12.10. Sessions
A multimedia session consists of a set of multimedia senders and receivers and the data streams that flow between them. Sessions use SIP dialogs and follow SIP rules for sending requests within dialogs.
The role SIP plays in establishing a multimedia session revolves around its ability to carry SDP media descriptions in its message bodies. SDP is used to describe the session, and the offer/answer model is employed [RFC3264]. SDP and the offer/answer model are described in Chapter 14. Section 12.10.1 describes the restrictions SIP has on such a model.
The session is initiated using the INVITE method, the request line and headers, which are populated with the UAC (see Section 12.7.4.1). The body is populated with an SDP offer. The answer may arrive in a provisional response or in the 2xx response.
INVITE requests follow a three-way handshake model: this means that the UAC, after receiving a final response to an INVITE request, must send an ACK request. The ACK request does not require a response; in fact, a response must never be sent to an ACK request.
If the UAC wants to cancel an invitation to a session after it sends the INVITE request, it sends a CANCEL request. The CANCEL request is constructed in a similar way to that in which the request-URI, the To header, the From header, the Call-ID header and the numeric part of the CSeq header are copied from the INVITE request. The method part of the CSeq header holds the CANCEL method. A UAS receiving a CANCEL responds ...
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