MMUSIC O. Levin Internet-Draft Microsoft Corporation Expires:
March 29, 2005July 1, 2004 G. Camarillo Ericsson September 28,January 2004 The SDP (Session Description Protocol) Label Attribute draft-ietf-mmusic-sdp-media-label-00.txtdraft-ietf-mmusic-sdp-media-label-01.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with RFC 3668. 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 March 29, 2005.July 1, 2004. Copyright Notice Copyright (C) The Internet Society (2004). Abstract This document defines a new Session Description Protocol (SDP) media-level attribute: "label". The "label" attribute carries a pointer to a media stream in the context of an arbitrary network application that uses SDP. The sender of the SDP document can attach the "label" attribute to a particular media stream or streams. The application can then use the provided pointer to refer to each particular media stream in its context. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Motivation for the New label Attribute . . . . . . . . . . . . 3 4. The Label Attribute . . . . . . . . . . . . . . . . . . . . . 4 5. The Label Attribute in the Offer/Answer Model . . . . . . . . 4 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Security Considerations . . . . . . . . . . . . . . . . . . . 5 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 10.1 Normative References . . . . . . . . . . . . . . . . . . . . 6 10.2 Informative References . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . 7 1. Introduction SDP is being used by a variety of distributed over the network applications. These applications deal with multiple sessions being described by SDP  and serving multiple users or services in the context of a single application instance. Applications of this kind need a means to identify a particular media stream across multiple SDP descriptions exchanged with different users. The XCON framework is an example of a centralized conference architecture that uses SDP according to the offer/answer mechanism defined in  to establish media streams with each of the conference participants. Additionally, XCON identifies the need to uniquely identify a media stream in terms of its role in a conference regardless of its media type, transport protocol, and media format. This can be acomplished by using an external document that points to the appropriate media stream and provides information (e.g., the media stream's role in the conference) about it. This specification defines the SDP  "label" media-level attribute, which provides a pointer to a media stream which is described by an m='m' line in an SDP session description. 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  and indicate requirement levels for compliant implementations. 3. Motivation for the New label Attribute Even though SDP and its extensions already provide a few ways to refer to a media stream, none of them is appropriate to be used in the context of external documents that may be created before the session description itself and need to be handled by automata. The "i"'i' SDP attribute, defined in RFC 2327 ,, can be used to label media streams. Nevertheless, values of the "i"'i' attribute are intended for human users and not for automata. The "mid"'mid' SDP attribute, defined in RFC 3388 ,, can be used to identify media streams as well. Nevertheless, the scope of "mid"'mid' is too limited to be used by applications dealing with multiple SDP sessions. This is due to the fact that values of the "mid"'mid' attribute are meaningful in the context of a single SDP session, not in the context of a broader application (e.g., a multiparty application). Another way of referring to a media stream is by using the order of the m-line'm' line in the SDP session document (e.g., the 5th media stream in the session description). This is the mechanism used in the offer/answer model . The problem with this mechanism is that it can only be used to refer to media streams in session descriptions that exist already. There are scenarios where a static document needs to refer, using a pointer, to a media stream that will be negotiated by SDP means and created in the future. When the media stream is eventually created, the application needs to label the media stream so that the pointer in the static document points to the proper media stream in the session description. 4. The Label Attribute This specification defines a new media-level value attribute: "label".'label'. Its formatting in SDP is described by the following BNF:BNF : label-attribute = "a=label:" pointer pointer = token The "label"'label' attribute contains a token which is defined by an application and is used in its context. The new attribute can be attached to m-lines'm' lines in multiple SDP documents allowing the application to logically group the media streams across SDP sessions when necessary. 5. The Label Attribute in the Offer/Answer Model This specification does not define a means to discover whether or not the peer endpoint understands the "label"'label' attribute because "label"'label' values are informative only at the offer/answer model level. At the offer/answer level, it means that the fact that an offer does not contain label attributes does not imply that the answer should not have them. It also means that the fact that an offer contains label attributes does not imply that the answer should have them too. In addition to the basic offer/answer rule above, applications that use "label"'label' as a pointer to media streams MUST specify its usage constraints. For example, such applications MAY mandate support for "label".'label'. In this case, the application will define means for negotiation of the "label"'label' attribute support as a part of its specification. 6. Example The following is an example of an SDP session description that uses the "label"'label' attribute: v=0 o=bob 280744730 28977631 IN IP4 host.example.com s= c=IN IP4 192.0.2.2 t=0 0 m=audio 6886 RTP/AVP 0 a=label:1 m=audio 22334 RTP/AVP 0 a=label:2 7. Security Considerations An attacker may attempt to add, modify, or remove "label"'label' attributes from a session description. This could result in an application behaving in a non-desirable way. So, it is strongly RECOMMENDED that integrity protection be applied to the SDP session descriptions. For session descriptions carried in SIP , 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. IANA Considerations Contact name: Orit Levin email@example.com. Attribute name: "label". Type of attribute Media level. Subject to charset: Not. Purpose of attribute: The "Label"'label' attribute associates a media stream with a label. This label allows the media stream to be referenced by external documents. Allowed attribute values: Any octet string.A token. 9. Acknowledgements Robert Sparks, Adam Roach, and Rohan Mahy provided useful comments on this document. 10. References 10.1 Normative References  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Handley, M.Crocker, D. and V. Jacobson, "SDP: Session Description Protocol",P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2327, April 1998.2234, November 1997.  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.  Camarillo, G., Eriksson, G., Holler, J.Handley, M., Jacobson, V. and H. Schulzrinne, "Grouping of Media Lines in theC. Perkins, "SDP: Session Description Protocol (SDP)", RFC 3388, December 2002.Protocol", draft-ietf-mmusic-sdp-new-21 (work in progress), October 2004. 10.2 Informative References  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.  Koskelainen, P.Camarillo, G., Eriksson, G., Holler, J. and H. Khartabil, "Requirements for Conference Policy Control Protocol", draft-ietf-xcon-cpcp-reqs-04 (workSchulzrinne, "Grouping of Media Lines in progress), August 2004.the Session Description Protocol (SDP)", RFC 3388, December 2002. Authors' Addresses Orit Levin Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA EMail: firstname.lastname@example.org Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland EMail: Gonzalo.Camarillo@ericsson.com 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 email@example.com. 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 (2004). 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.