6.2 Multicast Streams

6.2 Multicast Streams

Unlike unicast, where there is a two-sided view of the stream, there is only a single view of the stream for multicast. As such, generating an answer to a multicast offer generally involves modifying a limited set of aspects of the stream.

If a multicast stream is accepted, the address and port information in the answer MUST match that of the offer. Similarly, the directionality information in the answer (sendonly, recvonly, or sendrecv) MUST equal that of the offer. This is because all participants in a multicast session need to have equivalent views of the parameters of the session, an underlying assumption of the multicast bias of RFC 2327.

The set of media formats in the answer MUST be equal to or be a subset of those in the offer. Removing a format is a way for the answerer to indicate that the format is not supported.

The ptime and bandwidth attributes in the answer MUST equal the ones in the offer, if present. If not present, a non-zero ptime MAY be added to the answer.