7. Offerer Processing of the Answer

7. Offerer Processing of the Answer

When the offerer receives the answer, it MAY send media on the accepted stream(s) (assuming it is listed as sendrecv or recvonly in the answer). It MUST send using a media format listed in the answer, and it SHOULD use the first media format listed in the answer when it does send.

The reason this is a SHOULD, and not a MUST (its also a SHOULD, and not a MUST, for the answerer), is because there will oftentimes be a need to change codecs on the fly. For example, during silence periods, an agent might like to switch to a comfort noise codec. Or, if the user presses a number on the keypad, the agent might like to send that using RFC 2833 [9]. Congestion control might necessitate changing to a lower rate codec based on feedback.

The offerer SHOULD send media according to the value of any ptime and bandwidth attribute in the answer.

The offerer MAY immediately cease listening for media formats that were listed in the initial offer, but not present in the answer.