MMUSIC Working Group F. Andreasen Internet-Draft Cisco Systems Document: draft-andreasen-mmusic-sdp-simcap-reqts-00.txt February 2001 Category: Informational SDP Simple Capability Negotiation Requirements Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. 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. 1. Abstract This document presents a set of requirements for defining Session Description Protocol (SDP) attributes that will allow SDP to provide a minimal and backwards compatible capability negotiation mechanism. The mechanism is intended as a simple and limited solution to the general capability negotiation problem being addressed by ongoing work on the next generation of SDP, also known as SDPng. 2. Conventions used in this document 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 [2]. 3. Introduction The Session Description Protocol (SDP) [3] describes multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. SDP was not intended to provide capability negotiation, however as the need for this has become increasingly important, work has begun on a "next generation SDP" (SDPng) [4] that supports both session Andreasen Informational - Expires August 2001 [Page 1] Internet-Draft SDP Simple Capability Negotiation Reqts. February 2001 description and capability negotiation. SDPng is not anticipated to be backwards compatible with SDP and work on SDPng is currently only in the requirements phase. However, several other protocols, e.g. SIP [5] and MGCP [6], use SDP, and are likely to continue doing so for the foreseeable future. Nevertheless, in many cases these protocols have an urgent need for some limited form of capability negotiation. For example, an endpoint may support G.711 audio (over RTP) as well as T.38 fax relay (over UDP or TCP [8]). However, with current SDP, this can only be expressed by describing two separate media streams, which the endpoint must then support at the same time. Another example involves support for multiple codecs. An endpoint indicates this by including all the codecs in the "m=" line in the session description. However, the endpoint thereby also commits to simultaneous support for each of those codecs. In practice, DSP memory and processing power limitations may not make this feasible. As noted in [4], the problem with SDP is, that media descriptions are used to describe session parameters as well as capabilities without a clear distinction between the two. In this document, we provide a set of requirements for providing a minimal and backwards compatible capability negotiation feature in SDP. It should be noted, that this mechanism is not intended to solve the general capability negotiation problem targeted by SDPng. It is only intended as a simple and limited solution to the most urgent real world problems facing current users of SDP. 4. Requirements In the following sections, we provide requirements for the simple capability negotiation mechanism. 4.1 Backwards Compatibility The solution must be backwards compatible with SDP. In particular, it must adhere to the current SDP grammar. Furthermore, implementations that do not support it must be able to ignore and skip capability information provided without affecting the semantics of the remaining SDP. 4.2 Simplicity and Limited Scope The solution must be simple both in terms of syntax and semantics. In line with this, the scope of the solution should only be to solve the most common and pressing real world capability negotiation problems encountered by current users of SDP. 4.3 Capabilities and Capability Set The following provides a set of more detailed requirements. Andreasen Informational - Expires August 2001 [Page 2] Internet-Draft SDP Simple Capability Negotiation Reqts. February 2001 In order to do capability negotiation, it must be possible to provide one or more capabilities. Each capability must be independent and the capabilities provided form the capability set. It must be possible to provide a capability set at the session-level and the media level. A capability set provided at the session-level must apply to the entire session where as a capability set provided at the media level must only apply to the particular media stream within which the capability set was provided. Providing a capability must imply a willingness and ability to support that capability, but not an actual commitment. In line with this principle and for reasons of simplicity, it must be permissible to provide a (potentially static) capability set that is independent of the actual media stream parameters provided for the session. It is thus possible that a subsequent attempt to use a given capability can not be honored, e.g. due to a change in available resources. A capability set should contain a handle that allows for easy referencing of the capability set. Each capability within the capability set should similarly contain a handle that allows for easy referencing of the capability within that capability set. Each capability must at a minimum contain a media description with the media type, transport protocol, and media format for the capability as defined in [3]. Furthermore, it must be possible to provide additional capability parameters for each capability provided. In particular, it must be possible to provide one or more of the following capability parameters: * Bandwidth information for the capability. * Attribute information for the capability. For each capability parameter, it must be possible to provide: * One or more alternative values for the capability parameter. * One or more allowable numerical ranges for capability parameters that otherwise contain a single numerical value. Finally, the encoding of a capability should be straightforward and well-defined based on its encoding in the session description itself. Neither the syntax nor semantics of a particular parameter should thus affect this encoding, although of course only numerical value attributes can use the numerical range description. 4.4 Rejected Requirements In addition to the requirements provided above, the following requirements were considered and rejected, as they are seen as non- essential: * Capability interdependence, incl. - grouping capabilities, Andreasen Informational - Expires August 2001 [Page 3] Internet-Draft SDP Simple Capability Negotiation Reqts. February 2001 - expressing simultaneous capability sets, - expressing alternative capability sets - constraining the number of uses of a certain capability (set) 5. Security Considerations The addition of the simple capability negotiation attributes to SDP is not believed to affect security. 6. References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 [3] M. Handley and V. Jacobson, "SDP: session description protocol," Request for Comments (Proposed Standard) 2327, Internet Engineering Task Force, Apr. 1998. [4] Kutscher, Ott, Bormann, "Requirements for Session Description and Capability Negotiation", draft-kutscher-mmusic-sdpng-req- 00.txt, July 14, 2000 [5] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP: session initiation protocol," Request for Comments (Proposed Standard) 2543, Internet Engineering Task Force, Mar. 1999. [6] Arango, M., Dugan, A., Elliott, I., Huitema, C. and S. Pickett, "Media Gateway Control Protocol (MGCP) Version 1.0", RFC 2705, October 1999. [7] J. Ott, J. Kutscher, C. Bormann, "Capability description for group cooperation", draft-ott-mmusic-cap-00.txt, June 1999 [8] PROPOSED T.38 AMENDMENT û REC. T.38 ANNEX D, Geneva, 2-10 February, 2000, (available from ftp://standards.nortelnetworks.com/itu_to_ietf/SG8/February00/Dr aft_T38_Annex_D.txt) [9] Beser, B., "Codec Capabilities Attribute for SDP", Internet Draft, draft-beser-mmusic-capabilities-00.txt, March 2000. Andreasen Informational - Expires August 2001 [Page 4] Internet-Draft SDP Simple Capability Negotiation Reqts. February 2001 7. Acknowledgments This work draws upon the ongoing work on SDPng; in particular [4], as well as discussions in the MMUSIC working group. Furthermore, this work was inspired by [7] and the CableLabs PacketCable project. Related work can be found in [9] as well. 8. Author's Addresses Flemming Andreasen Cisco Systems 499 Thornall Street, 8th floor Edison, NJ Email: fandreas@cisco.com Andreasen Informational - Expires August 2001 [Page 5] Internet-Draft SDP Simple Capability Negotiation Reqts. February 2001 Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. 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. 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