Media negotiation and the handling of preconditions, which will be described in Section 11.6.4, are closely related concepts in IMS. Both are more concerned with the description of the session parameters in SDP. Nevertheless, they have a major influence on SIP signalling.
During media negotiation the two items of UE agree on the set of media they want to use for the session and the codes which will be used for the different media types. Therefore, the SDP offer/answer mechanism is used, which – in IMS – basically works in the following way (Figure 11.5):
The calling UE sends a first SDP offer in the INVITE request to the called UE. This SDP lists all media types (e.g., audio, video or certain applications like whiteboard or chat) the caller wants to use for this session and lists the different codecs that the caller supports for encoding these different media types.
The called UE responds with a first SDP answer, in which it may reject some of the proposed media types. It also reduces the list of codecs by dropping those that it does not support, such that only the codecs that are supported on both ends remain.
After receiving the first answer the caller has to make the final decision on the used codecs. It sends a second offer to the called user, which indicates a single codec for every media type that will be used during the session.
The called UE accepts the second offer and sends an answer back as confirmation.