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Network Working Group                                       G. CamarilloRequest for Comments: 5369                                      EricssonCategory: Informational                                     October 2008Framework 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.Abstract   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   individuals.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Discovery of the Need for Transcoding Services  . . . . . . . .23.  Transcoding Services Invocation . . . . . . . . . . . . . . . .43.1.  Third-Party Call Control Transcoding Model  . . . . . . . .43.2.  Conference Bridge Transcoding Model . . . . . . . . . . . .64.  Security Considerations . . . . . . . . . . . . . . . . . . . .75.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . . .86.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .86.1.  Normative References  . . . . . . . . . . . . . . . . . . .86.2.  Informative References  . . . . . . . . . . . . . . . . . .9Camarillo                    Informational                      [Page 1]

RFC 5369                 Transcoding Framework              October 20081.  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 Sections3.1 and3.2, respectively.  Both   models meet the requirements regarding transcoding services   invocation inRFC 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 inRFC 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 inRFC3264 [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   INVITE.   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 20083.  Transcoding Services Invocation   Once the need for transcoding for a particular session has been   identified as described inSection 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, andSection 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   stream.   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   accesses).   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.  References6.1.  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 2008Author's Address   Gonzalo Camarillo   Ericsson   Hirsalantie 11   Jorvas  02420   Finland   EMail: Gonzalo.Camarillo@ericsson.comCamarillo                    Informational                      [Page 9]

RFC 5369                 Transcoding Framework              October 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 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   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Camarillo                    Informational                     [Page 10]