RFC1209

Network Working Group D. Piscitello Request for Comments: 1209 J. Lawrence

                                           Bell Communications Research
                                                             March 1991

        The Transmission of IP Datagrams over the SMDS Service

Status of this Memo

  This memo defines a protocol for the transmission of IP and ARP
  packets over a Switched Multi-megabit Data Service Network configured
  as a logical IP subnetwork.  This RFC specifies an IAB standards
  track protocol for the Internet community, and requests discussion
  and suggestions for improvements.  Please refer to the current
  edition of the "IAB Official Protocol Standards" for the
  standardization state and status of this protocol.  Distribution of
  this memo is unlimited.

Abstract

  This memo describes an initial use of IP and ARP in an SMDS service
  environment configured as a logical IP subnetwork, LIS (described
  below).  The encapsulation method used is described, as well as
  various service-specific issues.  This memo does not preclude
  subsequent treatment of the SMDS Service in configurations other than
  LIS; specifically, public or inter-company, inter-enterprise
  configurations may be treated differently and will be described in
  future documents.  This document considers only directly connected IP
  end-stations or routers; issues raised by MAC level bridging are
  beyond the scope of this paper.

Acknowledgment

  This memo draws heavily in both concept and text from [4], written by
  Jon Postel and Joyce K. Reynolds of ISI and [5], written by David
  Katz of Merit, Inc.  The authors would also like to acknowledge the
  contributions of the IP Over SMDS Service working group of the
  Internet Engineering Task Force.

Conventions

  The following language conventions are used in the items of
  specification in this document:

     o MUST, SHALL, or MANDATORY -- the item is an absolute
       requirement of the specification.

IP over SMDS Working Group

RFC 1209 IP and ARP over the SMDS Service March 1991

     o SHOULD or RECOMMENDED -- the item should generally be followed
       for all but exceptional circumstances.

     o MAY or OPTIONAL -- the item is truly optional and may be
       followed or ignored according to the needs of the implementor.

Introduction

  The goal of this specification is to allow compatible and
  interoperable implementations for transmitting IP datagrams and ARP
  requests and replies.

  The characteristics of the SMDS Service and the SMDS Interface
  Protocol (SIP) are presented in [3], [6], and in [7].  Briefly, the
  SMDS Service is a connectionless, public, packet-switched data
  service.  The operation and features of the SMDS Service are similar
  to those found in high-speed data networks such as LANs:

     o The SMDS Service provides a datagram packet transfer, where each
       data unit is handled and switched separately without the prior
       establishment of a network connection.

     o The SMDS Service exhibits high throughput and low delay, and
       provides the transparent transport and delivery of up to 9188
       octets of user information in a single transmission.

     o No explicit flow control mechanisms are provided; instead, the
       rate of information transfer on the access paths is controlled
       both in the subscriber-to-network direction and in the network-
       to-subscriber direction through the use of an access class
       enforcement mechanism.

     o Both individually and group-addressed (multicast) packets can
       be transferred.

     o In addition to these LAN-like features, a set of addressing-
       related service features (source address validation, source and
       destination address screening) are provided to enable a
       subscriber or set of subscribers to create a logical private
       network, or closed user group, over the SMDS Service.  The
       access control provided by the closed user group mechanism is
       supplied by the SMDS provider according to the specifications
       stated in [3].

     o SMDS addresses are 60 bits plus a 4 bit Address Type.  The
       Address Type subfield occupies the 4 most significant bits of
       the destination and source address fields of the SIP Level 3
       Protocol Data Unit (PDU).  It contains the value 1100 to

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RFC 1209 IP and ARP over the SMDS Service March 1991

       indicate an individual address and the value 1110 for a 60-bit
       group address.

  The SMDS Interface Protocol is based on the IEEE Standard 802.6,
  Distributed Queue Dual Bus (DQDB) Connectionless MAC protocol [8].
  The SMDS service layer corresponds to the IEEE 802 MAC sublayer.  The
  remainder of the Data Link Service is provided by the IEEE 802.2
  Logical Link Control (LLC) service [9].  The resulting stack of
  services is illustrated in Figure 1:

                          +--------------------+
                          |      IP/ARP        |
                          +--------------------+
                          |IEEE 802.2 LLC/SNAP |
                          +--------------------+
                          | SIP LEVEL 3 (MAC)  |
                          +--------------------+
                          | SIP LEVELS 1 & 2   |
                          +--------------------+

           Figure 1.  Protocol stack for IP over SMDS Service

  This memo describes an initial use of IP and ARP in an SMDS Service
  environment configured as a logical IP subnetwork (described below).
  It does not preclude subsequent treatment of SMDS Service in
  configurations other than logical IP subnetworks; specifically,
  public or inter-company, inter-enterprise configurations may be
  treated differently and will be described in future documents.  This
  document does not address issues related to transparent data link
  layer interoperability.

Logical IP Subnetwork Configuration

  This section describes the scenario for an SMDS Service that is
  configured with multiple logical IP subnetworks, LIS (described
  below).  The scenario considers only directly connected IP end-
  stations or routers; issues raised by MAC level bridging are beyond
  the scope of this paper.

  In the LIS scenario, each separate administrative entity configures
  its hosts within a closed logical IP subnetwork.  Each LIS operates
  and communicates independently of other LISs over the same network
  providing SMDS.  Hosts connected to SMDS communicate directly to
  other hosts within the same LIS.  Communication to hosts outside of
  an individual LIS is provided via an IP router.  This router would
  simply be a station attached to the SMDS Service that has been
  configured to be a member of both logical IP subnetworks.  This
  configuration results in a number of disjoint LISs operating over the

IP over SMDS Working Group

RFC 1209 IP and ARP over the SMDS Service March 1991

  same network supporting the SMDS Service.  It is recognized that with
  this configuration, hosts of differing IP networks would communicate
  via an intermediate router even though a direct path over the SMDS
  Service may be possible.

  It is envisioned that the service will evolve to provide a more
  public interconnection, allowing machines directly connected to the
  SMDS Service to communicate without an intermediate router.  However,
  the issues raised by such a large public interconnection, such as
  scalability of address resolution or propagation of routing updates,
  are beyond the scope of this paper.  We anticipate that future RFCs
   will address these issues.

  The following is a list of the requirements for a LIS configuration:

     o All members have the same IP network/subnet number.

     o All stations within a LIS are accessed directly over SMDS.

     o All stations outside of the LIS are accessed via a router.

     o For each LIS a single SMDS group address has been configured
       that identifies all members of the LIS.  Any packet transmitted
       with this address is delivered by SMDS Service to all members
       of the LIS.

  The following list identifies a set of SMDS Service specific
  parameters that MUST be implemented in each IP station which would
  connect to the SMDS Service.  The parameter values will be determined
  at SMDS subscription time and will be different for each LIS.  Thus
  these parameters MUST be user configurable.

     o SMDS Hardware Address (smds$ha).  The SMDS Individual address
       of the IP station as determined at subscription time.  Each
       host MUST be configured to accept datagrams destined for this
       address.

     o SMDS LIS Group Address(smds$lis-ga).  The SMDS Group address
       that has been configured at subscription time to identify the
       SMDS Subscriber Network Interfaces (SNI) of all members of the
       LIS connected to the SMDS Service.  All members of the LIS MUST
       be prepared to accept datagrams addressed to smds$lis-ga.

     o SMDS Arp Request Address (smds$arp-req).  The SMDS address
       (individual or group) to which arp requests are to be sent.  In
       the initial LIS configuration this value is set to smds$lis-ga.
       It is conceivable that in other configurations this value would
       be set to some address other than that of smds$lis-ga (see

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RFC 1209 IP and ARP over the SMDS Service March 1991

       section on Address Resolution).

  It is RECOMMENDED that routers providing LIS functionality over the
  SMDS service also support the ability to interconnect differing LISs.
  Routers that wish to provide interconnection of differing LISs MUST
  be able to support multiple sets of these parameters (one set for
  each connected LIS) and be able to associate each set of parameters
  with a specific IP network/subnet number.  In addition, it is
  RECOMMENDED that a router be able to provide this multiple LIS
  support with a single physical SMDS interface that may have one or
  more individual SMDS addresses.

  The following list identifies LIS specific parameters that MUST be
  configured in the network supporting the SMDS Service.  For each LIS,
  the IP network administrator MUST request the configuration of these
  parameters at subscription time.  The administrator of each LIS MUST
  update these parameters as each new station is added to the LIS.

     o SMDS LIS Group Address(smds$lis-ga).  An SMDS Group address MUST
       be configured at subscription time to identify the SMDS
       Subscriber Network Interfaces (SNI) of all members of the LIS
       connected to the SMDS Service.

     o SMDS Address Screening Tables (Source and Destination).  The use
       of SMDS screening tables is not necessary for the operation of
       IP over SMDS Service.  If the SMDS screening tables are to be
       used, both source and destination tables for each SNI MUST be
       configured to allow, at minimum, both the direct communication
       between all hosts in the same LIS and the use of the SMDS LIS
       Group Address.

Packet Format

     Service SHALL be encapsulated within the IEEE 802.2 LLC and IEEE
     802.1A Sub-Network Access Protocol (SNAP) [10] Data Link layers
     and the 3-level SIP.  The SNAP MUST be used with an
     Organizationally Unique Identifier Code indicating that the SNAP
     header contains the EtherType code as listed in Assigned Numbers
     [11] (see Figure 2).  Note that values specified in this document
     follow Internet conventions: multi-byte fields are described in
     big-endian order and bits within bytes are described as most
     significant first [11].

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RFC 1209 IP and ARP over the SMDS Service March 1991

                                                      +-------+
                                                      |IP/ARP | IP/ARP
                             +----+----+----+----+----+-------+
                             |   Org Code   |Ethertype|       | SNAP
              +----+----+----+----+----+----+----+----+-------+
              |DSAP|SSAP|Ctrl|                                | LLC

+-----+----+-+-+----+----+----+----+----+----+----+----+-------+ |SIP..|HLPI|...| | SIP L3 +-----+----+-+-+----+----+----+----+----+----+----+----+-------+

                   Figure 2.  Data Link Encapsulation

     o The value of HLPI in the SIP L3 Header is 1.

     o The total length of the LLC Header and the SNAP header is 8
       octets.

     o The value of DSAP and SSAP in the LLC header is 170 (decimal),
       AA (Internet hexadecimal).

     o The Ctrl (Control) value in the LLC header is 3 (Indicates Type
       One Unnumbered Information).

     o The Org Code in the SNAP header is zero (000000 Internet
       hexadecimal).

     o The EtherType for IP is 2048 (decimal), 0800 (Internet
       hexadecimal).  The EtherType for ARP is 2054 (decimal), 0806
       (Internet hexadecimal).

  IEEE 802.2 LLC Type One Unnumbered Information (UI) communication
  (which must be implemented by all conforming IEEE 802.2 stations) is
  used exclusively.  The Higher Layer Protocol Id (HLPI) field in the
  SIP L3_PDU header MUST be set to the IEEE 802.6 assigned Protocol Id
  value for LLC (decimal 1) [8].  All frames MUST be transmitted in
  standard IEEE 802.2 LLC Type 1 Unnumbered Information format, with
  the DSAP and the SSAP fields of the IEEE 802.2 header set to the
  assigned global SAP value for SNAP (decimal 170) [10].  The 24-bit
  Org Code (Organizationally Unique Identifier Code) in the SNAP MUST
  be set to a value of zero, and the remaining 16 bits are set to the
  EtherType value from Assigned Numbers [11] (2048 for IP, 2054 for
  ARP).

  The data link encapsulation for IP packets is shown in Figure 3 and
  for ARP in Figure 4.  All values shown are in Internet hexadecimal
  format.

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RFC 1209 IP and ARP over the SMDS Service March 1991

    +--------------+---------------------------------------+-------+
    |      SIP     |             LLC / SNAP                |  IP   |
    |              |                                       |       |
    |SIP..|HLPI|...|DSAP|SSAP|Ctrl|   Org Code   |Ethertype|       |
    +-----+----+-+-+----+----+----+----+----+----+----+----+-------+
    |SIP..| 01 |...| AA | AA | 03 |    000000    |  0800   | IP... |
    +-----+----+-+-+----+----+----+----+----+----+----+----+-------+

            Figure 3.  IP Data Link Encapsulation and Values

    +--------------+---------------------------------------+-------+
    |      SIP     |             LLC / SNAP                |  ARP  |
    |              |                                       |       |
    |SIP..|HLPI|...|DSAP|SSAP|Ctrl|   Org Code   |Ethertype|       |
    +-----+----+-+-+----+----+----+----+----+----+----+----+-------+
    |SIP..| 01 |...| AA | AA | 03 |    000000    |  0806   | ARP...|
    +-----+----+-+-+----+----+----+----+----+----+----+----+-------+

            Figure 4.  ARP Data Link Encapsulation and Values

Address Resolution

  The dynamic mapping of 32-bit Internet addresses to SMDS addresses
  SHALL be done via the dynamic discovery procedure of the Address
  Resolution Protocol (ARP) [2].

  Internet addresses are assigned independent of SMDS addresses.  Each
  host implementation MUST know its own Internet address and SMDS
  address and respond to Address Resolution requests appropriately.
  Hosts MUST also use ARP to map Internet addresses to SMDS addresses
  when needed.

  The ARP protocol has several fields that parameterize its use in any
  specific context [2].  These fields are:

          ar$hrd   16 - bits     The Hardware Type Code
          ar$pro   16 - bits     The Protocol Type Code
          ar$hln    8 - bits     Octets in each hardware address
          ar$pln    8 - bits     Octets in each protocol address
          ar$op    16 - bits     Operation Code

     o The hardware type code assigned to SMDS addresses is 14
       (decimal), 0E (Internet hexadecimal) [11].

     o The protocol type code for IP is 2048 (decimal), 0800
       (Internet hexadecimal) [11].

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RFC 1209 IP and ARP over the SMDS Service March 1991

     o The hardware address length for SMDS is 8.

     o The protocol address length for IP is 4.

     o The operation code is 1 for request and 2 for reply.

  The SMDS hardware addresses in ARP packets (ar$sha, ar$tha) MUST be
  carried in SMDS native address format, with the most significant bit
  of the Address Type sub-field as the high order bit of the first
  octet.  Although outside the scope of this document, it is
  RECOMMENDED that SMDS addresses be represented in this format in all
  higher layer Internet protocols (e.g., SNMP).

  Traditionally, ARP requests are broadcast to all directly connected
  stations.  For the SMDS Service, the ARP request packet is
  transmitted to the smds$arp-req hardware address.  In the LIS
  configuration, the smds$arp-req address is set to smds$lis-ga, (the
  SMDS group address that identifies all members of the LIS).  It is
  conceivable that in a larger scale, public configuration, the
  smds$arp-req address would be configured to the address of some ARP-
  server(s) instead of the group address that identifies the entire
  LIS.

IP Broadcast Address

  There is no facility for complete hardware broadcast addressing over
  the SMDS Service.  As discussed in the "LIS Configuration" section,
  an SMDS group address (smds$lis-ga) SHALL be configured to include
  all stations in the same LIS.  The broadcast Internet address (the
  address on that network with a host part of all binary ones) MUST be
  mapped to smds$lis-ga (see also [12]).

IP Multicast Support

  A method of supporting IP multicasting is specified in [13].  It
  would be desirable to fully utilize the SMDS group address
  capabilities to support IP multicasting.  However, the method in [13]
  requires a Network Service Interface which provides multicast-like
  ability to provide dynamic access to the local network service
  interface operations:

     o JoinLocalGroup (group-address)

     o LeaveLocalGroup (group-address)

  The SMDS group address ability does not currently support dynamic
  subscription and removal from group address lists.  Therefore, it is
  RECOMMENDED that in the LIS configuration, if IP multicasting is to

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RFC 1209 IP and ARP over the SMDS Service March 1991

  be supported, the method of IP multicasting described for pure
  broadcast media, such as the Experimental Ethernet, be used.  For
  this method, all Multicast IP addresses are mapped to the same SMDS
  address which the broadcast Internet address is mapped for a given
  LIS.  Thus all Multicast IP addresses are mapped to smds$lis-ga.
  Filtering of multicast packets MUST be performed in the destination
  host.

Trailer Formats

  Some versions of Unix 4.x BSD use a different encapsulation method in
  order to get better network performance with the VAX virtual memory
  architecture.  Trailers SHALL not be used over the SMDS Service.

Byte Order

  As described in Appendix B of the Internet Protocol specification
  [1], the IP datagram is transmitted over the SMDS Service as a series
  of 8-bit bytes.  The byte order of the IP datagram shall be mapped
  directly onto the native SMDS byte order.

MAC Sublayer Details

Packet Size

  The SMDS Service defines a maximum service data unit size of 9188
  information octets.  This leaves 9180 octets for user data after the
  LLC/SNAP header is taken into account.  Therefore, the MTU for IP
  stations operating over the network supporting the SMDS Service SHALL
  be 9180 octets.

  There is no minimum packet size restriction defined for the SMDS
  Service.

Other MAC Sublayer Issues

  The SMDS Service requires that the publicly administered 60-bit
  address plus 4-bit type field format SHALL be used in both source and
  destination address fields of the SIP L3_PDU [3].

IEEE 802.2 Details

  While not necessary for supporting IP and ARP, all implementations
  MUST support IEEE 802.2 standard Class I service in order to be
  compliant with IEEE 802.2.  Some of the functions are not related
  directly to the support of the SNAP SAP (e.g., responding to XID and
  TEST commands directed to the null or global SAP addresses), but are
  part of a general LLC implementation.  Both [4] and [5] describe the

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RFC 1209 IP and ARP over the SMDS Service March 1991

  minimum functionality necessary for a conformant station.
  Implementors should also consult IEEE Std. 802.2 [14] for details.

REFERENCES

   1. Postel, J., "Internet Protocol", RFC 791, USC/Information
      Sciences Institute, September 1981.

   2. Plummer, D., "An Ethernet Address Resolution Protocol - or -
      Converting Network Protocol Addresses to 48.bit Ethernet Address
      for Transmission on Ethernet Hardware", RFC 826, MIT, November
      1982.

   3. "Generic Systems Requirements in support of Switched Multi-
      megabit Data Service", Technical Advisory TA-TSY-000772, Bellcore
      Technical Advisory, Issue 3, October 1989.

   4. Postel, J., and J. Reynolds, "A Standard for the Transmission of
      IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information
      Sciences Institute, February 1988.

   5. Katz, D., "A Proposed Standard for the Transmission of IP
      Datagrams over FDDI Networks", RFC 1188, Merit/NSFNET, October
      1990.

   6. Dix, F., Kelly, M., and R. Klessig, "Access to a Public Switched
      Multi-Megabit Data Service Offering", ACM SIGCOMM CCR, July 1990.

   7. Hemrick, C. and L. Lang, "Introduction to Switched Multi-megabit
      Data Service (SMDS), an Early Broadband Service", publication
      pending in the Proceedings of the XIII International Switching
      Symposium (ISS 90), May 27, 1990 - June 1, 1990.

   8. Institute of Electrical & Electronic Engineers, Inc. IEEE
      Standard 802.6, "Distributed Queue Dual Bus (DQDB) Subnetwork of
      a Metropolitan Area Network (MAN) Standard", December 1990.

   9. IEEE, "IEEE Standards for Local Area Networks: Logical Link
      Control", IEEE, New York, New York, 1985.

  10. IEEE, "Draft Standard P802.1A--Overview and Architecture", 1989.

  11. Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,
      USC/Information Sciences Institute, March 1990.

  12. Braden, R., and J. Postel, "Requirements for Internet Gateways",
      RFC 1009, USC/Information Sciences Institute, June 1987.

IP over SMDS Working Group

RFC 1209 IP and ARP over the SMDS Service March 1991

  13. Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
      Stanford University, August 1989.

  14. IEEE,"ANSI/IEEE Std 802.2-1985, ISO Draft International Standard
      8802/2", IEEE, New York, New York, 1985.

Security Considerations

  Security issues are not discussed in this memo.

Authors' Addresses

  Dave Piscitello
  Bell Communications Research
  331 Newman Springs Road
  Red Bank, NJ  07701

  Phone: (908) 758-2286

  EMail: [email protected]

  Joseph Lawrence
  Bell Communications Research
  331 Newman Springs Road
  Red Bank, NJ  07701

  Phone: (908) 758-4146

  EMail: [email protected]

IP over SMDS Working Group