SIPPING Working Group G. Camarillo Internet-Draft Ericsson Expires: December 3, 2005 June 1, 2005 The Session Initiation Protocol (SIP) Conference Bridge Transcoding Model draft-ietf-sipping-transc-conf-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 3, 2005. Copyright Notice Copyright (C) The Internet Society (2005). 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. Camarillo Expires December 3, 2005 [Page 1]
Internet-Draft Conference Transcoding Model June 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Caller's Invocation . . . . . . . . . . . . . . . . . . . . . 4 3.1 Unsuccessful Session Establishment . . . . . . . . . . . . 6 4. Callee's Invocation . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1 Normative References . . . . . . . . . . . . . . . . . . . 8 8.2 Informational References . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . 10 Camarillo Expires December 3, 2005 [Page 2]
Internet-Draft Conference Transcoding Model June 2005 1. Introduction The Framework for Transcoding with SIP [6] describes how two SIP [4] UAs (User Agents) 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. The transcoding framework introduces two models to invoke transcoding services: the 3pcc (third-party call control) model [7] 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 Section 3 and Section 4 specify how the caller A or the callee B, respectively, can use the conference bridge model to invoke transcoding services from T. Camarillo Expires December 3, 2005 [Page 3]
Internet-Draft Conference Transcoding Model June 2005 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 [3] and indicate requirement levels for compliant implementations. 3. Caller's Invocation A needs to perform two operations to invoke transcoding services from T for a session between A and B. A needs to establish a session with T and provide T with B's URI so that T can generate an INVITE towards B. A uses the procedures for Conference Establishment Using Request- Contained Lists in SIP [9] to provide T with B's URI using the same INVITE that establishes the session between A and T. 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 [9], A adds a body part whose disposition type is recipient-list [8]. This body part consists of a URI-list that MUST contain a single URI: B's URI. If a trancoder receives a URI-list with more than one URI, it SHOULD return a 488 (Max 1 URI allowed in URI-list) response. 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 Expires December 3, 2005 [Page 4]
Internet-Draft Conference Transcoding Model June 2005 The following example shows an INVITE with two body parts: an SDP [10] 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 Conten-Type: multipart/mixed;boundary="boundary1" Content-Length: xxx --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 20000 RTP/AVP 0 a=rtpmap:0 PCMU/8000 --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 Camarillo Expires December 3, 2005 [Page 5]
Internet-Draft Conference Transcoding Model June 2005 than the INVITE (1) received by the transcoder. 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) SHOULD have the same status code as the one received in the response from the callee (4). 3.1 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) 404 Not Found-----| | | | | |---------(5) ACK---------->| |<----(6) 404 Not Found-----| | | | | |---------(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 404 (Not Found) response means that the initial INVITE (1) did not reach the transcoder or that the INVITE generated by the transcoder (4) did not reach the callee. To resolve this ambiguity, the callee can either require the use of the reliable provisional responses [5] SIP extension or send an OPTIONS request to the transcoder to check whether it is reachable. 4. Callee's Invocation If a UA receives an INVITE with an offer 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 Camarillo Expires December 3, 2005 [Page 6]
Internet-Draft Conference Transcoding Model June 2005 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. <t> Please view in a fixed-width font such as Courier. 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 A does not necessarily need to be the one performing the recursion on the 302 (Moved Temporarily) response. Any proxy in the path between A and B may perform such a recursion. 5. Security Considerations TBD. Need to mention how consent applies to this work when consent is more mature. Need to mention TLS [1] and S/MIME [2]. Camarillo Expires December 3, 2005 [Page 7]
Internet-Draft Conference Transcoding Model June 2005 6. IANA Considerations This document does not contain any IANA actions. 7. 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. 8. References 8.1 Normative References [1] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [2] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Certificate Handling", RFC 3850, July 2004. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] 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. [5] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [6] Camarillo, G., "Framework for Transcoding with the Session Initiation Protocol", draft-camarillo-sipping-transc-framework-00 (work in progress), August 2003. [7] Camarillo, G., "Transcoding Services Invocation in the Session Initiation Protocol (SIP) Using Third Party Call Control (3pcc)", draft-ietf-sipping-transc-3pcc-02 (work in progress), September 2004. [8] Camarillo, G. and A. Roach, "Requirements and Framework for Session Initiation Protocol (SIP)Uniform Resource Identifier (URI)-List Services", draft-ietf-sipping-uri-services-02 (work in progress), December 2004. [9] Camarillo, G. and A. Johnston, "Conference Establishment Using Request-Contained Lists in the Session Initiation Protocol Camarillo Expires December 3, 2005 [Page 8]
Internet-Draft Conference Transcoding Model June 2005 (SIP)", draft-ietf-sipping-uri-list-conferencing-02 (work in progress), December 2004. 8.2 Informational References [10] Handley, M., "SDP: Session Description Protocol", draft-ietf-mmusic-sdp-new-24 (work in progress), February 2005. Author's Address Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: Gonzalo.Camarillo@ericsson.com Camarillo Expires December 3, 2005 [Page 9]
Internet-Draft Conference Transcoding Model June 2005 Intellectual Property Statement 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 in BCP 78 and BCP 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 at http://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. Disclaimer of Validity 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 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. Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Camarillo Expires December 3, 2005 [Page 10]
