MMUSIC J. Lennox Internet-Draft Vidyo Intended status: Standards Track July 6, 2009 Expires: January 7, 2010 A Session Description Protocol (SDP) Attribute for Maximum Media Source Count Indication draft-lennox-mmusic-sdp-max-sources-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 January 7, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract The Real-Time Transport Protocol (RTP) is a multi-point protocol, supporting multiple simultaneous sending sources in an RTP session. Lennox Expires January 7, 2010 [Page 1] Internet-Draft Max Media Source Count Indication in SDP July 2009 However, many existing RTP endpoints cannot usefully receive more than one simultaneous source. This document provides a Session Description Protocol (SDP) attribute that allows endpoints to indicate the maximum number of sources they can usefully receive. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. The "max-sources" Media Attribute . . . . . . . . . . . . . . . 3 4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 4 5. Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 7.1. New SDP Media-Level Attribute . . . . . . . . . . . . . . . 5 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 8.1. Normative References . . . . . . . . . . . . . . . . . . . 5 8.2. Informative References . . . . . . . . . . . . . . . . . . 6 Appendix A. Open issues . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 Lennox Expires January 7, 2010 [Page 2] Internet-Draft Max Media Source Count Indication in SDP July 2009 1. Introduction The Real-Time Transport Protocol (RTP) [RFC3550] is a multi-point protocol. It supports multiple simultaneous media senders, and, as documented in RTP Topologies [RFC5117], scales from two-party point- to-point sessions to large, wide-area sessions. However, many existing systems do not make use of this capability. Especially for Voice-over-IP and multimedia conferencing systems using SIP [RFC3261], many existing terminals will not decode more than a single RTP stream at a time per RTP session. Indeed, many deployed systems are known to misbehave badly upon receiving more than one simultaneous RTP stream in an RTP session. Thus, even if endpoints can receive, decode, and usefully present multiple sources in an RTP session, there is no portable way for their peers to know this and take advantage of it. To remedy this situation, this document defines a new SDP attribute, "max-sources", which specifies the maximum number of sources that an endpoint in an RTP session can usefully process. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119] and indicate requirement levels for compliant implementations. 3. The "max-sources" Media Attribute This section defines a new SDP media-level attribute, "max-sources", which indicates the maximum number of sources supported in an RTP session. a=max-sources: a=max-sources:unlimited The SDP media attribute "max-sources" indicates the maximum number of RTP sources that an RTP receiver supports within an RTP session. is the maximum number of sources that can usefully be decoded, expressed as a positive decimal integer or the string "unlimited". Thus, it indicates the maximum number of RTP senders that should be present in the session, other than those that the SDP Lennox Expires January 7, 2010 [Page 3] Internet-Draft Max Media Source Count Indication in SDP July 2009 author itself will send. The "max-sources" media attribute MAY be used for any RTP-based media transport. It is not defined for other transports. The "max-sources" attribute is only defined for SDP Offer/Answer [RFC3264] messages, not declarative SDP. A receiver of a "max- sources" attribute in an offer or answer SHOULD restrict the number of sources that it transmits on the RTP session to the number of sources mentioned in the SDP message. The "max-sources" attribute does not support the value of "0". If no sources are supported on a session, an appropriate SDP sendrecv attribute (e.g. "sendonly") SHOULD be used instead. The "max-sources" attribute SHOULD NOT be used with RTP sessions sent over any-source multicast (those that RTP Topologies [RFC5117] describes as "Topo-Multicast", or the multicast side of "Topo- Translator"), because new participants cannot immediately determine how many existing senders are present in the RTP session. For the unicast portion of translator-based topologies, it is the translator's responsibility to forward only the appropriate number of sources along the unicast path. Figure 1 in Section 5 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for the "max-sources" attribute. The "max-sources" media attribute is not dependent on charset. 4. Backward Compatibility As mentioned in the introduction, it is unclear with existing endpoints whether they can usefully support multiple sources on an RTP session. In most circumstances, therefore, existing endpoints will default to sending only a single source per session. This will be compatible with any requested value of "max-sources" even if the receiver of the SDP does not support the attribute. 5. Formal Grammar This section gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for each the "max-sources" media attribute. Lennox Expires January 7, 2010 [Page 4] Internet-Draft Max Media Source Count Indication in SDP July 2009 max-sources-attr = "max-sources:" ( 1*DIGIT / "unlimited" ) attribute =/ max-sources-attr Figure 1: Syntax of the "max-sources" media attribute 6. Security Considerations All the security implications of RTP [RFC3550] and of SDP [RFC4566] apply. Explicitly indicating the number of RTP sources supported in an RTP media stream does not appear to add further security issues. 7. IANA Considerations 7.1. New SDP Media-Level Attribute This document defines a new SDP media-level attribute, "max-sources". This attribute should be registered by IANA under "Session Description Protocol (SDP) Parameters" under "att-field (media level only)". Its format is defined in Section 3 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. Lennox Expires January 7, 2010 [Page 5] Internet-Draft Max Media Source Count Indication in SDP July 2009 8.2. Informative References [RFC3261] 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. [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, January 2008. Appendix A. Open issues o Does "max-sources" need to be defined for declarative SDP (e.g. for RTSP)? Author's Address Jonathan Lennox Vidyo, Inc. 433 Hackensack Avenue Sixth Floor Hackensack, NJ 07601 US Email: jonathan@vidyo.com Lennox Expires January 7, 2010 [Page 6]