Simple definition
A SIP transaction contains a request and its associated responses.
Purpose and use
A SIP transaction tracks a request and associated responses from transmission through completion under the method's rules.
How does it work?
Identifiers such as the Via branch parameter and request method associate messages; INVITE transactions have specific handling rules.
Example and verification
For repeated retransmissions, check whether the response reaches the client and matches the correct transaction before initiating another call attempt.
Points to watch
A transaction does not necessarily cover an entire call. ACK handling differs according to the INVITE result.
In short
Separating transactions from dialogs makes timers, retransmissions and late responses easier to diagnose.