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rfc:rfc5370

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

Camarillo Standards Track [Page 1] RFC 5370 Conference Transcoding Model October 2008

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.

Camarillo Standards Track [Page 2] RFC 5370 Conference Transcoding Model October 2008

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.

Camarillo Standards Track [Page 3] RFC 5370 Conference Transcoding Model October 2008

 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

Camarillo Standards Track [Page 4] RFC 5370 Conference Transcoding Model October 2008

 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
  1. -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
  1. -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.

Camarillo Standards Track [Page 5] RFC 5370 Conference Transcoding Model October 2008

 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.

Camarillo Standards Track [Page 6] RFC 5370 Conference Transcoding Model October 2008

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.

Camarillo Standards Track [Page 7] RFC 5370 Conference Transcoding Model October 2008

 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.

Camarillo Standards Track [Page 8] RFC 5370 Conference Transcoding Model October 2008

 [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.

Camarillo Standards Track [Page 9] RFC 5370 Conference Transcoding Model October 2008

Author's Address

 Gonzalo Camarillo
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: Gonzalo.Camarillo@ericsson.com

Camarillo Standards Track [Page 10] RFC 5370 Conference Transcoding Model October 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Camarillo Standards Track [Page 11]

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