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Network Working Group G. Camarillo Request for Comments: 5369 Ericsson Category: Informational October 2008

Framework for Transcoding with the Session Initiation Protocol (SIP)

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.


 This document defines a framework for transcoding with SIP.  This
 framework includes how to discover the need for transcoding services
 in a session and how to invoke those transcoding services.  Two
 models for transcoding services invocation are discussed: the
 conference bridge model and the third-party call control model.  Both
 models meet the requirements for SIP regarding transcoding services
 invocation to support deaf, hard of hearing, and speech-impaired

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Discovery of the Need for Transcoding Services  . . . . . . . . 2
 3.  Transcoding Services Invocation . . . . . . . . . . . . . . . . 4
   3.1.  Third-Party Call Control Transcoding Model  . . . . . . . . 4
   3.2.  Conference Bridge Transcoding Model . . . . . . . . . . . . 6
 4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
 5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . 8
 6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
   6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
   6.2.  Informative References  . . . . . . . . . . . . . . . . . . 9

Camarillo Informational [Page 1] RFC 5369 Transcoding Framework October 2008

1. Introduction

 Two user agents involved in a SIP [RFC3261] dialog may find it
 impossible to establish a media session due to a variety of
 incompatibilities.  Assuming that both user agents understand the
 same session description format (e.g., SDP [RFC4566]),
 incompatibilities can be found at the user agent level and at the
 user level.  At the user agent level, both terminals may not support
 any common codec or may not support common media types (e.g., a text-
 only terminal and an audio-only terminal).  At the user level, a deaf
 person will not understand anything said over an audio stream.
 In order to make communications possible in the presence of
 incompatibilities, user agents need to introduce intermediaries that
 provide transcoding services to a session.  From the SIP point of
 view, the introduction of a transcoder is done in the same way to
 resolve both user level and user agent level incompatibilities.  So,
 the invocation mechanisms described in this document are generally
 applicable to any type of incompatibility related to how the
 information that needs to be communicated is encoded.
    Furthermore, although this framework focuses on transcoding, the
    mechanisms described are applicable to media manipulation in
    general.  It would be possible to use them, for example, to invoke
    a server that simply increases the volume of an audio stream.
 This document does not describe media server discovery.  That is an
 orthogonal problem that one can address using user agent provisioning
 or other methods.
 The remainder of this document is organized as follows.  Section 2
 deals with the discovery of the need for transcoding services for a
 particular session.  Section 3 introduces the third-party call
 control and conference bridge transcoding invocation models, which
 are further described in Sections 3.1 and 3.2, respectively.  Both
 models meet the requirements regarding transcoding services
 invocation in RFC 3351 [RFC3351], which support deaf, hard of
 hearing, and speech-impaired individuals.

2. Discovery of the Need for Transcoding Services

 According to the one-party consent model defined in RFC 3238
 [RFC3238], services that involve media manipulation invocation are
 best invoked by one of the endpoints involved in the communication,
 as opposed to being invoked by an intermediary in the network.
 Following this principle, one of the endpoints should be the one
 detecting that transcoding is needed for a particular session.

Camarillo Informational [Page 2] RFC 5369 Transcoding Framework October 2008

 In order to decide whether or not transcoding is needed, a user agent
 needs to know the capabilities of the remote user agent.  A user
 agent acting as an offerer [RFC3264] typically obtains this knowledge
 by downloading a presence document that includes media capabilities
 (e.g., Bob is available on a terminal that only supports audio) or by
 getting an SDP description of media capabilities as defined in RFC
 3264 [RFC3264].
 Presence documents are typically received in a NOTIFY request
 [RFC3265] as a result of a subscription.  SDP media capabilities
 descriptions are typically received in a 200 (OK) response to an
 OPTIONS request or in a 488 (Not Acceptable Here) response to an
 In the absence of presence information, routing logic that involves
 parallel forking to several user agents may make it difficult (or
 impossible) for the caller to know which user agent will answer the
 next call attempt.  For example, a call attempt may reach the user's
 voicemail while the next one may reach a SIP phone where the user is
 available.  If both terminating user agents have different
 capabilities, the caller cannot know, even after the first call
 attempt, whether or not transcoding will be necessary for the
 session.  This is a well-known SIP problem that is referred to as
 HERFP (Heterogeneous Error Response Forking Problem).  Resolving
 HERFP is outside the scope of this document.
 It is recommended that an offerer does not invoke transcoding
 services before making sure that the answerer does not support the
 capabilities needed for the session.  Making wrong assumptions about
 the answerer's capabilities can lead to situations where two
 transcoders are introduced (one by the offerer and one by the
 answerer) in a session that would not need any transcoding services
 at all.
    An example of the situation above is a call between two GSM
    (Global System for Mobile Communications) phones (without using
    transcoding-free operation).  Both phones use a GSM codec, but the
    speech is converted from GSM to PCM (Pulse Code Modulation) by the
    originating MSC (Mobile Switching Center) and from PCM back to GSM
    by the terminating MSC.
 Note that transcoding services can be symmetric (e.g., speech-to-text
 plus text-to-speech) or asymmetric (e.g., a one-way speech-to-text
 transcoding for a hearing-impaired user that can talk).

Camarillo Informational [Page 3] RFC 5369 Transcoding Framework October 2008

3. Transcoding Services Invocation

 Once the need for transcoding for a particular session has been
 identified as described in Section 2, one of the user agents needs to
 invoke transcoding services.
 As stated earlier, transcoder location is outside the scope of this
 document.  So, we assume that the user agent invoking transcoding
 services knows the URI of a server that provides them.
 Invoking transcoding services from a server (T) for a session between
 two user agents (A and B) involves establishing two media sessions;
 one between A and T and another between T and B.  How to invoke T's
 services (i.e., how to establish both A-T and T-B sessions) depends
 on how we model the transcoding service.  We have considered two
 models for invoking a transcoding service.  The first is to use
 third-party call control [RFC3725], also referred to as 3pcc.  The
 second is to use a (dial-in and dial-out) conference bridge that
 negotiates the appropriate media parameters on each individual leg
 (i.e., A-T and T-B).
 Section 3.1 analyzes the applicability of the third-party call
 control model, and Section 3.2 analyzes the applicability of the
 conference bridge transcoding invocation model.

3.1. Third-Party Call Control Transcoding Model

 In the 3pcc transcoding model, defined in [RFC4117], the user agent
 invoking the transcoding service has a signalling relationship with
 the transcoder and another signalling relationship with the remote
 user agent.  There is no signalling relationship between the
 transcoder and the remote user agent, as shown in Figure 1.

Camarillo Informational [Page 4] RFC 5369 Transcoding Framework October 2008

        |       |
        |   T   |**
        |       |  **
        +-------+    **
          ^   *        **
          |   *          **
          |   *            **
         SIP  *              **
          |   *                **
          |   *                  **
          v   *                    **
        +-------+               +-------+
        |       |               |       |
        |   A   |<-----SIP----->|   B   |
        |       |               |       |
        +-------+               +-------+
         <-SIP-> Signalling
         ******* Media
               Figure 1: Third-Party Call Control Model
 This model is suitable for advanced endpoints that are able to
 perform third party call control.  It allows endpoints to invoke
 transcoding services on a stream basis.  That is, the media streams
 that need transcoding are routed through the transcoder while the
 streams that do not need it are sent directly between the endpoints.
 This model also allows invoking one transcoder for the sending
 direction and a different one for the receiving direction of the same
 Invoking a transcoder in the middle of an ongoing session is also
 quite simple.  This is useful when session changes occur (e.g., an
 audio session is upgraded to an audio/video session) and the
 endpoints cannot cope with the changes (e.g., they had common audio
 codecs but no common video codecs).
 The privacy level that is achieved using 3pcc is high, since the
 transcoder does not see the signalling between both endpoints.  In
 this model, the transcoder only has access to the information that is
 strictly needed to perform its function.

3.2. Conference Bridge Transcoding Model

 In a centralized conference, there are a number of media streams
 between the conference server and each participant of a conference.
 For a given media type (e.g., audio) the conference server sends,

Camarillo Informational [Page 5] RFC 5369 Transcoding Framework October 2008

 over each individual stream, the media received over the rest of the
 streams, typically performing some mixing.  If the capabilities of
 all the endpoints participating in the conference are not the same,
 the conference server may have to send audio to different
 participants using different audio codecs.
 Consequently, we can model a transcoding service as a two-party
 conference server that may change not only the codec in use, but also
 the format of the media (e.g., audio to text).
 Using this model, T behaves as a B2BUA (Back-to-Back User Agent) and
 the whole A-T-B session is established as described in [RFC5370].
 Figure 2 shows the signalling relationships between the endpoints and
 the transcoder.
                  |       |**
                  |   T   |  **
                  |       |\   **
                  +-------+ \\   **
                    ^   *     \\   **
                    |   *       \\   **
                    |   *         SIP  **
                   SIP  *           \\   **
                    |   *             \\   **
                    |   *               \\   **
                    v   *                 \    **
                  +-------+               +-------+
                  |       |               |       |
                  |   A   |               |   B   |
                  |       |               |       |
                  +-------+               +-------+
                   <-SIP-> Signalling
                   ******* Media
                   Figure 2: Conference Bridge Model
 In the conferencing bridge model, the endpoint invoking the
 transcoder is generally involved in less signalling exchanges than in
 the 3pcc model.  This may be an important feature for endpoints using
 low-bandwidth or high-delay access links (e.g., some wireless
 On the other hand, this model is less flexible than the 3pcc model.
 It is not possible to use different transcoders for different streams
 or for different directions of a stream.

Camarillo Informational [Page 6] RFC 5369 Transcoding Framework October 2008

 Invoking a transcoder in the middle of an ongoing session or changing
 from one transcoder to another requires the remote endpoint to
 support the Replaces [RFC3891] extension.  At present, not many user
 agents support it.
 Simple endpoints that cannot perform 3pcc and thus cannot use the
 3pcc model, of course, need to use the conference bridge model.

4. Security Considerations

 The specifications of the 3pcc and the conferencing transcoding
 models discuss security issues directly related to the implementation
 of those models.  Additionally, there are some considerations that
 apply to transcoding in general.
 In a session, a transcoder has access to at least some of the media
 exchanged between the endpoints.  In order to avoid rogue transcoders
 getting access to those media, it is recommended that endpoints
 authenticate the transcoder.  TLS [RFC5246] and S/MIME [RFC3850] can
 be used for this purpose.
 To achieve a higher degree of privacy, endpoints following the 3pcc
 transcoding model can use one transcoder in one direction and a
 different one in the other direction.  This way, no single transcoder
 has access to all the media exchanged between the endpoints.
 The fact that transcoders need to access media exchanged between the
 endpoints implies that endpoints cannot use end-to-end media security
 mechanisms.  Media encryption would not allow the transcoder to
 access the media, and media integrity protection would not allow the
 transcoder to modify the media (which is obviously necessary to
 perform the transcoding function).  Nevertheless, endpoints can still
 use media security between the transcoder and themselves.

5. 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.

6. References

6.1. Normative References

 [RFC3238]  Floyd, S. and L. Daigle, "IAB Architectural and Policy
            Considerations for Open Pluggable Edge Services",
            RFC 3238, January 2002.

Camarillo Informational [Page 7] RFC 5369 Transcoding Framework October 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.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            June 2002.
 [RFC3265]  Roach, A.B., "Session Initiation Protocol (SIP)-Specific
            Event Notification", RFC 3265, June 2002.
 [RFC3351]  Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A.
            van Wijk, "User Requirements for the Session Initiation
            Protocol (SIP) in Support of Deaf, Hard of Hearing and
            Speech-impaired Individuals", RFC 3351, August 2002.
 [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
            Camarillo, "Best Current Practices for Third Party Call
            Control (3pcc) in the Session Initiation Protocol (SIP)",
            BCP 85, RFC 3725, April 2004.
 [RFC3850]  Ramsdell, B., "Secure/Multipurpose Internet Mail
            Extensions (S/MIME) Version 3.1 Certificate Handling",
            RFC 3850, July 2004.
 [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
            Protocol (SIP) "Replaces" Header", RFC 3891,
            September 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.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [RFC5370]  Camarillo, G., "The Session Initiation Protocol (SIP)
            Conference Bridge Transcoding Model", RFC 5370,
            October 2008.

6.2. Informative References

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

Camarillo Informational [Page 8] RFC 5369 Transcoding Framework October 2008

Author's Address

 Gonzalo Camarillo
 Hirsalantie 11
 Jorvas  02420

Camarillo Informational [Page 9] RFC 5369 Transcoding Framework October 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Camarillo Informational [Page 10]

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