule in the
+Let's now consider that the RuleID corresponds to a Rule in the
-  table.  With the CDAs defined in this document, the reconstructed
+table.  With the CDAs defined in this document, the reconstructed
-  packet is, at most, a constant number of bits bigger than the SCHC
+packet is, at most, a constant number of bits bigger than the SCHC
-  Packet that was received.  This assumes that the compute-*
+Packet that was received.  This assumes that the compute-*
-  decompression actions produce a bounded number of bits, irrespective
+decompression actions produce a bounded number of bits, irrespective
-  of the incoming SCHC Packet.  This property is true for IPv6 Length,
+of the incoming SCHC Packet.  This property is true for IPv6 Length,
-  UDP Length, and UDP Checksum, for which the compute-* CDA is
+UDP Length, and UDP Checksum, for which the compute-* CDA is
-  recommended by this document.
+recommended by this document.
-  As a consequence, SCHC decompression does not amplify attacks, beyond
+As a consequence, SCHC decompression does not amplify attacks, beyond
-  adding a bounded number of bits to the SCHC Packet received.  This
+adding a bounded number of bits to the SCHC Packet received.  This
-  bound is determined by the Rule stored in the receiving device.
+bound is determined by the Rule stored in the receiving device.
-  As a general safety measure, a SCHC decompressor should never
+As a general safety measure, a SCHC decompressor should never
-  reconstruct a packet larger than MAX_PACKET_SIZE (defined in a
+reconstruct a packet larger than MAX_PACKET_SIZE (defined in a
-  Profile, with 1500 bytes as generic default).
+Profile, with 1500 bytes as generic default).
 .1.2.  Compressed Packet Size as a Side Channel to Guess a Secret
-        Token
+      Token
-  Some packet compression methods are known to be susceptible to
+Some packet compression methods are known to be susceptible to
-  attacks, such as BREACH and CRIME.  The attack involves injecting
+attacks, such as BREACH and CRIME.  The attack involves injecting
-  arbitrary data into the packet and observing the resulting compressed
+arbitrary data into the packet and observing the resulting compressed
-  packet size.  The observed size potentially reflects correlation
+packet size.  The observed size potentially reflects correlation
-  between the arbitrary data and some content that was meant to remain
+between the arbitrary data and some content that was meant to remain
-  secret, such as a security token, thereby allowing the attacker to
+secret, such as a security token, thereby allowing the attacker to
-  get at the secret.
+get at the secret.
-  By contrast, SCHC compression takes place header field by header
+By contrast, SCHC compression takes place header field by header
-  field, with the SCHC Packet being a mere concatenation of the
+field, with the SCHC Packet being a mere concatenation of the
-  compression residues of each of the individual field.  Any
+compression residues of each of the individual field.  Any
-  correlation between header fields does not result in a change in the
+correlation between header fields does not result in a change in the
-  SCHC Packet size compressed under the same Rule.
+SCHC Packet size compressed under the same Rule.
-  If SCHC C/D is used to compress packets that include a secret
+If SCHC C/D is used to compress packets that include a secret
-  information field, such as a token, the Rule set should be designed
+information field, such as a token, the Rule set should be designed
-  so that the size of the compression residue for the field to remain
+so that the size of the compression residue for the field to remain
-  secret is the same irrespective of the value of the secret
+secret is the same irrespective of the value of the secret
-  information.  This is achieved by, e.g., sending this field in
+information.  This is achieved by, e.g., sending this field in
-  extenso with the "ignore" MO and the "value-sent" CDA.  This
+extenso with the "ignore" MO and the "value-sent" CDA.  This
-  recommendation is disputable if it is ascertained that the Rule set
+recommendation is disputable if it is ascertained that the Rule set
-  itself will remain secret.
+itself will remain secret.
 .1.3.  Decompressed Packet Different from the Original Packet
-  As explained in Section 7.2, using FPs with value 0 in Field
+As explained in Section 7.2, using FPs with value 0 in Field
-  Descriptors in a Rule may result in header fields appearing in the
+Descriptors in a Rule may result in header fields appearing in the
-  decompressed packet in an order different from that in the original
+decompressed packet in an order different from that in the original
-  packet.  Likewise, as stated in Section 7.4.3, using an "ignore" MO
+packet.  Likewise, as stated in Section 7.4.3, using an "ignore" MO
-  together with a "not-sent" CDA will result in the header field taking
+together with a "not-sent" CDA will result in the header field taking
-  the TV value, which is likely to be different from the original
+the TV value, which is likely to be different from the original
-  value.
+value.
-  Depending on the protocol, the order of header fields in the packet
+Depending on the protocol, the order of header fields in the packet
-  may or may not be functionally significant.
+may or may not be functionally significant.
-  Furthermore, if the packet is protected by a checksum or a similar
+Furthermore, if the packet is protected by a checksum or a similar
-  integrity protection mechanism, and if the checksum is transmitted
+integrity protection mechanism, and if the checksum is transmitted
-  instead of being recomputed as part of the decompression, these
+instead of being recomputed as part of the decompression, these
-  situations may result in the packet being considered corrupt and
+situations may result in the packet being considered corrupt and
-  dropped.
+dropped.
 .2.  Security Considerations for SCHC Fragmentation/Reassembly
@@ Line 2,692: / Line 2,686: @@
 .2.1.  Buffer Reservation Attack
-  Let's assume that an attacker is able to send a forged SCHC Fragment
+Let's assume that an attacker is able to send a forged SCHC Fragment
-  to a SCHC reassembler.
+to a SCHC reassembler.
-  A node can perform a buffer reservation attack: the receiver will
+A node can perform a buffer reservation attack: the receiver will
-  reserve buffer space for the SCHC Packet.  If the implementation has
+reserve buffer space for the SCHC Packet.  If the implementation has
-  only one buffer, other incoming fragmented SCHC Packets will be
+only one buffer, other incoming fragmented SCHC Packets will be
-  dropped while the reassembly buffer is occupied during the reassembly
+dropped while the reassembly buffer is occupied during the reassembly
-  timeout.  Once that timeout expires, the attacker can repeat the same
+timeout.  Once that timeout expires, the attacker can repeat the same
-  procedure, and iterate, thus, creating a denial-of-service attack.
+procedure, and iterate, thus, creating a denial-of-service attack.
-  An implementation may have multiple reassembly buffers.  The cost to
+An implementation may have multiple reassembly buffers.  The cost to
-  mount this attack is linear with the number of buffers at the target
+mount this attack is linear with the number of buffers at the target
-  node.  Better, the cost for an attacker can be increased if
+node.  Better, the cost for an attacker can be increased if
-  individual fragments of multiple SCHC Packets can be stored in the
+individual fragments of multiple SCHC Packets can be stored in the
-  reassembly buffer.  The finer grained the reassembly buffer (down to
+reassembly buffer.  The finer grained the reassembly buffer (down to
-  the smallest tile size), the higher the cost of the attack.  If
+the smallest tile size), the higher the cost of the attack.  If
-  buffer overload does occur, a smart receiver could selectively
+buffer overload does occur, a smart receiver could selectively
-  discard SCHC Packets being reassembled based on the sender behavior,
+discard SCHC Packets being reassembled based on the sender behavior,
-  which may help identify which SCHC Fragments have been sent by the
+which may help identify which SCHC Fragments have been sent by the
-  attacker.  Another mild countermeasure is for the target to abort the
+attacker.  Another mild countermeasure is for the target to abort the
-  fragmentation/reassembly session as early as it detects a non-
+fragmentation/reassembly session as early as it detects a non-
-  identical SCHC Fragment duplicate, anticipating for an eventual
+identical SCHC Fragment duplicate, anticipating for an eventual
-  corrupt SCHC Packet, so as to save the sender the hassle of sending
+corrupt SCHC Packet, so as to save the sender the hassle of sending
-  the rest of the fragments for this SCHC Packet.
+the rest of the fragments for this SCHC Packet.
 .2.2.  Corrupt Fragment Attack
-  Let's assume that an attacker is able to send a forged SCHC Fragment
+Let's assume that an attacker is able to send a forged SCHC Fragment
-  to a SCHC reassembler.  The malicious node is additionally assumed to
+to a SCHC reassembler.  The malicious node is additionally assumed to
-  be able to hear an incoming communication destined to the target
+be able to hear an incoming communication destined to the target
-  node.
+node.
-  It can then send a forged SCHC Fragment that looks like it belongs to
+It can then send a forged SCHC Fragment that looks like it belongs to
-  a SCHC Packet already being reassembled at the target node.  This can
+a SCHC Packet already being reassembled at the target node.  This can
-  cause the SCHC Packet to be considered corrupt and to be dropped by
+cause the SCHC Packet to be considered corrupt and to be dropped by
-  the receiver.  The amplification happens here by a single spoofed
+the receiver.  The amplification happens here by a single spoofed
-  SCHC Fragment rendering a full sequence of legitimate SCHC Fragments
+SCHC Fragment rendering a full sequence of legitimate SCHC Fragments
-  useless.  If the target uses ACK-Always or ACK-on-Error mode, such a
+useless.  If the target uses ACK-Always or ACK-on-Error mode, such a
-  malicious node can also interfere with the acknowledgement and
+malicious node can also interfere with the acknowledgement and
-  repetition algorithm of SCHC F/R.  A single spoofed ACK, with all
+repetition algorithm of SCHC F/R.  A single spoofed ACK, with all
-  Bitmap bits set to 0, will trigger the repetition of WINDOW_SIZE
+Bitmap bits set to 0, will trigger the repetition of WINDOW_SIZE
-  tiles.  This protocol loop amplification depletes the energy source
+tiles.  This protocol loop amplification depletes the energy source
-  of the target node and consumes the channel bandwidth.  Similarly, a
+of the target node and consumes the channel bandwidth.  Similarly, a
-  spoofed ACK REQ will trigger the sending of a SCHC ACK, which may be
+spoofed ACK REQ will trigger the sending of a SCHC ACK, which may be
-  much larger than the ACK REQ if WINDOW_SIZE is large.  These
+much larger than the ACK REQ if WINDOW_SIZE is large.  These
-  consequences should be borne in mind when defining profiles for SCHC
+consequences should be borne in mind when defining profiles for SCHC
-  over specific LPWAN technologies.
+over specific LPWAN technologies.
 .2.3.  Fragmentation as a Way to Bypass Network Inspection
-  Fragmentation is known for potentially allowing one to force through
+Fragmentation is known for potentially allowing one to force through
-  a Network Inspection device (e.g., firewall) packets that would be
+a Network Inspection device (e.g., firewall) packets that would be
-  rejected if unfragmented.  This involves sending overlapping
+rejected if unfragmented.  This involves sending overlapping
-  fragments to rewrite fields whose initial value led the Network
+fragments to rewrite fields whose initial value led the Network
-  Inspection device to allow the flow to go through.
+Inspection device to allow the flow to go through.
-  SCHC F/R is expected to be used over one LPWAN link, where no Network
+SCHC F/R is expected to be used over one LPWAN link, where no Network
-  Inspection device is expected to sit.  As described in Section 5.2,
+Inspection device is expected to sit.  As described in Section 5.2,
-  even if the SCHC F/R on the Network Infrastructure side is located in
+even if the SCHC F/R on the Network Infrastructure side is located in
-  the Internet, a tunnel is to be established between it and the NGW.
+the Internet, a tunnel is to be established between it and the NGW.
 .2.4.  Privacy Issues Associated with SCHC Header Fields
-  SCHC F/R allocates a DTag value to fragments belonging to the same
+SCHC F/R allocates a DTag value to fragments belonging to the same
-  SCHC Packet.  Concerns were raised that, if DTag is a wide counter
+SCHC Packet.  Concerns were raised that, if DTag is a wide counter
-  that is incremented in a predictable fashion for each new fragmented
+that is incremented in a predictable fashion for each new fragmented
-  SCHC Packet, it might lead to a privacy issue, such as enabling
+SCHC Packet, it might lead to a privacy issue, such as enabling
-  tracking of a device across LPWANs.
+tracking of a device across LPWANs.
-  However, SCHC F/R is expected to be used over exactly one LPWAN link.
+However, SCHC F/R is expected to be used over exactly one LPWAN link.
-  As described in Section 5.2, even if the SCHC F/R on the Network
+As described in Section 5.2, even if the SCHC F/R on the Network
-  Infrastructure side is located in the Internet, a tunnel is to be
+Infrastructure side is located in the Internet, a tunnel is to be
-  established between it and the NGW.  Therefore, assuming the tunnel
+established between it and the NGW.  Therefore, assuming the tunnel
-  provides confidentiality, neither the DTag field nor any other SCHC-
+provides confidentiality, neither the DTag field nor any other SCHC-
-  introduced field is visible over the Internet.
+introduced field is visible over the Internet.
 .  References
@@ Line 2,771: / Line 2,765: @@
 .1.  Normative References
-  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
+[[RFC2119]]  Bradner, S., "Key words for use in RFCs to Indicate
-              Requirement Levels", BCP 14, RFC 2119,
+          Requirement Levels", [[BCP14|BCP 14]], [[RFC2119|RFC 2119]],
-              DOI 10.17487/RFC2119, March 1997,
+          DOI 10.17487/RFC2119, March 1997,
-              <https://www.rfc-editor.org/info/rfc2119>.
+          <https://www.rfc-editor.org/info/rfc2119>.
-  [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
+[[RFC6936]]  Fairhurst, G. and M. Westerlund, "Applicability Statement
-              for the Use of IPv6 UDP Datagrams with Zero Checksums",
+          for the Use of IPv6 UDP Datagrams with Zero Checksums",
-              RFC 6936, DOI 10.17487/RFC6936, April 2013,
+          [[RFC6936|RFC 6936]], DOI 10.17487/RFC6936, April 2013,
-              <https://www.rfc-editor.org/info/rfc6936>.
+          <https://www.rfc-editor.org/info/rfc6936>.
-  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
+[[RFC8174]]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
-Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
+Key Words", [[BCP14|BCP 14]], [[RFC8174|RFC 8174]], DOI 10.17487/RFC8174,
-              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
+          May 2017, <https://www.rfc-editor.org/info/rfc8174>.
-  [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
+[[RFC8200]]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
-              (IPv6) Specification", STD 86, RFC 8200,
+          (IPv6) Specification", [[STD86|STD 86]], [[RFC8200|RFC 8200]],
-              DOI 10.17487/RFC8200, July 2017,
+          DOI 10.17487/RFC8200, July 2017,
-              <https://www.rfc-editor.org/info/rfc8200>.
+          <https://www.rfc-editor.org/info/rfc8200>.
-  [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
+[[RFC8376]]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
-              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
+          Overview", [[RFC8376|RFC 8376]], DOI 10.17487/RFC8376, May 2018,
-              <https://www.rfc-editor.org/info/rfc8376>.
+          <https://www.rfc-editor.org/info/rfc8376>.
 .2.  Informative References
-  [ETHERNET] IEEE, "IEEE Standard for Ethernet",
+[ETHERNET] IEEE, "IEEE Standard for Ethernet",
-              DOI 10.1109/IEEESTD.2012.6419735, IEEE
+          DOI 10.1109/IEEESTD.2012.6419735, IEEE
-              Standard 802.3-2012, December 2012,
+          Standard 802.3-2012, December 2012,
-              <https://ieeexplore.ieee.org/document/6419735>.
+          <https://ieeexplore.ieee.org/document/6419735>.
-  [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
+[[RFC4944]]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
-              "Transmission of IPv6 Packets over IEEE 802.15.4
+          "Transmission of IPv6 Packets over IEEE 802.15.4
-              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
+          Networks", [[RFC4944|RFC 4944]], DOI 10.17487/RFC4944, September 2007,
-              <https://www.rfc-editor.org/info/rfc4944>.
+          <https://www.rfc-editor.org/info/rfc4944>.
-  [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
+[[RFC5795]]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
-              Header Compression (ROHC) Framework", RFC 5795,
+          Header Compression (ROHC) Framework", [[RFC5795|RFC 5795]],
-              DOI 10.17487/RFC5795, March 2010,
+          DOI 10.17487/RFC5795, March 2010,
-              <https://www.rfc-editor.org/info/rfc5795>.
+          <https://www.rfc-editor.org/info/rfc5795>.
-  [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
+[[RFC6282]]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
-              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
+          Datagrams over IEEE 802.15.4-Based Networks", [[RFC6282|RFC 6282]],
-              DOI 10.17487/RFC6282, September 2011,
+          DOI 10.17487/RFC6282, September 2011,
-              <https://www.rfc-editor.org/info/rfc6282>.
+          <https://www.rfc-editor.org/info/rfc6282>.
-  [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
+[[RFC6437]]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
-              "IPv6 Flow Label Specification", RFC 6437,
+          "IPv6 Flow Label Specification", [[RFC6437|RFC 6437]],
-              DOI 10.17487/RFC6437, November 2011,
+          DOI 10.17487/RFC6437, November 2011,
-              <https://www.rfc-editor.org/info/rfc6437>.
+          <https://www.rfc-editor.org/info/rfc6437>.
-  [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
+[[RFC7136]]  Carpenter, B. and S. Jiang, "Significance of IPv6
-              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
+          Interface Identifiers", [[RFC7136|RFC 7136]], DOI 10.17487/RFC7136,
-              February 2014, <https://www.rfc-editor.org/info/rfc7136>.
+          February 2014, <https://www.rfc-editor.org/info/rfc7136>.
-  [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
+[[RFC8065]]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
-              Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
+          Layer Mechanisms", [[RFC8065|RFC 8065]], DOI 10.17487/RFC8065,
-              February 2017, <https://www.rfc-editor.org/info/rfc8065>.
+          February 2017, <https://www.rfc-editor.org/info/rfc8065>.
 Appendix A.  Compression Examples
-  This section gives some scenarios of the compression mechanism for
+This section gives some scenarios of the compression mechanism for
-  IPv6/UDP.  The goal is to illustrate the behavior of SCHC.
+IPv6/UDP.  The goal is to illustrate the behavior of SCHC.
-  The mechanisms defined in this document can be applied to a Dev that
+The mechanisms defined in this document can be applied to a Dev that
-  embeds some applications running over CoAP.  In this example, three
+embeds some applications running over CoAP.  In this example, three
-  flows are considered.  The first flow is for the device management
+flows are considered.  The first flow is for the device management
-  based on CoAP using Link Local IPv6 addresses and UDP ports 123 and
+based on CoAP using Link Local IPv6 addresses and UDP ports 123 and
 for Dev and App, respectively.  The second flow is a CoAP server
-  for measurements done by the Dev (using ports 5683) and Global IPv6
+for measurements done by the Dev (using ports 5683) and Global IPv6
-  Address prefixes alpha::IID/64 to beta::1/64.  The last flow is for
+Address prefixes alpha::IID/64 to beta::1/64.  The last flow is for
-  legacy applications using different ports numbers, the destination
+legacy applications using different ports numbers, the destination
-  IPv6 address prefix is gamma::1/64.
+IPv6 address prefix is gamma::1/64.
-  Figure 25 presents the protocol stack.  IPv6 and UDP are represented
+Figure 25 presents the protocol stack.  IPv6 and UDP are represented
-  with dotted lines since these protocols are compressed on the radio
+with dotted lines since these protocols are compressed on the radio
-  link.
+link.
-    Management  Data
+ Management  Data
-  +----------+---------+---------+
++----------+---------+---------+
-  |  CoAP  |  CoAP  | legacy  |
+|  CoAP  |  CoAP  | legacy  |
-  +----||----+---||----+---||----+
++----||----+---||----+---||----+
-  .  UDP    .  UDP    |  UDP  |
+.  UDP    .  UDP    |  UDP  |
-  ................................
-  .  IPv6  .  IPv6  .  IPv6  .
-  +------------------------------+
-  |    SCHC Header compression  |
-  |      and fragmentation      |
-  +------------------------------+
-  |      LPWAN L2 technologies  |
-  +------------------------------+
-            Dev or NGW
-              Figure 25: Simplified Protocol Stack for LP-WAN
+.  IPv6  .  IPv6  .  IPv6  .
++------------------------------+
+|    SCHC Header compression  |
+|      and fragmentation      |
++------------------------------+
+|      LPWAN L2 technologies  |
++------------------------------+
+        Dev or NGW
-  Rule 0
+          Figure 25: Simplified Protocol Stack for LP-WAN
-    Special RuleID used to tag an uncompressed UDP/IPV6 packet.
-  Rule 1
+Rule 0
-    +----------------+--+--+--+---------+--------+------------++------+
+   Special RuleID used to tag an uncompressed UDP/IPV6 packet.
-    |      FID      |FL|FP|DI|    TV  |  MO  |    CDA    || Sent |
-    |                |  |  |  |        |        |            ||[bits]|
-    +----------------+--+--+--+---------+---------------------++------+
-    |IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent  ||      |
-    |IPv6 Diffserv  |8 |1 |Bi|0       | equal  | not-sent  ||      |
-    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
-    |IPv6 Length    |16|1 |Bi|        | ignore | compute-*  ||      |
-    |IPv6 Next Header|8 |1 |Bi|17      | equal  | not-sent  ||      |
-    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent  ||      |
-    |IPv6 DevPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent  ||      |
-    |IPv6 DevIID    |64|1 |Bi|        | ignore | DevIID    ||      |
-    |IPv6 AppPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent  ||      |
-    |IPv6 AppIID    |64|1 |Bi|::1      | equal  | not-sent  ||      |
-    +================+==+==+==+=========+========+============++======+
-    |UDP DevPort    |16|1 |Bi|123      | equal  | not-sent  ||      |
-    |UDP AppPort    |16|1 |Bi|124      | equal  | not-sent  ||      |
-    |UDP Length      |16|1 |Bi|        | ignore | compute-*  ||      |
-    |UDP checksum    |16|1 |Bi|        | ignore | compute-*  ||      |
-    +================+==+==+==+=========+========+============++======+
-                Figure 26: Context Rules - Rule 0 and Rule 1
+Rule 1
+ +----------------+--+--+--+---------+--------+------------++------+
+ |      FID      |FL|FP|DI|    TV  |  MO  |    CDA    || Sent |
+ |                |  |  |  |        |        |            ||[bits]|
+ +----------------+--+--+--+---------+---------------------++------+
+ |IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent  ||      |
+ |IPv6 Diffserv  |8 |1 |Bi|0        | equal  | not-sent  ||      |
+ |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent  ||      |
+ |IPv6 Length    |16|1 |Bi|        | ignore | compute-*  ||      |
+ |IPv6 Next Header|8 |1 |Bi|17      | equal  | not-sent  ||      |
+ |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent  ||      |
+ |IPv6 DevPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent  ||      |
+ |IPv6 DevIID    |64|1 |Bi|        | ignore | DevIID    ||      |
+ |IPv6 AppPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent  ||      |
+ |IPv6 AppIID    |64|1 |Bi|::1      | equal  | not-sent  ||      |
+ +================+==+==+==+=========+========+============++======+
+ |UDP DevPort    |16|1 |Bi|123      | equal  | not-sent  ||      |
+ |UDP AppPort    |16|1 |Bi|124      | equal  | not-sent  ||      |
+ |UDP Length      |16|1 |Bi|        | ignore | compute-*  ||      |
+ |UDP checksum    |16|1 |Bi|        | ignore | compute-*  ||      |
+ +================+==+==+==+=========+========+============++======+
-    Rule 2
+            Figure 26: Context Rules - Rule 0 and Rule 1
-    +----------------+--+--+--+---------+--------+------------++------+
-    |      FID      |FL|FP|DI|    TV  |  MO  |    CDA    || Sent |
-    |                |  |  |  |        |        |            ||[bits]|
-    +----------------+--+--+--+---------+--------+------------++------+
-    |IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent  ||      |
-    |IPv6 Diffserv  |8 |1 |Bi|0       | equal  | not-sent  ||      |
-    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent  ||      |
-    |IPv6 Length    |16|1 |Bi|        | ignore | compute-*  ||      |
-    |IPv6 Next Header|8 |1 |Bi|17      | equal  | not-sent  ||      |
-    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent  ||      |
-    |IPv6 DevPrefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||  1  |
-    |                |  |  |  |fe80::/64] mapping|            ||      |
-    |IPv6 DevIID    |64|1 |Bi|        | ignore | DevIID    ||      |
-    |IPv6 AppPrefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||  2  |
-    |                |  |  |  |alpha/64,| mapping|            ||      |
-    |                |  |  |  |fe80::64]|        |            ||      |
-    |IPv6 AppIID    |64|1 |Bi|::1000  | equal  | not-sent  ||      |
-    +================+==+==+==+=========+========+============++======+
-    |UDP DevPort    |16|1 |Bi|5683    | equal  | not-sent  ||      |
-    |UDP AppPort    |16|1 |Bi|5683    | equal  | not-sent  ||      |
-    |UDP Length      |16|1 |Bi|        | ignore | compute-*  ||      |
-    |UDP checksum    |16|1 |Bi|        | ignore | compute-*  ||      |
-    +================+==+==+==+=========+========+============++======+
-                    Figure 27: Context Rules - Rule 2
+ Rule 2
+ +----------------+--+--+--+---------+--------+------------++------+
+ |      FID      |FL|FP|DI|    TV  |  MO  |    CDA    || Sent |
+ |                |  |  |  |        |        |            ||[bits]|
+ +----------------+--+--+--+---------+--------+------------++------+
+ |IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent  ||      |
+ |IPv6 Diffserv  |8 |1 |Bi|0        | equal  | not-sent  ||      |
+ |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent  ||      |
+ |IPv6 Length    |16|1 |Bi|        | ignore | compute-*  ||      |
+ |IPv6 Next Header|8 |1 |Bi|17      | equal  | not-sent  ||      |
+ |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent  ||      |
+ |IPv6 DevPrefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||  1  |
+ |                |  |  |  |fe80::/64] mapping|            ||      |
+ |IPv6 DevIID    |64|1 |Bi|        | ignore | DevIID    ||      |
+ |IPv6 AppPrefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||  2  |
+ |                |  |  |  |alpha/64,| mapping|            ||      |
+ |                |  |  |  |fe80::64]|        |            ||      |
+ |IPv6 AppIID    |64|1 |Bi|::1000  | equal  | not-sent  ||      |
+ +================+==+==+==+=========+========+============++======+
+ |UDP DevPort    |16|1 |Bi|5683    | equal  | not-sent  ||      |
+ |UDP AppPort    |16|1 |Bi|5683    | equal  | not-sent  ||      |
+ |UDP Length      |16|1 |Bi|        | ignore | compute-*  ||      |
+ |UDP checksum    |16|1 |Bi|        | ignore | compute-*  ||      |
+ +================+==+==+==+=========+========+============++======+
-    Rule 3
+                  Figure 27: Context Rules - Rule 2
-    +----------------+--+--+--+---------+--------+------------++------+
-    |      FID      |FL|FP|DI|    TV  |  MO  |    CDA    || Sent |
-    |                |  |  |  |        |        |            ||[bits]|
-    +----------------+--+--+--+---------+--------+------------++------+
-    |IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent  ||      |
-    |IPv6 Diffserv  |8 |1 |Bi|0        | equal  | not-sent  ||      |
-    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent  ||      |
-    |IPv6 Length    |16|1 |Bi|        | ignore | compute-*  ||      |
-    |IPv6 Next Header|8 |1 |Bi|17      | equal  | not-sent  ||      |
-    |IPv6 Hop Limit  |8 |1 |Up|255      | ignore | not-sent  ||      |
-    |IPv6 Hop Limit  |8 |1 |Dw|        | ignore | value-sent ||  8  |
-    |IPv6 DevPrefix  |64|1 |Bi|alpha/64 | equal  | not-sent  ||      |
-    |IPv6 DevIID    |64|1 |Bi|        | ignore | DevIID    ||      |
-    |IPv6 AppPrefix  |64|1 |Bi|gamma/64 | equal  | not-sent  ||      |
-    |IPv6 AppIID    |64|1 |Bi|::1000  | equal  | not-sent  ||      |
-    +================+==+==+==+=========+========+============++======+
-    |UDP DevPort    |16|1 |Bi|8720    | MSB(12)| LSB        ||  4  |
-    |UDP AppPort    |16|1 |Bi|8720    | MSB(12)| LSB        ||  4  |
-    |UDP Length      |16|1 |Bi|        | ignore | compute-*  ||      |
-    |UDP checksum    |16|1 |Bi|        | ignore | compute-*  ||      |
-    +================+==+==+==+=========+========+============++======+
-                    Figure 28: Context Rules - Rule 3
+ Rule 3
+ +----------------+--+--+--+---------+--------+------------++------+
+ |      FID      |FL|FP|DI|    TV  |  MO  |    CDA    || Sent |
+ |                |  |  |  |        |        |            ||[bits]|
+ +----------------+--+--+--+---------+--------+------------++------+
+ |IPv6 Version    |4 |1 |Bi|6        | ignore | not-sent  ||      |
+ |IPv6 Diffserv  |8 |1 |Bi|0        | equal  | not-sent  ||      |
+ |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent  ||      |
+ |IPv6 Length    |16|1 |Bi|        | ignore | compute-*  ||      |
+ |IPv6 Next Header|8 |1 |Bi|17      | equal  | not-sent  ||      |
+ |IPv6 Hop Limit  |8 |1 |Up|255      | ignore | not-sent  ||      |
+ |IPv6 Hop Limit  |8 |1 |Dw|        | ignore | value-sent ||  8  |
+ |IPv6 DevPrefix  |64|1 |Bi|alpha/64 | equal  | not-sent  ||      |
+ |IPv6 DevIID    |64|1 |Bi|        | ignore | DevIID    ||      |
+ |IPv6 AppPrefix  |64|1 |Bi|gamma/64 | equal  | not-sent  ||      |
+ |IPv6 AppIID    |64|1 |Bi|::1000  | equal  | not-sent  ||      |
+ +================+==+==+==+=========+========+============++======+
+ |UDP DevPort    |16|1 |Bi|8720    | MSB(12)| LSB        ||  4  |
+ |UDP AppPort    |16|1 |Bi|8720    | MSB(12)| LSB        ||  4  |
+ |UDP Length      |16|1 |Bi|        | ignore | compute-*  ||      |
+ |UDP checksum    |16|1 |Bi|        | ignore | compute-*  ||      |
+ +================+==+==+==+=========+========+============++======+
-  Figures 26 to 28 describe an example of a Rule set.
+                  Figure 28: Context Rules - Rule 3
-  In this example, 0 was chosen as the special RuleID that tags packets
+Figures 26 to 28 describe an example of a Rule set.
-  that cannot be compressed with any compression Rule.
-  All the fields described in Rules 1-3 are present in the IPv6 and UDP
+In this example, 0 was chosen as the special RuleID that tags packets
-  headers.  The DevIID value is inferred from the L2 header.
+that cannot be compressed with any compression Rule.
-  Rules 2-3 use global addresses.  The way the Dev learns the prefix is
+All the fields described in Rules 1-3 are present in the IPv6 and UDP
-  not in the scope of the document.
+headers.  The DevIID value is inferred from the L2 header.
-  Rule 3 compresses each port number to 4 bits.
+Rules 2-3 use global addresses.  The way the Dev learns the prefix is
+not in the scope of the document.
+Rule 3 compresses each port number to 4 bits.
 Appendix B.  Fragmentation Examples
-  This section provides examples for the various fragment reliability
+This section provides examples for the various fragment reliability
-  modes specified in this document.  In the drawings, Bitmaps are shown
+modes specified in this document.  In the drawings, Bitmaps are shown
-  in their uncompressed form.
+in their uncompressed form.
-  Figure 29 illustrates the transmission in No-ACK mode of a SCHC
+Figure 29 illustrates the transmission in No-ACK mode of a SCHC
-  Packet that needs 11 SCHC Fragments.  FCN is 1 bit wide.
+Packet that needs 11 SCHC Fragments.  FCN is 1 bit wide.
-          Sender              Receiver
+        Sender              Receiver
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-------FCN=0-------->|
+          |-------FCN=0-------->|
-            |-----FCN=1 + RCS --->| Integrity check: success
+          |-----FCN=1 + RCS --->| Integrity check: success
-          (End)
+        (End)
+              Figure 29: No-ACK Mode, 11 SCHC Fragments
+In the following examples, N (the size of the FCN field) is 3 bits.
+The All-1 FCN value is therefore 7.
+Figure 30 illustrates the transmission in ACK-on-Error mode of a SCHC
+Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
+WINDOW_SIZE=7 and no lost SCHC Fragment.
+        Sender              Receiver
+          |-----W=0, FCN=6----->|
+          |-----W=0, FCN=5----->|
+          |-----W=0, FCN=4----->|
+          |-----W=0, FCN=3----->|
+          |-----W=0, FCN=2----->|
+          |-----W=0, FCN=1----->|
+          |-----W=0, FCN=0----->|
+      (no ACK)
+          |-----W=1, FCN=6----->|
+          |-----W=1, FCN=5----->|
+          |-----W=1, FCN=4----->|
+          |--W=1, FCN=7 + RCS-->| Integrity check: success
+          |<-- ACK, W=1, C=1 ---| C=1
+        (End)
+      Figure 30: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
+                  Fragment, No Lost SCHC Fragment
+Figure 31 illustrates the transmission in ACK-on-Error mode of a SCHC
+Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
+WINDOW_SIZE=7, and three lost SCHC Fragments.
-                Figure 29: No-ACK Mode, 11 SCHC Fragments
+        Sender              Receiver
+          |-----W=0, FCN=6----->|
+          |-----W=0, FCN=5----->|
+          |-----W=0, FCN=4--X-->|
+          |-----W=0, FCN=3----->|
+          |-----W=0, FCN=2--X-->|
+          |-----W=0, FCN=1----->|
+          |-----W=0, FCN=0----->|        6543210
+          |<-- ACK, W=0, C=0 ---| Bitmap:1101011
+          |-----W=0, FCN=4----->|
+          |-----W=0, FCN=2----->|
+      (no ACK)
+          |-----W=1, FCN=6----->|
+          |-----W=1, FCN=5----->|
+          |-----W=1, FCN=4--X-->|
+          |- W=1, FCN=7 + RCS ->| Integrity check: failure
+          |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
+          |-----W=1, FCN=4----->| Integrity check: success
+          |<-- ACK, W=1, C=1 ---| C=1
+        (End)
-  In the following examples, N (the size of the FCN field) is 3 bits.
+      Figure 31: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
-  The All-1 FCN value is therefore 7.
+                    Fragment, Lost SCHC Fragments
-  Figure 30 illustrates the transmission in ACK-on-Error mode of a SCHC
+Figure 32 shows an example of a transmission in ACK-on-Error mode of
-  Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
+a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2,
-  WINDOW_SIZE=7 and no lost SCHC Fragment.
+and three lost SCHC Fragments.
-          Sender              Receiver
+  Sender              Receiver
-            |-----W=0, FCN=6----->|
+    |-----W=0, FCN=27----->| 4 tiles sent
-            |-----W=0, FCN=5----->|
+    |-----W=0, FCN=23----->| 4 tiles sent
-            |-----W=0, FCN=4----->|
+    |-----W=0, FCN=19----->| 4 tiles sent
-            |-----W=0, FCN=3----->|
+    |-----W=0, FCN=15--X-->| 4 tiles sent (not received)
-            |-----W=0, FCN=2----->|
+    |-----W=0, FCN=11----->| 4 tiles sent
-            |-----W=0, FCN=1----->|
+    |-----W=0, FCN=7 ----->| 4 tiles sent
-            |-----W=0, FCN=0----->|
+    |-----W=0, FCN=3 ----->| 4 tiles sent
-        (no ACK)
+    |-----W=1, FCN=27----->| 4 tiles sent
-            |-----W=1, FCN=6----->|
+    |-----W=1, FCN=23----->| 4 tiles sent
-            |-----W=1, FCN=5----->|
+    |-----W=1, FCN=19----->| 4 tiles sent
-            |-----W=1, FCN=4----->|
+    |-----W=1, FCN=15----->| 4 tiles sent
-            |--W=1, FCN=7 + RCS-->| Integrity check: success
+    |-----W=1, FCN=11----->| 4 tiles sent
-            |<-- ACK, W=1, C=1 ---| C=1
+    |-----W=1, FCN=7 ----->| 4 tiles sent
-          (End)
+    |-----W=1, FCN=3 --X-->| 4 tiles sent (not received)
+    |-----W=2, FCN=27----->| 4 tiles sent
+    |-----W=2, FCN=23----->| 4 tiles sent
+^  |-----W=2, FCN=19----->| 1 tile sent
+|  |-----W=2, FCN=18----->| 1 tile sent
+|  |-----W=2, FCN=17----->| 1 tile sent
+    |-----W=2, FCN=16----->| 1 tile sent
+s  |-----W=2, FCN=15----->| 1 tile sent
+m  |-----W=2, FCN=14----->| 1 tile sent
+a  |-----W=2, FCN=13--X-->| 1 tile sent (not received)
+l  |-----W=2, FCN=12----->| 1 tile sent
+l  |---W=2, FCN=31 + RCS->| Integrity check: failure
+e  |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111
+r  |-----W=0, FCN=15----->| 1 tile sent
+    |-----W=0, FCN=14----->| 1 tile sent
+L  |-----W=0, FCN=13----->| 1 tile sent
+  |-----W=0, FCN=12----->| 1 tile sent
+    |<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000
+M  |-----W=1, FCN=3 ----->| 1 tile sent
+T  |-----W=1, FCN=2 ----->| 1 tile sent
+U  |-----W=1, FCN=1 ----->| 1 tile sent
+    |-----W=1, FCN=0 ----->| 1 tile sent
+|  |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001
+|  |-----W=2, FCN=13----->| Integrity check: success
+V  |<--- ACK, W=2, C=1 ---| C=1
+  (End)
-        Figure 30: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
+              Figure 32: ACK-on-Error Mode, Variable MTU
-                      Fragment, No Lost SCHC Fragment
-  Figure 31 illustrates the transmission in ACK-on-Error mode of a SCHC
+In this example, the L2 MTU becomes reduced just before sending the
-  Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
+"W=2, FCN=19" fragment, leaving space for only one tile in each
-  WINDOW_SIZE=7, and three lost SCHC Fragments.
+forthcoming SCHC Fragment.  Before retransmissions, the 73 tiles are
+carried by a total of 25 SCHC Fragments, the last nine being of
+smaller size.
-          Sender              Receiver
+Note: other sequences of events (e.g., regarding when ACKs are sent
-            |-----W=0, FCN=6----->|
+by the Receiver) are also allowed by this specification.  Profiles
-            |-----W=0, FCN=5----->|
+may restrict this flexibility.
-            |-----W=0, FCN=4--X-->|
-            |-----W=0, FCN=3----->|
-            |-----W=0, FCN=2--X-->|
-            |-----W=0, FCN=1----->|
-            |-----W=0, FCN=0----->|        6543210
-            |<-- ACK, W=0, C=0 ---| Bitmap:1101011
-            |-----W=0, FCN=4----->|
-            |-----W=0, FCN=2----->|
-        (no ACK)
-            |-----W=1, FCN=6----->|
-            |-----W=1, FCN=5----->|
-            |-----W=1, FCN=4--X-->|
-            |- W=1, FCN=7 + RCS ->| Integrity check: failure
-            |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
-            |-----W=1, FCN=4----->| Integrity check: success
-            |<-- ACK, W=1, C=1 ---| C=1
-          (End)
-        Figure 31: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
+Figure 33 illustrates the transmission in ACK-Always mode of a SCHC
-                      Fragment, Lost SCHC Fragments
+Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with
+N=3, WINDOW_SIZE=7, and no loss.
-  Figure 32 shows an example of a transmission in ACK-on-Error mode of
+        Sender              Receiver
-  a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2,
+          |-----W=0, FCN=6----->|
-  and three lost SCHC Fragments.
+          |-----W=0, FCN=5----->|
+          |-----W=0, FCN=4----->|
+          |-----W=0, FCN=3----->|
+          |-----W=0, FCN=2----->|
+          |-----W=0, FCN=1----->|
+          |-----W=0, FCN=0----->|
+          |<-- ACK, W=0, C=0 ---| Bitmap:1111111
+          |-----W=1, FCN=6----->|
+          |-----W=1, FCN=5----->|
+          |-----W=1, FCN=4----->|
+          |--W=1, FCN=7 + RCS-->| Integrity check: success
+          |<-- ACK, W=1, C=1 ---| C=1
+        (End)
-      Sender              Receiver
+   Figure 33: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
-      |-----W=0, FCN=27----->| 4 tiles sent
+                              No Loss
-      |-----W=0, FCN=23----->| 4 tiles sent
-      |-----W=0, FCN=19----->| 4 tiles sent
-      |-----W=0, FCN=15--X-->| 4 tiles sent (not received)
-      |-----W=0, FCN=11----->| 4 tiles sent
-      |-----W=0, FCN=7 ----->| 4 tiles sent
-      |-----W=0, FCN=3 ----->| 4 tiles sent
-      |-----W=1, FCN=27----->| 4 tiles sent
-      |-----W=1, FCN=23----->| 4 tiles sent
-      |-----W=1, FCN=19----->| 4 tiles sent
-      |-----W=1, FCN=15----->| 4 tiles sent
-      |-----W=1, FCN=11----->| 4 tiles sent
-      |-----W=1, FCN=7 ----->| 4 tiles sent
-      |-----W=1, FCN=3 --X-->| 4 tiles sent (not received)
-      |-----W=2, FCN=27----->| 4 tiles sent
-      |-----W=2, FCN=23----->| 4 tiles sent
-  ^   |-----W=2, FCN=19----->| 1 tile sent
-  |  |-----W=2, FCN=18----->| 1 tile sent
-  |  |-----W=2, FCN=17----->| 1 tile sent
-      |-----W=2, FCN=16----->| 1 tile sent
-  s  |-----W=2, FCN=15----->| 1 tile sent
-  m  |-----W=2, FCN=14----->| 1 tile sent
-  a  |-----W=2, FCN=13--X-->| 1 tile sent (not received)
-  l  |-----W=2, FCN=12----->| 1 tile sent
-  l  |---W=2, FCN=31 + RCS->| Integrity check: failure
-  e  |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111
-  r  |-----W=0, FCN=15----->| 1 tile sent
-      |-----W=0, FCN=14----->| 1 tile sent
-  L  |-----W=0, FCN=13----->| 1 tile sent
-  |-----W=0, FCN=12----->| 1 tile sent
-      |<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000
-  M  |-----W=1, FCN=3 ----->| 1 tile sent
-  T  |-----W=1, FCN=2 ----->| 1 tile sent
-  U  |-----W=1, FCN=1 ----->| 1 tile sent
-      |-----W=1, FCN=0 ----->| 1 tile sent
-  |  |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001
-  |  |-----W=2, FCN=13----->| Integrity check: success
-  V  |<--- ACK, W=2, C=1 ---| C=1
-    (End)
-                Figure 32: ACK-on-Error Mode, Variable MTU
+Figure 34 illustrates the transmission in ACK-Always mode of a SCHC
+Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3,
+WINDOW_SIZE=7 and three lost SCHC Fragments.
-  In this example, the L2 MTU becomes reduced just before sending the
+        Sender              Receiver
-  "W=2, FCN=19" fragment, leaving space for only one tile in each
+          |-----W=0, FCN=6----->|
-  forthcoming SCHC Fragment.  Before retransmissions, the 73 tiles are
+          |-----W=0, FCN=5----->|
-  carried by a total of 25 SCHC Fragments, the last nine being of
+          |-----W=0, FCN=4--X-->|
-  smaller size.
+          |-----W=0, FCN=3----->|
+          |-----W=0, FCN=2--X-->|
+          |-----W=0, FCN=1----->|
+          |-----W=0, FCN=0----->|        6543210
+          |<-- ACK, W=0, C=0 ---| Bitmap:1101011
+          |-----W=0, FCN=4----->|
+          |-----W=0, FCN=2----->|
+          |<-- ACK, W=0, C=0 ---| Bitmap:1111111
+          |-----W=1, FCN=6----->|
+          |-----W=1, FCN=5----->|
+          |-----W=1, FCN=4--X-->|
+          |--W=1, FCN=7 + RCS-->| Integrity check: failure
+          |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
+          |-----W=1, FCN=4----->| Integrity check: success
+          |<-- ACK, W=1, C=1 ---| C=1
+        (End)
-  Note: other sequences of events (e.g., regarding when ACKs are sent
+  Figure 34: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
-  by the Receiver) are also allowed by this specification.  Profiles
+                      Three Lost SCHC Fragments
-  may restrict this flexibility.
-  Figure 33 illustrates the transmission in ACK-Always mode of a SCHC
+Figure 35 illustrates the transmission in ACK-Always mode of a SCHC
-  Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with
+Packet fragmented in six tiles, with one tile per SCHC Fragment, N=3,
-  N=3, WINDOW_SIZE=7, and no loss.
+WINDOW_SIZE=7, three lost SCHC Fragments, and only one retry needed
+to recover each lost SCHC Fragment.
           Sender               Receiver
              |-----W=0, FCN=6----->|
              |-----W=0, FCN=5----->|
-             |-----W=0, FCN=4----->|
+             |-----W=0, FCN=4--X-->|
-             |-----W=0, FCN=3----->|
+             |-----W=0, FCN=3--X-->|
-             |-----W=0, FCN=2----->|
+             |-----W=0, FCN=2--X-->|
-             |-----W=0, FCN=1----->|
+             |--W=0, FCN=7 + RCS-->| Integrity check: failure
-            |-----W=0, FCN=0----->|
+             |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
-             |<-- ACK, W=0, C=0 ---| Bitmap:1111111
+             |-----W=0, FCN=4----->| Integrity check: failure
-             |-----W=1, FCN=6----->|
+             |-----W=0, FCN=3----->| Integrity check: failure
-             |-----W=1, FCN=5----->|
+             |-----W=0, FCN=2----->| Integrity check: success
-             |-----W=1, FCN=4----->|
+             |<-- ACK, W=0, C=1 ---| C=1
-            |--W=1, FCN=7 + RCS-->| Integrity check: success
-             |<-- ACK, W=1, C=1 ---| C=1
            (End)
-    Figure 33: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
+      Figure 35: ACK-Always Mode, Six Tiles, One Tile per SCHC
-                                  No Loss
+                Fragment, Three Lost SCHC Fragments
-  Figure 34 illustrates the transmission in ACK-Always mode of a SCHC
+Figure 36 illustrates the transmission in ACK-Always mode of a SCHC
-  Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3,
+Packet fragmented in six tiles, with one tile per SCHC Fragment, N=3,
-  WINDOW_SIZE=7 and three lost SCHC Fragments.
+WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK
+lost.
           Sender               Receiver
              |-----W=0, FCN=6----->|
              |-----W=0, FCN=5----->|
              |-----W=0, FCN=4--X-->|
-             |-----W=0, FCN=3----->|
+             |-----W=0, FCN=3--X-->|
              |-----W=0, FCN=2--X-->|
-             |-----W=0, FCN=1----->|
+             |--W=0, FCN=7 + RCS-->| Integrity check: failure
-            |-----W=0, FCN=0----->|        6543210
+             |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
-             |<-- ACK, W=0, C=0 ---| Bitmap:1101011
+             |-----W=0, FCN=4----->| Integrity check: failure
-             |-----W=0, FCN=4----->|
+             |-----W=0, FCN=3----->| Integrity check: failure
-             |-----W=0, FCN=2----->|
+             |-----W=0, FCN=2----->| Integrity check: success
-            |<-- ACK, W=0, C=0 ---| Bitmap:1111111
+             |<-X-ACK, W=0, C=1 ---| C=1
-             |-----W=1, FCN=6----->|
+    timeout  |                    |
-            |-----W=1, FCN=5----->|
+             |--- W=0, ACK REQ --->| ACK REQ
-            |-----W=1, FCN=4--X-->|
+             |<-- ACK, W=0, C=1 ---| C=1
-            |--W=1, FCN=7 + RCS-->| Integrity check: failure
-             |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
-             |-----W=1, FCN=4----->| Integrity check: success
-             |<-- ACK, W=1, C=1 ---| C=1
            (End)
-    Figure 34: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
+      Figure 36: ACK-Always Mode, Six Tiles, One Tile per SCHC
-                        Three Lost SCHC Fragments
+                      Fragment, SCHC ACK Loss
-  Figure 35 illustrates the transmission in ACK-Always mode of a SCHC
+Figure 37 illustrates the transmission in ACK-Always mode of a SCHC
-  Packet fragmented in six tiles, with one tile per SCHC Fragment, N=3,
+Packet fragmented in six tiles, with N=3, WINDOW_SIZE=7, with three
-  WINDOW_SIZE=7, three lost SCHC Fragments, and only one retry needed
+lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.
-  to recover each lost SCHC Fragment.
-            Sender                Receiver
+          Sender                Receiver
-                |-----W=0, FCN=6----->|
+            |-----W=0, FCN=6----->|
-                |-----W=0, FCN=5----->|
+            |-----W=0, FCN=5----->|
-                |-----W=0, FCN=4--X-->|
+            |-----W=0, FCN=4--X-->|
-                |-----W=0, FCN=3--X-->|
+            |-----W=0, FCN=3--X-->|
-                |-----W=0, FCN=2--X-->|
+            |-----W=0, FCN=2--X-->|
-                |--W=0, FCN=7 + RCS-->| Integrity check: failure
+            |--W=0, FCN=7 + RCS-->| Integrity check: failure
-                |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
+            |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
-                |-----W=0, FCN=4----->| Integrity check: failure
+            |-----W=0, FCN=4----->| Integrity check: failure
-                |-----W=0, FCN=3----->| Integrity check: failure
+            |-----W=0, FCN=3----->| Integrity check: failure
-                |-----W=0, FCN=2----->| Integrity check: success
+            |-----W=0, FCN=2--X-->|
-                |<-- ACK, W=0, C=1 ---| C=1
+      timeout|                    |
-              (End)
+            |--- W=0, ACK REQ --->| ACK REQ
+            |<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
+            |-----W=0, FCN=2----->| Integrity check: success
+            |<-- ACK, W=0, C=1 ---| C=1
+          (End)
-          Figure 35: ACK-Always Mode, Six Tiles, One Tile per SCHC
+      Figure 37: ACK-Always Mode, Six Tiles, Retransmitted SCHC
-                    Fragment, Three Lost SCHC Fragments
+                        Fragment Lost Again
-  Figure 36 illustrates the transmission in ACK-Always mode of a SCHC
+Figure 38 illustrates the transmission in ACK-Always mode of a SCHC
-  Packet fragmented in six tiles, with one tile per SCHC Fragment, N=3,
+Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5,
-  WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK
+WINDOW_SIZE=24, and two lost SCHC Fragments.
-  lost.
-            Sender               Receiver
+      Sender               Receiver
-                |-----W=0, FCN=6----->|
+        |-----W=0, FCN=23----->|
-                |-----W=0, FCN=5----->|
+        |-----W=0, FCN=22----->|
-                |-----W=0, FCN=4--X-->|
+        |-----W=0, FCN=21--X-->|
-                |-----W=0, FCN=3--X-->|
+        |-----W=0, FCN=20----->|
-                |-----W=0, FCN=2--X-->|
+        |-----W=0, FCN=19----->|
-                |--W=0, FCN=7 + RCS-->| Integrity check: failure
+        |-----W=0, FCN=18----->|
-                |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
+        |-----W=0, FCN=17----->|
-                |-----W=0, FCN=4----->| Integrity check: failure
+        |-----W=0, FCN=16----->|
-                |-----W=0, FCN=3----->| Integrity check: failure
+        |-----W=0, FCN=15----->|
-                |-----W=0, FCN=2----->| Integrity check: success
+        |-----W=0, FCN=14----->|
-                |<-X-ACK, W=0, C=1 ---| C=1
+        |-----W=0, FCN=13----->|
-      timeout  |                     |
+        |-----W=0, FCN=12----->|
-                |--- W=0, ACK REQ --->| ACK REQ
+        |-----W=0, FCN=11----->|
-                |<-- ACK, W=0, C=1 ---| C=1
+        |-----W=0, FCN=10--X-->|
-              (End)
+        |-----W=0, FCN=9 ----->|
+        |-----W=0, FCN=8 ----->|
-          Figure 36: ACK-Always Mode, Six Tiles, One Tile per SCHC
+        |-----W=0, FCN=7 ----->|
-                          Fragment, SCHC ACK Loss
+        |-----W=0, FCN=6 ----->|
+        |-----W=0, FCN=5 ----->|
+        |-----W=0, FCN=4 ----->|
+        |-----W=0, FCN=3 ----->|
+        |-----W=0, FCN=2 ----->|
+        |-----W=0, FCN=1 ----->|
+        |-----W=0, FCN=0 ----->|
+        |                      |
+        |<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111
+        |-----W=0, FCN=21----->|
+        |-----W=0, FCN=10----->|
+        |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
+        |-----W=1, FCN=23----->|
+        |-----W=1, FCN=22----->|
+        |-----W=1, FCN=21----->|
+        |--W=1, FCN=31 + RCS-->| Integrity check: success
+        |<--- ACK, W=1, C=1 ---| C=1
+      (End)
-  Figure 37 illustrates the transmission in ACK-Always mode of a SCHC
+  Figure 38: ACK-Always Mode, 28 Tiles, One Tile per SCHC Fragment,
-  Packet fragmented in six tiles, with N=3, WINDOW_SIZE=7, with three
+                        Lost SCHC Fragments
-  lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.
-              Sender                Receiver
-                |-----W=0, FCN=6----->|
-                |-----W=0, FCN=5----->|
-                |-----W=0, FCN=4--X-->|
-                |-----W=0, FCN=3--X-->|
-                |-----W=0, FCN=2--X-->|
-                |--W=0, FCN=7 + RCS-->| Integrity check: failure
-                |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
-                |-----W=0, FCN=4----->| Integrity check: failure
-                |-----W=0, FCN=3----->| Integrity check: failure
-                |-----W=0, FCN=2--X-->|
-        timeout|                    |
-                |--- W=0, ACK REQ --->| ACK REQ
-                |<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
-                |-----W=0, FCN=2----->| Integrity check: success
-                |<-- ACK, W=0, C=1 ---| C=1
-              (End)
-        Figure 37: ACK-Always Mode, Six Tiles, Retransmitted SCHC
-                            Fragment Lost Again
-  Figure 38 illustrates the transmission in ACK-Always mode of a SCHC
-  Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5,
-  WINDOW_SIZE=24, and two lost SCHC Fragments.
-        Sender              Receiver
-          |-----W=0, FCN=23----->|
-          |-----W=0, FCN=22----->|
-          |-----W=0, FCN=21--X-->|
-          |-----W=0, FCN=20----->|
-          |-----W=0, FCN=19----->|
-          |-----W=0, FCN=18----->|
-          |-----W=0, FCN=17----->|
-          |-----W=0, FCN=16----->|
-          |-----W=0, FCN=15----->|
-          |-----W=0, FCN=14----->|
-          |-----W=0, FCN=13----->|
-          |-----W=0, FCN=12----->|
-          |-----W=0, FCN=11----->|
-          |-----W=0, FCN=10--X-->|
-          |-----W=0, FCN=9 ----->|
-          |-----W=0, FCN=8 ----->|
-          |-----W=0, FCN=7 ----->|
-          |-----W=0, FCN=6 ----->|
-          |-----W=0, FCN=5 ----->|
-          |-----W=0, FCN=4 ----->|
-          |-----W=0, FCN=3 ----->|
-          |-----W=0, FCN=2 ----->|
-          |-----W=0, FCN=1 ----->|
-          |-----W=0, FCN=0 ----->|
-          |                      |
-          |<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111
-          |-----W=0, FCN=21----->|
-          |-----W=0, FCN=10----->|
-          |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
-          |-----W=1, FCN=23----->|
-          |-----W=1, FCN=22----->|
-          |-----W=1, FCN=21----->|
-          |--W=1, FCN=31 + RCS-->| Integrity check: success
-          |<--- ACK, W=1, C=1 ---| C=1
-        (End)
-    Figure 38: ACK-Always Mode, 28 Tiles, One Tile per SCHC Fragment,
-                            Lost SCHC Fragments
 Appendix C.  Fragmentation State Machines
-  The fragmentation state machines of the sender and the receiver, one
+The fragmentation state machines of the sender and the receiver, one
-  for each of the different reliability modes, are described in the
+for each of the different reliability modes, are described in the
-  following figures:
+following figures:
-                +===========+
+            +===========+
-  +------------+  Init    |
++------------+  Init    |
-  |  FCN=0    +===========+
+|  FCN=0    +===========+
-  |  No Window
+|  No Window
-  |  No Bitmap
+|  No Bitmap
-  |                  +-------+
+|                  +-------+
-  |          +========+==+    | More Fragments
+|          +========+==+    | More Fragments
-  |          |          | <--+ 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
+|          |          | <--+ 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-  +--------> |  Send    |      send Fragment (FCN=0)
++--------> |  Send    |      send Fragment (FCN=0)
-              +===+=======+
+          +===+=======+
-                  |  last fragment
+              |  last fragment
-                  |  21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
+              |  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-                  |  FCN = 1
+              |  FCN = 1
-                  v  send fragment+RCS
+              v  send fragment+RCS
-              +============+
+          +============+
-              |    END    |
+          |    END    |
-              +============+
+          +============+
-            Figure 39: Sender State Machine for the No-ACK Mode
+        Figure 39: Sender State Machine for the No-ACK Mode
-                        +------+ Not All-1
+                      +------+ Not All-1
-              +==========+=+    | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)
+          +==========+=+    | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)
-              |            + <--+ set Inactivity Timer
+          |            + <--+ set Inactivity Timer
-              |  RCV Frag  +-------+
+          |  RCV Frag  +-------+
-              +=+===+======+      |All-1 &
+          +=+===+======+      |All-1 &
-      All-1 &  |  |              |RCS correct
+  All-1 &  |  |              |RCS correct
-    RCS wrong  |  |Inactivity    |
+ RCS wrong  |  |Inactivity    |
-                |  |Timer Exp.    |
+            |  |Timer Exp.    |
-                v  |              |
+            v  |              |
-    +==========++  |              v
+  +==========++  |              v
-    |  Error  |<-+    +========+==+
+  |  Error  |<-+    +========+==+
-    +===========+        |    END    |
+  +===========+        |    END    |
-                          +===========+
+                      +===========+
-          Figure 40: Receiver State Machine for the No-ACK Mode
+        Figure 40: Receiver State Machine for the No-ACK Mode
-                +=======+
+              +=======+
-                | INIT  |      FCN!=0 & more frags
+              | INIT  |      FCN!=0 & more frags
-                |      |      21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
+              |      |      21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-                +======++  +--+ send Window + frag(FCN)
+              +======++  +--+ send Window + frag(FCN)
-                    W=0 |  |  | FCN-
+                W=0 |  |  | FCN-
-    Clear lcl_bm      |  |  v set lcl_bm
+  Clear lcl_bm      |  |  v set lcl_bm
-          FCN=max value |  ++==+========+
+      FCN=max value |  ++==+========+
-                        +> |            |
+                    +> |            |
-  +---------------------> |    SEND    |
++---------------------> |    SEND    |
-  |                      +==+===+=====+
+|                      +==+===+=====+
-  |      FCN==0 & more frags |  | last frag
+|      FCN==0 & more frags |  | last frag
-  |    21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~ |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
+|    21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-  |              set lcl_bm |  | set lcl_bm
+|              set lcl_bm |  | set lcl_bm
-  |  send wnd + frag(all-0) |  | send wnd+frag(all-1)+RCS
+|  send wnd + frag(all-0) |  | send wnd+frag(all-1)+RCS
-  |      set Retrans_Timer  |  | set Retrans_Timer
+|      set Retrans_Timer  |  | set Retrans_Timer
-  |                          |  |
+|                          |  |
-  |Recv_wnd == wnd &        |  |
+|Recv_wnd == wnd &        |  |
-  |lcl_bm==recv_bm &        |  |  +----------------------+
+|lcl_bm==recv_bm &        |  |  +----------------------+
-  |more frag                |  |  | lcl_bm!=rcv-bm      |
+|more frag                |  |  | lcl_bm!=rcv-bm      |
-  |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~    |  |  | 21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)            |
+|21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~    |  |  | 21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)            |
-  |Stop Retrans_Timer        |  |  | Attempt++            v
+|Stop Retrans_Timer        |  |  | Attempt++            v
-  |clear lcl_bm              v  v  |                +=====+=+
+|clear lcl_bm              v  v  |                +=====+=+
-  |window=next_window  +====+===+==+===+            |Resend |
+|window=next_window  +====+===+==+===+            |Resend |
-  +---------------------+              |            |Missing|
++---------------------+              |            |Missing|
-                    +----+    Wait      |            |Frag  |
+                +----+    Wait      |            |Frag  |
-  not expected wnd |    |    Bitmap    |            +=======+
+not expected wnd |    |    Bitmap    |            +=======+
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~ +--->+              ++Retrans_Timer Exp  |
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ +--->+              ++Retrans_Timer Exp  |
-      discard frag      +==+=+===+=+==+=+| 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~ |
+    discard frag      +==+=+===+=+==+=+| 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~ |
-                            | |  | ^  ^  |reSend(empty)All-* |
+                        | |  | ^  ^  |reSend(empty)All-* |
-                            | |  | |  |  |Set Retrans_Timer  |
+                        | |  | |  |  |Set Retrans_Timer  |
-                            | |  | |  +--+Attempt++          |
+                        | |  | |  +--+Attempt++          |
-    C_bit==1 &            | |  | +-------------------------+
+  C_bit==1 &            | |  | +-------------------------+
-  Recv_window==window &    | |  |  all missing frags sent
+Recv_window==window &    | |  |  all missing frags sent
-                no more frag| |  |  21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
+            no more frag| |  |  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)| |  |  Set Retrans_Timer
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)| |  |  Set Retrans_Timer
-          Stop Retrans_Timer| |  |
+      Stop Retrans_Timer| |  |
-    +=============+        | |  |
+ +=============+        | |  |
-    |    END    +<--------+ |  |
+ |    END    +<--------+ |  |
-    +=============+          |  | Attempt > MAX_ACK_REQUESTS
+ +=============+          |  | Attempt > MAX_ACK_REQUESTS
-              All-1 Window & |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
+            All-1 Window & |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
-                  C_bit ==0 & |  v Send Abort
+              C_bit ==0 & |  v Send Abort
-            lcl_bm==recv_bm  | +=+===========+
+          lcl_bm==recv_bm  | +=+===========+
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~ +>|    ERROR    |
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~ +>|    ERROR    |
-                  Send Abort  +=============+
+                Send Abort  +=============+
-          Figure 41: Sender State Machine for the ACK-Always Mode
+      Figure 41: Sender State Machine for the ACK-Always Mode
-    Not All- & w=expected +---+  +---+w = Not expected
+ Not All- & w=expected +---+  +---+w = Not expected
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~ |  |  |  |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ |  |  |  |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-    Set lcl_bm(FCN)      |  v  v  |discard
+ Set lcl_bm(FCN)      |  v  v  |discard
-                        ++===+===+===+=+
+                      ++===+===+===+=+
-  +---------------------+    Rcv      +--->* ABORT
++---------------------+    Rcv      +--->* ABORT
-  |  +------------------+  Window    |
+|  +------------------+  Window    |
-  |  |                  +=====+==+=====+
+|  |                  +=====+==+=====+
-  |  |      All-0 & w=expect |  ^ w =next & not-All
+|  |      All-0 & w=expect |  ^ w =next & not-All
-  |  |    21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) |  |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
+|  |    21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) |  |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-  |  |    set lcl_bm(FCN)    |  |expected = next window
+|  |    set lcl_bm(FCN)    |  |expected = next window
-  |  |      send lcl_bm      |  |Clear lcl_bm
+|  |      send lcl_bm      |  |Clear lcl_bm
-  |  |                        |  |
+|  |                        |  |
-  |  | w=expected & not-All  |  |
+|  | w=expected & not-All  |  |
-  |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])    |  |
+|  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])    |  |
-  |  |    set lcl_bm(FCN)+-+ |  | +--+ w=next & All-0
+|  |    set lcl_bm(FCN)+-+ |  | +--+ w=next & All-0
-  |  |    if lcl_bm full | | |  | |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
+|  |    if lcl_bm full | | |  | |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-  |  |    send lcl_bm    | | |  | |  | expected = nxt wnd
+|  |    send lcl_bm    | | |  | |  | expected = nxt wnd
-  |  |                    v | v  | |  | Clear lcl_bm
+|  |                    v | v  | |  | Clear lcl_bm
-  |  |w=expected& All-1 +=+=+=+==+=++ | set lcl_bm(FCN)
+|  |w=expected& All-1 +=+=+=+==+=++ | set lcl_bm(FCN)
-  |  |  21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~  +->+    Wait  +<+ send lcl_bm
+|  |  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~  +->+    Wait  +<+ send lcl_bm
-  |  |    discard    +--|    Next  |
+|  |    discard    +--|    Next  |
-  |  | All-0  +---------+  Window  +--->* ABORT
+|  | All-0  +---------+  Window  +--->* ABORT
-  |  | 21:56, 22 September 2020 (UTC)  +-------->+========+=++
+|  | 21:37, 5 May 2021 (UTC)  +-------->+========+=++
-  |  | snd lcl_bm  All-1 & w=next| |  All-1 & w=nxt
+|  | snd lcl_bm  All-1 & w=next| |  All-1 & w=nxt
-  |  |                & RCS wrong| |  & RCS right
+|  |                & RCS wrong| |  & RCS right
-  |  |          21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~| | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
+|  |          21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~| | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
-  |  |            set lcl_bm(FCN)| |set lcl_bm(FCN)
+|  |            set lcl_bm(FCN)| |set lcl_bm(FCN)
-  |  |                send lcl_bm| |send lcl_bm
+|  |                send lcl_bm| |send lcl_bm
-  |  |                          | +----------------------+
+|  |                          | +----------------------+
-  |  |All-1 & w=expected        |                        |
+|  |All-1 & w=expected        |                        |
-  |  |& RCS wrong                v  +---+ w=expected &  |
+|  |& RCS wrong                v  +---+ w=expected &  |
-  |  |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)  +====+=====+ | RCS wrong      |
+|  |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)  +====+=====+ | RCS wrong      |
-  |  |set lcl_bm(FCN)      |          +<+ 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC) |
+|  |set lcl_bm(FCN)      |          +<+ 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC) |
-  |  |send lcl_bm          | Wait End |  set lcl_bm(FCN)|
+|  |send lcl_bm          | Wait End |  set lcl_bm(FCN)|
-  |  +--------------------->+          +--->* ABORT      |
+|  +--------------------->+          +--->* ABORT      |
-  |                        +===+====+=+-+ All-1&RCS wrong|
+|                        +===+====+=+-+ All-1&RCS wrong|
-  |                            |    ^  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)|
+|                            |    ^  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)|
-  |      w=expected & RCS right |    +---+  send lcl_bm  |
+|      w=expected & RCS right |    +---+  send lcl_bm  |
-  |      21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~ |                        |
+|      21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~ |                        |
-  |      set lcl_bm(FCN)      | +-+ Not All-1          |
+|      set lcl_bm(FCN)      | +-+ Not All-1          |
-  |        send lcl_bm          | | | 21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)          |
+|        send lcl_bm          | | | 21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)          |
-  |                            | | |  discard            |
+|                            | | |  discard            |
-  |All-1&w=expected & RCS right | | |                    |
+|All-1&w=expected & RCS right | | |                    |
-  |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) v | v +----+All-1        |
+|21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) v | v +----+All-1        |
-  |set lcl_bm(FCN)            +=+=+=+=+==+ |21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)    |
+|set lcl_bm(FCN)            +=+=+=+=+==+ |21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)    |
-  |send lcl_bm                |          +<+Send lcl_bm  |
+|send lcl_bm                |          +<+Send lcl_bm  |
-  +-------------------------->+    END  |                |
++-------------------------->+    END  |                |
-                              +==========+<---------------+
+                            +==========+<---------------+
-          --->* ABORT
+      --->* ABORT
-          In any state
+      In any state
-            on receiving a SCHC ACK REQ
+          on receiving a SCHC ACK REQ
-                Send a SCHC ACK for the current window
+            Send a SCHC ACK for the current window
-        Figure 42: Receiver State Machine for the ACK-Always Mode
+      Figure 42: Receiver State Machine for the ACK-Always Mode
-                    +=======+
+                  +=======+
-                    |      |
+                  |      |
-                    | INIT  |
+                  | INIT  |
-                    |      |      FCN!=0 & more frags
+                  |      |      FCN!=0 & more frags
-                    +======++      21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
+                  +======++      21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-        Frag RuleID trigger |  +--+ Send cur_W + frag(FCN);
+    Frag RuleID trigger |  +--+ Send cur_W + frag(FCN);
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC) |  |  | FCN--;
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC) |  |  | FCN--;
-    cur_W=0; FCN=max_value;|  |  | set [cur_W, cur_Bmp]
+  cur_W=0; FCN=max_value;|  |  | set [cur_W, cur_Bmp]
-      clear [cur_W, Bmp_n];|  |  v
+    clear [cur_W, Bmp_n];|  |  v
-            clear rcv_Bmp  |  ++==+==========+      **BACK_TO_SEND
+          clear rcv_Bmp  |  ++==+==========+      **BACK_TO_SEND
-                            +->+              |  cur_W==rcv_W &
+                        +->+              |  cur_W==rcv_W &
-        **BACK_TO_SEND        |    SEND    |  [cur_W,Bmp_n]==rcv_Bmp
+      **BACK_TO_SEND        |    SEND    |  [cur_W,Bmp_n]==rcv_Bmp
-  +-------------------------->+              |  & more frags
++-------------------------->+              |  & more frags
-  |  +----------------------->+              |  21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
+|  +----------------------->+              |  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-  |  |                        ++==+==========+  cur_W++;
+|  |                        ++==+==========+  cur_W++;
-  |  |      FCN==0 & more frags|  |last frag    clear [cur_W, Bmp_n]
+|  |      FCN==0 & more frags|  |last frag    clear [cur_W, Bmp_n]
-  |  |  21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])|  |21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)
+|  |  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])|  |21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)
-  |  |        set cur_Bmp;    |  |set [cur_W, Bmp_n];
+|  |        set cur_Bmp;    |  |set [cur_W, Bmp_n];
-  |  |send cur_W + frag(All-0);|  |send cur_W + frag(All-1)+RCS;
+|  |send cur_W + frag(All-0);|  |send cur_W + frag(All-1)+RCS;
-  |  |        set Retrans_Timer|  |set Retrans_Timer
+|  |        set Retrans_Timer|  |set Retrans_Timer
-  |  |                        |  | +---------------------------------+
+|  |                        |  | +---------------------------------+
-  |  |                        |  | |cur_W ==                        |
+|  |                        |  | |cur_W ==                        |
-  |  |Retrans_Timer expires &  |  | |  rcv_W & [cur_W,Bmp_n]!=rcv_Bmp|
+|  |Retrans_Timer expires &  |  | |  rcv_W & [cur_W,Bmp_n]!=rcv_Bmp|
-  |  |more Frags              |  | |  21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)            |
+|  |more Frags              |  | |  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)            |
-  |  |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)    |  | |  Attempts++; W=cur_W            |
+|  |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)    |  | |  Attempts++; W=cur_W            |
-  |  |stop Retrans_Timer;      |  | | +--------+          rcv_W==Wn &|
+|  |stop Retrans_Timer;      |  | | +--------+          rcv_W==Wn &|
-  |  |[cur_W,Bmp_n]==cur_Bmp;  v  v | |        v  [Wn,Bmp_n]!=rcv_Bmp|
+|  |[cur_W,Bmp_n]==cur_Bmp;  v  v | |        v  [Wn,Bmp_n]!=rcv_Bmp|
-  |  |cur_W++            +=====+==+=+=+==+  +=+=========+ 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~|
+|  |cur_W++            +=====+==+=+=+==+  +=+=========+ 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~|
-  |  +-------------------+              |  | Resend    | Attempts++;|
+|  +-------------------+              |  | Resend    | Attempts++;|
-  +----------------------+  Wait x ACK  |  | Missing  |      W=Wn |
++----------------------+  Wait x ACK  |  | Missing  |      W=Wn |
-  +--------------------->+              |  | Frags(W)  +<-----------+
++--------------------->+              |  | Frags(W)  +<-----------+
-  |        rcv_W==Wn &+-+              |  +======+====+
+|        rcv_W==Wn &+-+              |  +======+====+
-  | [Wn,Bmp_n]!=rcv_Bmp| ++=+===+===+==+=+          |
+| [Wn,Bmp_n]!=rcv_Bmp| ++=+===+===+==+=+          |
-  |      21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)|  ^ |  |  |  ^            |
+|      21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)|  ^ |  |  |  ^            |
-  |        send (cur_W,+--+ |  |  |  +------------+
+|        send (cur_W,+--+ |  |  |  +------------+
-  |        ALL-0-empty)    |  |  |    all missing frag sent(W)
+|        ALL-0-empty)    |  |  |    all missing frag sent(W)
-  |                        |  |  |    21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
+|                        |  |  |    21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-  |  Retrans_Timer expires &|  |  |    set Retrans_Timer
+|  Retrans_Timer expires &|  |  |    set Retrans_Timer
-  |            No more Frags|  |  |
+|            No more Frags|  |  |
-  |          21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)|  |  |
+|          21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)|  |  |
-  |      stop Retrans_Timer;|  |  |
+|      stop Retrans_Timer;|  |  |
-  |(re)send frag(All-1)+RCS |  |  |
+|(re)send frag(All-1)+RCS |  |  |
-  +-------------------------+  |  |
++-------------------------+  |  |
-                    cur_W==rcv_W&|  |
+                cur_W==rcv_W&|  |
-          [cur_W,Bmp_n]==rcv_Bmp&|  | Attempts > MAX_ACK_REQUESTS
+      [cur_W,Bmp_n]==rcv_Bmp&|  | Attempts > MAX_ACK_REQUESTS
-    No more Frags & RCS flag==OK|  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
+  No more Frags & RCS flag==OK|  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])|  | send Abort
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])|  | send Abort
-    +=========+stop Retrans_Timer|  |  +===========+
+ +=========+stop Retrans_Timer|  |  +===========+
-    |  END  +<-----------------+  +->+  ERROR  |
+ |  END  +<-----------------+  +->+  ERROR  |
-    +=========+                        +===========+
+ +=========+                        +===========+
-        Figure 43: Sender State Machine for the ACK-on-Error Mode
+      Figure 43: Sender State Machine for the ACK-on-Error Mode
-  This is an example only.  It is not normative.  The specification in
+This is an example only.  It is not normative.  The specification in
-  Section 8.4.3.1 allows for sequences of operations different from the
+Section 8.4.3.1 allows for sequences of operations different from the
-  one shown here.
+one shown here.
-                    +=======+        New frag RuleID received
+                +=======+        New frag RuleID received
-                    |      |        21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
+                |      |        21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])
-                    | INIT  +-------+cur_W=0;clear([cur_W,Bmp_n]);
+                | INIT  +-------+cur_W=0;clear([cur_W,Bmp_n]);
-                    +=======+      |sync=0
+                +=======+      |sync=0
-                                    |
+                                |
-      Not All* & rcv_W==cur_W+---+ | +--+
+    Not All* & rcv_W==cur_W+---+ | +--+
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC) |  | | | (E)
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC) |  | | | (E)
-        set[cur_W,Bmp_n(FCN)]|  v v v  |
+      set[cur_W,Bmp_n(FCN)]|  v v v  |
-                            ++===+=+=+==+=+
+                          ++===+=+=+==+=+
-      +----------------------+            +--+ All-0&Full[cur_W,Bmp_n]
+  +----------------------+            +--+ All-0&Full[cur_W,Bmp_n]
-      |          ABORT *<---+  Rcv Window |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
+  |          ABORT *<---+  Rcv Window |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-      |  +-------------------+            +<-+ cur_W++;set Inact_timer;
+  |  +-------------------+            +<-+ cur_W++;set Inact_timer;
-      |  |                +->+=+=+=+=+=+===+    clear [cur_W,Bmp_n]
+  |  |                +->+=+=+=+=+=+===+    clear [cur_W,Bmp_n]
-      |  | All-0 empty(Wn)|    | | | ^ ^
+  |  | All-0 empty(Wn)|    | | | ^ ^
-      |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC) +----+ | | | |rcv_W==cur_W & sync==0;
+  |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC) +----+ | | | |rcv_W==cur_W & sync==0;
-      |  | sendACK([Wn,Bmp_n])  | | | |& Full([cur_W,Bmp_n])
+  |  | sendACK([Wn,Bmp_n])  | | | |& Full([cur_W,Bmp_n])
-      |  |                      | | | |& All* || last_miss_frag
+  |  |                      | | | |& All* || last_miss_frag
-      |  |                      | | | |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~
+  |  |                      | | | |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~
-      |  |    All* & rcv_W==cur_W|(C)| |sendACK([cur_W,Bmp_n]);
+  |  |    All* & rcv_W==cur_W|(C)| |sendACK([cur_W,Bmp_n]);
-      |  |              & sync==0| | | |cur_W++; clear([cur_W,Bmp_n])
+  |  |              & sync==0| | | |cur_W++; clear([cur_W,Bmp_n])
-      |  |&no_full([cur_W,Bmp_n])| |(E)|
+  |  |&no_full([cur_W,Bmp_n])| |(E)|
-      |  |      21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~ | | | |              +========+
+  |  |      21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~ | | | |              +========+
-      |  | sendACK([cur_W,Bmp_n])| | | |              | Error/ |
+  |  | sendACK([cur_W,Bmp_n])| | | |              | Error/ |
-      |  |                      | | | |  +----+    | Abort  |
+  |  |                      | | | |  +----+    | Abort  |
-      |  |                      v v | |  |    |    +===+====+
+  |  |                      v v | |  |    |    +===+====+
-      |  |                  +===+=+=+=+===+=+ (D)        ^
+  |  |                  +===+=+=+=+===+=+ (D)        ^
-      |  |                +--+    Wait x    |  |        |
+  |  |                +--+    Wait x    |  |        |
-      |  | All-0 empty(Wn)+->| Missing Frags |<-+        |
+  |  | All-0 empty(Wn)+->| Missing Frags |<-+        |
-      |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)    +=============+=+            |
+  |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)    +=============+=+            |
-      |  | sendACK([Wn,Bmp_n])            +--------------+
+  |  | sendACK([Wn,Bmp_n])            +--------------+
-      |  |                                      *ABORT
+  |  |                                      *ABORT
-      v  v
+  v  v
-    (A)(B)
+  (A)(B)
-                                      (D) All* || last_miss_frag
+                                  (D) All* || last_miss_frag
-      (C) All* & sync>0                  & rcv_W!=cur_W & sync>0
+    (C) All* & sync>0                  & rcv_W!=cur_W & sync>0
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~                  & Full([rcv_W,Bmp_n])
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~                  & Full([rcv_W,Bmp_n])
-          Wn=oldest[not full(W)];        21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
+        Wn=oldest[not full(W)];        21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-          sendACK([Wn,Bmp_n])            Wn=oldest[not full(W)];
+        sendACK([Wn,Bmp_n])            Wn=oldest[not full(W)];
-                                          sendACK([Wn,Bmp_n]);sync--
+                                      sendACK([Wn,Bmp_n]);sync--
-                                ABORT-->* Uplink Only &
+                            ABORT-->* Uplink Only &
-                                          Inact_Timer expires
+                                      Inact_Timer expires
-      (E) Not All* & rcv_W!=cur_W        || Attempts > MAX_ACK_REQUESTS
+    (E) Not All* & rcv_W!=cur_W        || Attempts > MAX_ACK_REQUESTS
-:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)          21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~
+:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)          21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~
-          sync++; cur_W=rcv_W;          send Abort
+        sync++; cur_W=rcv_W;          send Abort
-          set[cur_W,Bmp_n(FCN)]
+        set[cur_W,Bmp_n(FCN)]
-    (A)(B)
+  (A)(B)
-      |  |
+  |  |
-      |  | All-1 & rcv_W==cur_W & RCS!=OK        All-0 empty(Wn)
+  |  | All-1 & rcv_W==cur_W & RCS!=OK        All-0 empty(Wn)
-      |  | 21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])    +-+  21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)
+  |  | 21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]])    +-+  21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)
-      |  | sendACK([cur_W,Bmp_n],C=0)      | v  sendACK([Wn,Bmp_n])
+  |  | sendACK([cur_W,Bmp_n],C=0)      | v  sendACK([Wn,Bmp_n])
-      |  |                      +===========+=++
+  |  |                      +===========+=++
-      |  +--------------------->+  Wait End  +-+
+  |  +--------------------->+  Wait End  +-+
-      |                        +=====+=+====+=+ | All-1
+  |                        +=====+=+====+=+ | All-1
-      |    rcv_W==cur_W & RCS==OK    | |    ^  | & rcv_W==cur_W
+  |    rcv_W==cur_W & RCS==OK    | |    ^  | & rcv_W==cur_W
-      |    21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)~~    | |    +---+ & RCS!=OK
+  |    21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)~~    | |    +---+ & RCS!=OK
-      |  sendACK([cur_W,Bmp_n],C=1)  | |          21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)
+  |  sendACK([cur_W,Bmp_n],C=1)  | |          21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)
-      |                              | | sendACK([cur_W,Bmp_n],C=0);
+  |                              | | sendACK([cur_W,Bmp_n],C=0);
-      |                              | |          Attempts++
+  |                              | |          Attempts++
-      |All-1 & Full([cur_W,Bmp_n])    | |
+  |All-1 & Full([cur_W,Bmp_n])    | |
-      |& RCS==OK & sync==0            | +-->* ABORT
+  |& RCS==OK & sync==0            | +-->* ABORT
-      |21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)21:56, 22 September 2020 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:56, 22 September 2020 (UTC)            v
+  |21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)21:37, 5 May 2021 (UTC)[[User:Admin|Admin]] ([[User talk:Admin|talk]]) 21:37, 5 May 2021 (UTC)            v
-      |sendACK([cur_W,Bmp_n],C=1)  +=+=========+
+  |sendACK([cur_W,Bmp_n],C=1)  +=+=========+
-      +---------------------------->+    END    |
+  +---------------------------->+    END    |
-                                    +===========+
+                                +===========+
-        Figure 44: Receiver State Machine for the ACK-on-Error Mode
+    Figure 44: Receiver State Machine for the ACK-on-Error Mode
 Appendix D.  SCHC Parameters
-  This section lists the information that needs to be provided in the
+This section lists the information that needs to be provided in the
-  LPWAN technology-specific documents.
+LPWAN technology-specific documents.
-  *  Most common uses cases, deployment scenarios.
+*  Most common uses cases, deployment scenarios.
-  *  Mapping of the SCHC architectural elements onto the LPWAN
+*  Mapping of the SCHC architectural elements onto the LPWAN
-      architecture.
+  architecture.
-  *  Assessment of LPWAN integrity checking.
+*  Assessment of LPWAN integrity checking.
-  *  Various potential channel conditions for the technology and the
+*  Various potential channel conditions for the technology and the
-      corresponding recommended use of SCHC C/D and SCHC F/R.
+  corresponding recommended use of SCHC C/D and SCHC F/R.
-  This section lists the parameters that need to be defined in the
+This section lists the parameters that need to be defined in the
-  Profile.
+Profile.
-  *  RuleID numbering scheme, fixed-size or variable-size RuleIDs,
+*  RuleID numbering scheme, fixed-size or variable-size RuleIDs,
-      number of Rules, the way the RuleID is transmitted.
+  number of Rules, the way the RuleID is transmitted.
-  *  maximum packet size that should ever be reconstructed by SCHC
+*  maximum packet size that should ever be reconstructed by SCHC
-      decompression (MAX_PACKET_SIZE).  See Section 12.
+  decompression (MAX_PACKET_SIZE).  See Section 12.
-  *  Padding: size of the L2 Word (for most LPWAN technologies, this
+*  Padding: size of the L2 Word (for most LPWAN technologies, this
-      would be a byte; for some technologies, a bit).
+  would be a byte; for some technologies, a bit).
-  *  Decision to use SCHC fragmentation mechanism or not.  If yes, the
+*  Decision to use SCHC fragmentation mechanism or not.  If yes, the
-      document must describe:
+  document must describe:
-      -  reliability mode(s) used, in which cases (e.g., based on link
+  -  reliability mode(s) used, in which cases (e.g., based on link
-        channel condition).
+      channel condition).
-      -  RuleID values assigned to each mode in use.
+  -  RuleID values assigned to each mode in use.
-      -  presence and number of bits for DTag (T) for each RuleID value,
+  -  presence and number of bits for DTag (T) for each RuleID value,
-        lifetime of DTag at the receiver.
+      lifetime of DTag at the receiver.
-      -  support for interleaved packet transmission, to what extent.
+  -  support for interleaved packet transmission, to what extent.
-      -  WINDOW_SIZE, for modes that use windows.
+  -  WINDOW_SIZE, for modes that use windows.
-      -  number of bits for W (M) for each RuleID value, for modes that
+  -  number of bits for W (M) for each RuleID value, for modes that
-        use windows.
+      use windows.
-      -  number of bits for FCN (N) for each RuleID value, meaning of
+  -  number of bits for FCN (N) for each RuleID value, meaning of
-        the FCN values.
+      the FCN values.
-      -  what makes an All-0 SCHC Fragment and a SCHC ACK REQ
+  -  what makes an All-0 SCHC Fragment and a SCHC ACK REQ
-        distinguishable (see Section 8.3.1.1).
+      distinguishable (see Section 8.3.1.1).
-      -  what makes an All-1 SCHC Fragment and a SCHC Sender-Abort
+  -  what makes an All-1 SCHC Fragment and a SCHC Sender-Abort
-        distinguishable (see Section 8.3.1.2).
+      distinguishable (see Section 8.3.1.2).
-      -  for RuleIDs that use ACK-on-Error mode: when the last tile of a
+  -  for RuleIDs that use ACK-on-Error mode: when the last tile of a
-        SCHC Packet is to be sent in a Regular SCHC Fragment, alone in
+      SCHC Packet is to be sent in a Regular SCHC Fragment, alone in
-        an All-1 SCHC Fragment or with any of these two methods.
+      an All-1 SCHC Fragment or with any of these two methods.
-      -  for RuleIDs that use ACK-on-Error mode: if the penultimate tile
+  -  for RuleIDs that use ACK-on-Error mode: if the penultimate tile
-        of a SCHC Packet is of the regular size only or if it can also
+      of a SCHC Packet is of the regular size only or if it can also
-        be one L2 Word shorter.
+      be one L2 Word shorter.
-      -  for RuleIDs that use ACK-on-Error mode: times at which the
+  -  for RuleIDs that use ACK-on-Error mode: times at which the
-        sender must listen for SCHC ACKs.
+      sender must listen for SCHC ACKs.
-      -  size of RCS and algorithm for its computation, for each RuleID,
+  -  size of RCS and algorithm for its computation, for each RuleID,
-        if different from the default CRC32.  Byte fill-up with zeroes
+      if different from the default CRC32.  Byte fill-up with zeroes
-        or other mechanism, to be specified.  Support for UDP checksum
+      or other mechanism, to be specified.  Support for UDP checksum
-        elision.
+      elision.
-      -  Retransmission Timer duration for each RuleID value, if
+  -  Retransmission Timer duration for each RuleID value, if
-        applicable to the SCHC F/R mode.
+      applicable to the SCHC F/R mode.
-      -  Inactivity Timer duration for each RuleID value, if applicable
+  -  Inactivity Timer duration for each RuleID value, if applicable
-        to the SCHC F/R mode.
+      to the SCHC F/R mode.
-      -  MAX_ACK_REQUESTS value for each RuleID value, if applicable to
+  -  MAX_ACK_REQUESTS value for each RuleID value, if applicable to
-        the SCHC F/R mode.
+      the SCHC F/R mode.
-  *  if L2 Word is wider than a bit and SCHC fragmentation is used,
+*  if L2 Word is wider than a bit and SCHC fragmentation is used,
-      value of the padding bits (0 or 1).
+  value of the padding bits (0 or 1).
-  A Profile may define a delay to be added after each SCHC message
+A Profile may define a delay to be added after each SCHC message
-  transmission for compliance with local regulations or other
+transmission for compliance with local regulations or other
-  constraints imposed by the applications.
+constraints imposed by the applications.
-  *  In some LPWAN technologies, as part of energy-saving techniques,
+*  In some LPWAN technologies, as part of energy-saving techniques,
-      Downlink transmission is only possible immediately after an Uplink
+  Downlink transmission is only possible immediately after an Uplink
-      transmission.  In order to avoid potentially high delay in the
+  transmission.  In order to avoid potentially high delay in the
-      Downlink transmission of a fragmented SCHC Packet, the SCHC
+  Downlink transmission of a fragmented SCHC Packet, the SCHC
-      Fragment receiver may perform an Uplink transmission as soon as
+  Fragment receiver may perform an Uplink transmission as soon as
-      possible after reception of a SCHC Fragment that is not the last
+  possible after reception of a SCHC Fragment that is not the last
-      one.  Such Uplink transmission may be triggered by the L2 (e.g.,
+  one.  Such Uplink transmission may be triggered by the L2 (e.g.,
-      an L2 ACK sent in response to a SCHC Fragment encapsulated in a L2
+  an L2 ACK sent in response to a SCHC Fragment encapsulated in a L2
-      PDU that requires an L2 ACK) or it may be triggered from an upper
+  PDU that requires an L2 ACK) or it may be triggered from an upper
-      layer.  See Appendix F.
+  layer.  See Appendix F.
-  *  the following parameters need to be addressed in documents other
+*  the following parameters need to be addressed in documents other
-      than this one but not necessarily in the LPWAN technology-specific
+  than this one but not necessarily in the LPWAN technology-specific
-      documents:
+  documents:
-      -  The way the Contexts are provisioned.
+  -  The way the Contexts are provisioned.
-      -  The way the Rules are generated.
+  -  The way the Rules are generated.
 Appendix E.  Supporting Multiple Window Sizes for Fragmentation
-  For ACK-Always or ACK-on-Error, implementers may opt to support a
+For ACK-Always or ACK-on-Error, implementers may opt to support a
-  single window size or multiple window sizes.  The latter, when
+single window size or multiple window sizes.  The latter, when
-  feasible, may provide performance optimizations.  For example, a
+feasible, may provide performance optimizations.  For example, a
-  large WINDOW_SIZE should be used for packets that need to be split
+large WINDOW_SIZE should be used for packets that need to be split
-  into a large number of tiles.  However, when the number of tiles
+into a large number of tiles.  However, when the number of tiles
-  required to carry a packet is low, a smaller WINDOW_SIZE and, thus, a
+required to carry a packet is low, a smaller WINDOW_SIZE and, thus, a
-  shorter Bitmap, may be sufficient to provide reception status on all
+shorter Bitmap, may be sufficient to provide reception status on all
-  tiles.  If multiple window sizes are supported, the RuleID signals
+tiles.  If multiple window sizes are supported, the RuleID signals
-  what WINDOW_SIZE is in use for a specific packet transmission.
+what WINDOW_SIZE is in use for a specific packet transmission.
 Appendix F.  ACK-Always and ACK-on-Error on Quasi-Bidirectional Links
-  The ACK-Always and ACK-on-Error modes of SCHC F/R are bidirectional
+The ACK-Always and ACK-on-Error modes of SCHC F/R are bidirectional
-  protocols: they require a feedback path from the reassembler to the
+protocols: they require a feedback path from the reassembler to the
-  fragmenter.
+fragmenter.
-  Some LPWAN technologies provide quasi-bidirectional connectivity,
+Some LPWAN technologies provide quasi-bidirectional connectivity,
-  whereby a Downlink transmission from the Network Infrastructure can
+whereby a Downlink transmission from the Network Infrastructure can
-  only take place right after an Uplink transmission by the Dev.
+only take place right after an Uplink transmission by the Dev.
-  When using SCHC F/R to send fragmented SCHC Packets Downlink over
+When using SCHC F/R to send fragmented SCHC Packets Downlink over
-  these quasi-bidirectional links, the following situation may arise:
+these quasi-bidirectional links, the following situation may arise:
-  if an Uplink SCHC ACK is lost, the SCHC ACK REQ message by the sender
+if an Uplink SCHC ACK is lost, the SCHC ACK REQ message by the sender
-  could be stuck indefinitely in the Downlink queue at the Network
+could be stuck indefinitely in the Downlink queue at the Network
-  Infrastructure, waiting for a transmission opportunity.
+Infrastructure, waiting for a transmission opportunity.
-  There are many ways by which this deadlock can be avoided.  The Dev
+There are many ways by which this deadlock can be avoided.  The Dev
-  application might be sending recurring Uplink messages such as keep-
+application might be sending recurring Uplink messages such as keep-
-  alive, or the Dev application stack might be sending other recurring
+alive, or the Dev application stack might be sending other recurring
-  Uplink messages as part of its operation.  However, these are out of
+Uplink messages as part of its operation.  However, these are out of
-  the control of this generic SCHC specification.
+the control of this generic SCHC specification.
-  In order to cope with quasi-bidirectional links, a SCHC-over-foo
+In order to cope with quasi-bidirectional links, a SCHC-over-foo
-  specification may want to amend the SCHC F/R specification to add a
+specification may want to amend the SCHC F/R specification to add a
-  timer-based retransmission of the SCHC ACK.  Below is an example of
+timer-based retransmission of the SCHC ACK.  Below is an example of
-  the suggested behavior for ACK-Always mode.  Because it is an
+the suggested behavior for ACK-Always mode.  Because it is an
-  example, [RFC2119] language is deliberately not used here.
+example, [[RFC2119]] language is deliberately not used here.
-  For Downlink transmission of a fragmented SCHC Packet in ACK-Always
+For Downlink transmission of a fragmented SCHC Packet in ACK-Always
-  mode, the SCHC Fragment receiver may support timer-based SCHC ACK
+mode, the SCHC Fragment receiver may support timer-based SCHC ACK
-  retransmission.  In this mechanism, the SCHC Fragment receiver
+retransmission.  In this mechanism, the SCHC Fragment receiver
-  initializes and starts a timer (the UplinkACK Timer) after the
+initializes and starts a timer (the UplinkACK Timer) after the
-  transmission of a SCHC ACK, except when the SCHC ACK is sent in
+transmission of a SCHC ACK, except when the SCHC ACK is sent in
-  response to the last SCHC Fragment of a packet (All-1 fragment).  In
+response to the last SCHC Fragment of a packet (All-1 fragment).  In
-  the latter case, the SCHC Fragment receiver does not start a timer
+the latter case, the SCHC Fragment receiver does not start a timer
-  after transmission of the SCHC ACK.
+after transmission of the SCHC ACK.
-  If, after transmission of a SCHC ACK that is not an All-1 fragment,
+If, after transmission of a SCHC ACK that is not an All-1 fragment,
-  and before expiration of the corresponding UplinkACK timer, the SCHC
+and before expiration of the corresponding UplinkACK timer, the SCHC
-  Fragment receiver receives a SCHC Fragment that belongs to the
+Fragment receiver receives a SCHC Fragment that belongs to the
-  current window (e.g., a missing SCHC Fragment from the current
+current window (e.g., a missing SCHC Fragment from the current
-  window) or to the next window, the UplinkACK timer for the SCHC ACK
+window) or to the next window, the UplinkACK timer for the SCHC ACK
-  is stopped.  However, if the UplinkACK timer expires, the SCHC ACK is
+is stopped.  However, if the UplinkACK timer expires, the SCHC ACK is
-  resent and the UplinkACK timer is reinitialized and restarted.
+resent and the UplinkACK timer is reinitialized and restarted.
-  The default initial value for the UplinkACK Timer, as well as the
+The default initial value for the UplinkACK Timer, as well as the
-  maximum number of retries for a specific SCHC ACK, denoted
+maximum number of retries for a specific SCHC ACK, denoted
-  MAX_ACK_REQUESTS, is to be defined in a Profile.  The initial value
+MAX_ACK_REQUESTS, is to be defined in a Profile.  The initial value
-  of the UplinkACK timer is expected to be greater than that of the
+of the UplinkACK timer is expected to be greater than that of the
-  Retransmission timer, in order to make sure that a (buffered) SCHC
+Retransmission timer, in order to make sure that a (buffered) SCHC
-  Fragment to be retransmitted finds an opportunity for that
+Fragment to be retransmitted finds an opportunity for that
-  transmission.  One exception to this recommendation is the special
+transmission.  One exception to this recommendation is the special
-  case of the All-1 SCHC Fragment transmission.
+case of the All-1 SCHC Fragment transmission.
-  When the SCHC Fragment sender transmits the All-1 SCHC Fragment, it
+When the SCHC Fragment sender transmits the All-1 SCHC Fragment, it
-  starts its Retransmission Timer with a large timeout value (e.g.,
+starts its Retransmission Timer with a large timeout value (e.g.,
-  several times that of the initial UplinkACK Timer).  If a SCHC ACK is
+several times that of the initial UplinkACK Timer).  If a SCHC ACK is
-  received before expiration of this timer, the SCHC Fragment sender
+received before expiration of this timer, the SCHC Fragment sender
-  retransmits any lost SCHC Fragments as reported by the SCHC ACK, or
+retransmits any lost SCHC Fragments as reported by the SCHC ACK, or
-  if the SCHC ACK confirms successful reception of all SCHC Fragments
+if the SCHC ACK confirms successful reception of all SCHC Fragments
-  of the last window, the transmission of the fragmented SCHC Packet is
+of the last window, the transmission of the fragmented SCHC Packet is
-  considered complete.  If the timer expires, and no SCHC ACK has been
+considered complete.  If the timer expires, and no SCHC ACK has been
-  received since the start of the timer, the SCHC Fragment sender
+received since the start of the timer, the SCHC Fragment sender
-  assumes that the All-1 SCHC Fragment has been successfully received
+assumes that the All-1 SCHC Fragment has been successfully received
-  (and possibly, the last SCHC ACK has been lost: this mechanism
+(and possibly, the last SCHC ACK has been lost: this mechanism
-  assumes that the Retransmission Timer for the All-1 SCHC Fragment is
+assumes that the Retransmission Timer for the All-1 SCHC Fragment is
-  long enough to allow several SCHC ACK retries if the All-1 SCHC
+long enough to allow several SCHC ACK retries if the All-1 SCHC
-  Fragment has not been received by the SCHC Fragment receiver, and it
+Fragment has not been received by the SCHC Fragment receiver, and it
-  also assumes that it is unlikely that several ACKs become all lost).
+also assumes that it is unlikely that several ACKs become all lost).
 Acknowledgements
-  Thanks to (in alphabetical order) Sergio Aguilar Romero, David Black,
+Thanks to (in alphabetical order) Sergio Aguilar Romero, David Black,
-  Carsten Bormann, Deborah Brungard, Brian Carpenter, Philippe Clavier,
+Carsten Bormann, Deborah Brungard, Brian Carpenter, Philippe Clavier,
-  Alissa Cooper, Roman Danyliw, Daniel Ducuara Beltran, Diego Dujovne,
+Alissa Cooper, Roman Danyliw, Daniel Ducuara Beltran, Diego Dujovne,
-  Eduardo Ingles Sanchez, Rahul Jadhav, Benjamin Kaduk, Arunprabhu
+Eduardo Ingles Sanchez, Rahul Jadhav, Benjamin Kaduk, Arunprabhu
-  Kandasamy, Suresh Krishnan, Mirja Kuehlewind, Barry Leiba, Sergio
+Kandasamy, Suresh Krishnan, Mirja Kuehlewind, Barry Leiba, Sergio
-  Lopez Bernal, Antoni Markovski, Alexey Melnikov, Georgios
+Lopez Bernal, Antoni Markovski, Alexey Melnikov, Georgios
-  Papadopoulos, Alexander Pelov, Charles Perkins, Edgar Ramos, Alvaro
+Papadopoulos, Alexander Pelov, Charles Perkins, Edgar Ramos, Alvaro
-  Retana, Adam Roach, Shoichi Sakane, Joseph Salowey, Pascal Thubert,
+Retana, Adam Roach, Shoichi Sakane, Joseph Salowey, Pascal Thubert,
-  and Eric Vyncke for useful design considerations, reviews and
+and Eric Vyncke for useful design considerations, reviews and
-  comments.
+comments.
-  Carles Gomez has been funded in part by the Spanish Government
+Carles Gomez has been funded in part by the Spanish Government
-  (Ministerio de Educacion, Cultura y Deporte) through the Jose
+(Ministerio de Educacion, Cultura y Deporte) through the Jose
-  Castillejo grant CAS15/00336 and by the ERDF and the Spanish
+Castillejo grant CAS15/00336 and by the ERDF and the Spanish
-  Government through project TEC2016-79988-P.  Part of his contribution
+Government through project TEC2016-79988-P.  Part of his contribution
-  to this work has been carried out during his stay as a visiting
+to this work has been carried out during his stay as a visiting
-  scholar at the Computer Laboratory of the University of Cambridge.
+scholar at the Computer Laboratory of the University of Cambridge.
 Authors' Addresses
-  Ana Minaburo
+Ana Minaburo
-  Acklio
+Acklio
 A avenue des Champs Blancs
 Cesson-Sevigne Cedex
-  France
+France
-  Email: [email protected]
-  Laurent Toutain
-  IMT Atlantique
-rue de la Chataigneraie
-  CS 17607
-Cesson-Sevigne Cedex
-  France
-  Email: Laurent.Toutain@imt-atlantique.fr
+Email: ana@ackl.io
+Laurent Toutain
+IMT Atlantique
+rue de la Chataigneraie
+CS 17607
+Cesson-Sevigne Cedex
+France
-  Carles Gomez
+Email: [email protected]
-  Universitat Politecnica de Catalunya
-  C/Esteve Terradas, 7
-Castelldefels
-  Spain
-  Email: [email protected]
+Carles Gomez
+Universitat Politecnica de Catalunya
+C/Esteve Terradas, 7
+Castelldefels
+Spain
+Email: [email protected]
-  Dominique Barthel
+Dominique Barthel
-  Orange Labs
+Orange Labs
 chemin du Vieux Chene
 Meylan
-  France
+France
-  Email: [email protected]
+Email: [email protected]
+Juan Carlos Zuniga
+SIGFOX
+rue Jean Rostand
+Labege
+France
-  Juan Carlos Zuniga
+Email: JuanCarlos.Zuniga@sigfox.com
-  SIGFOX
-rue Jean Rostand
-Labege
-  France
-  Email: [email protected]
+[[Category:Standards Track]]

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Difference between revisions of "RFC8724"

Latest revision as of 21:37, 5 May 2021

Contents

Introduction

Requirements Notation

LPWAN Architecture

Terminology

SCHC Overview

SCHC Packet Format

Functional Mapping

RuleID

Compression/Decompression

SCHC C/D Rules

Packet Processing

Matching Operators

Compression/Decompression Actions (CDA)

Processing Fixed-Length Fields

Processing Variable-Length Fields

Not-Sent CDA

Value-Sent CDA

Mapping-Sent CDA

LSB CDA

DevIID, AppIID CDA

Compute-*

Fragmentation/Reassembly

Overview

SCHC F/R Protocol Elements

Messages

Tiles, Windows, Bitmaps, Timers, Counters

Tiles

Windows

Bitmaps

Timers and Counters

Integrity Checking

Header Fields

SCHC F/R Message Formats

SCHC Fragment Format

Regular SCHC Fragment

All-1 SCHC Fragment

SCHC ACK Format

Bitmap Compression

SCHC ACK REQ Format

SCHC Sender-Abort Format

SCHC Receiver-Abort Format

SCHC F/R Modes

No-ACK Mode

Sender Behavior

Receiver Behavior

ACK-Always Mode

Sender Behavior

Receiver Behavior

ACK-on-Error Mode

Sender Behavior

Receiver Behavior

Padding Management

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