Skip to main content

8.6. Mapping to SDP

The information carried in the MIME media type specification has a specific mapping to fields in SDP [5], which is commonly used to describe RTP sessions. When SDP is used to specify retransmissions for an RTP stream, the mapping is done as follows:

  • The MIME types ("video"), ("audio"), ("text"), and ("application") go in the SDP "m=" as the media name.

  • The MIME subtype ("rtx") goes in SDP "a=rtpmap" as the encoding name. The RTP clockrate in "a=rtpmap" MUST be that of the retransmission payload type. See Section 4 for details on this.

  • The AVPF profile-specific parameters "ack" and "nack" go in SDP "a=rtcp-fb". Several SDP "a=rtcp-fb" are used for several types of feedback. See the AVPF profile [1] for details.

  • The retransmission payload-format-specific parameters "apt" and "rtx-time" go in the SDP "a=fmtp" as a semicolon-separated list of parameter=value pairs.

  • Any remaining parameters go in the SDP "a=fmtp" attribute by copying them directly from the MIME media type string as a semicolon-separated list of parameter=value pairs.

In the following sections, some example SDP descriptions are presented. In some of these examples, long lines are folded to meet the column width constraints of this document; the backslash ("\") at the end of a line and the carriage return that follows it should be ignored.

Last updated on