Internet Engineering Task Force SIP WG Internet Draft G. Camarillo Ericsson H. Schulzrinne Columbia University E. Burger SnowShore Networks draft-camarillo-mmusic-source-sink-01.txt August 28, 2003 Expires: February 2004 The Source and Sink Attributes for the Session Description Protocol 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 defines two media level SDP attributes, namely source and sink. They are intended to be used to invoke services that involve media manipulation, such as transcoding services. G. Camarillo et. al. [Page 1] Internet Draft SIP August 28, 2003 Table of Contents 1 Introduction ........................................ 3 1.1 Terminology ......................................... 3 2 Applicability ....................................... 3 3 Syntax of Source and Sink ........................... 3 4 SDP Example ......................................... 4 5 Use of Source and Sink with SIP ..................... 4 6 IANA Considerations ................................. 5 7 Security Considerations ............................. 5 8 Acknowledgements .................................... 5 9 Authors' Addresses .................................. 5 10 Normative References ................................ 5 11 Informative References .............................. 6 G. Camarillo et. al. [Page 2] Internet Draft SIP August 28, 2003 1 Introduction Servers performing media manipulations, such as transcoding or mixing, take the contents of one or several media streams as input and send their output over another media stream. A client requesting this type of service from a server needs to identify which media streams are to be used as input and which ones will be used to send the output of the media manipulation process. This document defines two SDP media level attributes, namely source and sink, that can be used to explicitly convey this information in an SDP session description. 1.1 Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and indicate requirement levels for compliant SIP implementations. 2 Applicability A server that provides simple media manipulation services between a single unidirectional input (recvonly) stream and a single unidirectional output (sendonly) stream, such as a text-to-speech server, does not need to specify source and sink attributes in the SDP. However, a server that needs to correlate more than the simple media manipulation service needs a mechanism to specify which media descriptions refer to which directionality of the input streams. Thus, servers that use SDP [2] to provide more complex services that involve more media streams (like some of the ones described by [5]) SHOULD make use of the source and sink attributes. The source and sink attributes MUST NOT be used to perform media alignment between SIP [3] user agents. The nth matching rules defined by the offer/answer model [4] must be used regardless of the presence or absence of the sink and source attributes. 3 Syntax of Source and Sink We define the following media level SDP attributes: source-attribute = "a=source:" identification-tag sink-attribute = "a=sink:" identification-tag identification-tag = token G. Camarillo et. al. [Page 3] Internet Draft SIP August 28, 2003 An SDP session description that contains a media stream with a particular identification tag in a source attribute MUST have the same identification tag in, at least, one sink attribute. An SDP session description that contains a media stream with a particular identification tag in a sink attribute MUST have the same identification tag in, at least, one source attribute. If an entity receives a session description that breaks the rules stated above, it MUST act as if it had received a malformed session description. 4 SDP Example The SDP session description below sent to a server indicates that incoming audio from the first stream has to be sent over the second audio stream and over the text stream. Incoming text has to be sent over the first audio stream (but not over the second one). The exact media manipulations to be applied are typically identified by the URI that identifies the service (e.g., sip:texttospeech@domain.com). m=audio 40000 RTP/AVP 0 c=IN IP4 B.domain.com a=source:1 a=sink:2 m=audio 20000 RTP/AVP 0 c=IN IP4 A.domain.com a=recvonly a=sink:1 m=text 20002 RTP/AVP t140 c=IN IP4 A.domain.com a=source:2 a=sink:1 5 Use of Source and Sink with SIP A user agent that wishes to use the source and sink attributes adds them to an offer. The answerer SHOULD copy the same source and sink attributes in its answer for all the streams that were accepted (i.e., their port number is different than zero). An answerer that understand the source and sink attributes MUST NOT add or remove any of them from a stream that was accepted. The offerer knows whether the answerer understands these attributes because the answer will contain source and sink attributes. If the G. Camarillo et. al. [Page 4] Internet Draft SIP August 28, 2003 answerer does not understand them, the answer will not contain source and sink attributes. 6 IANA Considerations This document defines two media level SDP attributes: "source" and "sink". They should be registered in SDP parameters registry. http://www.iana.org/assignments/sdp-parameters 7 Security Considerations An attacker adding, removing or modifying source or sink a= lines could change the expected behavior from a media manipulation service. It is thus STRONGLY RECOMMENDED that integrity protection be applied to the SDP session descriptions. For session descriptions carried in SIP [3], S/MIME is the natural choice to provide such end-to-end integrity protection, as described in RFC 3261. Other applications MAY use a different form of integrity protection. 8 Acknowledgements Jeff Van Dyke provided useful comments on this document. 9 Authors' Addresses Gonzalo Camarillo Ericsson Advanced Signalling Research Lab. FIN-02420 Jorvas Finland electronic mail: Gonzalo.Camarillo@ericsson.com Henning Schulzrinne Dept. of Computer Science Columbia University 1214 Amsterdam Avenue New York, NY 10027 USA electronic mail: schulzrinne@cs.columbia.edu Eric W. Burger SnowShore Networks, Inc. Chelmsford, MA USA electronic mail: eburger@snowshore.com 10 Normative References G. Camarillo et. al. [Page 5] Internet Draft SIP August 28, 2003 [1] S. Bradner, "Key words for use in RFCs to indicate requirement levels," RFC 2119, Internet Engineering Task Force, Mar. 1997. [2] M. Handley and V. Jacobson, "SDP: session description protocol," RFC 2327, Internet Engineering Task Force, Apr. 1998. [3] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J. Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session initiation protocol," RFC 3261, Internet Engineering Task Force, June 2002. [4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with session description protocol (SDP)," RFC 3264, Internet Engineering Task Force, June 2002. 11 Informative References [5] N. Charlton, M. Gasson, G. Gybels, M. Spanner, 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, Internet Engineering Task Force, Aug. 2002. 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. G. Camarillo et. al. [Page 6] Internet Draft SIP August 28, 2003 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 et. al. [Page 7]