Session Establishment Flows

Referencing RFC3665, there are 11 basic session establishment flows that illustrate the diverse scenarios encountered in SIP communications, ranging from successful session establishments to various unsuccessful attempts due to no answers or busy signals. These flows, including "Successful Session Establishment" and "Session Establishment Through Two Proxies," provide a framework for understanding the best practices in SIP communication setups. Further exploration into call forwarding scenarios and their specifics will be addressed in dedicated chapters.

We'll delve deeper into the technological aspects now. Typically, SIP operates over UDP, but it's important for SIP devices to also support TCP. A SIP message is divided into two main components: