RFC 3407 (rfc3407) - Page 2 of 10
Session Description Protocol (SDP) Simple Capability Declaration
Alternative Format: Original Text Document
RFC 3407 SDP Simple Capability Declaration October 2002 continue doing so for the foreseeable future. Nevertheless, in many cases these signaling 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 cannot 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 these codecs. In practice, Digital Signal Processor (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 define a minimal and backwards compatible capability declaration feature in SDP by defining a set of new SDP attributes. Together, these attributes define a capability set, which consists of a capability set sequence number followed by one or more capability descriptions. Each capability description in the set contains information about supported media formats, but the endpoint is not committing to use any of these. In order to actually use a declared capability, session negotiation will have to be done by means outside the scope of this document, e.g., using the offer/answer model [8]. 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. 3. Simple Capability Declaration Attributes The SDP Simple Capability Declaration (simcap) is defined by a set of SDP attributes. Together, these attributes form a capability set which describes the complete media capabilities of the endpoint. Any previous capability sets issued by the endpoint for the session in question no longer apply. The capability set consists of a sequence number and one or more capability descriptions. Each such capability description describes the media type and media formats supported and may include one or more capability parameters to further define the capability. A session description MUST NOT contain more than one capability set, however the capability set can describe capabilities at both the session and media level. Capability descriptions provided at the session level apply to all media streams of the media Andreasen Standards Track
