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Versions: 00 01                                                         
Internet Engineering Task Force                                   SIP WG
Internet Draft                                              G. Camarillo
                                                                Ericsson
draft-camarillo-sipping-transc-b2bua-00.txt
August 28, 2003
Expires: February, 2004


  The Session Initiation Protocol Conference Bridge Transcoding Model

STATUS OF THIS MEMO

   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
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   http://www.ietf.org/ietf/1id-abstracts.txt

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   http://www.ietf.org/shadow.html.


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.













G. Camarillo                                                  [Page 1]


Internet Draft                    SIP                    August 28, 2003





                           Table of Contents



   1          Introduction ........................................    3
   2          Caller's Invocation .................................    3
   3          Callee's Invocation .................................    3
   4          Security Considerations .............................    4
   5          Contributors ........................................    5
   6          OPEN ISSUES .........................................    5
   7          Authors' Addresses ..................................    5
   8          Bibliography ........................................    5




































G. Camarillo                                                  [Page 2]


Internet Draft                    SIP                    August 28, 2003


1 Introduction

   The framework for transcoding with SIP (draft-camarillo-sipping-
   transc-framework) describes how two SIP UAs can discover
   imcompatibilities 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. Using the
   conference bridge model is one way to perform such invocation.

   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 between both UAs and is
   identified by a URI.

2 Caller's Invocation

   Figure 1 shows the message flow for the caller's invocation of a
   transcoder T. The caller (A) sends an INVITE (1) to the URI of the
   transcoder (T) to establish the session A-T.


   Once this session is established, A sends a REFER (4) to T intructing
   T to INVITE B. The session description that T will use in this INVITE
   to B (6) depends on the service provided by T. A speech-to-text
   text-to-speech transcoder, for instance, would generate a session
   description with an audio stream or with a text stream depending on
   the incoming session description in the first INVITE (1).

   This INVITE from T to B establishes the session T-B. During session
   establishment, following standard REFER procedures, A receives
   NOTIFIES from T informing A about the progress of the establishment
   of this session (T-B).

3 Callee's Invocation

   If a UA receives an INVITE with an offer that is not acceptable it
   can only invoke a transcoder if the caller supports the Replaces [1]
   extension. This support is indicated by the Supported header field in
   the INVITE.

   If the caller (A) does not support Replaces, the callee (B) can
   always reject the session and attempt to establish a new session with
   A following the procedures in Section 2. This way, B would act as a
   caller and, consequently, it would follow the procedures for caller's
   invocation of transcoders.

   Assuming that the caller (A) supports Replaces, the callee (B)



G. Camarillo                                                  [Page 3]


Internet Draft                    SIP                    August 28, 2003




      A                            T                            B

      |                            |                            |
      |------(1) INVITE SDP A----->|                            |
      |                            |                            |
      |<----(2) 200 OK SDP TA------|                            |
      |                            |                            |
      |----------(3) ACK---------->|                            |
      |                            |                            |
      |                            |                            |
      |---------(4) REFER--------->|                            |
      |                            |                            |
      |<--------(5) 200 OK---------|                            |
      |                            |                            |
      |                            |-----(6) INVITE SDP TB----->|
      |                            |                            |
      |                            |<-----(7) 200 OK SDP B------|
      |                            |                            |
      |                            |----------(8) ACK---------->|
      |                            |                            |
      |<--------(9) NOTIFY---------|                            |
      |                            |                            |
      |---------(10) 200 OK------->|                            |
      |                            |                            |
      | ************************** | ************************** |
      |*          MEDIA           *|*          MEDIA           *|
      | ************************** | ************************** |
      |



   Figure 1: Caller's Invocation of a Transcoder


   follows the steps shown in Figure 2 to invoke a transcoder. B sends
   an INVITE to the transcoder (T). Once the session B-T is established,
   B sends a REFER to T instructing T to send an INVITE to A with a
   Replaces header field. When A receives this INVITE, it will replace
   the original dialog (1) with this new dialog.


4 Security Considerations

   This document describes how to use the REFER method invoke
   transcoding services. It does not introduce new security
   considerations besides the ones discussed in [2].




G. Camarillo                                                  [Page 4]


Internet Draft                    SIP                    August 28, 2003


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 OPEN ISSUES

   Now we have A send an INVITE and then a REFER to T. If we combined
   those two transactions in one somehow, we would be saving one RTT and
   some traffic over A's access.

   It is an open issue to decide whether we want to optimize it this way
   or not. In case we decide to do it, this is a similar problem to the
   1:N MESSAGE, where A asks a B2BUA in the network to send a MESSAGE to
   a set of N receivers.

   It is also similar to presence lists, where a B2BUA in the network
   sends a bunch of SUBSCRIBEs on behalf of the UA. There, a URI at the
   B2BUA identifies a set of destinations. In the transcoding case, the
   URI at the B2BUA would identify only one destination, and the binding
   between that URI and the URI of the destination would need to be
   created on the fly (the binding for presence lists is created in
   advance).

   Session policies is something we may want to look at as well,
   although, in principle, it deals with proxies rather than with
   B2BUAs.

   In SIP, the Route header field is used to traverse proxies, but is
   seems that using it for traversing B2BUAs would be stretching its
   semantics too much.

7 Authors' Addresses

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

8 Bibliography

   [1] B. Biggs, R. W. Dean, and R. Mahy, "The session inititation
   protocol (SIP) Engineering Task Force, Aug. 2003.  Work in progress.

   [2] R. Sparks, "The SIP refer method," internet draft, Internet



G. Camarillo                                                  [Page 5]


Internet Draft                    SIP                    August 28, 2003




      A                            T                            B

      |                            |                            |
      |-------------------(1) INVITE SDP A--------------------->|
      |                            |                            |
      |                            |<-----(2) INVITE SDP B------|
      |                            |                            |
      |                            |------(3) 200 OK SDP TB---->|
      |                            |                            |
      |                            |<--------- (4) ACK ---------|
      |                            |                            |
      |                            |                            |
      |                            |<--------(5) REFER----------|
      |                            |                            |
      |                            |---------(6) 200 OK-------->|
      |                            |                            |
      |<-----(7) INVITE SDP TA-----|                            |
      |                            |                            |
      |------(8) 200 OK SDP A----->|                            |
      |                            |                            |
      |<----------(9) ACK----------|                            |
      |                            |                            |
      |                            |---------(10) NOTIFY------->|
      |                            |                            |
      |                            |<--------(11) 200 OK--------|
      |                            |                            |
      |---------------------(12) CANCEL------------------------>|
      |                            |                            |
      |<--------------------(13) 200 OK-------------------------|
      |                            |                            |
      |<-------------(14) 487 Request Terminated----------------|
      |                            |                            |
      |-----------------------(15) ACK------------------------->|
      |                            |                            |
      | ************************** | ************************** |
      |*          MEDIA           *|*          MEDIA           *|
      | ************************** | ************************** |
      |                            |                            |








   Figure 2: Caller's Invocation of a Transcoder



G. Camarillo                                                  [Page 6]


Internet Draft                    SIP                    August 28, 2003


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G. Camarillo                                                  [Page 7]


Internet Draft                    SIP                    August 28, 2003





















































G. Camarillo                                                  [Page 8]