MMUSIC Working Group F. Andreasen Internet-Draft Cisco Systems Document: draft-andreasen-mmusic-sdp-simcap-00.txt November 2000 Category: Informational SDP Simple Capability Negotiation 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 proposes a set of Session Description Protocol (SDP) attributes that 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 Andreasen Informational - Expires May 2001 [Page 1] Internet-Draft SDP Simple Capability Negotiation November 2000 "next generation SDP" (SDPng) [4] that supports both session 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). Unless the endpoint is willing to support two media streams at the same time, this can not currently be expressed in SDP. 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 propose a minimal and backwards compatible capability negotiation feature in SDP by defining a set of new SDP attributes. It should be noted, that the mechanism is not intended to solve the general capability negotiation problem targeted by SDPng. It is merely intended as a simple and limited solution to the most urgent problems facing current users of SDP. 4. Requirements In the following sections, we list and discuss requirements for the simple capability negotiation. 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 capability negotiation problems encountered by current users of SDP. A more precise definition of this particular requirement is desirable, however the details of it will invariably be subjective. Andreasen Informational - Expires May 2001 [Page 2] Internet-Draft SDP Simple Capability Negotiation November 2000 Nevertheless, the following provides some additional detail based on [4], [7] and previous discussion within the MMUSIC working group: The following are considered minimum requirements: * It must be possible to describe each capability independently. * Lists of alternative values for a capability must be supported. * Supplying a capability should simply imply a willingness to support that capability, but not an actual commitment. * The description of a capability should be straightforward from its representation in the session description itself (do not want to come up with elaborate new syntax). The following list a set of requirements that were considered, but where further discussion is felt to be needed: * The ability to express ranges of values for a particular capability, possibly with "step" values within the range, e.g. "between 10 and 100 in increments of 10". The following list a set of requirements that were considered, but seen as non-essential: * Capability interdependence, incl. - grouping capabilities, - expressing simultaneous capability sets, - expressing alternative capability sets - constraining the number of uses of a certain capability (set) 5. Simple Capability Negotiation Attributes In this section, we provide a list of SDP attributes enabling the simple SDP capability negotiation we are looking for. The attributes form a capability set which describes the media capabilities of the endpoint. The capability set begins with a single sequence number followed by one or more capability descriptions listing all media formats the endpoint is currently able and willing to support. A subsequent request to use one of these media formats is however not guaranteed to succeed, e.g. due to limited DSP processing power, or bandwidth constraints. The sequence number is on the form a=sqn: where is an integer between 0 and 255 (both included). The initial sequence number is 0 and increments by 1 modulo 256 with each new capability set from the endpoint. The sequence number may either be provided as a session- or media-level attribute. Each capability description in the capability set is on the form: a=cdsc: Andreasen Informational - Expires May 2001 [Page 3] Internet-Draft SDP Simple Capability Negotiation November 2000 where is an integer between 1 and 255 (both included) identifying the capability, and , , and are defined as in the SDP ôm=ö line. The capability number should start with 1 in the first capability description, and be incremented by the number of capabilities in the for each subsequent capability description. A capability description may include one or more capability parameter lines on the form: a=cpar: where is either bandwidth information (ôb=ö) or an attribute (ôa=ö). A capability parameter line provides additional parameters for the preceding capability description. Capability parameter lines MUST immediately follow the "cdsc" line they refer to, thus a capability description ends at the first non "cpar" line that follows the "cdsc" attribute line. Capability descriptions may be provided at the session- or media- level. A capability description provided at the session-level applies to all the media streams specified, where as a capability description provided at the media-level only applies to that particular media stream. Below we show an example session description using the above capability negotiation attributes: v=0 o=- 25678 753849 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 18 96 a=rtpmap:96 telephone-event a=fmtp:96 0-15,32-35 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 96 a=cpar: a=fmtp:96 0-16,32-35 a=cdsc: 4 image udptl t38 a=cdsc: 5 image tcp t38 The sender of this session description is currently prepared to send and receive G.729 audio as well as telephone-events 0-15 and 32-35. The sender is furthermore capable of supporting: * media streams using PCMU encoding * telephone events 0-16 and 32-35 * T.38 fax relay using udp or tcp (see [8]) Andreasen Informational - Expires May 2001 [Page 4] Internet-Draft SDP Simple Capability Negotiation November 2000 Note, that the first capability number is 1, where as the next is 4, since three media formats were included in the first capability. Also note, that the rtpmap for payload type 96 was not included in the capability description again, as it was already specified for the media (m=) line. 6. Security Considerations The addition of the simple capability negotiation attributes to SDP is not believed to affect security. 7. 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/Dra ft_T38_Annex_D.txt) 9 Beser, B., "Codec Capabilities Attribute for SDP", Internet Draft, draft-beser-mmusic-capabilities-00.txt, March 2000. 8. Acknowledgments Andreasen Informational - Expires May 2001 [Page 5] Internet-Draft SDP Simple Capability Negotiation November 2000 This work draws upon the ongoing work on SDPng; in particular [4]. Furthermore, this work was inspired by [7] and the CableLabs PacketCable project. Related work can be found in [9] as well. 9. Author's Addresses Flemming Andreasen Cisco Systems 499 Thornall Street, 8th floor Edison, NJ Email: fandreas@cisco.com Andreasen Informational - Expires May 2001 [Page 6] Internet-Draft SDP Simple Capability Negotiation November 2000 Full Copyright Statement Copyright (C) The Internet Society (2000). 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. 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. Acknowledgement Funding for the RFC editor function is currently provided by the Internet Society. Andreasen Informational - Expires May 2001 [Page 7]