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Versions: 00 01                                                         
Internet Engineering Task Force                                   SIP WG
Internet Draft                                              G. Camarillo
May 28, 2002
Expires: November 2002

              Compressing the Session Initiation Protocol


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   This document describes a mechanism to signal that compression is
   desired for one or more SIP messages. It also states when it is
   appropriate to send compressed SIP messages to a SIP entity.

G. Camarillo                                                  [Page 1]

Internet Draft                    SIP                       May 28, 2002

                           Table of Contents

   1          Introduction ........................................    3
   2          Overview of operation ...............................    4
   3          Sending a Request to a Server .......................    4
   4          Sending a Response to a Client ......................    5
   5          Error Situations ....................................    5
   6          Augmented BNF .......................................    6
   7          Example .............................................    6
   8          Security Considerations .............................    9
   9          Acknowledges ........................................    9
   10         Authors' Addresses ..................................    9
   11         Normative References ................................   10
   12         Informative References ..............................   10

G. Camarillo                                                  [Page 2]

Internet Draft                    SIP                       May 28, 2002

1 Introduction

   A SIP [1] client sending a request to a SIP server typically performs
   a DNS lookup for the domain name of the server. When NAPTR [3] or SRV
   [4] records are available for the server, the client can specify the
   type of service it wants. The service in this context is the
   transport protocol to be used by SIP (e.g., UDP, TCP or SCTP). A SIP
   server that support, for instance, three different transport
   protocols, will have three different DNS entries.

   Since it is foreseen that the number of transport protocols supported
   by a particular application layer protocol is not going to grow
   dramatically, having a DNS entry per transport seems like a scalable
   enough solution.

   However, sometimes it is necessary to include new layers between the
   transport protocol and the application layer protocol. Examples of
   these layers are transport layer security and compression. If DNS was
   used to discover the availability of these layers for a particular
   server, the number of DNS entries needed for that server would grow

   A server that, for example, supported TCP and SCTP as transports, TLS
   for transport security and Sigcomp for signaling compression, would
   need the 8 DNS entries listed below:

        1.   TCP, no security, no compression

        2.   TCP, no security, SigComp

        3.   TCP, TLS, no compression

        4.   TCP, TLS, SigComp

        5.   SCTP, no security, no compression

        6.   SCTP, no security, SigComp

        7.   SCTP, TLS, no compression

        8.   SCTP, TLS, SigComp

   It is clear that this way of using DNS is not scalable. Therefore, an
   application layer mechanism to express support of signalling
   compression is needed.

        Note that for historical reasons both HTTP and SIP use a
        different port for TLS on top of TCP than for TCP alone,

G. Camarillo                                                  [Page 3]

Internet Draft                    SIP                       May 28, 2002

        although at present this solution is not considered
        scalable any longer.

   A SIP element that supports compression will need to be prepared to
   receive compressed and uncompressed messages on the same port. It
   will perform demultiplexing based on the cookie in the topmost bits
   of every compressed message.

2 Overview of operation

   There are two types of SIP messages; SIP requests and SIP responses.
   Clients send SIP requests to the host part of a URI and servers send
   responses to the host in the sent-by parameter of the Via header

   We define two parameters, one for SIP URIs and the other for the Via
   header field. The format of both parameters is the same, as shown in
   the examples below:

   Via: SIP/2.0/UDP server1.foo.com:5060;branch=z9hG4bK87a7;comp=sigcomp

   The presence of this parameter in a URI indicates that the request
   has to be compressed using Sigcomp, as defined in [2]. The presence
   of comp=sigcomp in a Via header field indicates that the response has
   to be compressed using Sigcomp.

        Note that, as stated in , every implementation that
        supports Sigcomp needs to implement the static dictionary
        for SIP and SDP.

3 Sending a Request to a Server

   A request is sent to the host part of a URI. This URI, referred to as
   the next-hop URI, is the Request-URI of the request or an entry in
   the Route header field.

   If the next-hop URI contains the parameter comp=sigcomp, the client
   SHOULD compress the request using Sigcomp as defined in [2]. The
   next-hop URI can contain such a parameter due to a number of reasons:

        o The URI was obtained from a Record-Route header field when the
          dialog was established.

        o The URI was previously registered in a registrar using a
          REGISTER request.

G. Camarillo                                                  [Page 4]

Internet Draft                    SIP                       May 28, 2002

        o The URI was obtained from a Contact header field in a 3xx or
          485 response to the request.

        o The URI was obtained out of band or statically configured.

   A client MUST NOT send a compressed request to a server if it does
   not know whether or not the server supports Sigcomp.

   If a client does not know whether or not the server supports Sigcomp,
   but in case the server supported it, it would like to receive
   compressed responses, this client SHOULD add the parameter
   comp=sigcomp to the topmost entry of the Via header field. The
   request, however, as stated above, will not be compressed.

   Regardless of whether the request is sent compressed or not, if a
   client would like to receive subsequent requests within the same
   dialog compressed, this client SHOULD add the parameter comp=sigcomp
   to the URI in the Contact header field if it is a user agent client.
   If the client is a proxy, it SHOULD add the parameter comp=sigcomp to
   its URI in the Record-Route header field.

4 Sending a Response to a Client

   A response is sent to the host in the sent-by parameter of the Via
   header field. If the topmost Via header field contains the parameter
   comp=sigcomp, the response SHOULD be compressed. Otherwise, the
   response MUST NOT be compressed.

   A proxy performing Record-Route inspects the Record-Route header
   field in the response and the Contact header field in the request
   that triggered this response (see example in Section 7). It looks for
   the URI of the next upstream (closer to the user agent client) hop in
   the route set. If this URI contains the parameter comp=sigcomp, the
   proxy SHOULD add comp=sigcomp to its entry in the Record-Route header

   The same way, a user agent server SHOULD add comp=sigcomp to the
   Contact header field of the response if the URI of the next upstream
   hop in the route set contained the parameter comp=sigcomp.

5 Error Situations

   If a compressed SIP request arrives to a SIP server that does not
   understand SigComp, the server will not have any means to indicate
   the error to the client. The message will be impossible to parse, and
   there will be no Via header field indicating an address to send the
   error response.

G. Camarillo                                                  [Page 5]

Internet Draft                    SIP                       May 28, 2002

   If a SIP client sends a compressed request and the client transaction
   times out without having received any response, the client SHOULD
   retry the same request without using compression. If the compressed
   request was sent over a TCP connection, the client SHOULD close that
   connection and open a new one to send the uncompressed request.
   Otherwise the server would not be able to detect the beginning of the
   new message.

6 Augmented BNF

   This section provides the augmented Backus-Naur Form (BNF) of both
   parameters described above.

   The compression URI parameter is a "uri-parameter", as defined by the
   SIP ABNF (Section 25.1 of [1]):

        compression-param  =  "comp=" ("sigcomp" / other-compression)
        other-compression  =  token

   The Via compression parameter is a "via-extension", as defined by the
   SIP ABNF (Section 25.1 of [1]):

        via-compression    =  "comp" EQUAL ("sigcomp" / other-compression)
        other-compression  =  token

7 Example

   The following example illustrates the use of the parameters defined
   above. The call flow of Figure 1 shows an INVITE-200 OK-ACK handshake
   between a UAC and a UAS through two proxies. Proxy P1 does not
   Record-Route but proxy P2 does. Both proxies support compression, but
   they do not use it by default.

    UAC            P1            P2           UAS

     |(1)INVITE(c) |             |             |
     |------------>| (2) INVITE  |             |
     |             |------------>| (3) INVITE  |
     |             |             |------------>|
     |             |             | (4) 200 OK  |
     |             | (5) 200 OK  |<------------|
     |(6)200 OK(c) |<------------|             |
     |<------------|             |             |

G. Camarillo                                                  [Page 6]

Internet Draft                    SIP                       May 28, 2002

     |             |  (7)ACK(c)  |             |
     |-------------------------->|   (8) ACK   |
     |             |             |------------>|
     |             |             |             |
     |             |             |             |

   Figure 1: INVITE transaction through two proxies

   Messages (1), (6) and (7) are compressed (c).

   We provide a partial description of the messages involved in this
   call flow below. Only some parts of each message are shown, namely
   the Method name, the Request-URI and the Via, Route, Record-Route and
   Contact header fields. We have not used a correct format for these
   header fields. We have rather focus on the contents of the header
   fields and on the presence (or absence) of the "comp=sigcomp"

       Via: UAC;comp=sigcomp
       Contact: UAC;comp=sigcomp

   P1 is the outbound proxy of the UAC, and it supports Sigcomp. The UAC
   is configured to send compressed traffic to P1, and therefore, it
   compresses the INVITE (1). In addition, the UAC wants to receive
   future requests and responses for this dialog compressed. Therefore,
   it adds the comp=Sigcomp parameter to the Via and to the Contact
   header fields.

       Via: P1
       Via: UAC;comp=sigcomp
       Contact: UAC;comp=sigcomp

   P1 forwards the INVITE (2) to P2. P1 does not use compression by
   default, so it sends the INVITE uncompressed to P2.

       Via: P2

G. Camarillo                                                  [Page 7]

Internet Draft                    SIP                       May 28, 2002

       Via: P1
       Via: UAC;comp=sigcomp
       Contact: UAC;comp=sigcomp

   P2 forwards the INVITE (3) to the UAS. P2 supports compression, but
   it does not use it by default. Therefore, it sends the INVITE
   uncompressed. P2 wishes to remain in the signalling path and
   therefore it Record-Routes.

   (4) 200 OK
       Via: P2
       Via: P1
       Via: UAC;comp=sigcomp
       Contact: UAS

   The UAS generates a 200 OK response and sends it to the host in the
   topmost Via, which is P2.

   (5) 200 OK
       Via: P1
       Via: UAC;comp=sigcomp
       Contact: UAS

   P2 receives the 200 OK response. P2 Record-Routed, so it inspects the
   Route set for this dialog. For requests from the UAS towards the UAC
   (the opposite direction than the first INVITE), the next hop will be
   the Contact header field of the INVITE, because P1 did not Record-
   Route. That Contact identified the UAC:

       Contact: UAC;comp=sigcomp

   Since the UAC wants to receive compressed requests (Contact of the
   INVITE), P2 assumes that the UAC would also like to send compressed
   requests (Record-Route of the 200 OK). Therefore, P2 modifies its
   entry in the Record-Route header field of the 200 OK (5). In the
   INVITE (3), P2 did not used the comp=sigcomp parameter. Now it adds
   it in the 200 OK (5). This will allow the UAC sending compressed
   requests within this dialog.

G. Camarillo                                                  [Page 8]

Internet Draft                    SIP                       May 28, 2002

   (6) 200 OK
       Via: UAC;comp=sigcomp
       Contact: UAS

   P1 sends the 200 OK (6) compressed to the UAC because the Via header
   field contained the comp=Sigcomp parameter.

   (7) ACK UAS
       Via: UAC;comp=sigcomp
       Contact: UAC;comp=sigcomp

   The UAC sends the ACK (7) compressed directly to P2 (P1 did not

   (8) ACK UAS
       Via: P2
       Via: UAC;comp=sigcomp
       Contact: UAC;comp=sigcomp

   P2 sends the ACK (8) uncompressed to the UAS.

8 Security Considerations

   A SIP entity receiving a compressed message has to decompress it and
   to parse it. This requires slightly more processing power than only
   parsing a message. This implies that a denial of service attack using
   compressed messages would be slightly worse than an attack with
   uncompressed messages.

   An attacker inserting the parameter comp=sigcomp in a SIP message
   could make a SIP entity send compressed messages to another SIP
   entity that did not support Sigcomp. Appropriate integrity mechanisms
   should be used to avoid this attack.

9 Acknowledges

   Jonathan Rosenberg and Miguel Angel Garcia provided valuable comments
   on this memo.

10 Authors' Addresses

G. Camarillo                                                  [Page 9]

Internet Draft                    SIP                       May 28, 2002

   Gonzalo Camarillo
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   electronic mail:  Gonzalo.Camarillo@ericsson.com

11 Normative References

   [1] J. Rosenberg, H. Schulzrinne,  et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [2] R. Price, J. Rosenberg, C. Bormann, H. Hannu, and Z. Liu,
   "Universal decompressor virtual machine (udvm)," Internet Draft,
   Internet Engineering Task Force, Jan. 2002.  Work in progress.

12 Informative References

   [3] M. Mealling and R. Daniel, "The naming authority pointer (NAPTR)
   DNS resource record," RFC 2915, Internet Engineering Task Force,
   Sept. 2000.

   [4] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying
   the location of services (DNS SRV)," RFC 2782, Internet Engineering
   Task Force, Feb. 2000.

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G. Camarillo                                                 [Page 10]

Internet Draft                    SIP                       May 28, 2002

   This document and the information contained herein is provided on an

G. Camarillo                                                 [Page 11]