Editing Session Initiation Protocol (section)

==Transactions==
[[Image:SIP signaling.png|right|thumb|Example: User1's UAC uses an ''invite client transaction'' to send the initial INVITE (1) message. If no response is received after a timer-controlled wait period the UAC may choose to terminate the transaction or retransmit the INVITE. Once a response is received, User1 is confident the INVITE was delivered reliably. User1's UAC must then acknowledge the response. On delivery of the ACK (2), both sides of the transaction are complete. In this case, a dialog may have been established.<ref>{{cite web|first=James|last=Wright |publisher=Konnetic |url=http://www.konnetic.com/Documents/KonneticSIPIntroduction.pdf |title=SIP - An Introduction |access-date=2011-01-11}}</ref>]]

SIP defines a transaction mechanism to control the exchanges between participants and deliver messages reliably. A transaction is a state of a session, which is controlled by various timers. Client transactions send requests and server transactions respond to those requests with one or more responses. The responses may include provisional responses with a response code in the form ''1xx'', and one or multiple final responses (2xx – 6xx).

Transactions are further categorized as either type ''invite'' or type ''non-invite''. Invite transactions differ in that they can establish a long-running conversation, referred to as a ''dialog'' in SIP, and so include an acknowledgment (ACK) of any non-failing final response, e.g., ''200 OK''.