Session Description Protocol
In Chapter 11, we looked at the SIP protocol for establishing and tearing down sessions. In this chapter, we will look at the use of the Session Description Protocol (SDP) in describing parameters carried within the SIP messages to describe multimedia sessions. SDP is based on RFC 4566 whose opening paragraph provides a concise purpose statement:
When initiating multimedia teleconference, Voice-over-IP calls, streaming video, or other sessions, there is a requirement to convey media details, transport addresses, and other session description metadata to the participants. 
Thus, SDP will allow the originating and terminating parties1 to describe their respective capabilities in establishing a session along with any desired media formats and port addresses.