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Versions: 00 01 02 03 04 05 06 07 08 RFC 5049

Robust Header Compression                                     C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Expires: May 30, 2007                                             Z. Liu
                                                   Nokia Research Center
                                                                R. Price
                                             Cogent Defence and Security
                                                                Networks
                                                            G. Camarillo
                                                                Ericsson
                                                       November 26, 2006


   Applying Signaling Compression (SigComp) to the Session Initiation
                             Protocol (SIP)
                   draft-ietf-rohc-sigcomp-sip-04.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on May 30, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes some specifics that apply when Signaling



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   Compression (SigComp) is applied to the Session Initiation Protocol
   (SIP), such as default minimum values of SigComp parameters,
   compartment and state management, and a few issues on SigComp over
   TCP.  Any implementation of SigComp for use with SIP must conform to
   this document, in addition to SigComp and support of the SIP and
   Session Description Protocol (SDP) static dictionary.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Minimum Values of SigComp Parameters for SIP/SigComp . . . . .  3
     3.1.  decompression_memory_size (DMS) for SIP/SigComp  . . . . .  4
     3.2.  state_memory_size (SMS) for SIP/SigComp  . . . . . . . . .  4
     3.3.  cycles_per_bit (CPB) for SIP/SigComp . . . . . . . . . . .  5
     3.4.  SigComp_version (SV) for SIP/SigComp . . . . . . . . . . .  5
     3.5.  locally available state (LAS) for SIP/SigComp  . . . . . .  5
   4.  Delimiting SIP Messages and SigComp Messages on the Same
       Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Continuous Mode over TCP . . . . . . . . . . . . . . . . . . .  6
   6.  Too Large SIP Messages . . . . . . . . . . . . . . . . . . . .  6
   7.  SIP Retransmissions  . . . . . . . . . . . . . . . . . . . . .  6
   8.  Compartment and State Management for SIP/SigComp . . . . . . .  7
     8.1.  Remote Application Identification  . . . . . . . . . . . .  7
     8.2.  Identifier Comparison Rules  . . . . . . . . . . . . . . . 10
     8.3.  Compartment Opening and Closure  . . . . . . . . . . . . . 10
     8.4.  Compartment Valid During a Registration  . . . . . . . . . 11
     8.5.  Lack of a Compartment  . . . . . . . . . . . . . . . . . . 12
   9.  Recommendations for Network Administrators . . . . . . . . . . 12
   10. Private Agreements . . . . . . . . . . . . . . . . . . . . . . 13
   11. Backwards Compatibility  . . . . . . . . . . . . . . . . . . . 13
   12. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     16.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20










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1.  Introduction

   SigComp [RFC3320] is a solution for compressing messages generated by
   application protocols.  Although its primary driver is to compress
   SIP [RFC3261] messages, the solution itself has been intentionally
   designed to be application agnostic so that it can be applied to any
   application protocol; this is denoted as ANY/SigComp.  Consequently,
   many application dependent specifics are left out of the base
   standard.  It is intended that a separate specification is used to
   describe those specifics when SigComp is applied to a particular
   application protocol.

   This document binds SigComp and SIP; this is denoted as SIP/SigComp.
   Any SigComp implementation that is used for the compression of SIP
   messages must conform to this document, as well as to [RFC3320].
   Additionally, it must support the SIP/SDP static dictionary as
   specified in [RFC3485] and the mechanism for discovering SigComp
   support at the SIP layer as specified in [RFC3486].


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14 [RFC2119].


3.  Minimum Values of SigComp Parameters for SIP/SigComp

   In order to support a wide range of capabilities among endpoints
   implementing SigComp, SigComp defines a few parameters to describe
   SigComp behavior (see section 3.3 of [RFC3320]).  For each parameter,
   [RFC3320] specifies a minimum value that any SigComp endpoint MUST
   support for ANY/SigComp.  Those minimum values were determined with
   the consideration of all imaginable devices in which SigComp may be
   implemented.  Scalability was also considered as a key factor.

   However, some of the minimum values specified in [RFC3320] are too
   small to allow good performance for SIP message compression.
   Therefore, they are increased for SIP/SigComp as specified in the
   following sections.  For completeness, those parameters that are the
   same for SIP/SigComp as they are for ANY/SigComp are also listed.

   Note: the new minimum values are specific to SIP/SigComp.  They do
   not apply to any other application protocols.

   Note: a SigComp endpoint MAY offer additional resources if available;
   these resources can be advertised to remote endpoints as described in



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   section 9.4.9 of [RFC3320].

3.1.  decompression_memory_size (DMS) for SIP/SigComp

   Minimum value for ANY/SigComp: 2048 bytes, as specified in section
   3.3.1 of [RFC3320].

   Minimum value for SIP/SigComp: 8192 bytes.

   Reason: a DMS of 2048 bytes is too small for SIP message compression
   as it seriously limits the compression ratio and even makes
   compression impossible for certain messages.  For example, the
   condition set by [RFC3320] for SigComp over UDP means: C + 2*B + R +
   2*S + 128 < DMS (each term is described below).  On the other hand,
   8KB additional memory should not cause any problem for an endpoint
   that already implements SIP, SigComp, and applications that use SIP
   as DMS is memory only temporarily needed during decompression of a
   SigComp message (the memory can be reclaimed when the message has
   been decompressed).

   C  size of compressed application message, depending on R
   B  size of bytecode.  Note: two copies -- one as part of the SigComp
      message and one in UDVM (Universal Decompressor Virtual Machine)
      memory.
   R  size of ring buffer in UDVM memory
   S  any additional state uploaded other than that created from the
      content of the ring buffer at the end of decompression (similar to
      B, two copies of S are needed)
   128 the smallest address in UDVM memory to copy bytecode to

3.2.  state_memory_size (SMS) for SIP/SigComp

   Minimum value for ANY/SigComp: 0 (zero) bytes, as specified in
   section 3.3.1 of [RFC3320].

   Minimum value for SIP/SigComp: 2048 bytes.

   Reason: a non-zero SMS allows an endpoint to upload a state in the
   first SIP message sent to a remote endpoint without the uncertainty
   of whether or not it can be created in the remote endpoint.  A non-
   zero SMS obviously requires the SIP/SigComp implementation to keep
   state.  Based on the observation that there is little gain from
   stateless SigComp compression, the assumption is that purely
   stateless SIP implementations are unlikely to provide a SigComp
   function.  Stateful implementations should have little problem to
   keep 2K additional state for each compartment (see Section 8).

   Note: SMS is a parameter that applies to each individual compartment.



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   An endpoint MAY offer different SMS values for different compartments
   as long as the SMS value is not less than 2048 bytes.

3.3.  cycles_per_bit (CPB) for SIP/SigComp

   Minimum value for ANY/SigComp: 16, as specified in section 3.3.1 of
   [RFC3320].

   Minimum value for SIP/SigComp: 16 (same as above)

3.4.  SigComp_version (SV) for SIP/SigComp

   For ANY/SigComp: 0x01, as specified in section 3.3.2 of [RFC3320].

   For SIP/SigComp: >= 0x02 (at least SigComp + NACK)

3.5.  locally available state (LAS) for SIP/SigComp

   Minimum LAS for ANY/SigComp: none, see section 3.3.3 of [RFC3320].

   Minimum LAS for SIP/SigComp: the SIP/SDP static dictionary as defined
   in [RFC3485].


4.  Delimiting SIP Messages and SigComp Messages on the Same Port

   In order to limit the number of ports required by a SigComp-aware
   endpoint, it is possible to allow both SigComp messages and 'vanilla'
   SIP messages (i.e. uncompressed SIP messages with no SigComp header)
   to arrive on the same port.

   For a message-based transport such as UDP or SCTP, this can be done
   per message.  The receiving endpoint checks the first octet of the
   UDP/SCTP payload to determine whether the message has been compressed
   using SigComp.  If the MSBs (Most Significant Bits) of the octet are
   "11111" then the message is considered to be a SigComp message and is
   parsed as per [RFC3320].  If the MSBs of the octet take any other
   value, then the message is assumed to be an uncompressed SIP message,
   and is passed directly to the application with no further effect on
   the SigComp layer.

   For a stream-based transport such as TCP, the distinction is per
   connection.  The receiving endpoint checks the first octet of the TCP
   data stream to determine whether the stream has been compressed using
   SigComp.  If the MSBs of the octet are "11111" then the stream is
   considered to contain SigComp messages and is parsed as per
   [RFC3320].  If the MSBs of the octet take any other value, then the
   stream is assumed to contain uncompressed SIP messages, and is passed



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   directly to the application with no further effect on the SigComp
   layer.  Note that SigComp message delimiters MUST NOT be used if the
   stream contains uncompressed SIP messages.

   Applications MUST NOT mix SIP messages and SigComp messages on a
   single TCP connection.  If the TCP connection is used to carry
   SigComp messages then all messages sent over the connection MUST have
   a SigComp header and be delimited by the use of 0xFFFF as described
   in [RFC3320].

   [I-D.ietf-rohc-sigcomp-impl-guide] shows how to send uncompressed
   messages in a SigComp structured TCP connection using a "well-known
   shim header".  Should it for any reason not be desirable to set up
   more than one TCP connection to a SIP implementation, but the
   flexibility to send both compressed and uncompressed SIP messages be
   required, the compressor can set up a SigComp structured connection
   and send any uncompressed SIP messages using the well-known shim
   header.


5.  Continuous Mode over TCP

   Continuous Mode is a special feature of SigComp, which is designed to
   improve the overall compression ratio for long-lived connections.
   Its use requires pre-agreement between the SigComp compressor and
   decompressor.  Continuous mode is not used with SIP/SigComp.

   Reason: continuous mode requires the transport itself to provide a
   certain level of protection against denial of service attacks.  TCP
   alone is not considered to provide enough protection.


6.  Too Large SIP Messages

   SigComp does not support the compression of messages larger than 64k.
   Therefore, if a SIP application sending compressed SIP messages to
   another SIP application over a transport connection (e.g., a TCP
   connection) needs to send a SIP message larger than 64k, the SIP
   application SHOULD establish a new transport connection and send the
   (uncompressed) SIP message over the new connection.


7.  SIP Retransmissions

   SIP retransmissions need to be compressed again before being sent.
   That is, SIP applications MUST NOT retransmit already-compressed
   information.




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   The reason for this behavior is that it is impossible to know whether
   the failure causing the retransmission occurred to the message being
   retransmitted or to the response to that message.  If the loss
   occurred to the response, any state changes effected by the first
   instance of the retransmitted message would already have taken place.
   If these state changes removed a state that the previously-
   transmitted message relied upon, then retransmission of the same
   compressed message would lead to a decompression failure.


8.  Compartment and State Management for SIP/SigComp

   An application exchanging compressed traffic with a remote
   application has a compartment that contains state information needed
   to compress outgoing messages and to decompress incoming messages.
   To increase the compression efficiency, the application must assign
   distinct compartments to distinct remote applications.

8.1.  Remote Application Identification

   SIP/SigComp applications identify remote applications by their SIP/
   SigComp identifiers.  Each SIP/SigComp application MUST have a SIP/
   SigComp identifier URN (Uniform Resource Name) that uniquely
   identifies the application.  Usage of a URN provides a persistent and
   unique name for the SIP/SigComp identifier.  It also provides an easy
   way to guarantee uniqueness.  This URN MUST be persistent as long as
   the application stores compartment state related to other SIP/SigComp
   applications.

   A SIP/Sigcomp application SHOULD use a UUID (Universally Unique
   IDentifier) URN as its SIP/SigComp identifier.  The UUID URN
   [RFC4122] allows for non-centralized computation of a URN based on
   time, unique names (such as a MAC address), or a random number
   generator.  If a URN scheme other than UUID is used, the URN MUST be
   selected such that the application can be certain that no other SIP/
   SigComp application would choose the same URN value.

   Note that the definition of SIP/SigComp identifier is similar to the
   definition of instance identifier in [I-D.ietf-sip-outbound].  One
   difference is that instance identifiers are only required to be
   unique within their AoR (Address of Record) while SIP/SigComp
   identifiers are required to be globally unique.

   Even if instance identifiers are only required to be unique within
   their AoR, devices may choose to generate globally unique instance
   identifiers.  A device with a globally unique instance identifier
   SHOULD use its instance identifier as its SIP/SigComp identifier.




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      Using the same value for an entity's instance and SIP/SigComp
      identifiers improves the compression ratio of header fields that
      carry both identifiers (e.g., a Contact header field in a REGISTER
      request).

   Server farms that share SIP/SigComp state across servers MUST use the
   same SIP/SigComp identifier for all their servers.

   SIP/SigComp identifiers are carried in the 'sigcomp-id' SIP URI
   (Uniform Resource Identifier) or Via header field parameter.  The
   'sigcomp-id' SIP URI parameter is a 'uri-parameter', as defined by
   the SIP ABNF (Augmented Backus-Naur Form, Section 25.1 of [RFC3261]).
   The following is its ABNF [RFC4234]:

      uri-sip-sigcomp-id = "sigcomp-id=" 1*paramchar

   The SIP URI 'sigcomp-id' parameter MUST contain a URN [RFC2141].

   The Via 'sigcomp-id' parameter is a 'via-extension', as defined by
   the SIP ABNF (Section 25.1 of [RFC3261]).  The following is its ABNF
   [RFC4234]:

      via-sip-sigcomp-id = "sigcomp-id" EQUAL
                      LDQUOT *( qdtext / quoted-pair ) RDQUOT

   The Via 'sigcomp-id' parameter MUST contain a URN [RFC2141].

   The following is an example of a Via header field with a 'sigcomp-id'
   parameter:

      Via: SIP/2.0/UDP server1.example.com:5060
         ;branch=z9hG4bK87a7
         ;comp=sigcomp
         ;sigcomp-id="urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128"

   Note that some characters that are allowed to appear in a Via header
   field parameter, such as ':' (colon), are not allowed to appear in a
   SIP URI parameter.  Those characters need to be escaped when they
   appear in a SIP URI parameter.

      The need to escape characters in parameters could be avoided by
      defining Contact, Route, Record-Route, Path, and Service-Route
      header field 'sigcomp-id' parameters instead of the 'sigcomp-id'
      SIP URI parameter.  For example, instance identifiers typically
      appear in '+sip.instance' Contact header field parameters, and not
      in SIP URI parameters.  We have chosen to define 'sigcomp-id' as a
      SIP URI parameter to be consistent with the use of the already-in-
      use 'comp=sigcomp' parameter, which is a SIP URI parameter as



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      well.

   The following is an example of a 'sigcomp-id' SIP URI parameter:

      sigcomp-id=urn%3auuid%3a0C67446E-F1A1-11D9-94D3-000A95A0E128

   SIP messages are matched with remote application identifiers as
   follows.

   Outgoing requests: the remote application identifier is the SIP/
      SigComp identifier of the URI to which the request is sent.  If
      the URI does not contain a SIP/SigComp identifier, the remote
      application identifier is the IP address plus port of the datagram
      carrying the request for connection-less transport protocols, and
      the transport connection (e.g., a TCP connection) carrying the
      request for connection-oriented transport protocols (this is to
      support legacy SIP/SigComp applications).

   Incoming responses: the remote application identifier is the same as
      the one of the previously-sent request that initiated the
      transaction the response belongs to.

   Incoming requests: the remote application identifier is the SIP/
      SigComp identifier of the top-most Via entry.  If the Via header
      field does not contain a SIP/SigComp identifier, the remote
      application identifier is the source IP address plus port of the
      datagram carrying the request for connection-less transport
      protocols, and the transport connection (e.g., a TCP connection)
      carrying the request for connection-oriented transport protocols
      (this is to support legacy SIP/SigComp applications).

   Outgoing responses: the remote application identifier is the same as
      the previously-received request that initiated the transaction the
      response belongs to.  Note that, due to standard SIP Via header
      field processing, this identifier will be present in the top-most
      Via entry in such responses (as long as it was present in the top-
      most Via entry of the previously-received request).

   A SIP/SigComp application placing its URI with the 'comp=sigcomp'
   parameter in a header field MUST add a 'sigcomp-id' parameter with
   its SIP/SigComp identifier to that URI.

   A SIP/SigComp application generating its own Via entry containing the
   'comp=sigcomp' parameter MUST add a 'sigcomp-id' parameter with its
   SIP/SigComp identifier to that Via entry.

   A given remote application identifier is mapped to a particular
   SigComp compartment ID following the rules given in Section 8.3 and



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   Section 8.4.

8.2.  Identifier Comparison Rules

   Equality comparisons between SIP/SigComp identifiers are performed
   using the rules for URN equality that are specific to the scheme in
   the URN.  If the element performing the comparisons does not
   understand the URN scheme, it performs the comparisons using the
   lexical equality rules defined in RFC 2141 [RFC2141].  Lexical
   equality may result in two URNs being considered unequal when they
   are actually equal.  In this specific usage of URNs, the only element
   which provides the URN is the SIP/SigComp application identified by
   that URN.  As a result, the SIP/SigComp application SHOULD provide
   lexically equivalent URNs in each registration it generates.  This is
   likely to be normal behavior in any case; applications are not likely
   to modify the value of their SIP/SigComp identifiers so that they
   remain functionally equivalent yet lexigraphically different from
   previous identifiers.

8.3.  Compartment Opening and Closure

   SIP applications need to know when to open a new compartment and when
   to close it.  The lifetime of SIP/SigComp compartments is linked to
   registration state.  Compartments are opened at SIP registration time
   and are typically closed when the registration expires or is
   canceled.

      Previous revisions of this document also defined compartments
      valid during a SIP transaction or a SIP dialog.  It was decided to
      eliminate those types of compartments because the complexity they
      introduced was higher than the benefits they brought in most
      deployment scenarios.

   Usually, any states created during the lifetime of a compartment will
   be "logically" deleted when the compartment is closed.  As described
   in section 6.2 of [RFC3320], a logical deletion can become a physical
   deletion only when no compartment continues to exist that created the
   (same) state.

   A SigComp endpoint may offer to keep a state created upon request
   from a SigComp peer endpoint beyond the default lifetime of a
   compartment (i.e., beyond the duration of its associated
   registration).  This may be used to improve compression efficiency of
   subsequent SIP messages generated by the same remote application at
   the SigComp peer endpoint.  To indicate that such state will continue
   to be available, the SigComp endpoint can inform its peer SigComp
   endpoint by announcing the (partial) state ID in the returned SigComp
   parameters at the end of the registration, dialog, or transaction



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   that was supposed to limit the lifetime of the SigComp state.  That
   signals the state will be maintained.  The mandatory support for the
   SigComp Negative Acknowledgement (NACK) Mechanism [RFC4077] in SIP/
   SigComp ensures that it is possible to recover from synchronization
   errors regarding comparment lifetimes.

   As an operational concern, bugs in the compartment management
   implementation are likely to lead to sporadic, hard to diagnose
   failures.  Decompressors may therefore want to cache old state and,
   if still available, allow access while logging diagnostic
   information.  Both compressors and decompressors use the SigComp
   Negative Acknowledgement (NACK) Mechanism [RFC4077] to recover from
   situations where such old state may no longer be available.

8.4.  Compartment Valid During a Registration

   A REGISTER transaction causes an application to open a new
   compartment to be valid for the duration of the registration
   established by the REGISTER transaction.

   A SIP application that needs to send a compressed SIP REGISTER (i.e.,
   a user agent generating a REGISTER or a proxy server relaying one to
   its next hop) SHOULD open a compartment for the request's remote
   application identifier.  A SIP application that receives a compressed
   SIP REGISTER (i.e., the registrar or a proxy relaying the REGISTER to
   its next-hop) SHOULD open a compartment for the request's remote
   application identifier.

   These compartments MAY be closed if the REGISTER request is responded
   with a non-2xx final response, or when the registration expires or is
   canceled.  However, applications MAY also choose to keep these
   compartments open for a longer period of time, as discussed
   previously.  For a given successful registration, applications SHOULD
   NOT close their associated compartments until the registration is
   over.

      A SIP network can be configured so that regular SIP traffic to and
      from a user agent traverses a different set of proxies than the
      initial REGISTER transaction.  The path the REGISTER transaction
      follows is typically determined by configuration data.  The path
      subsequent requests traverse is determined by the Path [RFC3327]
      and the Service-Route [RFC3308] header fields in the REGISTER
      transaction and by the Record-Route and the Route header fields in
      dialog-creating transactions.  Previous revisions of this document
      supported the use of different paths for different types of
      traffic.  However, for simplicity reasons, this document now
      assumes that networks using compression are configured so that
      subsequent requests follow the same path as the initial REGISTER



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      transaction.  Section 9 provides network administrators with
      recommendations so that they configure they networks properly.

   If following the previous rules, a SIP application is supposed to
   open a compartment for a remote application identifier for which it
   already has a compartment, the SIP application MUST use the already
   existing compartment.  That is, the SIP application MUST NOT open a
   new compartment.

8.5.  Lack of a Compartment

   The use of stateless compression (i.e., compression without a
   compartment) is not typically worthwhile and may even result in
   message expansion.  Therefore, if a SIP application does not have a
   compartment for a message it needs to send, it SHOULD NOT compress it
   even in the presence of the comp=sigcomp parameter.  Note that RFC
   3486 [RFC3486] states the following:

      "If the next-hop URI contains the parameter comp=sigcomp, the
      client SHOULD compress the request using SigComp"

   Experience since RFC 3486 [RFC3486] was written has shown that
   stateless compression is not worthwhile.  That is why now it is not
   recommended to use it any longer.


9.  Recommendations for Network Administrators

   Network administrators can configure their networks so that the
   compression efficiency achieved is increased.  The following
   recommendations help network administrators perform their task.

   For a given user agent, the route sets for incoming requests (created
   by a Path header field) and for outgoing requests (created by a
   Service-Route header field) are typically the same.  However,
   registrars can, if they wish, insert proxies in the latter route that
   do not appear in the former route and vice versa.  It is RECOMMENDED
   that registrars are configured so that proxies performing SigComp
   compression appear in both routes.

   The routes described previously apply to requests sent outside a
   dialog.  Requests inside a dialog follow a route constructed using
   Record-Route header fields.  It is RECOMMENDED that the proxies
   performing SigComp that are in the route for requests outside a
   dialog are configured to place themselves (by inserting themselves in
   the Record-Route header fields) in the routes used for requests
   inside dialogs.




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   When a user agent's registration expires, proxy servers performing
   compression may close their associated SIP/SigComp compartment.  If
   the user agent is involved in a dialog that was established before
   the registration expired, subsequent requests within the dialog may
   not be compressed any longer.  In order to avoid this situation, it
   is RECOMMENDED that user agents are registered as long as they are
   involved in a dialog.


10.  Private Agreements

   SIP/SigComp implementations that are subject to private agreements
   MAY deviate from this specification, if the private agreements
   unambiguously specify so.  Plausible candidates for such deviations
   include:

   o  Minimum values (Section 3).
   o  Use of continuous mode (Section 5).
   o  Compartment definition (Section 8).


11.  Backwards Compatibility

   SigComp has a number of parameters that can be configured per
   endpoint.  This document specifies a profile for SigComp when used
   for SIP compression that further constrains the range that some of
   these parameters may take.  Examples of this are Decompressor Memory
   Size, State Memory Size, and SigComp Version (support for NACK).
   Additionally, this document specifies how SIP/SigComp applications
   should perform compartment mapping.

   When this document was written, there already were a few existing
   SIP/SigComp deployments.  The rules in this document have been
   designed to maximize interoperability with those legacy SIP/SigComp
   implementations.  Nevertheless, implementers should be aware that
   legacy SIP/SigComp implementations may not conform to this
   specification.  Examples of problems with legacy applications would
   be smaller DMS than mandated in this document, lack of NACK support,
   or a different comparment mapping.


12.  Example

   Figure 1 shows an example message flow where the user agent and the
   outbound proxy exchange compressed SIP traffic.  Compressed messages
   are marked with a (c).





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        User Agent      Outbound Proxy       Registrar

             |(1) REGISTER (c) |                 |
             |---------------->|                 |
             |                 |(2) REGISTER     |
             |                 |---------------->|
             |                 |(3) 200 OK       |
             |                 |<----------------|
             |(4) 200 OK (c)   |                 |
             |<----------------|                 |
             |(5) INVITE (c)   |                 |
             |---------------->|                 |
             |                 |(6) INVITE       |
             |                 |------------------------------>
             |                 |(7) 200 OK       |
             |                 |<------------------------------
             |(8) 200 OK (c)   |                 |
             |<----------------|                 |
             |(9) ACK (c)      |                 |
             |---------------->|                 |
             |                 |(10) ACK         |
             |                 |------------------------------>
             |(11) BYE (c)     |                 |
             |---------------->|                 |
             |                 |(12) BYE         |
             |                 |------------------------------>
             |                 |(13) 200 OK      |
             |                 |<------------------------------
             |(14) 200 OK (c)  |                 |
             |<----------------|                 |

   Figure 1: Example message flow

   The user agent in Figure 1 is initialy configured (e.g., using the
   SIP configuration framework [I-D.ietf-sipping-config-framework]) with
   the URI of its outbound proxy.  That URI contains the outbound's
   proxy SIP/SigComp identifier, referred to as 'Outbound-id', in a
   'sigcomp-id' parameter.

   When the user agent sends an initial REGISTER request (1) to the
   outbound proxy's URI, the user agent opens a new compartment for
   'Outbound-id'.  This compartment will be valid, at least, for the
   duration of the registration.

   On receiving this REGISTER request (1), the outbound proxy opens a
   new compartment for the SIP/SigComp identifier that appears in the
   'sigcomp-id' parameter of the top-most Via entry.  This identifier,
   which is the user agent's SIP/SigComp identifier, is referred to as



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   'UA-id'.  The compartment opened by the outbound proxy will be valid,
   at least, for the duration of the registration.  The outbound proxy
   adds Path header field with its own URI, which contains the
   'Outbound-id' SIP/SigComp identifier, to the REGISTER request and
   relays it to the registrar (2).

   When the registrar receives the REGISTER request (2), it constructs
   the route future incoming requests (to the user agent) will follow
   using the Contact and the Path header fields.  Future incoming
   requests will traverse the outbound proxy before reaching the user
   agent.

   The registrar also constructs the route future outgoing requests
   (from the user agent) and places it in a Service-Route header field
   in a 200 (OK) response (3).  Future outgoing requests will always
   traverse the outbound proxy.  The registrar has ensured that the
   outbound proxy performing compression handles both incoming and
   outgoing requests.

   When the outbound proxy receives a 200 (OK) response (3), it inspects
   the top-most Via entry.  This entry's SIP/SigComp identifier 'UA-id'
   matches that of the compartment created before.  Therefore, the
   outbound proxy uses that compartment to compress it and relay it to
   the user agent.

   On receiving the 200 (OK) response (4), the user agent stores the
   Service-Route header field in order to use it to send future outgoing
   requests.  The Service-Route header field contains the outbound
   proxy's URI, which contains the 'Outbound-id' SIP/SigComp identifier.

   At a later point, the user agent needs to send an INVITE request (5).
   According to the Service-Route header field received previously, the
   user agent sends the INVITE request (5) to the outbound proxy's URI.
   Since this URI's SIP/SigComp identifier 'Outbound-id' matches that of
   the compartment created before, this compartment is used to compress
   the INVITE request.

   On receiving the INVITE request (5), the outbound proxy Record Routes
   and relays the INVITE request (6) forward.  The outbound proxy Record
   Routes to ensure that all SIP messages related to this new dialog are
   routed through the outbound proxy.

   Finally the dialog is terminated by a BYE transaction (11) that also
   traverses the outbound proxy.


13.  Security Considerations




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   The same security considerations as described in [RFC3320] apply to
   this document.  Note that keeping SigComp states longer than the
   duration of a SIP dialog should not pose new security risks for two
   reasons: a) the state has been allowed to be created in the first
   place; and b) this is on voluntary basis and a SigComp endpoint can
   choose not to offer it.


14.  IANA Considerations

   The IANA is requested to register the 'sigcomp-id' Via header field
   parameter, which is defined in Section 8.1, under the Header Field
   Parameters and Parameter Values subregistry within the SIP Parameters
   registry:


                                                  Predefined
   Header Field                  Parameter Name     Values     Reference
   ----------------------------  ---------------   ---------   ---------
   Via                           sigcomp-id           No       [RFCxxxx]

   The IANA is requested to register the 'sigcomp-id' SIP URI parameter,
   which is defined in Section 8.1, under the SIP/SIPS URI Parameters
   subregistry within the SIP Parameters registry:


   Parameter Name     Predefined Values     Reference
   --------------     -----------------     ---------
   sigcomp-id         No                    [RFCxxxx]

   Note to the RFC Editor: please, substitute RFCxxxx with the RFC
   number this document will get.


15.  Acknowledgements

   The authors would like to thank the following people for their
   comments and suggestions: Jan Christoffersson, Joerg Ott, Mark West,
   Pekka Pessi, Robert Sugar, Jonathan Rosenberg, and Robert Sparks.
   Abigail Surtees and Adam Roach performed thorough reviews of this
   document.


16.  References







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16.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3308]  Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer
              Two Tunneling Protocol (L2TP) Differentiated Services
              Extension", RFC 3308, November 2002.

   [RFC3320]  Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
              Liu, Z., and J. Rosenberg, "Signaling Compression
              (SigComp)", RFC 3320, January 2003.

   [RFC3327]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol
              (SIP) Extension Header Field for Registering Non-Adjacent
              Contacts", RFC 3327, December 2002.

   [RFC3485]  Garcia-Martin, M., Bormann, C., Ott, J., Price, R., and A.
              Roach, "The Session Initiation Protocol (SIP) and Session
              Description Protocol (SDP) Static Dictionary for Signaling
              Compression (SigComp)", RFC 3485, February 2003.

   [RFC3486]  Camarillo, G., "Compressing the Session Initiation
              Protocol (SIP)", RFC 3486, February 2003.

   [RFC4077]  Roach, A., "A Negative Acknowledgement Mechanism for
              Signaling Compression", RFC 4077, May 2005.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [I-D.ietf-sip-outbound]
              Jennings, C. and R. Mahy, "Managing Client Initiated
              Connections in the Session Initiation Protocol  (SIP)",
              draft-ietf-sip-outbound-04 (work in progress), June 2006.

   [I-D.ietf-rohc-sigcomp-impl-guide]



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              Surtees, A., "Implementer's Guide for SigComp",
              draft-ietf-rohc-sigcomp-impl-guide-06 (work in progress),
              March 2006.

16.2.  Informative References

   [I-D.ietf-sipping-config-framework]
              Petrie, D., "A Framework for Session Initiation Protocol
              User Agent Profile Delivery",
              draft-ietf-sipping-config-framework-08 (work in progress),
              March 2006.








































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Authors' Addresses

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28334
   Germany

   Phone: +49 421 218 7024
   Fax:   +49 421 218 7000
   Email: cabo@tzi.org


   Zhigang Liu
   Nokia Research Center
   6000 Connection Drive
   Irving, TX  75039
   USA

   Phone: +1 972 894-5935
   Email: zhigang.c.liu@nokia.com


   Richard Price
   Cogent Defence and Security Networks
   Queensway Meadows Industrial Estate
   Meadows Road
   Newport, Gwent  NP19 4SS

   Phone: +44 (0)1794 833681
   Email: richard.price@cogent-dsn.com
   URI:   http://www.cogent-dsn.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com










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