SIP quick reference: methods, responses, headers, and transports
A compact SIP reference for common methods, response classes, headers, transports, and troubleshooting questions.
Topics
SIP signalling
SIP timers: retransmissions, timeouts, and session refreshes
Understand SIP transaction timers, retransmissions, session timers, and why slow or lossy networks can cause calls to fail.
SIP methods and call flow: INVITE, ACK, BYE, and CANCEL
Follow the main SIP methods and understand the normal sequence for registration, call setup, cancellation, transfer, and teardown.
SIP headers explained: the fields that control a call
Learn what Via, From, To, Contact, Call-ID, CSeq, Route, Record-Route, and Authorization do in a SIP message.
SIP response codes: a practical troubleshooting guide
Understand common SIP status codes, what they mean, and what to check when registration or call setup fails.