Robust Header Compression                                     C. Bormann, Ed. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Expires: August 18, 2006 April 15, 2007                                           Z. Liu
                                                   Nokia Research Center
                                                                R. Price
                                             Cogent Defence and Security
                                                                Networks
                                                            G. Camarillo
                                                                Ericsson
                                                       February 14,
                                                        October 12, 2006

   Applying Signaling Compression (SigComp) to the Session Initiation
                             Protocol (SIP)
                   draft-ietf-rohc-sigcomp-sip-02.txt
                   draft-ietf-rohc-sigcomp-sip-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes some specifics that apply when Signaling
   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.  Compartment and State Management for SIP/SigComp . . . . . . .  7  6
     6.1.  Remote Application Identifiers . . Identification  . . . . . . . . . . . .  7
     6.2.  Identifier Comparison Rules  . . . . . . . . . . . . . . .  9
     6.3.  Compartment Opening and Closure  . . . . . . . . . . . . .  7
     6.3. 10
     6.4.  Compartment Valid During a Transaction . . . . . . . . . .  9
     6.4. 11
     6.5.  Compartment Valid During a Registration  . . . . . . . . .  9
     6.5. 11
     6.6.  Compartment Valid During a Dialog  . . . . . . . . . . . . 10 12
   7.  Recommendations for Network Administrators . . . . . . . . . . 10 12
   8.  Security Considerations  Private Agreements . . . . . . . . . . . . . . . . . . . 11
   9.  Private Agreements . . . 13
   9.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . . 11 13
   10. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   11. 16
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   12. 17
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     14.2. Informative References . . 11 . . . . . . . . . . . . . . . . 18
   Appendix A.  Shim header for sending uncompressed messages . . . . 12 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 15 21

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 (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] and [RFC3320].
   Additionally, it must support the SIP/SDP static dictionary as
   specified in [RFC3485].

   Note: [RFC3485] and the mechanism of for discovering SigComp
   support at the SIP layer
   is 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
   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 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 6).

   Note: SMS is a parameter that applies to each individual compartment.
   An endpoint MAY offer different SMS values for different compartments
   as long as the SMS value is not less than 2048 bytes.

   Compressors that make use of initial state memory MUST implement the
   SigComp Negative Acknowledgement (NACK) Mechanism [I-D.ietf-rohc-
   sigcomp-nack]. [RFC4077].  (Note
   that there is no such requirement on decompressors, but see also
   Section 6.)  For this requirement, initial state memory is defined as
   the assumption of a non-zero SMS value before having received an
   advertisement of non-zero SMS (e.g., via returned parameters as
   specified in section 9.4.9 of [RFC3320]); ANY/SigComp as defined in
   [RFC3320] does not have initial state memory.

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].

   OPEN ISSUE: what SigComp version(s) should be required for SIP/
   SigComp?

      Version >= 0x01 (i.e. any SigComp)
      Version

   For SIP/SigComp: >= 0x02 (i.e. at (at least SigComp + NACK)
      Version == 0x01 | 0x02 (base SigComp or SigComp + NACK)
      Version == 0x02 (only SigComp with NACK)

   For SIP/SigComp: 0x01 (same as above)

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 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
   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].

   Note: Appendix A 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.  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.

6.1.  Remote Application Identifiers Identification

   SIP/SigComp applications identify remote applications by their FQDN
   (Fully Qualified Domain Name) or by their IP address.  For outgoing
   requests, the remote SIP/
   SigComp identifiers.  Each SIP/SigComp application MUST have SIP/
   SigComp identifier is URN (Uniform Resource Name) that uniquely
   identifies the host part application.  Usage of a URN provides a persistent and
   unique name for the
   URI SIP/SigComp identifier.  It also provides an easy
   way to which the request is sent.  For incoming responses, guarantee uniqueness.  This URN MUST be persistent across
   power cycles of the remote
   application identifier is device or devices hosting the same SIP/SigComp
   application.  The SIP/ SigComp identifier MUST NOT change as the
   application moves from one network to another.

   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 previously-sent
   request URN MUST be
   selected such that initiated the transaction the response belongs to.  For
   incoming requests and outgoing responses, the remote application
   identifier is can be certain that no other SIP/
   SigComp application would choose the sent-by parameter of same URN value.

   Note that the top-most Via entry.

   A given remote application definition of SIP/SigComp identifier is mapped similar to a particular
   SigComp compartment ID following the rules given
   definition of instance identifier in the following
   sections.

      OPEN ISSUE: this [I-D.ietf-sip-outbound].  One
   difference is an implicit way of identifying remote
      applications.  It assumes that two remote applications instance identifiers are
      different if the host parts of only required to be
   unique within their URIs AoR (Address of Record) while SIP/SigComp
   identifiers are different.  However,
      if a proxy farm shares dictionary state among its proxies and
      these proxies use different host parts (e.g., proxy1.example.com
      and proxy2.example.com), they will required to be considered like different
      remote applications, when they should have been considered a
      single remote application.  If implementers intend globally unique.

   Nevertheless, devices may choose to implement generate globally unique instance
   identifiers.  A device with a globally unique instance identifier
   SHOULD use its instance identifier as its SIP/SigComp identifier.

   Server farms that share SIP/SigComp state sharing this way, we could across servers MUST use explicit application
      identifiers instead.  These the
   same SIP/SigComp identifier for all their servers.

   SIP/SigComp identifiers could be placed are carried in a the 'sigcomp-id' SIP URI parameter (e.g., sip:p1.example.net;id="12wsfeQ45") and in a
   (Uniform Resource Identifier) or Via header field parameter.

6.2.  Compartment Opening and Closure

   SIP applications need to know when to open a new compartment and when
   to close it.  The lifetime of
   'sigcomp-id' SIP URI parameter is a compartment depends on how 'uri-parameter', as defined by
   the SIP
   application obtained the remote application identifier (e.g., in a
   Record-Route header field ABNF (Augmented Backus-Naur Form, Section 25.1 of an incoming SIP message).  There are
   compartments that are valid for the duration of a registration, of a
   dialog, and of a single transaction. [RFC3261]).
   The following sections specify
   how is its ABNF [RFC4234]:

      sip-sigcomp-id = "sigcomp-id=" instance-val

      instance-val   = *uric ; defined in RFC 2396

   The Via 'sigcomp-id' parameter is a SIP application decides 'via-extension', as defined by
   the lifetime SIP ABNF (Section 25.1 of a particular
   compartment.

   If [RFC3261]).  The following the rules is its ABNF
   [RFC4234]:

      via-sip-sigcomp-id = "sigcomp-id" EQUAL
                           LDQUOT "<" instance-val ">" RDQUOT

      instance-val   = *uric ; defined in the RFC 2396

   The following sections, a SIP application is supposed to open an example of a compartment for Via header field with a remote application identifier
   for which it already has '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 compartment, the SIP application MUST use
   the already existing compartment.  That is, the SIP application MUST
   NOT open Via header
   field parameter, such as ':' (colon), are not allowed to appear in a new compartment.  Additionally, the
   SIP application MUST
   adjust the closure time for the compartment so that it is only closed URI parameter.  Those characters need to be escaped when the they
   appear in a SIP application does not URI parameter.

      The need it any longer. 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, a instance identifiers typically
      appear in '+sip.instance' Contact header field parameters, and not
      in SIP application may open URI parameters.  We have chosen to define 'sigcomp-id' as a compartment valid for
      SIP URI parameter to be consistent with the
   duration use of the already-in-
      use 'comp=sigcomp' parameter, which is a registration for SIP URI parameter as
      well.

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

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

   SIP messages are matched with remote application
   identifier.  At a later point, identifiers as
   follows.

   Outgoing requests: the remote application identifier is supposed to open a
   new compartment for the duration SIP/
      SigComp identifier of a particular dialog for the same
   remote application identifier.  Following URI to which the previous rule, request is sent.  If
      the SIP
   application URI does not open contain a new compartment SIP/SigComp identifier but use is
      associated with an instance identifier (e.g., the URI appears in a
      Contact header field with a '+sip.instance' parameter), the
      instance identifier is used as the already
   existing one for that remote application identifier.  However,
      In other cases, the
   SIP remote application must not close that compartment until both, the
   registration and the dialog are over.  So, if the registration
   finishes before identifier is the dialog, host part
      of the compartment will not be closed
   (because URI to which the dialog request is still active) even though sent (this is to support legacy
      SIP/SigComp applications).

   Incoming responses: the compartment was
   originally open for remote application identifier is the registration.

   Usually, any states created during same as
      the lifetime one 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 previously-sent request that created initiated the
   (same) state.)

   A SigComp endpoint may offer to keep a state created upon request
   from a SigComp peer endpoint beyond
      transaction the default lifetime of a
   compartment.  This may be used to improve compression efficiency of
   subsequent SIP messages generated by response belongs to.

   Incoming requests: the same remote application at identifier is the SIP/
      SigComp peer endpoint.  To indicate that such state will continue
   to be available, identifier of the SigComp endpoint can inform its peer SigComp
   endpoint by announcing top-most Via entry.  If the (partial) state ID in Via header
      field does not contain a SIP/SigComp identifier, the returned SigComp
   parameters at remote
      application identifier is the end sent-by parameter of the registration, dialog, or transaction
   that was supposed top-most
      Via entry (this is to limit the lifetime of the SigComp state.  That
   signals support legacy SIP/SigComp applications).

   Outgoing responses: the state will be maintained.  As there remote application identifier is no way to signal
   any limit to the lifetime of this state, both decompressors same as
      the previously-received request that
   intend to offer state initiated the transaction the
      response belongs to.

   A SIP/SigComp application placing its URI with possibly limited lifetimes as well as
   compressors the 'comp=sigcomp'
   parameter in a header field MUST add a 'sigcomp-id' parameter with
   its SIP/SigComp identifier to that make use of such state SHOULD implement URI unless the SigComp
   Negative Acknowledgement (NACK) Mechanism [I-D.ietf-rohc-sigcomp-
   nack].

   As URI is associated
   with an operational concern, bugs instance identifier (e.g., the URI appears in a Contact
   header field with a '+sip.instance' parameter).  If the compartment management
   implementation are likely to lead URI is
   associated with an instance identifier, the SIP/SigComp application
   SHOULD NOT add a 'sigcomp-id' parameter to sporadic, hard the 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 diagnose
   failures.  Decompressors may therefore want that Via entry.

   A given remote application identifier is mapped to cache old state and,
   if still available, allow access while logging diagnostic
   information.  Both compressors and decompressors a particular
   SigComp compartment ID following the rules given in Section 6.3,
   Section 6.4, Section 6.5, Section 6.6.

6.2.  Identifier Comparison Rules

   Equality comparisons between SIP/SigComp identifiers are RECOMMENDED performed
   using the rules for URN equality that are specific to
   implement the SigComp Negative Acknowledgement (NACK) Mechanism
   [I-D.ietf-rohc-sigcomp-nack], which facilitates recovery scheme in a
   situation where such old state
   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 no longer be available.

6.3.  Compartment Valid During a Transaction

   A SIP application that needs to send a compressed SIP 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.

   Comparisons between SIP/SigComp identifiers and instance identifiers
   are performed following the same rules.

6.3.  Compartment Opening and Closure

   SIP applications need to know when to open a new compartment and when
   to close it.  The lifetime of a compartment depends on how the SIP
   application obtained the remote application identifier (e.g., in a
   Record-Route header field of an incoming SIP message).  There are
   compartments that are valid for the duration of a registration, of a
   dialog, and of a single transaction.  The following sections specify
   how a SIP application decides the lifetime of a particular
   compartment.

   If following the rules in the following sections, 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.  Additionally, the SIP application MUST
   adjust the closure time for the compartment so that it is only closed
   when the SIP application does not need it any longer.

   For example, a SIP application may open a compartment valid for the
   duration of a registration for a particular remote application
   identifier.  At a later point, the application is supposed to open a
   new compartment for the duration of a particular dialog for the same
   remote application identifier.  Following the previous rule, the SIP
   application does not open a new compartment but use the already
   existing one for that remote application identifier.  However, the
   SIP application must not close that compartment until both, the
   registration and the dialog are over.  So, if the registration
   finishes before the dialog, the compartment will not be closed
   (because the dialog is still active) even though the compartment was
   originally opened for the registration.

   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.  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
   that was supposed to limit the lifetime of the SigComp state.  That
   signals the state will be maintained.  As there is no way to signal
   any limit to the lifetime of this state, both decompressors that
   intend to offer state with possibly limited lifetimes as well as
   compressors that make use of such state use the SigComp Negative
   Acknowledgement (NACK) Mechanism [RFC4077] to recover from
   synchronization errors.

   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.

6.4.  Compartment Valid During a Transaction

   A SIP application that needs to send a compressed SIP request SHOULD
   open a compartment for the request's remote application identifier.
   This compartment will be used to receive compressed responses for the
   request.  The application SHOULD NOT close the compartment until the
   transaction is over.

   A SIP application that receives a compressed SIP request SHOULD open
   a compartment for the request's remote application identifier.  This
   compartment will be used to send compressed responses for the
   request.  The application SHOULD NOT close the compartment until the
   transaction is over.

   The previous rules ensure that SIP applications always have a
   compartment to send and receive responses.

6.5.  Compartment Valid During a Registration

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

   A 200 (OK) response for a register may contain a Path [RFC3327] and a
   Service-Route [RFC3308] header field.  These header fields indicate
   the route future incoming and outgoing requests will follow.

   A SIP application generating a 200 (OK) response for a REGISTER or
   receiving such a response proceeds as follows.  If the application
   inserted itself in the Contact (i.e., because it is the user agent)
   or in the Path header field of the REGISTER, or it appears in the
   Service-Route header field, the application constructs the route
   future incoming requests will follow (using the Contact and the Path
   header fields) and the route future outgoing requests will follow
   (using the Contact and the Service-Route header fields).  The
   application checks whether the URIs of its adjacent applications in
   both routes have the 'comp=sigcomp' parameter.  The application
   SHOULD open a new compartment for the remote application identifier
   of the URIs with that parameter.  The application SHOULD NOT close
   the compartments until the registration is over.

   Note that the route for incoming requests is typically the same
   (although traversed in the opposite direction) as the route for
   outgoing requests.

6.6.  Compartment Valid During a Dialog

   A transaction that establishes a dialog can cause an application to
   open a new compartment to be valid for the duration of the dialog
   established by the transaction.

   A SIP message that establishes a dialog (e.g., a 2xx response for an
   INVITE) may contain a Record-Route header field.  This header field
   indicates the route future requests within the dialog will follow.

   On generating or receiving a SIP message that establishes a dialog, a
   SIP application that inserted itself in the Contact (i.e., because it
   is the user agent) or in the Record-Route header field of the
   request, constructs (using the Contact, and the Record-Route header
   fields) the route requests within the dialog will follow.  The
   application checks whether the URIs of its adjacent applications in
   that route have the 'comp=sigcomp' parameter.  The application SHOULD
   open a new compartment for the remote application identifier of the
   URIs with that parameter.  The application SHOULD NOT close the
   compartments until the dialog is over.

7.  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.

8.  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  Compartment definition (Section 6).
   o  Use of continuous mode (Section 5).

9.  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.

10.  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).

        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 SHOULD
   open (1) to the
   outbound proxy's URI, the user agent opens a new compartment for the request's remote application identifier.
   'Outbound-id'.  This compartment will will, in principle, be used to receive compressed responses valid for the
   request.  The application can close the compartment when
   duration of the
   transaction is over.

   A SIP application receives a compressed SIP REGISTER transaction, as discussed in Section 6.4.

   On receiving this REGISTER request SHOULD open (1), the outbound proxy opens a
   new compartment for the request's remote application identifier.  This
   compartment will be used to send compressed responses for SIP/SigComp identifier that appears in the
   request.  The application SHOULD NOT close
   'sigcomp-id' parameter of the compartment until top-most Via entry.  This identifier,
   which is the
   transaction user agent's SIP/SigComp identifier, is over.

   The previous rules ensure that SIP applications always have an
   already existing compartment to send and receive responses.

6.4.  Compartment Valid During a Registration

   A REGISTER transaction can cause an application referred to open a new as
   'UA-id'.  The compartment to opened by the outbound proxy will, in
   principle, be valid for the duration of the registration
   established by REGISTER transaction, as
   discussed in Section 6.4.  The outbound proxy adds Path header field
   with its own URI to the REGISTER transaction.

   A request and relays it to the
   registrar (2).

   When the outbound proxy receives a 200 (OK) response (3) for a register may contain a Path [RFC3327] and a
   Service-Route [RFC3308] header field.  These header fields indicate the
   REGISTER request, it constructs the route future incoming requests
   will follow (using the Contact and the Path header fields) and the
   route future outgoing requests will follow.

   On receiving follow (using the Contact and the
   Service-Route header fields).  Both the Path and the Service-Route
   header fields contain the outbound proxy's URI.  The Contact header
   field contains the user agent's URI, which carries the user agent's
   SIP/SigComp identifier 'UA-id'.

   Consequently, the outbound proxy is supposed to open a new
   compartment for 'UA-id' for the duration of the registration, as
   discussed in Section 6.4.  However, since the outbound proxy has
   already a 200 (OK) response compartment for a REGISTER, a SIP application 'UA-id', it reuses that inserted itself comparment, as
   discussed in Section 6.3.

   On receiving the Contact (i.e., because it is 200 (OK) response (4), the user
   agent) or in the Path header field of the REGISTER, agent constructs the
   route future incoming requests will follow (using the Contact and the Path header fields)
   field) and the route future outgoing requests will follow (using the Contact and the
   Service-Route header fields).  The
   application checks whether the URIs of its adjacent applications in
   both routes have the comp=sigcomp parameter. field).  The application SHOULD user agent is supposed to open a
   new compartment for the remote application identifier of the
   URIs with that parameter.  The application SHOULD NOT close the
   compartments until the registration is over.

   Note that the route 'Outbound-id' for incoming requests is typically the same
   (although traversed in duration of the opposite direction)
   registration, as the route for
   outgoing requests.

   Note that some user agents use several registration discussed in parallel to
   improve service reliability.  Different registration typically have
   different associated route vectors.  Messages sent to different
   remote application identifiers will use different compartments, even
   if those messages are generated by Section 6.4.  However, since the same user agent.  It is
   assumed that the remote applications do not share SIP/SigComp state
   among them.

6.5.  Compartment Valid During
   agent has already a Dialog

   A transaction compartment for 'Outbound-id', it reuses that establishes
   comparment, as discussed in Section 6.3.

   At a dialog can cause later point, the user agent needs to send an application INVITE request (5).
   The user agent is supposed to open a new compartment to be valid for
   'Outbound-id' for the duration of the dialog
   established by INVITE transaction, as
   discussed in Section 6.4.  However, since the transaction.

   A SIP message that establishes a dialog (e.g., user agent has already
   a 2xx response compartment for an
   INVITE) may contain 'Outbound-id', it reuses that comparment, as
   discussed in Section 6.3.

   On receiving the INVITE request (5), the outbound proxy is supposed
   to open a Record-Route header field.  This header field
   indicates new compartment for 'UA-id' for the duration of the route future requests within INVITE
   transaction, as discussed in Section 6.4.  However, since the dialog will follow.

   On receiving such a SIP message,
   outbound proxy has already a SIP application compartment for 'UA-id', it reuses that inserted
   itself
   comparment, as discussed in Section 6.3.  The outbound proxy Record-
   Routes and relays the Contact (i.e., because it is the user agent) or in INVITE request (6) forward.

   When the
   Record-Route header field of outbound proxy receives a dialog-establishing 200 (OK)
   response (7) for the INVITE request, it constructs (using the
   Contact, and the Record-Route header fields) the route future
   requests within the dialog will follow.  The application checks whether the
   URIs of its adjacent applications in that route have follow (using the
   "comp=sigcomp" parameter. Contact and Route
   header fields).  The application SHOULD outbound proxy is supposed to open a new
   compartment for 'UA-id' for the remote application identifier duration of the URIs with
   that parameter.  The application SHOULD NOT close the compartments
   until dialog, as discussed
   in Section 6.6.  However, since the dialog is over.

7.  Recommendations outbound proxy has already a
   compartment for Network Administrators

   Network administrators can configure their networks so 'UA-id', it reuses that comparment, as discussed in
   Section 6.3.

   On receiving the 200 (OK) response (8), the
   compression efficiency achieved is increased.  The following
   recommendations help network administrators perform their task.

   For a given user agent, agent constructs the
   route sets for incoming requests (created
   by a Path header field) and for outgoing future requests (created by a
   Service-Route header field) are typically the same.  However,
   registrars can, if they wish, insert proxies in within the latter route that
   do not appear in dialog will follow (using the former route and vice versa.  It is RECOMMENDED
   that registrars are configured so that proxies performing SigComp
   compression appear in both routes. Route
   header field).  The routes described previously apply user agent is supposed to requests sent outside open a
   dialog.  Requests inside new compartment
   for 'Outbound-id' for the duration of the dialog, as discussed in
   Section 6.4.  However, since the user agent has already a compartment
   for 'Outbound-id', it reuses that comparment, as discussed in
   Section 6.3.

   When the dialog follow a route constructed using
   Record-Route header fields.  It is RECOMMENDED that terminated by a BYE transaction (11), the proxies
   performing SigComp that are in user
   agent is supposed to close the route compartment for requests outside a
   dialog are configured 'Outbound-id' and the
   outbound proxy is supposed to place themselves (by inserting themselves in close the Record-Route header fields) compartment for 'UA-id', as
   discussed in Section 6.6.  However, both compartments should be kept
   open until the routes used for requests
   inside dialogs.

8. current registration expires.  Therefore, none of them
   close their compartments yet.

11.  Security Considerations

   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.

9.  Private Agreements

   SIP/SigComp implementations that are subject

12.  IANA Considerations

   The IANA is requested to private agreements
   MAY deviate from this specification, if register the private agreements
   unambiguously specify so.  Plausible candidates for such deviations
   include:

   o  Minimum values (Section 3).
   o  Compartment definition (Section 6).
   o  Use of continuous mode (Section 5).

10. 'sigcomp-id' Via header field
   parameter, which is defined in Section 6.1, 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 Considerations

   This specification does not require any actions from is requested to register the IANA.

11. 'sigcomp-id' SIP URI parameter,
   which is defined in Section 6.1, 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.

13.  Acknowledgements

   Abigail Surtees provided the code and text for Appendix A.

   The authors would like to thank the following people for their
   comments and suggestions: Abigail Surtees, Jan Christoffersson, Joerg
   Ott, Mark West, Pekka Pessi, Robert Sugar, and Adam Roach.

12. Roach, Jonathan
   Rosenberg, and Robert Sparks.

14.  References

14.1.  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.

   [I-D.ietf-rohc-sigcomp-nack]

   [RFC4077]  Roach, A., "A Negative Acknowledgement Mechanism for
              Signaling Compression", draft-ietf-rohc-sigcomp-nack-02 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), October 2004. June 2006.

14.2.  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.

Appendix A.  Shim header for sending uncompressed messages

   This appendix presents bytecode that simply instructs the
   decompressor to output the entire message (effectively sending it
   uncompressed but within a SigComp message).

   The mnemonic code is:

      at (0)
      :udvm_memory_size         pad (2)
      :cycles_per_bit           pad (2)
      :sigcomp_version          pad (2)
      :partial_state_id_length  pad (2)
      :state_length             pad (2)
      :reserved                 pad (2)
      at (64)
      :byte_copy_left           pad (2)
      :byte_copy_right          pad (2)
      :input_bit_order          pad (2)
      :stack_location           pad (2)

      ; Simple loop
      ;       Read a byte
      ;       Output a byte
      ; Until there are no more bytes!

      at (128)
      :start
      INPUT-BYTES (1, byte_copy_left, end)
      OUTPUT (byte_copy_left, 1)
      JUMP (start)

      :end
      END-MESSAGE (0,0,0,0,0,0,0)

   which translates to give the following initial 13 bytes of the
   SigComp message (in hexadecimal):

   f8 00 a1 1c 01 86 09 22 86 01 16 f9 23

   As an implementation optimization, a SigComp implementation MAY
   compare the initial 13 bytes of each incoming message with the 13
   bytes given (the "well-known shim header"), and, in case of a match,
   simply copy the SigComp message data that follow the shim header
   without even setting up a UDVM.  (Note that, before a SigComp message
   is formed from the incoming TCP data, the record marking protocol
   defined in section 4.2.2 of [RFC3320] has to be performed.)

   To obtain the maximum benefit from this optimization, compressors
   SHOULD employ exactly the well-known shim header given (and none of
   the other conceivable byte code sequences for just copying input to
   output) to send uncompressed data in a SigComp channel.

Authors' Addresses

   Carsten Bormann (editor)
   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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