The examples so far have focused on various SIP requests, responses, and header fields. Although some mention has been made of the message body and entity header fields, the message body has not been described in any detail. Although a SIP message body can carry many different types of information (such as an image or text to display), the most common mes- sage body will be session information describing the media to be exchanged between the parties. The session description will include media information such as the RTP payload type, addresses, and ports. The format of that description will normally be according to SDP.

SIP uses SDP in an answer/offer mode. A caller sends an INVITE with an SDP description that describes the set of media formats, addresses, and ports that the caller is willing to use. This set of media formats comprises an offer by the caller. The called party responds with an SDP description that aligns with the offered SDP description, but which includes an acceptance or rejection of each of the offered media formats. The result of this exchange is an agreement between the two parties as to the types of media they are willing to share.