





Network Working Group                                       G. Camarillo
Request for Comments: 5370                                      Ericsson
Category: Standards Track                                   October 2008


                 The Session Initiation Protocol (SIP)
                  Conference Bridge Transcoding Model

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes how to invoke transcoding services using the
   conference bridge model.  This way of invocation meets the
   requirements for SIP regarding transcoding services invocation to
   support deaf, hard of hearing, and speech-impaired individuals.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Caller's Invocation .............................................3
      3.1. Procedures at the User Agent ...............................3
      3.2. Procedures at the Transcoder ...............................3
      3.3. Example ....................................................4
      3.4. Unsuccessful Session Establishment .........................6
   4. Callee's Invocation .............................................7
   5. Security Considerations .........................................7
   6. Contributors ....................................................8
   7. References ......................................................8
      7.1. Normative References .......................................8
      7.2. Informative References .....................................9













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1.  Introduction

   RFC 5369 [RFC5369] describes how two SIP [RFC3261] UAs (User Agents)
   can discover incompatibilities that prevent them from establishing a
   session (e.g., lack of support for a common codec or for a common
   media type).  When such incompatibilities are found, the UAs need to
   invoke transcoding services to successfully establish the session.
   The transcoding framework introduces two models to invoke transcoding
   services: the 3pcc (third-party call control) model [RFC4117] and the
   conference bridge model.  This document specifies the conference
   bridge model.

   In the conference bridge model for transcoding invocation, a
   transcoding server that provides a particular transcoding service
   (e.g., speech-to-text) behaves as a B2BUA (Back-to-Back User Agent)
   between both UAs and is identified by a URI.  As shown in Figure 1,
   both UAs, A and B, exchange signalling and media with the transcoder
   T.  The UAs do not exchange any traffic (signalling or media)
   directly between them.

                  +-------+
                  |       |**
                  |   T   |  **
                  |       |\   **
                  +-------+ \\   **
                    ^   *     \\   **
                    |   *       \\   **
                    |   *         SIP  **
                   SIP  *           \\   **
                    |   *             \\   **
                    |   *               \\   **
                    v   *                 \    **
                  +-------+               +-------+
                  |       |               |       |
                  |   A   |               |   B   |
                  |       |               |       |
                  +-------+               +-------+

                   <-SIP-> Signalling
                   ******* Media

                  Figure 1: Conference bridge model

   Sections 3 and 4 specify how the caller A or the callee B,
   respectively, can use the conference bridge model to invoke
   transcoding services from T.





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2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [RFC2119], and indicate requirement
   levels for compliant implementations.

3.  Caller's Invocation

   User agent A needs to perform two operations to invoke transcoding
   services from T for a session between user agent A and user agent B.
   User agent A needs to establish a session with T and provide T with
   user agent B's URI so that T can generate an INVITE towards user
   agent B.

3.1.  Procedures at the User Agent

   User agent A uses the procedures for RFC 5366 [RFC5366] to provide T
   with B's URI using the same INVITE that establishes the session
   between A and T.  That is, user agent A adds to the INVITE a body
   part whose disposition type is recipient-list [RFC5363].  This body
   part consists of a URI-list that contains a single URI: user agent
   B's URI.

      Note that, as described in the transcoding framework [RFC5369],
      the transcoding model described in this document is modeled as a
      two-party conference server.  Consequently, this document focuses
      on two-party sessions that need transcoding.  Multi-party sessions
      can be established using INVITE requests with multiple URIs in
      their bodies, as specified in [RFC5366].

3.2.  Procedures at the Transcoder

   On receiving an INVITE with a URI-list body, the transcoder follows
   the procedures in [RFC5366] to generate an INVITE request towards the
   URI contained in the URI-list body.  Note that the transcoder acts as
   a B2BUA, not as a proxy.

   Additionally, the transcoder MUST generate the From header field of
   the outgoing INVITE request using the same value as the From header
   field included in the incoming INVITE request, subject to the privacy
   requirements (see [RFC3323] and [RFC3325]) expressed in the incoming
   INVITE request.  Note that this does not apply to the "tag"
   parameter.






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   The session description the transcoder includes in the outgoing
   INVITE request depends on the type of transcoding service that
   particular transcoder provides.  For example, a transcoder resolving
   audio codec incompatibilities would generate a session description
   listing the audio codecs the transcoder supports.

   When the transcoder receives a final response for the outgoing INVITE
   requests, it generates a new final response for the incoming INVITE
   request.  This new final response SHOULD have the same status code as
   the one received in the response for the outgoing INVITE request.

   If a transcoder receives an INVITE request with a URI-list with more
   than one URI, it SHOULD return a 488 (Max 1 URI allowed in URI-list)
   response.

3.3.  Example

   Figure 2 shows the message flow for the caller's invocation of a
   transcoder T.  The caller A sends an INVITE (1) to the transcoder (T)
   to establish the session A-T.  Following the procedures in [RFC5366],
   the caller A adds a body part whose disposition type is recipient-
   list [RFC5363].

        A                           T                           B
        |                           |                           |
        |-----(1) INVITE SDP A----->|                           |
        |                           |                           |
        |<-(2) 183 Session Progress-|                           |
        |                           |-----(3) INVITE SDP TB---->|
        |                           |                           |
        |                           |<-----(4) 200 OK SDP B-----|
        |                           |                           |
        |                           |---------(5) ACK---------->|
        |<----(6) 200 OK SDP TA-----|                           |
        |                           |                           |
        |---------(7) ACK---------->|                           |
        |                           |                           |
        | ************************* | ************************* |
        |**        Media          **|**        Media          **|
        | ************************* | ************************* |
        |                           |                           |

      Figure 2: Successful invocation of a transcoder by the caller








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   The following example shows an INVITE with two body parts: an SDP
   [RFC4566] session description and a URI-list.

   INVITE sip:transcoder@example.com SIP/2.0
   Via: SIP/2.0/TCP client.chicago.example.com
       ;branch=z9hG4bKhjhs8ass83
   Max-Forwards: 70
   To: Transcoder <sip:transcoder@example.org>
   From: A <sip:A@chicago.example.com>;tag=32331
   Call-ID: d432fa84b4c76e66710
   CSeq: 1 INVITE
   Contact: <sip:A@client.chicago.example.com>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
        SUBSCRIBE, NOTIFY
   Allow-Events: dialog
   Accept: application/sdp, message/sipfrag
   Require: recipient-list-invite
   Content-Type: multipart/mixed;boundary="boundary1"
   Content-Length: 556

   --boundary1
   Content-Type: application/sdp

   v=0
   o=example 2890844526 2890842807 IN IP4 chicago.example.com
   s=-
   c=IN IP4 192.0.2.1
   t=0 0
   m=audio 50000 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   --boundary1
   Content-Type: application/resource-lists+xml
   Content-Disposition: recipient-list

   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
                  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
     <list>
       <entry uri="sip:B@example.org" />
     </list>
   </resource-lists>
   --boundary1--

   On receiving the INVITE, the transcoder generates a new INVITE
   towards the callee.  The transcoder acts as a B2BUA, not as a proxy.
   Therefore, this new INVITE (3) belongs to a different transaction
   than the INVITE (1) received by the transcoder.



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   When the transcoder receives a final response (4) from the callee, it
   generates a new final response (6) for INVITE (1).  This new final
   response (6) has the same status code as the one received in the
   response from the callee (4).

3.4.  Unsuccessful Session Establishment

   Figure 3 shows a similar message flow as the one in Figure 3.
   Nevertheless, this time the callee generates a non-2xx final response
   (4).  Consequently, the transcoder generates a non-2xx final response
   (6) towards the caller as well.

   A                           T                           B
   |                           |                           |
   |-----(1) INVITE SDP A----->|                           |
   |                           |                           |
   |<-(2) 183 Session Progress-|                           |
   |                           |-----(3) INVITE SDP TB---->|
   |                           |                           |
   |                           |<----(4) 603 Decline-------|
   |                           |                           |
   |                           |---------(5) ACK---------->|
   |<----(6) 603 Decline-------|                           |
   |                           |                           |
   |---------(7) ACK---------->|                           |
   |                           |                           |

         Figure 3: Unsuccessful session establishment

   The ambiguity in this flow is that, if the provisional response (2)
   gets lost, the caller does not know whether the 603 (Decline)
   response means that the initial INVITE (1) was rejected by the
   transcoder or that the INVITE generated by the transcoder (4) was
   rejected by the callee.  The use of the "History-Info" header field
   [RFC4244] between the transcoder and the caller resolves the previous
   ambiguity.

   Note that this ambiguity problem could also have been resolved by
   having transcoders act as a pure conference bridge.  The transcoder
   would respond with a 200 (OK) to the INVITE request from the caller,
   and it would generate an outgoing INVITE request towards the callee.
   The caller would get information about the result of the latter
   INVITE request by subscribing to the conference event package
   [RFC4575] at the transcoder.  Although this flow would have resolved
   the ambiguity problem without requiring support for the "History-
   Info" header field, it is more complex, requires a higher number of
   messages, and introduces higher session setup delays.  That is why it
   was not chosen to implement transcoding services.



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4.  Callee's Invocation

   If a UA receives an INVITE with a session description that is not
   acceptable, it can redirect it to the transcoder by using a 302
   (Moved Temporarily) response.  The Contact header field of the 302
   (Moved Temporarily) response contains the URI of the transcoder plus
   a "?body=" parameter.  This parameter contains a recipient-list body
   with B's URI.  Note that some escaping (e.g., for Carriage Returns
   and Line Feeds) is needed to encode a recipient-list body in such a
   parameter.  Figure 4 shows the message flow for this scenario.

   A                           T                           B
   |                           |                           |
   |-------------------(1) INVITE SDP A------------------->|
   |                           |                           |
   |<--------------(2) 302 Moved Temporarily---------------|
   |                           |                           |
   |-----------------------(3) ACK------------------------>|
   |                           |                           |
   |-----(4) INVITE SDP A----->|                           |
   |                           |                           |
   |<-(5) 183 Session Progress-|                           |
   |                           |-----(6) INVITE SDP TB---->|
   |                           |                           |
   |                           |<-----(7) 200 OK SDP B-----|
   |                           |                           |
   |                           |---------(8) ACK---------->|
   |<----(9) 200 OK SDP TA-----|                           |
   |                           |                           |
   |--------(10) ACK---------->|                           |
   |                           |                           |
   | ************************* | ************************* |
   |**        Media          **|**        Media          **|
   | ************************* | ************************* |

       Figure 4: Callee's invocation of a transcoder

   Note that the syntax resulting from encoding a body into a URI as
   described earlier is quite complex.  It is actually simpler for
   callees to invoke transcoding services using the 3pcc transcoding
   model [RFC4117] instead.

5.  Security Considerations

   Transcoders implementing this specification behave as a URI-list
   service as described in [RFC5366].  Therefore, the security
   considerations for URI-list services discussed in [RFC5363] apply
   here as well.



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   In particular, the requirements related to list integrity and
   unsolicited requests are important for transcoding services.  User
   agents SHOULD integrity protect URI-lists using mechanisms such as
   S/MIME [RFC3850] or TLS [RFC5246], which can also provide URI-list
   confidentiality if needed.  Additionally, transcoders MUST
   authenticate and authorize users and MAY provide information about
   the identity of the original sender of the request in their outgoing
   requests by using the SIP identity mechanism [RFC4474].

   The requirement in [RFC5363] to use opt-in lists (e.g., using RFC
   5360 [RFC5360]) deserves special discussion.  The type of URI-list
   service implemented by transcoders following this specification does
   not produce amplification (only one INVITE request is generated by
   the transcoder on receiving an INVITE request from a user agent) and
   does not involve a translation to a URI that may be otherwise unknown
   to the caller (the caller places the callee's URI in the body of its
   initial INVITE request).  Additionally, the identity of the caller is
   present in the INVITE request generated by the transcoder.
   Therefore, there is no requirement for transcoders implementing this
   specification to use opt-in lists.

6.  Contributors

   This document is the result of discussions amongst the conferencing
   design team.  The members of this team include Eric Burger, Henning
   Schulzrinne, and Arnoud van Wijk.

7.  References

7.1.  Normative References

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [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.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.







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   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3850]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Certificate Handling", RFC
              3850, July 2004.

   [RFC4117]  Camarillo, G., Burger, E., Schulzrinne, H., and A. van
              Wijk, "Transcoding Services Invocation in the Session
              Initiation Protocol (SIP) Using Third Party Call Control
              (3pcc)", RFC 4117, June 2005.

   [RFC5369]  Camarillo, G., "Framework for Transcoding with the Session
              Initiation Protocol", RFC 5369, October 2008.

   [RFC5363]  Camarillo, G. and A.B. Roach, "Framework and Security
              Considerations for Session Initiation Protocol (SIP) URI-
              List Services", RFC 5363, October 2008.

   [RFC5366]  Camarillo, G. and A. Johnston, "Conference Establishment
              Using Request-Contained Lists in the Session Initiation
              Protocol (SIP)", RFC 5366, October 2008.

   [RFC4244]  Barnes, M., Ed., "An Extension to the Session Initiation
              Protocol (SIP) for Request History Information", RFC 4244,
              November 2005.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

7.2.  Informative References

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
              Session Initiation Protocol (SIP) Event Package for
              Conference State", RFC 4575, August 2006.

   [RFC5360]  Rosenberg, J., "A Framework for Consent-Based
              Communications in the Session Initiation Protocol (SIP)",
              RFC 5360, October 2008.






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Author's Address

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com










































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