Message Session Relay Protocol
In Chapter 11, we saw how Session Initiation Protocol (SIP) is used to set up and tear down sessions. Chapter 12 covered the Offer/Answer Model. We summarize a few essentials from those chapters as follows:
Three messages are present in a successful ◾ INVITE transaction: INVITE, 200 OK, and ACK. INVITE/200 OK/ACK is sometimes called the threeway handshake. More messages (e.g., 100 Trying, 180 Ringing) may be present. In the Offer/Answer Model, Session Description Protocol (SDP) session ◾ descriptions are exchanged during the 3-way handshake. In the most common approach, the offer is an SDP session description comprising the body of the INVITE request, and the answer is a session description comprising the body of the 200 OK response.