https://tools.ietf.org/html/rfc5285#section-5 Stream or media-level attribute used to indicate the presence of RTP Header Extensions, and the mapping of local identifiers used in the header extension to a larger namespace.
A usable mapping MUST use IDs in the valid range, and each ID in this range MUST be used only once for each media (or only once if the mappings are session level). Mappings that do not conform to these rules MAY be presented, for instance, during offer/answer negotiation as described in the next section, but remapping to conformant values is necessary before they can be applied.
Each extension is named by a URI. Each local identifier potentially used in the stream is mapped to a string using an attribute of the form: a=extmap:VALUE["/"DIRECTION] URI EXTENSIONATTRIBUTES Where URI is a URI, as above, VALUE is the local identifier (ID) of this extension and is an integer in the valid range inclusive (0 is reserved for padding in both forms, and 15 is reserved in the one-byte header form), and direction is one of "sendonly", "recvonly", "sendrecv", or "inactive" (without the quotes). Example:
This attribute maps from an RTP payload type number (as used in an "m=" line) to an format name denoting the payload format to be used. It also provides information on the clock rate and format parameters. It is a media-level attribute that is not dependent on charset.
An SDP media description with a transport protocol of "RTP/AVP" or "RTP/SAVP".
The SDP media attribute "rid" specifies restrictions defining a unique RTP payload configuration.
The allowed directions for an RID attribute.
A restriction for an RID attribute.
The SDP media attribute "simulcast" describes, independently for send and receive directions, the number of simulcast RTP streams as well as potential alternative formats for each simulcast RTP stream.
The allowed directions for a simulcast attribute.
A stream for a simulcast stream description.
A stream description for a simulcast attribute.
A simulcast stream identifier.
The SDP media attribute "ssrc" indicates a property (known as a "source-level attribute") of a media source (RTP stream) within an RTP session.
SSRC atribute names.
The SDP media attribute "ssrc-group" expresses a relationship among several sources of an RTP session.