3.1.5.34.2 User agent behavior for negotiating multiplexed media channels

If a user agent wishes to include multiplexed media streams in an SDP offer, it MUST take care in how it forms the offer. The peer user agent might not be able to parse the multiplexed media descriptions and reject the entire offer.

To interoperate with the broadest set of peer user agents, the offering user agent SHOULD<62> construct a MIME structure containing multiple SDP content parts for the SIP INVITE request body, as described in [MS-SIPRE] section 3.15.4.1, with one of the SDP content parts omitting the multiplexed media streams. An SDP part containing the multiplexed media streams SHOULD be placed as the last part in the multi-part MIME structure.

If a set of media streams are multiplexed in an SDP offer, the corresponding media streams in the SDP answer (not including those rejected by the answerer) MUST also be multiplexed. A user agent that does support multiplexing does not have to accept all the multiplexed media streams in an SDP offer. For example, if the SDP offer includes seven multiplexed "main-video" media descriptions, but the receiving user agent supports at most five such "main-video" multiplexed streams, it SHOULD accept the first five "main-video" media descriptions in the offer and reject the remaining.