9. Indicating Capabilities

Before an agent sends an offer, it is helpful to know if the media formats in that offer would be acceptable to the answerer. Certain protocols, like SIP, provide a means to query for such capabilities. SDP can be used in responses to such queries to indicate capabilities. This section describes how such an SDP message is formatted. Since SDP has no way to indicate that the message is for the purpose of capability indication, this is determined from the context of the higher layer protocol. The ability of baseline SDP to indicate capabilities is very limited. It cannot express allowed parameter ranges or values, and can not be done in parallel with an offer/answer itself. Extensions might address such limitations in the future.

An SDP constructed to indicate media capabilities is structured as follows. It MUST be a valid SDP, except that it MAY omit both "e=" and "p=" lines. The "t=" line MUST be equal to "0 0". For each media type supported by the agent, there MUST be a corresponding media description of that type. The session ID in the origin field MUST be unique for each SDP constructed to indicate media capabilities. The port MUST be set to zero, but the connection address is arbitrary. The usage of port zero makes sure that an SDP formatted for capabilities does not cause media streams to be established if it is interpreted as an offer or answer.

The transport component of the "m=" line indicates the transport for that media type. For each media format of that type supported by the agent, there SHOULD be a media format listed in the "m=" line. In the case of RTP, if dynamic payload types are used, an rtpmap attribute MUST be present to bind the type to a specific format. There is no way to indicate constraints, such as how many simultaneous streams can be supported for a particular codec, and so on.

v=0
o=carol 28908764872 28908764872 IN IP4 100.3.6.6
s=-
t=0 0
c=IN IP4 192.0.2.4
m=audio 0 RTP/AVP 0 1 3
a=rtpmap:0 PCMU/8000
a=rtpmap:1 1016/8000
a=rtpmap:3 GSM/8000
m=video 0 RTP/AVP 31 34
a=rtpmap:31 H261/90000
a=rtpmap:34 H263/90000

Figure 1: SDP Indicating Capabilities

The SDP of Figure 1 indicates that the agent can support three audio codecs (PCMU, 1016, and GSM) and two video codecs (H.261 and H.263).

9. Indicating Capabilities​

9. Indicating Capabilities