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Network Working Group                                           O. LevinRequest for Comments: 4574                         Microsoft CorporationCategory: Standards Track                                   G. Camarillo                                                                Ericsson                                                             August 2006The Session Description Protocol (SDP) Label AttributeStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).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 Contents1. Introduction ....................................................22. Terminology .....................................................23. Motivation for the New label Attribute ..........................24. The Label Attribute .............................................35. The Label Attribute in the Offer/Answer Model ...................46. Example .........................................................47. Security Considerations .........................................48. IANA Considerations .............................................59. Acknowledgements ................................................510. References .....................................................610.1. Normative References ......................................610.2. Informative References ....................................6Levin & Camarillo           Standards Track                     [Page 1]

RFC 4574                  SDP Label Attribute                August 20061.  Introduction   SDP is being used by a variety of distributed-over-the-network   applications.  These applications deal with multiple sessions being   described by SDP [4] 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 [3] 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 accomplished 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.  The SIP Event   Package for Conference State [7] defines and uses a concrete format   for such external documents.   This specification defines the SDP [4] "label" media-level attribute,   which provides a pointer to a media stream that is described by an   '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 inBCP 14,RFC 2119 [1] 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' SDP attribute, defined inRFC 2327 [4], can be used to label   media streams.  Nevertheless, values of the 'i' attribute are   intended for human users and not for automata.Levin & Camarillo           Standards Track                     [Page 2]

RFC 4574                  SDP Label Attribute                August 2006   The 'mid' SDP attribute, defined inRFC 3388 [6], can be used to   identify media streams as well.  Nevertheless, the scope of 'mid' is   too limited to be used by applications dealing with multiple SDP   sessions.  This is because values of the '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 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 [3].   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'.  Its formatting in SDP is described by the following ABNF   [2]:      label-attribute    = "a=label:" pointer      pointer            = token      token              = 1*(token-char)      token-char         = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39                           / %x41-5A / %x5E-7E   The token-char and token elements are defined in [4] but included   here to provide support for the implementor of this SDP feature.   The 'label' attribute contains a token that is defined by an   application and is used in its context.  The new attribute can be   attached to 'm' lines in multiple SDP documents allowing the   application to logically group the media streams across SDP sessions   when necessary.Levin & Camarillo           Standards Track                     [Page 3]

RFC 4574                  SDP Label Attribute                August 20065.  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' attribute because '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' as a pointer to media streams MUST specify its usage   constraints.  For example, such applications MAY mandate support for   'label'.  In this case, the application will define means for   negotiation of the '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' attribute:      v=0      o=bob 280744730 28977631 IN IP4 host.example.com      s=      i=A Seminar on the session description protocol      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:27.  Security Considerations   An attacker may attempt to add, modify, or remove '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 [5], S/MIME is the natural choice   to provide such end-to-end integrity protection, as described inRFC3261 [5].  Other applications MAY use a different form of integrity   protection.Levin & Camarillo           Standards Track                     [Page 4]

RFC 4574                  SDP Label Attribute                August 20068.  IANA Considerations   The IANA has registered the following new SDP attribute:   Contact name:          Orit Levin oritl@microsoft.com.   Attribute name:        "label".   Type of attribute:     Media level.   Subject to charset:    Not.   Purpose of attribute:  The 'label' attribute associates a media   stream with a label.  This label allows the media stream to be   referenced by external documents.   Allowed attribute values:  A token.9.  Acknowledgements   Robert Sparks, Adam Roach, and Rohan Mahy provided useful comments on   this document.Levin & Camarillo           Standards Track                     [Page 5]

RFC 4574                  SDP Label Attribute                August 200610.  References10.1.  Normative References   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax        Specifications: ABNF",RFC 4234, October 2005.   [3]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with        Session Description Protocol (SDP)",RFC 3264, June 2002.   [4]  Handley, M., Jacobson, V. and C. Perkins, "SDP: Session        Description Protocol",RFC 4566, July 2006.10.2.  Informative References   [5]  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.   [6]  Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,        "Grouping of Media Lines in the Session Description Protocol        (SDP)",RFC 3388, December 2002.   [7]  Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session        Initiation Protocol (SIP) Event Package for Conference State",RFC 4575, August 2006.Levin & Camarillo           Standards Track                     [Page 6]

RFC 4574                  SDP Label Attribute                August 2006Authors' Addresses   Orit Levin   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052   USA   EMail: oritl@microsoft.com   Gonzalo Camarillo   Ericsson   Hirsalantie 11   Jorvas  02420   Finland   EMail: Gonzalo.Camarillo@ericsson.comLevin & Camarillo           Standards Track                     [Page 7]

RFC 4574                  SDP Label Attribute                August 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Levin & Camarillo           Standards Track                     [Page 8]