SDP Simple Capability Negotiation (SDP Simcap) draft-andreasen-mmusic-sdp-simcap-00.txt 49th IETF - December 13, 2000 Flemming Andreasen fandreas@cisco.com

Situation SDP currently used for two things without a clear distinction: Describe media parameters for the session Describe capabilites Capability description limited to media parameters leads to problems with, e.g. Different media types Multiple codecs Real problems today.

Multiple Media Types A single media line can only express a single media type. Two “m=“ lines implies simultaneous support of two separate media streams. Example (audio and T.38 fax relay): m=audio 3456 RTP/AVP 0 m=image 3458 udptl t38 Similar issue with upcoming modem relay standards, and possibly other.

Multiple Codecs Supported codecs listed in media line implies commitment to use any of the codecs. But DSP’s have limited resources, especially memory: Real-time codec switch on the fly problematic. Thus cannot list all possible codecs in media line: Limits initial negotiation ability as well as mid-call changes.

SDPng SDPng will take care of the previous problems and much more, but: SDPng not backwards compatible with SDP SDPng is work in progress Existing base of existing SDP users which is unlikely to go away any time soon: SIP, MGCP, Megaco, RTSP, etc. Need a simple and limited solution we can use with SDP: SDP Simcap proposal to address this

SDP Simcap Simple and limited backwards compatible SDP capability negotiation - general capability negotiation problem addressed by SDPng. How simple and how limited ? Proposed minimum requirements in draft: Each capability description independent Alternative values supported Capability does not imply commitment (no reserve) Capability syntax follows media description (simple) Further discussion needed

Syntax Define additional SDP attributes. Sequence number Capability Capability description Capability parameter(s) Example 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

Next Steps Soliciting input and comments: Working Group Item ? Look further at scope and requirements first Working Group Item ? Informational ?