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 Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt To view the list Internet-Draft Shadow Directories, see 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 The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication 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 implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Full Copyright Statement Copyright (c) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. G. Camarillo [Page 7] Internet Draft SIP August 28, 2003 G. Camarillo [Page 8]