SIP Transaction

A single request-response exchange in SIP identified by the branch parameter in the Via header, with built-in retransmission timers for unreliable transports like UDP. A client transaction sends a request and handles retransmissions until a final response is received; a server transaction receives the request and returns responses. The ACK for non-2xx INVITE responses is part of the transaction; the ACK for 2xx is a separate transaction. Multiple transactions compose a SIP dialog.