RFC4540
Network Working Group M. Stiemerling Request for Comments: 4540 J. Quittek Category: Experimental NEC
C. Cadar
May 2006
NEC's Simple Middlebox Configuration (SIMCO) Protocol Version 3.0
Status of This Memo
This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
IESG Note
The content of this RFC was at one time considered by the IETF, and therefore it may resemble a current IETF work in progress or a published IETF work. This RFC is not a candidate for any level of Internet Standard. The IETF disclaims any knowledge of the fitness of this RFC for any purpose and in particular notes that the decision to publish is not based on IETF review for such things as security, congestion control, or inappropriate interaction with deployed protocols. The RFC Editor has chosen to publish this document at its discretion. Readers of this RFC should exercise caution in evaluating its value for implementation and deployment. See RFC 3932 RFC3932 for more information.
Abstract
This document describes a protocol for controlling middleboxes such as firewalls and network address translators. It is a fully compliant implementation of the Middlebox Communications (MIDCOM) semantics described in RFC 3989. Compared to earlier experimental versions of the SIMCO protocol, this version (3.0) uses binary message encodings in order to reduce resource requirements.
4.2.2. Message Sub-types for Requests and Positive
Contents
- 1 Introduction
- 2 Compliance with MIDCOM Protocol Semantics
- 3 SIMCO Sessions
- 4 SIMCO Message Components
- 4.1 Message Types
- 4.2 The SIMCO Header
- 4.3 The SIMCO Payload
- 4.3.1 SIMCO Protocol Version Attribute
- 4.3.2 Authentication Attributes
- 4.3.3 Middlebox Capabilities Attribute
- 4.3.4 Policy Rule Identifier Attribute
- 4.3.5 Group Identifier Attribute
- 4.3.6 Policy Rule Lifetime Attribute
- 4.3.7 Policy Rule Owner Attribute
- 4.3.8 Address Tuple Attribute
- 4.3.9 PRR Parameter Set Attribute
- 5 SIMCO Message Formats
- 6 Message Format Checking
- 7 Session Control Message Processing
- 8 Policy Rule Control Message Processing
- 9 Security Considerations
Introduction
The Simple Middlebox Configuration (SIMCO) protocol is used to control firewalls and Network Address Translators (NATs). As defined in RFC3234, firewalls and NATs are classified as middleboxes. A middlebox is a device on the datagram path between the source and destination that performs other functions than just IP routing. As outlined in RFC3303, firewalls and NATs are potential obstacles to packet streams, for example, if dynamically negotiated UDP or TCP port numbers are used, as in many peer-to-peer communication applications.
SIMCO allows applications to communicate with middleboxes on the datagram path in order to request a dynamic configuration at the middlebox that enables datagram streams to pass the middlebox. Applications can request pinholes at firewalls and address bindings at NATs.
The semantics for the SIMCO protocol are described in RFC3989.
Terminology
The terminology used in this document is fully aligned with the terminology defined in RFC3989. In the remainder of the text, the term SIMCO refers to SIMCO version 3.0. The term "prefix-length" is used as described in RFC4291 and RFC1519. With respect to wildcarding, the prefix length determines the part of an IP address that will be used in address match operations.
Binary Encodings
Previous experimental versions of SIMCO used simple ASCII encodings with augmented BNF for syntax specification. This encoding requires more resources than binary encodings do for generation and parsing of messages. This applies to resources for coding agents and middleboxes as well as to resources for executing a SIMCO stack.
Low resource requirements are important properties for two main reasons:
- For many applications (for example, IP telephony), session setup
times are critical. Users do accept setup times only up to some
limit, and some signaling protocols start retransmitting
messages if setup is not completed within a certain time.
- Many middleboxes are rather small and relatively low-cost
devices. For these, support of resource-intensive protocols
might be a problem. The acceptance of a protocol on these
devices depends, among other things, on the cost of implementing
the protocol and of its hardware requirements.
Therefore, we decided to use a simple and efficient binary encoding for SIMCO version 3.0, which is described in this document.
Compliance with MIDCOM Protocol Semantics
SIMCO version 3 is fully compliant with the MIDCOM protocol semantics defined by RFC3989. SIMCO implements protocol transactions as defined in Section 2.1.1 of RFC3989. All message types defined in Section 2.1.2 of RFC3989 are supported by SIMCO, and all mandatory transactions are implemented. SIMCO does not implement the optional group transactions. For all implemented transactions, SIMCO implements all parameters concerning the information contained.
SIMCO defines a few new terms to reference functionality in the semantics. Among these terms are Session Authentication (SA) and Policy Enable Rule After reservation (PEA) messages. SA is used to model the state transition given in Figure 2 of RFC3989 from NOAUTH to OPEN. PEA is used to model the state transition given in Figure 4 of RFC3989 from RESERVED to ENABLED.
SIMCO implements one additional transaction, the Policy Disable Rule (PDR) transaction, to those defined in RFC3989. PDR transactions are used by security functions such as intrusion detection and attack detection. They allow the agent to block a specified kind of traffic. PDRs have priority above Policy Enable Rules (PERs). When a PDR is established, all conflicting PERs (including PERs with just a partial overlap) are terminated, and no new conflicting PER can be established before the PDR is terminated. Support of the PDR transaction by SIMCO is optional. For a detailed description of the PDR transaction semantics, see Section 8.8.
SIMCO Sessions
The SIMCO protocol uses a session model with two parties: an agent representing one or more applications and a middlebox. Both parties may participate in multiple sessions. An agent may simultaneously communicate with several middleboxes using one session per middlebox. A middlebox may simultaneously communicate with several agents using one session per agent.
+-------+ SIMCO protocol +-----------+
| agent +------------------+ middlebox |
+-------+ +-----------+
Figure 1: Participants in a SIMCO session
SIMCO sessions must run over a reliable transport layer protocol and are initiated by the agent. SIMCO implementations must support TCP, while other reliable transport protocols can be used as transport for SIMCO as well. When using TCP as transport, middleboxes must wait for agents to connect on port 7626. This port is assigned to SIMCO servers by IANA (see http://www.iana.org/assignments/port-numbers). The session may be secured, if required, by either IPsec or TLS RFC4346 to guarantee authentication, message integrity and confidentiality. The use of IPsec is outlined in Section 9, "Security Considerations".
The transaction semantics of sessions is explained in RFC3989 Section 2.2.
SIMCO Message Components
All SIMCO messages from agent to middlebox and from middlebox to agent are sent over the transport protocol as specified in Section 3. SIMCO messages are Type-Length-Value (TLV) encoded using big endian (network ordered) binary data representations.
All SIMCO messages start with the SIMCO header containing message type, message length, and a message identifier. The rest of the message, the payload, contains zero, one, or more TLV message attributes.
Message Types
The message type in the SIMCO header is divided into a basic type and a sub-type. There are four basic types of SIMCO messages:
- request, - positive reply, - negative reply, - notification.
Messages sent from the agent to the middlebox are always of basic type 'request message', while the basic type of messages sent from the middlebox to the agent is one of the three other types. Request messages and positive and negative reply messages belong to request transactions. From the agent's point of view, notification messages belong to notification transactions only. From the middlebox's point of view, a notification message may also belong to a request transaction. See section 2.3.4. of RFC3989 for a detailed discussion of this issue.
The message sub-type gives further information on the message type within the context of the basic message type. Only the combination of basic type and sub-type clearly identify the type of a message.
The SIMCO Header
The SIMCO header is the first part of all SIMCO messages. It contains four fields: the basic message type, the message sub-type, the message length (excluding the SIMCO header) in octets, and the transaction identifier. The SIMCO header has a size of 64 bits. Its layout is defined in Figure 2.
Message Type _______________^_______________ / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Basic Type | Sub-Type | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transaction Identifier (TID) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: The SIMCO header
Basic Message Types
For the basic type field, the following values are defined:
0x01 : Request Message 0x02 : Positive Reply Message 0x03 : Negative Reply Message 0x04 : Notification Message
Message Sub-types for Requests and Positive Replies
For basic types 0x01 (request) and 0x02 (positive reply), a common set of values for the sub-type field is defined. Most of the sub- types can be used for both basic types. Restricted sub-types are marked accordingly.
0x01 : (SE) session establishment 0x02 : (SA) session authentication 0x03 : (ST) session termination 0x11 : (PRR) policy reserve rule 0x12 : (PER) policy enable rule 0x13 : (PEA) PER after reservation (request only) 0x14 : (PDR) policy disable rule 0x15 : (PLC) policy rule lifetime change 0x16 : (PRD) policy rule deletion (positive reply only) 0x21 : (PRS) policy rule status 0x22 : (PRL) policy rule list 0x23 : (PES) policy enable rule status (positive reply only) 0x24 : (PDS) policy disable rule status (positive reply only)
Message Sub-types for Negative Replies
For basic type 0x03 (negative reply message), the following values of the sub-type field are defined:
Replies concerning general message handling 0x10 : wrong basic request message type 0x11 : wrong request message sub-type 0x12 : badly formed request 0x13 : reply message too big
Replies concerning sessions 0x20 : request not applicable 0x21 : lack of resources 0x22 : protocol version mismatch 0x23 : authentication failed 0x24 : no authorization 0x25 : transport protocol problem
0x26 : security of underlying protocol layers insufficient
Replies concerning policy rules 0x40 : transaction not supported 0x41 : agent not authorized for this transaction 0x42 : no resources available for this transaction 0x43 : specified policy rule does not exist 0x44 : specified policy rule group does not exist 0x45 : not authorized for accessing specified policy 0x46 : not authorized for accessing specified group 0x47 : requested address space not available 0x48 : lack of IP addresses 0x49 : lack of port numbers 0x4A : middlebox configuration failed 0x4B : inconsistent request 0x4C : requested wildcarding not supported 0x4D : protocol type doesn't match 0x4E : NAT mode not supported 0x4F : IP version mismatch 0x50 : conflict with existing rule 0x51 : not authorized to change lifetime 0x52 : lifetime can't be extended 0x53 : illegal IP Address 0x54 : protocol type not supported 0x55 : illegal port number 0x56 : illegal number of subsequent ports (NOSP) 0x57 : already enable PID 0x58 : parity doesn't match
Message Sub-types for Notifications
For basic type 0x04, the following values of the sub-type field are defined:
0x01 : (BFM) badly formed message received 0x02 : (AST) asynchronous session termination 0x03 : (ARE) asynchronous policy rule event
Transaction Identifier
The transaction identifier (TID) is an arbitrary number identifying the transaction. In a request message, the agent chooses an agent- unique TID, such that the same agent never uses the same TID in two different request messages belonging to the same session. Reply messages must contain the same TID as the corresponding request message. In a notification message, the middlebox chooses a
middlebox-unique TID, such that the same middlebox never uses the same TID in two different notification messages belonging to the same session.
The SIMCO Payload
A SIMCO payload consists of zero, one, or more type-length-value (TLV) attributes. Each TLV attribute starts with a 16-bit type field and a 16-bit length field, as shown in Figure 3.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | attribute type | attribute length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | value +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Structure of TLV attribute
The attribute length field contains the length of the value field in octets.
The following attribute types are defined:
type description length ---------------------------------------------------- 0x0001 : SIMCO protocol version 32 bits 0x0002 : authentication challenge <= 4096 octets 0x0003 : authentication token <= 4096 octets 0x0004 : middlebox capabilities 64 bits 0x0005 : policy rule identifier 32 bits 0x0006 : group identifier 32 bits 0x0007 : policy rule lifetime 32 bits 0x0008 : policy rule owner <= 255 octets 0x0009 : address tuple 32, 96 or 192 bits 0x000A : PRR parameter set 32 bits 0x000B : PER parameter set 32 bits
SIMCO Protocol Version Attribute
The SIMCO protocol version attribute has a length of four octets. The first two octets contain the version number, one the major version number and the other the minor version number. Two remaining octets are reserved.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0001 | 0x0004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |major version #|minor version #| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Protocol version attribute
The SIMCO protocol specified within this document is version 3.0. The version numbers carried in the protocol version attribute are 3 for major version number and 0 for minor version number.
Authentication Attributes
The authentication challenge attribute and the authentication token attribute have the same format. Both contain a single value field with variable length. For both, the maximum length is limited to 4096 octets. Please note that the length of these attributes may have values that are not multiples of 4 octets. In case of an authentication challenge attribute, the value field contains an authentication challenge sent from one peer to the other, requesting that the other peer authenticate itself.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0002 | challenge length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | challenge +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Authentication challenge attribute
The authentication token attribute is used for transmitting an authentication token.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0003 | authentication length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | authentication token +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Authentication attribute
Middlebox Capabilities Attribute
The middlebox capabilities attribute has a length of eight octets.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0004 | 0x0008 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MB type |I|E|P|S|IIV|EIV| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | max policy rule lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Capabilities attribute
The first parameter field carries a bit field called MB type and provides information about the middlebox type. The following bits within the field are defined. The remaining ones are reserved.
0x80 : packet filter firewall 0x40 : network address translator 0x10 : support of PDR transaction 0x01 : port translation (requires 0x40 set) 0x02 : protocol translation (requires 0x40 set) 0x04 : twice NAT support (requires 0x40 set)
For middleboxes that implement combinations of NAT and firewalls, combinations of those flags are possible. For instance, for a Network Address and Port Translator (NAPT) with packet filter and PDR transaction support, the value of the MB type parameter field is 0xD1.
The following four parameters fields are binary flags with a size of one bit:
I : internal IP address wildcard support E : external IP address wildcard support P : port wildcard support S : persistent storage of policy rules
The supported IP version for the internal and external network are coded into the IIV (Internal IP version) and EIV (external IP version) parameter fields. They both have a size of two bits. Allowed values are 0x1 for IP version 4 (IPv4), 0x2 for IP version 6 (IPv6), and the combination of both (0x3) for IPv4 and IPv6 dual stack.
The next parameter field with a length of 16 bits is reserved.
The max policy rule lifetime parameter field specifies the maximum lifetime a policy rule may have.
Policy Rule Identifier Attribute
The policy rule identifier (PID) attribute contains an identifier of a policy rule. The identifier has a length of four octets.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0005 | 0x0004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | policy rule identifier (PID) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Policy rule identifier attribute
Group Identifier Attribute
The group identifier (GID) attribute contains an identifier of a policy rule group. The identifier has a length of four octets.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0006 | 0x0004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | group identifier (GID) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Group identifier attribute
Policy Rule Lifetime Attribute
The policy rule lifetime attribute specifies the requested or actual remaining lifetime of a policy rule, in seconds. Its value field contains a 32-bit unsigned integer.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0007 | 0x0004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | policy rule lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Policy rule lifetime attribute
Policy Rule Owner Attribute
The policy rule owner attribute specifies the authenticated agent that created and owns the policy rule. Its value field does not have a fixed length, but its length is limited to 255 octets. Please note that the length of this attribute may have values that are not multiples of 4 octets. The owner is set by the middlebox.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0008 | owner length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | owner +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Policy rule owner attribute
Address Tuple Attribute
The address tuple attribute contains a set of parameters specifying IP and transport addresses. The length of this attribute is 32, 96, or 192 bits.
The first parameter field has a length of 4 bits. It indicates whether the contained parameters specify just the used protocols or also concrete addresses. Defined values for this field are:
0x0 : full addresses 0x1 : protocols only
The second parameter field also has a length of 4 bits. It specifies the IP version number. Defined values for this field are:
0x1 : IPv4 0x2 : IPv6
The third parameter field has a length of 8 bits. It specifies a prefix length to be used for IP address wildcarding (see Section 1.1).
The fourth parameter field has also a length of 8 bits. It specifies the transport protocol. Defined values for this field are all values that are allowed in the 'Protocol' field of the IPv4 header RFC791 or in the 'Next Header field' of the IPv6 header RFC2460. The set of defined numbers for these fields is maintained by the Internet Assigned Numbers Authority (IANA) under the label 'PROTOCOL NUMBERS'.
The fifth parameter field has also a length of 8 bits. It specifies the location of the address. Defined values for this field are:
0x00 : internal (A0) 0x01 : inside (A1) 0x02 : outside (A2) 0x03 : external (A3)
Port and address wildcarding can only be used in PER and PEA transactions. The address tuple attribute carries a port number of 0 if the port should be wildcarded. For IPv4, a prefix length less than 0x20 is IP address wildcarding. For IPv6, a prefix length less than 0x80 is IP address wildcarding.
The port range field must be always greater than zero, but at least 1.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0009 | 0x0004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x1 |IP ver.| prefix length |trnsp. protocol| location | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0009 | 0x000C | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 | 0x1 | prefix length |trnsp. protocol| location | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | port number | port range | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0009 | 0x0018 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 | 0x2 | prefix length |trnsp. protocol| location | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | port number | port range | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + IPv6 address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Address tuple attributes
PRR Parameter Set Attribute
The policy reserve rule (PRR) parameter set attribute contains all parameters of the PRR request except the group identifier:
- NAT mode - port parity - requested inside IP version - requested outside IP version - transport protocol - port range
The attribute value field has a total size of 32 bits. It is sub- divided into six parameter fields.
The first parameter field, called NM, has a length of 2 bits and specifies the requested NAT mode of the middlebox at which a reservation is requested. Defined values for this field are:
01 : traditional 10 : twice
The second parameter field, called PP, has also a length of 2 bits. It specifies the requested port parity. Defined values for this field are:
00 : any 01 : odd 10 : even
The third and the fourth parameter fields are called IPi and IPo, respectively. Both have a length of 2 bits. They specify the requested version of the IP protocol at the inside (IPi) or outside (IPo) of the middlebox, respectively. Defined values for these fields are:
00 : any 01 : IPv4 10 : IPv6
The fifth parameter field has a length of 8 bits. It specifies the transport protocol for which the reservation should be made. A value of zero indicates that the reservation is requested for an IP address without specific selection of a protocol and a port number. Allowed non-zero values are the defined values for the 'protocol' field in the IPv4 header and IPv6 extension headers. The set of defined numbers for these fields is maintained by the Internet Assigned Numbers Authority (IANA) under the label 'PROTOCOL NUMBERS'.
The sixth parameter field has a length of 16 bits. It contains an unsigned integer specifying the length of the port range that should be supported. A value of 0xFFFF indicates that the reservation should be made for all port numbers of the specified transport protocol. A port range field with the value of 0x0001 specifies that only a single port number should be reserved. Values greater than one indicate the number of consecutive port numbers to be reserved. A value of zero is not valid for this field.
Please note that the wildcarding value 0xFFFF can only be used in the port range field in the PRR parameter set attribute. In the address tuple attribute, wildcarding of port numbers is specified by the port number field having a value of zero (see Section 4.3.8).
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x000A | 0x0004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |NM |PP |IPi|IPo|trnsp. protocol| port range | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: PRR parameter set attribute
4.3.10. PER Parameter Set Attribute
The policy enable rule (PER) parameter set attribute contains two parameters: the requested port parity, and the direction of the enabled data stream. The attribute value field has a total size of 32 bits, and it is sub-divided into 3 parameter fields.
The first parameter field has a length of 8 bits. It specifies the requested port parity. Defined values for this field are:
0x00 : any 0x03 : same
The second parameter field has a length of 8 bits. It specifies the direction of the enabled data stream. Defined values for this field are:
0x01 : inbound 0x02 : outbound 0x03 : bi-directional
The third parameter field has a length of 16 bits and is reserved.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x000B | 0x0004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | port parity | direction | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: PER parameter set attribute
SIMCO Message Formats
In the following, the formats of the different SIMCO message types are defined. The definitions are grouped into protocol error messages, session control messages, and policy rule control messages.
Protocol Error Replies and Notifications
When processing a received message, the middlebox might run into message processing problems before it can identify whether the message concerns session control or policy rule control. Also, it might not be possible to determine the message type, or it might detect a wrong message format. In these cases, the Badly Formed Message (BFM) notification or one of the following negative replies is sent:
0x0401 : BFM notification 0x0310 : wrong basic request message type 0x0311 : wrong request message sub-type 0x0312 : badly formed request
BFM Notification
The Badly Formed Message (BFM) notification message is sent from the middlebox to the agent after a message was received that does not comply to the SIMCO message format definition. The BFM notification has no attributes and contains the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 15: BFM notification structure
Protocol Error Negative Replies
Protocol error negative replies are sent from the middlebox to the agent if a message cannot be clearly interpreted, as it does not comply with any defined message format. Protocol error negative replies include 'wrong basic request message type' (0x0310), 'wrong request message sub-type' (0x0311), and 'badly formed request' (0x0312). These replies have no attributes. They consist of the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 16: Protocol error negative reply structure
Session Control Messages
Session control messages include the following list of message types (composed of basic type and sub-type):
0x0101 : SE request
0x0102 : SA request
0x0103 : ST request
0x0201 : SE positive reply
0x0202 : SA positive reply
0x0203 : ST positive reply
0x0310 : negative reply: wrong basic request message type
0x0311 : negative reply: wrong request message sub-type
0x0312 : negative reply: badly formed request
0x0320 : negative reply: request not applicable
0x0321 : negative reply: lack of resources
0x0322 : negative reply: protocol version mismatch
0x0323 : negative reply: authentication failed
0x0324 : negative reply: no authorization
0x0325 : negative reply: transport protocol problem
0x0326 : negative reply: security of underlying protocol layers
insufficient
0x0401 : BFM notification
0x0402 : AST notification
SE Request
The Session Establishment (SE) request message is sent from the agent to the middlebox to request establishment of a session. The SE request message contains one or two attributes: a mandatory SIMCO version number attribute and an optional authentication challenge attribute requesting that the middlebox authenticate itself.
+--------------------------+
| SIMCO header |
+--------------------------+
| SIMCO protocol version |
+--------------------------+
| authentication challenge | optional
+--------------------------+
Figure 17: Structure of SE request
SE Positive Reply
The Session Establishment (SE) reply message indicates completion of session establishment. It contains a single mandatory attribute: the middlebox capabilities attribute.
+--------------------------+
| SIMCO header |
+--------------------------+
| middlebox capabilities |
+--------------------------+
Figure 18: Structure of SE positive reply
SA Positive Reply
If the agent requested middlebox authentication, or if the middlebox wants the agent to authenticate itself, then the middlebox replies on the SE request with a Session Authentication (SA) reply message instead of an SE reply message. The SA reply message contains two optional attributes, but at least one of them needs to be present. The first one is an authentication challenge attribute requesting that the agent authenticate itself. The second one is an authentication token attribute authenticating the middlebox as the reply to an authentication request by the agent.
+--------------------------+
| SIMCO header |
+--------------------------+
| authentication challenge | optional
+--------------------------+
| authentication token | optional
+--------------------------+
Figure 19: Structure of SA positive reply
SA Request
The Session Authentication (SA) request message is sent from the agent to the middlebox after an initial SE request was answered by an SA reply. The SE request message contains one optional attribute: an authentication token attribute authenticating the agent as the response to an authentication challenge sent by the middlebox in an SA reply.
+--------------------------+
| SIMCO header |
+--------------------------+
| authentication token | optional
+--------------------------+
Figure 20: Structure of SA request
ST Request and ST Positive Reply
The Session Termination (ST) request message is sent from the agent to the middlebox to request termination of a session. The ST positive reply is returned, acknowledging the session termination. Both messages have no attributes and contain the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 21: Structure of ST request and positive reply
SE Negative Replies
There are nine different negative reply messages that can be sent from a middlebox to the agent if the middlebox rejects an SE request. Three of them are protocol error negative replies (0x031X) already covered in Section 4.1.2.
The remaining six negative replies are specific to session establishment. One of them, the 'protocol version mismatch' negative reply (0x0322), contains a single attribute: the protocol version attribute.
+--------------------------+
| SIMCO header |
+--------------------------+
| SIMCO protocol version |
+--------------------------+
Figure 22a: Structure of SE negative replies
The remaining three replies include 'request not applicable' (0x0320), 'lack of resources' (0x0321), 'authentication failed' (0x0323), 'no authorization' (0x0324), 'transport protocol problem' (0x0325), and 'security of underlying protocol layers insufficient' (0x0326). They consist of the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 22b: Structure of SE negative replies
AST Notification
The Asynchronous Session Termination (AST) notification message is sent from the middlebox to the agent, if the middlebox wants to terminate a SIMCO session. It has no attributes and contains the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 22a: Structure of AST notifications
Policy Rule Control Messages
Policy Rule control messages include the following list of message types (composed of basic type and sub-type):
0x0111 : PRR request 0x0112 : PER request 0x0113 : PEA request 0x0114 : PDR request 0x0115 : PLC request 0x0121 : PRS request 0x0122 : PRL request 0x0211 : PRR positive reply 0x0212 : PER positive reply 0x0214 : PDR positive reply 0x0215 : PLC positive reply 0x0216 : PRD positive reply 0x0221 : PRS positive reply 0x0223 : PES positive reply 0x0224 : PDS positive reply 0x0222 : PRL positive reply 0x0310 : negative reply: wrong basic request message type 0x0311 : negative reply: wrong request message sub-type 0x0312 : negative reply: badly formed request 0x0340 : negative reply: transaction not supported 0x0341 : negative reply: agent not authorized for this transaction 0x0342 : negative reply: no resources available for this
transaction
0x0343 : negative reply: specified policy rule does not exist
0x0344 : negative reply: specified policy rule group does not exist 0x0345 : negative reply: not authorized for accessing this policy 0x0346 : negative reply: not authorized for accessing specified
group
0x0347 : negative reply: requested address space not available 0x0348 : negative reply: lack of IP addresses 0x0349 : negative reply: lack of port numbers 0x034A : negative reply: middlebox configuration failed 0x034B : negative reply: inconsistent request 0x034C : negative reply: requested wildcarding not supported 0x034D : negative reply: protocol type doesn't match 0x034E : negative reply: NAT mode not supported 0x034F : negative reply: IP version mismatch 0x0350 : negative reply: conflict with existing rule 0x0351 : negative reply: not authorized to change lifetime 0x0352 : negative reply: lifetime can't be extended 0x0353 : negative reply: illegal IP Address 0x0354 : negative reply: protocol type not supported 0x0355 : negative reply: illegal port number 0x0356 : negative reply: illegal NOSP 0x0357 : negative reply: already enable PID 0x0358 : negative reply: parity doesn't match 0x0401 : negative reply: BFM notification 0x0403 : negative reply: ARE notification
Policy Events and Asynchronous Notifications
SIMCO maintains an owner attribute for each policy rule at the middlebox. Depending on the configuration of the middlebox, several agents may access the same policy rule; see also RFC3989, Sections 2.1.5 and 2.3.4.
To keep all agents synchronized about the state of their policy rules, SIMCO generates Asynchronous Rule Event (ARE) notifications. When an agent is reserving or enabling a policy rule, the middlebox sends an ARE to all agents that are authorized to access this policy rule. The middlebox sends an ARE to all agents authorized to access this policy rule when the rule lifetime is modified or if the rule is deleted.
PRR Request
The Policy Reserve Rule (PRR) request message is sent from the agent to the middlebox to request reservation of an IP address (and potentially also a range of port numbers) at the middlebox. Besides the SIMCO header, the request message contains two or three attributes. The first one is the PRR parameter set attribute specifying all parameters of the request except the requested policy
rule lifetime and the group identifier. The missing parameters are covered by the following two attributes. The last attribute, the group identifier, is optional.
+--------------------------+
| SIMCO header |
+--------------------------+
| PRR parameter set |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| group identifier | optional
+--------------------------+
Figure 23: Structure of PRR request
PER Request
The Policy Enable Rule (PER) request message is sent from the agent to the middlebox to request enabling of data communication between an internal and an external address. Besides the SIMCO header, the request message contains four or five attributes. The first one is the PER parameter set attribute specifying all parameters of the request except the internal address, the external address, the requested policy rule lifetime, and the group identifier. The missing parameters are covered by the following four attributes. Two address tuple parameters specify internal and external address tuples. Much like the PRR request, the last two attributes specify the requested lifetime and group identifier. The group identifier attribute is optional.
+--------------------------+
| SIMCO header |
+--------------------------+
| PER parameter set |
+--------------------------+
| address tuple (internal) |
+--------------------------+
| address tuple (external) |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| group identifier | optional
+--------------------------+
Figure 24: Structure of PER request
PEA Request
The Policy Enable rule After reservation (PEA) request message is sent from the agent to the middlebox to request enabling of data communication between an internal and an external address. It is similar to the PER request. There is just one difference. The optional group identifier attribute of the PER request is replaced by a mandatory policy rule identifier attribute referencing an already established policy reserve rule established by a PRR transaction.
+--------------------------+
| SIMCO header |
+--------------------------+
| PER parameter set |
+--------------------------+
| address tuple (internal) |
+--------------------------+
| address tuple (external) |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| policy rule identifier |
+--------------------------+
Figure 25: Structure of PEA request
The group identifier attribute is not included in the PEA request, since the group membership of the policy enable rule is inherited of the policy reserve rule.
PLC Request
The Policy Rule Lifetime Change (PLC) request message is sent from the agent to the middlebox to request a change of the remaining policy lifetime. Besides the SIMCO header, the request message contains two attributes specifying the policy rule to which the change should be applied and specifying the requested remaining lifetime.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| policy rule lifetime |
+--------------------------+
Figure 26: Structure of PLC request
PRS Request
The Policy Rule Status (PRS) request message is sent from the agent to the middlebox to request a report on the status of a specified policy rule. Besides the SIMCO header, the request message contains just one attribute specifying the policy rule for which the report is requested.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
Figure 27: Structure of PRS request
PRL Request
The Policy Rule List (PRL) request message is sent from the agent to the middlebox to request a list of all policy rules accessible to the agent. The message consists of the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 28: Structure of PRL request
PDR Request
The Policy Disable Rule (PDR) request message is sent from the agent to the middlebox to request a disable rule. The message consists of the SIMCO header, an internal address tuple, an external address tuple, and a lifetime attribute.
+--------------------------+
| SIMCO header |
+--------------------------+
| address tuple (internal) |
+--------------------------+
| address tuple (external) |
+--------------------------+
| policy rule lifetime |
+--------------------------+
Figure 29: Structure of PDR request
PRR Positive Reply
The Policy Reserve Rule (PRR) positive reply is sent after successful reservation of an address at the inside or outside of the middlebox. The message contains four mandatory attributes and an optional attribute: the policy rule identifier of the new policy reserve rule, the corresponding group identifier, the remaining lifetime of the policy rule, the reserved outside address tuple, and the optional reserved inside address tuple. The reserved inside address tuple is only returned when the middlebox is of type twice-NAT.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| group identifier |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| address tuple (outside) |
+--------------------------+
| address tuple (inside) | optional
+--------------------------+
Figure 30: Structure of PRR positive reply
5.3.10. PER Positive Reply
The Policy Enable Rule (PER) positive reply is sent after the middlebox successfully enables data transfer between an internal and an external address (by using a PER or PEA request message). The message contains five attributes: the policy rule identifier of the new policy enable rule, the corresponding group identifier, the remaining lifetime of the policy rule, the address tuple at the outside of the middlebox, and the address tuple at the inside of the middlebox.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| group identifier |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| address tuple (outside) |
+--------------------------+
| address tuple (inside) |
+--------------------------+
Figure 31: Structure of PER positive reply
5.3.11. PLC Positive Reply
The Policy Lifetime Change (PLC) positive reply is sent after the middlebox changes the lifetime of a policy rule to a positive (non- zero) value. The message contains just a single attribute: the remaining lifetime of the policy rule.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule lifetime |
+--------------------------+
Figure 32: Structure of PLC positive reply
5.3.12. PRD Positive Reply
The Policy Rule Deleted (PRD) positive reply is sent after the middlebox changes the remaining lifetime of a policy rule to zero, which means that it terminates the policy rule. The message consists of the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 33: Structure of PRD positive reply
5.3.13. PRS Positive Reply
The Policy Reserve Rule Status (PRS) positive reply is used for reporting the status of a policy reserve rule. The message format is identical with the format of the PRR positive reply except that it contains, in addition, a policy rule owner attribute.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| group identifier |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| address tuple (outside) |
+--------------------------+
| address tuple (inside) | optional
+--------------------------+
| policy rule owner |
+--------------------------+
Figure 34: Structure of PRS positive reply
5.3.14. PES Positive Reply
The Policy Enable Rule Status (PES) positive reply is used for reporting the status of a policy enable rule.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| group identifier |
+--------------------------+
| PER parameter set |
+--------------------------+
| address tuple (internal) |
+--------------------------+
| address tuple (inside) |
+--------------------------+
| address tuple (outside) |
+--------------------------+
| address tuple (external) |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| policy rule owner |
+--------------------------+
Figure 35: Structure of PES positive reply
5.3.15. PDS Positive Reply
The Policy Disable Rule Status (PDS) positive reply is used for reporting the status of a policy disable rule. The message contains five attributes: the policy rule identifier, the internal and external address tuples, the policy disable rule lifetime, and the policy rule owner.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| address tuple (internal) |
+--------------------------+
| address tuple (external) |
+--------------------------+
| policy rule lifetime |
+--------------------------+
| policy rule owner |
+--------------------------+
Figure 36: Structure of PDS positive reply
3.5.16. PRL Positive Reply
The Policy Rule List (PRL) positive reply is used for reporting the list of all established policy rules. The number of attributes of this message is variable. The message contains one policy rule identifier attribute per established policy rule.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| policy rule identifier |
+--------------------------+
| |
. . .
| |
+--------------------------+
| policy rule identifier |
+--------------------------+
Figure 37: Structure of PRL positive reply
5.3.17. PDR Positive Replies
The Policy Disable Rule (PDR) positive reply is sent after the middlebox successfully enables the policy disable rule and removal of conflicting policy rules. The message contains two attributes: the policy rule identifier of the new policy disable rule, and the remaining lifetime of the policy rule.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| policy rule lifetime |
+--------------------------+
Figure 38: Structure of PDR positive reply
5.3.18. Policy Rule Control Negative Replies
Session establishment negative replies are sent from the middlebox to the agent if a middlebox rejects a policy rule control request. Beyond protocol error replies, a number of policy rule control- specific negative reply messages that can be sent. They are listed at the beginning of Section 5.3. They all have no attributes. They consist of the SIMCO header only.
+--------------------------+
| SIMCO header |
+--------------------------+
Figure 39: Structure of Policy rule control negative replies
5.3.19. ARE Notification
The Asynchronous Policy Rule Event (ARE) notification message is sent from the middlebox to the agent. All agents participating in an open SIMCO session that are authorized to access this policy rule and are not explicitly requesting an action (i.e., reserving, enabling, and changing lifetime) receive such an ARE notification, when:
- a policy rule is deleted (lifetime attribute = 0)
- a policy rule is reserved (lifetime attribute = lifetime)
- a policy rule is enabled (lifetime attribute = lifetime)
- a policy rule's lifetime changed (lifetime attribute = lifetime)
Besides the SIMCO header, the request message contains two attributes specifying the policy rule that is concerned and the current lifetime.
+--------------------------+
| SIMCO header |
+--------------------------+
| policy rule identifier |
+--------------------------+
| policy rule lifetime |
+--------------------------+
Figure 40: Structure of ARE notification
Message Format Checking
This section describes common processing of all messages that are received by a middlebox.
1) When a message arrives at a middlebox, the header is checked for
consistency before the payload is processed.
o If the header checks fail, the middlebox sends a BFM
notification.
o If a session is already established, then the middlebox also
sends an AST notification and closes the connection.
2) The middlebox waits until it has received as many octets from the
agent as specified by the message length plus 8 octets (the length of the SIMCO header).
o If the middlebox is still waiting and does not receive any more
octets from the agent for 60 seconds, it sends a BFM
notification.
o If a session is already established, then the middlebox also
sends an AST notification and closes the connection after
sending the BFM notification; otherwise, it closes the
connection without sending another message.
3) After receiving a sufficient number of octets, the middlebox reads
the transaction identifier and the basic message type.
o If the value of the basic message type fields does not equal
0x01 (request message), then the middlebox stops processing the
message and sends a negative reply of type 'wrong basic request
message type' (0x0310) to the agent.
o If no session is established, then the middlebox closes the
connection after sending the 0x0310 reply.
4) Then the middlebox checks the message sub-type.
o If no session is established, then only sub-type 'session
establishment' (0x01) is accepted. For all other sub-types,
the middlebox sends a reply of type 'wrong request message
sub-type' (0x0311) to the agent and closes the connection after
sending the reply.
o If a session is already established, then the middlebox checks
if the message sub-type is one of the sub-types defined in
Section 4.2.2. (excluding 'session establishment' (0x01),
'session termination' (0x03), and 'policy rule
deletion'(0x15)).
o If not, then the middlebox stops processing the message and
sends a reply of type 'wrong request message sub-type'
(0x0311) to the agent.
5) Then the middlebox checks the TLV-structured attributes in the
message.
o If their type or number does not comply with the defined format
for this message type, the middlebox stops processing the
message and sends a reply of type 'badly formed request'
(0x0312) to the agent.
o If no session is established, then the middlebox closes the
connection after sending the 0x0312 reply.
6) After all message format checks are passed, the middlebox
processes the content of the attributes as described in the following sections.
Session Control Message Processing
For session control, the agent can send SE, SA, and ST request messages. The middlebox then sends per request a single reply back to the agent. Additionally, the middlebox may send unsolicited AST notifications.
Session State Machine
For each session, there is a session state machine illustrated by the figure below.
SE/BFM
SE/0x031X
SE/0x032X
+-------+
| v
+----------+
| CLOSED |----------------+
+----------+ |
| ^ ^ |
| | | SA/BFM | SE/SA
| | | SA/0x031X |
| | | SA/0x032X |
SE/SE | | | ST/ST v
| | | AST +----------+
| | +------------| NOAUTH |
| | +----------+
| | AST |
v | ST/ST | SA/SE
+----------+ |
| OPEN |<---------------+
+----------+
Figure 41: Session state machine
The figure illustrates all possible state transitions of a session. Request transactions (SE, SA, ST) are denoted by a descriptor of the request message, a '/' symbol, and a descriptor of the reply message. Notification transactions are denoted just by the a notification descriptor. For example, a successful SE transaction is denoted by 'SE/SE', and an AST notification is denoted by 'AST'.
Initially, all sessions are in state CLOSED. From there, a successful SE transaction can change its state either to NOAUTH or to OPEN. From state NOAUTH, a successful SA transaction changes session state to OPEN. A failed SA transaction changes session state from
NOAUTH back to CLOSED. Successful ST transactions and AST notifications change sessions from state NOAUTH or from state OPEN to state CLOSED.
A SIMCO session is established in state OPEN, which is the only state in which the middlebox accepts requests other than SE, SA, and ST.
Processing SE Requests
The SE request is only applicable if the session is in state CLOSED. If a session is in state NOAUTH or OPEN, then the middlebox sends a negative reply message of type 'request not applicable' (0x0320) to the agent, leaving the state of the session unchanged.
Before processing the content of the SE request message, the middlebox may check its resources and decide that available resources are not sufficient to serve the agent. In such a case, the middlebox returns a negative reply of type 'lack of resources' (0x0321) and closes the connection. Furthermore, the middlebox may decide to reject the SE request if the selected network connection and its protocol specific parameters are not acceptable for the middlebox. In such a case, the middlebox returns a negative reply of type 'transport protocol problem' (0x0325) and closes the connection. The middlebox returns a negative reply of type 'security of underlying protocol layers insufficient' (0x0326) and closes the connection, if the security properties of the network connection do not match the middlebox's requirements.
Processing of an SE request message starts with checking the major and minor protocol version number in the protocol version attribute. If the middlebox does not support the specified version number, then the middlebox returns a negative reply message of type 'protocol version mismatch' (0x0322) with the protocol version attribute indicating a version number that is supported by the middlebox. After sending this reply, the middlebox closes the connection.
If the agent is already sufficiently authenticated by means of the underlying network connection (for instance, IPsec or TLS), then the middlebox checks whether the agent is authorized to configure the middlebox. If it is not, the middlebox returns a negative reply of type 'no authorization' (0x0324) and closes the connection.
A positive reply on the SE request may be of sub-type SE or SA. An SE request is sent after both parties sufficiently authenticate and authorize each other. An SA reply message is sent if explicit authentication is requested by any party. The agent requests explicit authentication by adding an authentication challenge attribute to the SE request message. The middlebox requests explicit
authentication by returning an SA reply message with an authentication challenge attribute to the agent. If both parties request explicit authentication, then the SA reply message contains both an authentication challenge attribute for the agent and an authentication token attribute authenticating the middlebox.
If the SE request message contains an authentication challenge attribute, then the middlebox checks if it can authenticate itself. If yes, it adds a corresponding authentication token attribute to the SA reply. If it cannot authenticate based on the authentication challenge attribute, it adds an authentication token attribute to the SA reply message with a value field of length zero.
If the middlebox wants the agent to explicitly authenticate itself, then the middlebox creates an authentication challenge attribute for the agent and adds it to the SA reply message.
If the middlebox replies to the SE request message with an SA reply message, then the session state changes from CLOSED to NO_AUTH.
If the SE request message did not contain an authentication challenge attribute and if the middlebox does not request the agent to explicitly authenticate itself, then the middlebox sends an SE reply message in response to the SE request message. This implies that the session state changes from CLOSED to OPEN.
The SE reply message contains a capabilities attribute describing the middlebox capabilities.
Processing SA Requests
The SA request is only applicable if the session is in state NOAUTH. If a session is in state CLOSED or OPEN, then the middlebox sends a negative reply message of type 'request not applicable' (0x0320) to the agent. The state of the session remains unchanged.
After receiving an SA request message in state NOAUTH, the middlebox checks if the agent is sufficiently authenticated. Authentication may be based on an authentication token attribute that is optionally contained in the SA request message. If the agent is not sufficiently authenticated, then the middlebox returns a negative reply of type 'authentication failed' (0x0323) and closes the connection.
If authentication of the agent is successful, the middlebox checks if the agent is authorized to configure the middlebox. If not, the middlebox returns a negative reply of type 'no authorization' (0x0324) and closes the connection.
If authorization is successful, then the session state changes from NOAUTH to OPEN, and the agent returns an SE reply message that concludes session setup. The middlebox states its capabilities in the capability attribute contained in the SE reply message.
Processing ST Requests
The ST request is only applicable if the session is in state NOAUTH or OPEN. If a session is in state CLOSED, then the middlebox sends a negative reply message of type 'request not applicable' (0x0320) to the agent. The state of the session remains unchanged.
The middlebox always replies to a correct ST request with a positive ST reply. The state of the session changes from OPEN or from NOAUTH to CLOSED. After sending the ST reply, the middlebox closes the connection. Requests received after receiving the ST request and before closing the connection are ignored by the middlebox.
Generating AST Notifications
At any time, the middlebox may terminate an established session and change the session state from OPEN or from NOAUTH to CLOSED. Session termination is indicated to the agent by sending an AST notification.
Before sending the notification, the middlebox ensures that for all requests that have been processed, according replies are returned to the agent, such that the agent exactly knows the state of the middlebox at the time of session termination. After sending the AST notification, the middlebox sends no more messages to the agent, and it closes the connection.
Session Termination by Interruption of Connection
Section 2.2.4 of RFC3989 describes the session behavior when the network connection is interrupted. The behavior is defined for the middlebox (i.e., the SIMCO server) only and does not consider the behavior of the SIMCO agent in such an event.
If the SIMCO agent detects an interruption of the underlying network connection, it can terminate the session. The detection of the interrupted network connection can be done by several means, for instance, feedback of the operating system or a connection timeout. The definition of this detection mechanism is out of the scope of this memo.
Policy Rule Control Message Processing
For policy rule control and monitoring, the agent can send the PRR, PER, PEA, PLC, PRS, and PRL requests. The middlebox then sends a single reply message per request message back to the agent. Additionally, the middlebox may send unsolicited ARE notifications at any time.
The transaction semantics of policy rule control messages is explained in detail in RFC3989, Section 2.3.
For examples about protocol operation, see Section 4 of RFC3989.
Policy Rule State Machine
Policy rules are established by successful PRR, PEA, or PER transactions. Each time a policy rule is created, an unused policy rule identifier (PID) is assigned to the new policy rule. For each policy rule identifier, a state machine exists at the middlebox. The state machine is illustrated by the figure below.
PRR/PRR +---------------+
+----+ +-----------------+ PID UNUSED |<-+
| | | +---------------+ |
| v v PLC(lt=0)/ ^ | |
| +-------------+ PRD | | PER/PER | ARE(lt=0)
| | RESERVED +------------+ | | PLC(lt=0)/
| +-+----+------+ ARE(lt=0) v | PRD
| | | +---------------+ |
+----+ +---------------->| ENABLED +--+
PLC(lt>0)/ PEA/PER +-+-------------+
PLC | ^
+-----------+
lt = lifetime PLC(lt>0)/PLC
Figure 42: Policy rule state machine
The figure illustrates all possible state transitions of a PID and its associated policy. Successful configuration request transactions (PER, PRR, PEA, PLC) are denoted by a descriptor of the request message, a '/' symbol, and a descriptor of the reply message. Failed configuration request transactions are not displayed, because they do not change the PID state. Notification transactions are denoted just by the a notification descriptor. For example, a successful PRR request transaction is denoted by 'PRR/PRR', and an ARE notification
is denoted by 'ARE'. For PLC request transactions, the descriptor for the request message is extended by an indication of the value of the lifetime parameter contained in the message.
A successful PRR transaction (PRR/PRR) picks a PID in state UNUSED and changes the state to RESERVED. A successful PER transitions picks a PID in state UNUSED and changes the state to ENABLED. A PID in state RESERVED is changed to ENABLED by a successful PEA transaction. In state RESERVED or UNUSED, a successful PLC transaction with a lifetime parameter greater than zero does not change the PID's state. A successful PLC transaction with a lifetime parameter equal to zero changes the state of a PID from RESERVED to UNUSED or from ENABLED to UNUSED.
A failed request transaction does not change state at the middlebox.
An ARE notification transaction with the lifetime attribute set to zero has the same effect as a successful PLC transaction with a lifetime parameter equal to zero.
Processing PRR Requests
Processing PRR requests is much simpler on pure firewalls than on middleboxes with NAT functions. Therefore, this section has three sub-sections: The first one describes initial checks that are performed in any case. The second sub-section describes processing of PRR requests on pure firewalls, and the third one describes processing on all devices with NAT functions.
Initial Checks
When a middlebox receives a PRR request message, it first checks if the authenticated agent is authorized for requesting reservations. If not, it returns a negative reply message of type 'agent not authorized for this transaction' (0x0341).
If the request contains the optional group identifier, then the middlebox checks if the group already exists. If not, the middlebox returns a negative reply message of type 'specified policy rule group does not exist' (0x0344).
If the request contains the optional group identifier, then the middlebox checks if the authenticated agent is authorized for adding members to this group. If not, the middlebox returns a negative reply message of type 'not authorized for accessing specified group' (0x0346).
The middlebox may then check the PRR parameter set. A negative reply of type 'IP version mismatch' (0x034F) is returned if the IPi field does not match the inside IP version of the address at the middlebox. A negative reply of type 'IP version mismatch' (0x034F) is returned if the IPo field does not match the outside IP version of the address at the middlebox. The requested transport protocol type is checked, and a negative reply of type 'protocol type not supported' (0x0354) is returned if it is not supported. The middlebox may return a negative reply of type 'requested address space not available' (0x0347) if the requested address space is completely blocked or not supported by the middlebox in any way; for example, if a UDP port number is requested and all UDP packets are blocked by a middlebox acting as firewall.
The latter check at the middlebox is optional. If the check would fail and is not performed at this transaction, then two superfluous transactions will follow. First, the agent will send a request message for a corresponding PER transaction and will receive a negative reply on this. Second, either the agent will send a corresponding PLC request message with lifetime set to zero in order to delete the reservation, or the reservation will time out and the middlebox will send an ARE notification message with the lifetime attribute set to zero. Both transactions can be avoided if the middlebox initially performs this check.
A reason for avoiding this check might be its complexity. If the check is passed, the same check will have to be performed again for a subsequent corresponding PEA request. If processing two more transactions is considered to consume less resources than performing the check twice, it might be desirable not to perform it during the PRR transaction.
After checking the PRR parameter set, the middlebox chooses a lifetime value for the new policy rule to be created, which is greater than or equal to zero and less than or equal to the minimum of the requested value and the maximum lifetime specified by the middlebox capabilities attribute at session setup. Formally, the lifetime is chosen such that
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
holds, where 'lt_granted' is the actual lifetime chosen by the middlebox, 'lt_requested' is the lifetime requested by the agent, and 'lt_maximum' is the maximum lifetime specified during capability exchange at session setup.
If there are further sessions in state OPEN with authenticated agents authorized to access the policy rule, then to each of these agents a corresponding ARE notification with lifetime set to lt_granted is sent.
If the chosen lifetime is zero, the middlebox sends a negative reply of type 'middlebox configuration failed' (0x034A) to the agent.
Processing on Pure Firewalls
If the middlebox is configured as a pure firewall, then it accepts the request after the initial checks. It establishes a new policy reserve rule and assigns to it a policy rule identifier in state RESERVED. It generates a positive PRR reply and sets the attributes as specified below. No configuration of the firewall function is required.
The identifier chosen for the new policy rule is reported in the policy rule identifier attribute of the PRR reply.
If a group identifier attribute is contained in the PRR request, then the middlebox adds the new policy rule to the members of this group. If the PRR request does not contain a group identifier attribute, then the middlebox creates a new group with the new policy rule as the only member. In any case, the middlebox reports the group of which the new policy rule is a member in the group identifier attribute of the PRR reply.
The chosen lifetime is reported in the lifetime attribute of the PRR reply.
In the address tuple (outside) attribute of the PRR reply, the first parameter field is set to 'protocols only' (0x1). Consequently, the attribute has a length of 32 bits. The IP version parameter field is set according to the IPo parameter field in the PRR parameter set attribute of the PRR request message. The prefix length parameter field is set to 0x00, and the transport protocol parameter field in the address tuple (outside) attribute of the PRR reply is set identically to the transport protocol attribute in the PRR parameter set attribute of the PRR request message. The location parameter field is set to 'outside' (0x02).
Processing on Network Address Translators
If the middlebox is configured as a Network Address Translator (NAT), then it tries to reserve a NAT binding.
The middlebox first checks the PRR parameter set further if the NM (NAT mode) parameter matches its configuration. A negative reply of type 'NAT mode not supported' (0x034E) is returned by the middlebox if the configuration is not matched.
The following actions are performed, depending on the middlebox NAT type:
- traditional NAT A NAT binding at the outside (A2) with the requested transport protocol, external IP version, port range, and port parity is reserved.
- twice NAT A NAT binding at the outside (A2) with the requested transport protocol, external IP version, port range, and port parity is reserved. Furthermore, the middlebox reserves an inside (A1) NAT binding with the requested transport protocol, internal IP version, port range, and port parity.
The identifier chosen for the new policy rule is reported in the policy rule identifier attribute of the PRR reply.
After the checks are successfully performed, the middlebox establishes a new policy reserve rule, with the requested PRR parameter set, and assigns to it a policy rule identifier in state RESERVED. It generates a positive PRR reply and sets the attributes as specified below.
If a group identifier attribute is contained in the PRR request, then the middlebox adds the new policy rule to the members of this group. If the PRR request does not contain a group identifier attribute, then the middlebox creates a new group with the new policy rule as the only member. In any case, the middlebox reports the group of which the new policy rule is a member in the group identifier attribute of the PRR reply.
The chosen lifetime is reported in the lifetime attribute of the PRR reply.
In the address tuple (outside) attribute of the PRR reply, the first parameter field is set to 'full addresses' (0x0). The location parameter field is set to 'outside' (0x02). The IP version parameter
field is set according to the IPo parameter field in the PRR parameter set attribute of the PRR request message. For IPv4 addresses, the prefix length field is set to 0x20 to indicate a full address, and the reserved outside IPv4 address is set in the address field. For IPv6 addresses, the prefix length field is set to 0x80 to indicate a full address, and the reserved outside IPv6 address is set in the address field. The transport protocol parameter field in the address tuple (outside) attribute of the PRR reply is set identically to the transport protocol attribute in the PRR parameter set attribute of the PRR request message. The reserved outside base port number (i.e., the lowest port number of the allocated range) is stored in the port number parameter field, and the allocated port range is stored in the port range parameter field.
If the NM (NAT mode) parameter in the PRR parameter set attribute of the PRR request message has the value 'traditional', then the PRR reply message does not contain an address tuple (inside) attribute. If otherwise (it has the value 'twice'), then the PRR reply message contains an address tuple (inside) attribute. In the address tuple (inside) attribute of the PRR reply, the first parameter field is set to 'full addresses' (0x0). The location parameter field is set to 'inside' (0x01). The IP version parameter field is set according to the IPi parameter field in the PRR parameter set attribute of the PRR request message. For IPv4 addresses, the prefix length field is set to 0x20 to indicate a full address, and the reserved inside IPv4 address is set in the address field. For IPv6 addresses, the prefix length field is set to 0x80 to indicate a full address, and the reserved inside IPv6 address is set in the address field. The transport protocol parameter field in the address tuple (inside) attribute of the PRR reply is set identically to the transport protocol attribute in the PRR parameter set attribute of the PRR request message. The reserved inside base port number (i.e., the lowest port number of the allocated range) is stored in the port number parameter field, and the allocated port range is stored in the port range parameter field.
Processing PER Requests
Processing PER requests is much simpler on pure firewalls than on middleboxes with NAT functions. Therefore, this section has three sub-sections: The first one describes initial checks that are performed in any case. The second sub-section describes processing of PER requests on pure firewalls, and the third one describes processing on all devices with NAT functions.
Initial Checks
When a middlebox receives a PER request message, it first checks if the authenticated agent is authorized for requesting middlebox configurations for enabling communication. If not, it returns a negative reply message of type 'agent not authorized for this transaction' (0x0341).
If the request contains the optional group identifier, then the middlebox checks if the group already exists. If not, the middlebox returns a negative reply message of type 'specified policy rule group does not exist' (0x0344).
If the request contains the optional group identifier, then the middlebox checks if the authenticated agent is authorized for adding members to this group. If not, the middlebox returns a negative reply message of type 'not authorized for accessing specified group' (0x0346).
Then the middlebox checks the contained address tuple attributes.
If the first one does not have the location parameter field set to 'internal' (0x00), or if the second one does not have the location parameter field set to 'external' (0x03), then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If the transport protocol parameter field does not have the same value in both address tuple attributes, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If both address tuple attributes contain a port range parameter field, if both port range parameter fields have values not equal to 0xFFFF, and if the values of both port range parameter fields are different, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
Then the agent checks if wildcarding is requested and if the requested wildcarding is supported by the middlebox. Wildcarding support may be different for internal address tuples and external address tuples. The following parameter fields of the address tuple attribute can indicate wildcarding:
- the first parameter field If it is set to 'protocols only' (0x1), then IP addresses and port numbers are completely wildcarded.
- the transport protocol field If it is set to 0x00, then the transport protocol is completely wildcarded. Please note that a completely wildcarded transport protocol might still support only a limited set of transport protocols according to the capabilities of the middlebox. For example, a typical NAT implementation may apply transport wildcarding to UDP and TCP transport only. Wildcarding the transport protocol implies wildcarding of port numbers. If this field is set to 0x00, then the values of the port number field and the port range field are irrelevant.
- the prefix length field If the IP version number field indicates IPv4 and the value of this field is less than 0x20, then IP addresses are wildcarding according to this prefix length. If the IP version number field indicates IPv6 and the value of this field is less than 0x80, then IP addresses are wildcarding according to this prefix length. If the first parameter field is set to 'protocols only' (0x1), then the value of the prefix length field is irrelevant.
- the port number field If it is set to zero, then port numbers are completely wildcarded. In this case, the value of the port range field is irrelevant.
If any of these kinds of wildcarding is used, and if this is in conflict with wildcarding support for internal or external addresses of the middlebox, then the middlebox returns a negative reply message of type 'requested wildcarding not supported' (0x034C).
Please note that the port range field cannot be used for wildcarding. If it is set to a value greater than one, then middlebox configuration is requested for all port numbers in the interval starting with the specified port number and containing as many consecutive port numbers as specified by the parameter.
If the direction parameter field in the PER parameter set attribute has the value 'bi-directional', then only transport protocol wildcarding is allowed. If any other kind of wildcarding is specified in one or both of the IP address tuple attributes, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If the PER request conflicts with any policy disable rule (see Section 8.8.1), then the middlebox returns a negative reply message of type 'conflict with existing rule' (0x0350).
After checking the address tuple attributes, the middlebox chooses a lifetime value for the new policy rule to be created, which is greater than or equal to zero and less than or equal to the minimum of the requested value and the maximum lifetime specified by the middlebox capabilities attribute at session setup. Formally, the lifetime is chosen such that
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
holds, where 'lt_granted' is the actual lifetime chosen by the middlebox, 'lt_requested' is the lifetime requested by the agent, and 'lt_maximum' is the maximum lifetime specified during capability exchange at session setup.
If there are further sessions in state OPEN with authenticated agents authorized to access the policy rule, then to each of these agents a corresponding ARE notification with lifetime set to lt_granted is sent.
If the chosen lifetime is zero, the middlebox sends a negative reply of type 'middlebox configuration failed' (0x034A) to the agent.
Processing on Pure Firewalls
If the middlebox is acting as a pure firewall, then it tries to configure the requested pinhole. The firewall configuration ignores the port parity parameter field in the PER parameter set attribute, but it considers the direction parameter field in this attribute. The pinhole is configured such that communication between the specified internal and external address tuples is enabled in the specified direction and covering the specified wildcarding. If the configuration fails (for example, because the pinhole would conflict with high-level firewall policies), then the middlebox returns a negative reply message of type 'middlebox configuration failed' (0x034A).
If the configuration was successful, the middlebox establishes a new policy enable rule and assigns to it a policy rule identifier in state ENABLED. It generates a positive PER reply and sets the attributes as specified below.
The identifier chosen for the new policy rule is reported in the policy rule identifier attribute of the PER reply.
If a group identifier attribute is contained in the PER request, then the middlebox adds the new policy rule to the members of this group. If the PRR request does not contain a group identifier attribute, then the middlebox creates a new group with the new policy rule as
the only member. In any case, the middlebox reports the group of which the new policy rule is a member in the group identifier attribute of the PER reply.
The chosen lifetime is reported in the lifetime attribute of the PER reply.
The address tuple (internal) attribute of the PER request is reported as address tuple (outside) attribute of the PER reply. The address tuple (external) attribute of the PER request is reported as address tuple (inside) attribute of the PER reply.
Processing on Network Address Translators
If the middlebox is configured as a NAT, then it tries to configure the requested NAT binding. The actions taken by the NAT are quite similar to the actions of the Policy Reserve Rule (PRR) request, but in the PER request a NAT binding is enabled.
The following actions are performed, depending on the middlebox NAT type:
- traditional NAT A NAT binding is established between the internal and external address tuple with the requested transport protocol, port range, direction, and port parity. The outside address tuple is created.
- twice NAT A NAT binding is established between the internal and external address tuple with the requested transport protocol, port range, and port parity. But two address tuples are created: an outside address tuple and an inside address tuple.
Should the configuration fail in either NAT case, a negative reply 'middlebox configuration failed' (0x034A) is returned.
If the configuration was successful, the middlebox establishes a new policy enable rule and assigns to it a policy rule identifier in state ENABLED. It generates a positive PER reply and sets the attributes as specified below.
The identifier chosen for the new policy rule is reported in the policy rule identifier attribute of the PER reply.
If a group identifier attribute is contained in the PER request, then the middlebox adds the new policy rule to the members of this group. If the PRR request does not contain a group identifier attribute,
then the middlebox creates a new group with the new policy rule as the only member. In any case, the middlebox reports the group of which the new policy rule is a member in the group identifier attribute of the PER reply.
The chosen lifetime is reported in the lifetime attribute of the PER reply.
In the address tuple (outside) attribute of the PER reply, the first parameter field is set to 'full addresses' (0x0). The location parameter field is set to 'outside' (0x02). The IP version parameter field is set according to the IP version parameter field in the PER parameter set attribute of the PER request message. For IPv4 addresses, the prefix length field is set to 0x20 to indicate a full address, and the reserved outside IPv4 address is set in the address field. For IPv6 addresses, the prefix length field is set to 0x80 to indicate a full address, and the reserved outside IPv6 address is set in the address field. The transport protocol parameter field in the address tuple (outside) attribute of the PER reply is set identically to the transport protocol attribute in the PER parameter set attribute of the PER request message. The reserved outside base port number (i.e., the lowest port number of the allocated range) is stored in the port number parameter field, and the allocated port range is stored in the port range parameter field.
The address tuple (inside) is only returned if the middlebox is a twice NAT; otherwise, it is omitted. In the address tuple (inside) attribute of the PER reply, the first parameter field is set to 'full addresses' (0x0). The location parameter field is set to 'inside' (0x01). The IP version parameter field is set according to the IP version parameter field in the PER parameter set attribute of the PER request message. For IPv4 addresses, the prefix length field is set to 0x20 to indicate a full address, and the reserved inside IPv4 address is set in the address field. For IPv6 addresses, the prefix length field is set to 0x80 to indicate a full address, and the reserved inside IPv6 address is set in the address field. The transport protocol parameter field in the address tuple (inside) attribute of the PER reply is set identically to the transport protocol attribute in the PER parameter set attribute of the PER request message. The reserved inside base port number (i.e., the lowest port number of the allocated range) is stored in the port number parameter field, and the allocated port range is stored in the port range parameter field.
Processing on Combined Firewalls and NATs
Middleboxes that are combinations of firewalls and NATs are configured in such a way that first the NAT bindings are configured and afterwards the firewall pinholes. This sequence is needed since the firewall rules must be configured according to the outside address tuples and for twice NATs the inside address tuples as well. This aspect of middlebox operation may be irrelevant to SIMCO, since some NATs already do firewall configuration on their own.
Processing PEA Requests
Processing PEA requests is much simpler on pure firewalls than on middleboxes with NAT functions. Therefore, this section has three sub-sections: The first one describes initial checks that are performed in any case. The second sub-section describes processing of PEA requests on pure firewalls, and the third one describes processing on all devices with NAT functions.
Initial Checks
When a middlebox receives a PEA request message, it first checks if the authenticated agent is authorized for requesting middlebox configurations for enabling communication. If not, it returns a negative reply message of type 'agent not authorized for this transaction' (0x0341).
Then the middlebox checks the policy rule identifier attribute contained in the PEA message. If no policy rule with this identifier exists, then the middlebox returns a negative reply message of type 'specified policy rule does not exist' (0x0343). If there exists a policy with this identifier and if it is in a state other than RESERVED, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If a policy rule with this identifier exists, but the authenticated agent is not authorized for terminating this policy reserve rule, then the middlebox returns a negative reply message of type 'agent not authorized for accessing this policy' (0x0345).
Then the middlebox checks the contained address tuple attributes.
If the first one does not have the location parameter field set to 'internal' (0x00) or if the second one does not have the location parameter field set to 'external' (0x03), then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If the transport protocol parameter field does not have the same value in both address tuple attributes, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If both address tuple attributes contain a port range parameter field, if both port range parameter fields have values not equal to 0xFFFF, and if the values of both port range parameter fields are different, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
Then the agent checks if wildcarding is requested and if the requested wildcarding is supported by the middlebox. Wildcarding support may be different for internal address tuples and external address tuples. The following parameter fields of the address tuple attribute can indicate wildcarding:
- the first parameter field If it is set to 'protocols only' (0x1), then IP addresses and port numbers are completely wildcarded.
- the transport protocol field If it is set to 0x00, then IP the transport protocol is completely wildcarded. Please note that a completely wildcarded transport protocol might still support only a limited set of transport protocols according to the capabilities of the middlebox. For example, a typical NAT implementation may apply transport wildcarding to UDP and TCP transport only.
- the prefix length field If the IP version number field indicates IPv4 and the value of this field is less than 0x20, then IP addresses are wildcarding according to this prefix length. If the IP version number field indicates IPv6 and the value of this field is less than 0x80, then IP addresses are wildcarding according to this prefix length. If the first parameter field is set to 'protocols only' (0x1), then the value of the prefix length field is irrelevant.
- the port number field If it is set to zero, then port numbers are completely wildcarded.
- the port range field If it is set to a value greater than one, then port numbers are wildcarded within an interval starting with the specified port number and containing as many consecutive port numbers as specified by the parameter.
If any of these kinds of wildcarding is used, and if this is in conflict with wildcarding support for internal or external addresses of the middlebox, then the middlebox returns a negative reply message of type 'requested wildcarding not supported' (0x034C).
If the PEA request conflicts with any policy disable rule (see Section 8.8.1), then the middlebox returns a negative reply message of type 'conflict with existing rule' (0x0350).
After checking the address tuple attributes, the middlebox chooses a lifetime value for the new policy enable rule to be created, which is greater than or equal to zero and less than or equal to the minimum of the requested value and the maximum lifetime specified by the middlebox capabilities attribute at session setup. Formally, the lifetime is chosen such that
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
holds, where 'lt_granted' is the actual lifetime chosen by the middlebox, 'lt_requested' is the lifetime requested by the agent, and 'lt_maximum' is the maximum lifetime specified during capability exchange at session setup.
If there are further sessions in state OPEN with authenticated agents authorized to access the policy rule, then to each of these agents a corresponding ARE notification with lifetime set to lt_granted is sent.
If the chosen lifetime is zero, the middlebox sends a negative reply of type 'middlebox configuration failed' (0x034A) to the agent.
Processing on Pure Firewalls
If the middlebox is configured as a pure firewall, then it tries to configure the requested pinhole. The firewall configuration ignores the port parity parameter field in the PER parameter set attribute, but it considers the direction parameter field in this attribute. The pinhole is configured such that communication between the specified internal and external address tuples is enabled in the specified direction and covering the specified wildcarding. If the configuration fails, then the middlebox returns a negative reply message of type 'middlebox configuration failed' (0x034A).
If the configuration was successful, the middlebox replaces the policy reserve rule referenced by the policy rule identifier attribute in the PEA request message with a new policy enable rule. The policy enable rule re-uses the policy rule identifier of the replaced policy reserve rule. The state of the policy rule
identifier changes from RESERVED to ENABLED. The policy reserve rule is a member of the same group as the replaced policy reserve rule was.
Then the middlebox generates a positive PER reply and sets the attributes as specified below.
The identifier chosen for the new policy rule is reported in the policy rule identifier attribute of the PER reply.
The group identifier is reported in the group identifier attribute of the PER reply.
The chosen lifetime is reported in the lifetime attribute of the PER reply.
The address tuple (internal) attribute of the PER request is reported as the address tuple (outside) attribute of the PER reply. The address tuple (external) attribute of the PER request is reported as the address tuple (inside) attribute of the PER reply.
Processing on Network Address Translators
If the middlebox is configured as a NAT, then it tries to configure the requested NAT binding, i.e., enabling the already reserved binding. The already reserved NAT binding from the PRR request is now enabled in the middlebox.
If the enable configuration was successful, the middlebox replaces the policy reserve rule referenced by the policy rule identifier attribute in the PEA request message with a new policy enable rule. The policy enable rule re-uses the policy rule identifier of the replaced policy reserve rule. The state of the policy rule identifier changes from RESERVED to ENABLED. The policy reserve rule is a member of the same group as the replaced policy reserve rule was.
Then the middlebox generates a positive PER reply and sets the attributes as specified below.
The reserved outside address tuple is reported as the address tuple (outside) attribute of the PER reply. The reserved inside address tuple is reported as the address tuple (inside) attribute of the PER reply. Both reserved outside and inside address tuples are taken from the reserve policy rule generated during the PRR transaction.
Processing PLC Requests
When a middlebox receives a PLC request message, it first checks if the authenticated agent is authorized for requesting policy rule lifetime changes. If not, it returns a negative reply message of type 'agent not authorized for this transaction' (0x0341).
Then the middlebox checks the policy rule identifier attribute contained in the PLC message. If no policy rule with this identifier exists, then the middlebox returns a negative reply message of type 'specified policy rule does not exist' (0x0343).
If a policy rule with this identifier exists, but the authenticated agent is not authorized for changing the lifetime of this policy rule, then the middlebox returns a negative reply message of type 'agent not authorized for accessing this policy' (0x0345).
Then the middlebox chooses a lifetime value for the new policy rule, which is greater than zero and less than or equal to the minimum of the requested value and the maximum lifetime specified by the middlebox capabilities attribute at session setup. Formally, the lifetime is chosen such that
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
holds, where 'lt_granted' is the actual lifetime chosen by the middlebox, 'lt_requested' is the lifetime requested by the agent, and 'lt_maximum' is the maximum lifetime specified during capability exchange at session setup. This procedure implies that the chosen lifetime is zero if the requested lifetime is zero.
If the chosen lifetime is greater than zero, the middlebox changes the lifetime of the policy rule to the chosen value and generates a PLC reply message. The chosen lifetime is reported in the lifetime attribute of the message.
If otherwise (the chosen lifetime is zero), then the middlebox terminates the policy rule and changes the PID state from ENABLED or RESERVED, respectively, to UNUSED.
The middlebox generates a PRD reply message and sends it to the requesting agent. If there are further sessions in state OPEN with authenticated agents authorized to access the policy rule, then to each of these agents a corresponding ARE notification with lifetime set to zero is sent.
Processing PRS Requests
When a middlebox receives a PRS request message, it first checks if the authenticated agent is authorized for receiving policy status information. If not, it returns a negative reply message of type 'agent not authorized for this transaction' (0x0341).
Then the middlebox checks the policy rule identifier attribute contained in the PRS message. If no policy rule with this identifier exists in state RESERVED or ENABLED, then the middlebox returns a negative reply message of type 'specified policy rule does not exist' (0x0343).
If a policy rule with this identifier exists, but the authenticated agent is not authorized to receive status information for this policy rule, then the middlebox returns a negative reply message of type 'agent not authorized for accessing this policy' (0x0345).
If the checks described above are passed, the middlebox accepts the requests and generates a reply. If the policy rule for which status information is requested is in state RESERVED, then a PRS reply is generated and sent to the agent. If otherwise (the policy rule is in state ENABLED), then a PES reply is generated and sent to the agent. For policy disable rules, a PDS reply is generated and sent to the agent.
In both message formats, the lifetime attribute reports the current remaining lifetime of the policy rule, and the owner attribute reports the owner of the policy rule for which status information is requested.
The PRS reply message format is identical to the PRR reply message format except for an appended owner attribute. In the PRS reply, the attributes that are common with the PRR reply (except for the lifetime attribute) have exactly the same values as the corresponding attributes of the PRR reply that was sent as part of the PRR transaction that established the policy reserve rule.
In the PES reply message, the PER parameter set attribute, the address tuple (internal) attribute, and the address tuple (external) attribute have exactly the same values as the corresponding attributes of the PER or PEA request that were sent as part of the corresponding transaction that established the policy enable rule.
In the PES reply message, the policy rule identifier attribute, the group identifier attribute, the address tuple (inside) attribute, and the address tuple (outside) attribute have exactly the same values as the corresponding attributes of the PER reply that was sent as part of the PER or PEA transaction that established the policy enable rule.
In the PDS reply message, the policy rule identifier attribute, the address tuple (internal) attribute, and the address tuple (external) attribute have exactly the same values as the corresponding attributes of the PDR request message.
This transaction does not change the state of any policy rule.
Processing PRL Requests
When a middlebox receives a PRL request message, it first checks if the authenticated agent is authorized for receiving policy information. If not, it returns a negative reply message of type 'agent not authorized for this transaction' (0x0341).
Then the middlebox generates a PRL reply message. For each policy rule at the middlebox in state RESERVED or ENABLED that the authenticated agent can access, a policy rule identifier attribute is generated and added to the PRL reply message before the message is sent to the agent. A negative reply message of type 'reply message too big' (0x0313) is generated if the number of policy rule attributes to be returned exceeds the maximum transport unit size of the underlying network connection or the maximum length of a SIMCO message. The total size of a SIMCO message is limited to 65,536 octets in total (see Section 4.2 for the SIMCO header).
This transaction does not change the state of any policy rule.
Processing PDR requests
Processing of PDR requests is structured into five sub-sections. The first one describes the general extension of the MIDCOM protocol semantics by PDR. The second sub-section describes the initial checks that are performed in any case. The third sub-section describes the processing of PDR requests on pure firewalls. The fourth one describes processing on devices with NATs, and the fifth describes processing of devices with combined firewall and NAT functions.
Extending the MIDCOM semantics
The Policy Disable Rule (PDR) extends the MIDCOM protocol semantics RFC3989 by another policy rule type. The PDR is intended to be used for dynamically blocking unwanted traffic, particularly in case of an attack, for example, a distributed denial of service attack.
PDR requests follow the same ownership concept as all other transactions do (see RFC3989, Section 2.1.5). However, PDR prioritization over PERs is independent of ownership. A PDR always overrules a conflicting PER, even if the respective owners are different. Typically, only a highly privileged agent will be allowed to issue PDR requests.
A PDR rule and PER rule conflict with each other if their address tuples overlap such that there exists at least one IP packet that matches address tuple A0 of both rules in the internal network and that matches address tuple A3 of both rules in the external network. Note that the packet may be translated from the internal to external network, or vice versa.
Let's assume, for instance, that a policy enable rule (PER) enables all traffic from any external host using any UDP port to a certain UDP port of a certain internal host:
PER A3={ any external IP address, UDP, any port }
PER A0={ internal IP address 10.1.8.3, UDP, port 12345 }
Then this conflicts with a policy disable rule (PDR) blocking all UDP traffic from a potentially attacking host:
PDR A3={ external IP address 192.0.2.100, UDP, any port }
PDR A0={ any internal IP address, UDP, any port }
If a new PDR is established, then all conflicting PERS are terminated immediately. A new PER can only be established if it does not conflict with any already existing PDR.
Initial Checks
When a middlebox receives a PDR request message, it first checks if the authenticated agent is authorized for requesting middlebox configurations for disabling communication. If not, it returns a negative reply message of type 'agent not authorized for this transaction' (0x0341).
Then the middlebox checks the contained address tuple attributes.
If the first one does not have the location parameter field set to 'internal' (0x00), or if the second one does not have the location parameter field set to 'external' (0x03), then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If the transport protocol parameter field does not have the same value in both address tuple attributes, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
If both address tuple attributes contain a port range parameter field, if both port range parameter fields have values not equal to 0xFFFF, and if the values of both port range parameter fields are different, then the middlebox returns a negative reply message of type 'inconsistent request' (0x034B).
Then the agent checks if wildcarding is requested and if the requested wildcarding is supported by the middlebox. Wildcarding support may be different for internal address tuples and external address tuples. The following parameter fields of the address tuple attribute can indicate wildcarding:
- the first parameter field If it is set to 'protocols only' (0x1), then IP addresses and port numbers are completely wildcarded.
- the transport protocol field If it is set to 0x00, then the transport protocol is completely wildcarded. Please note that a completely wildcarded transport protocol might still support only a limited set of transport protocols according to the capabilities of the middlebox. For example, a typical NAT implementation may apply transport wildcarding to UDP and TCP transport only. Wildcarding the transport protocol implies wildcarding of port numbers. If this field is set to 0x00, then the values of the port number field and the port range field are irrelevant.
- the prefix length field If the IP version number field indicates IPv4 and the value of this field is less than 0x20, then IP addresses are wildcarding according to this prefix length. If the IP version number field indicates IPv6 and the value of this field is less than 0x80, then IP addresses are wildcarding according to this prefix length. If the first parameter field is set to 'protocols only' (0x1), then the value of the prefix length field is irrelevant.
- the port number field If it is set to zero, then port numbers are completely wildcarded. In this case, the value of the port range field is irrelevant.
If any of these kinds of wildcarding is used, and if this is in conflict with wildcarding support for internal or external addresses of the middlebox, then the middlebox returns a negative reply message of type 'requested wildcarding not supported' (0x034C).
Please note that the port range field cannot be used for wildcarding. If it is set to a value greater than one, then middlebox configuration is requested for all port numbers in the interval starting with the specified port number and containing as many consecutive port numbers as specified by the parameter.
The specified policy disable rule is activated, and the middlebox will terminate any conflicting policy enable rule immediately. Conflicts are defined in Section 8.8.1. Agents with open sessions that have access to the policy rules to be terminated are notified via the ARE notification.
The middlebox will reject all requests for new policy enable rules that conflict with the just established PDR as long as the PDR is not terminated. In such a case, a negative 'conflict with existing rule' (0x0350) reply will be generated.
After checking the address tuple attributes, the middlebox chooses a lifetime value for the new policy rule to be created, which is greater than or equal to zero and less than or equal to the minimum of the requested value and the maximum lifetime specified by the middlebox capabilities attribute at session setup. Formally, the lifetime is chosen such that
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
holds, where 'lt_granted' is the actual lifetime chosen by the middlebox, 'lt_requested' is the lifetime requested by the agent, and 'lt_maximum' is the maximum lifetime specified during capability exchange at session setup.
If there are further sessions in state OPEN with authenticated agents authorized to access the policy rule, then to each of these agents a corresponding ARE notification with lifetime set to lt_granted is sent.
If the chosen lifetime is zero, the middlebox sends a negative reply of type 'middlebox configuration failed' (0x034A) to the agent.
Processing on Pure Firewalls
If the middlebox is acting as a pure firewall, then it tries to configure the requested disable rule, i.e., configuring a blocking rule at the firewall. The disable rule is configured such that communication between the specified internal and external address tuples is blocked, covering the specified wildcarding. If the configuration fails (for example, because the blocking rule would conflict with high-level firewall policies), then the middlebox returns a negative reply message of type 'middlebox configuration failed' (0x034A).
If the configuration was successful, the middlebox establishes a new policy disable rule and assigns to it a policy rule identifier in state ENABLED. It generates a positive PDR reply and sets the attributes as specified below.
The identifier chosen for the new policy rule is reported in the policy rule identifier attribute of the PDR reply.
The chosen lifetime is reported in the lifetime attribute of the PDR reply.
Processing on Network Address Translators
If the middlebox is configured as a NAT, then it tries to block the specified address tuple in the NAT. The mechanisms used for this depend on the implementation and capabilities of the NAT.
Should the configuration fail in either NAT case, a negative reply 'middlebox configuration failed' (0x034A) is returned.
If the configuration was successful, the middlebox establishes a new policy disable rule and assigns to it a policy rule identifier in state ENABLED. It generates a positive PDR reply and sets the attributes as specified below.
The identifier chosen for the new policy rule is reported in the policy rule identifier attribute of the PDR reply.
The chosen lifetime is reported in the lifetime attribute of the PDR reply.
Processing on Combined Firewalls and NATs
Middleboxes that are combinations of firewall and NAT are configured in such a way that first the firewall is configured with the blocking rule and afterwards the NAT is configured to block the address tuple. This aspect of middlebox operation may be irrelevant to SIMCO, since some NATs already do firewall configuration on their own.
Generating ARE Notifications
At any time, the middlebox may terminate a policy rule by deleting the configuration of the rule and by changing the corresponding PID state from ENABLED or from RESERVED, respectively, to UNUSED.
For each session in state OPEN with authenticated agents authorized to access the policy rule, the middlebox generates a corresponding ARE notification with the lifetime attribute set to zero and sends it to the respective agent. The identifier of the terminated policy rule is reported in the policy rule identifier attribute of the ARE notification.
After sending the notification, the middlebox will consider the policy rule non-existent. It will not process any further transaction on this policy rule.
In the case of PRR, PER, PEA, and PLC (reserving and enabling policy rules and changes of the lifetime), the middlebox generates an ARE notification after processing the request. This ARE notification is generated for each session in state OPEN with authenticated agents (other than the requesting agent) who are authorized to access the policy rule. Through this ARE notification all other agents are kept synchronized with the latest state of the policy rules.
Security Considerations
Possible Threats to SIMCO
Middleboxes, such as firewalls and NATs, are usually operated for improving the network security and for extending the IP address space (note that stand-alone NATs are not considered to improve security; see RFC2663). The configuration of middleboxes from an external entity looks quite counterproductive on the first glimpse, since an attacker using this can possibly configure the middlebox in such way that no filtering is applied anymore or that NAT bindings are configured for malicious use. So the middlebox is not performing the intended function anymore. Possible threats to SIMCO are:
- Man-in-the-middle attack A malicious host intercepts messages exchanged between then SIMCO agent and middlebox and can change the content of the messages on the fly. This man-in-the-middle attack would result, from the agent's view, in a proper middlebox configuration, but the middlebox would not be configured accordingly. The man in the middlebox could open pinholes that compromise the protected network's security.
- Changing content The message content could be changed in such a way that the requested policy rule configuration is not configured in the middlebox, but that any other unwanted configuration could be. That way, an attacker can open the firewall for his own traffic.
- Replaying Already sent messages could be re-sent in order to configure the middlebox in such a way that hosts could configure policy rules without the permission of an application-level gateway or system administrator.
- Wiretapping An already configured policy rule could be re-used by other hosts if the policy rule is configured with too broad a wildcarding (see below). These hosts could send unwanted traffic.
Securing SIMCO with IPsec
The previous subsection identifies several issues on security for SIMCO. SIMCO can rely on IPsec mechanisms, as defined in RFC4302 and RFC4303, for ensuring proper operations.
When SIMCO relies on IPsec, it uses IPsec in transport mode with an authentication header (AH) RFC4302 and an encapsulating security payload (ESP) RFC4303, so that IP traffic between SIMCO agent and middlebox is protected. The authentication header is used for protecting the whole packet against content changes and replaying. The ESP header is used to prevent wiretapping.
At either the agent or middlebox side, the following should be pre- configured: the IP addresses of the agent or middlebox, TCP (as the transport protocol), and the port numbers (if possible). Only packets from the pre-configured address of the agents or middlebox should be accepted.
The keys for authentication for both the SIMCO agent and middlebox are pre-configured at each side. For replay protection, the use of a key management system is recommended. For the Internet Key Exchange (IKE) protocol, see RFC4306.
10. IAB Considerations on UNSAF
UNilateral Self-Address Fixing (UNSAF) is described in RFC3424 as a process at originating endpoints that attempt to determine or fix the address (and port) by which they are known to another endpoint. UNSAF proposals, such as STUN RFC3489, are considered a general class of work-arounds for NAT traversal and solutions for scenarios with no middlebox communication (MIDCOM).
This document describes a protocol implementation of the MIDCOM semantics and thus implements a middlebox communication (MIDCOM) solution. MIDCOM is not intended as a short-term work-around, but more as a long-term solution for middlebox communication. In MIDCOM, endpoints are not involved in allocating, maintaining, and deleting addresses and ports at the middlebox. The full control of addresses and ports at the middlebox is located at the SIMCO server.
Therefore, this document addresses the UNSAF considerations in RFC3424 by proposing a long-term alternative solution.
11. Acknowledgements
The authors would like to thank Sebastian Kiesel and Andreas Mueller for valuable feedback from their SIMCO implementation and Mary Barnes for a thorough document review.
12. Normative References
RFC3989 Stiemerling, M., Quittek, J., and T. Taylor, "Middlebox
Communications (MIDCOM) Protocol Semantics", RFC 3989, February 2005.
RFC4302 Kent, S., "IP Authentication Header", RFC 4302, December
2005.
RFC4303 Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
RFC4346 Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
13. Informative References
RFC791 Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
RFC1519 Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC 1519, September 1993.
RFC2460 Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
RFC2663 Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
RFC3234 Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
RFC3303 Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A.,
and A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
RFC3424 Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
RFC3489 Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
RFC3932 Alvestrand, H., "The IESG and RFC Editor Documents:
Procedures", BCP 92, RFC 3932, October 2004.
RFC4291 Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
RFC4306 Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
Authors' Addresses
Martin Stiemerling NEC Europe Ltd. Network Laboratories Europe Kurfuersten-Anlage 36 69115 Heidelberg Germany
Phone: +49 6221 4342-113 EMail: [email protected]
Juergen Quittek NEC Europe Ltd. Network Laboratories Europe Kurfuersten-Anlage 36 69115 Heidelberg Germany
Phone: +49 6221 4342-115 EMail: [email protected]
Cristian Cadar Muelheimer Strasse 23 40239 Duesseldorf Germany
EMail: [email protected]
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