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SIP quick reference: methods, responses, headers, and transports

SIP creates, changes, and ends communication sessions. Use this page as a map, then open the detailed guide for the specific failure.

Methods

REGISTER associates a user with a contact. INVITE starts or changes a session. ACK confirms an INVITE result. BYE ends an established dialog. CANCEL stops a pending INVITE. OPTIONS checks capabilities. REFER requests a transfer.

Response classes

  • 1xx provisional progress
  • 2xx success
  • 3xx redirection
  • 4xx request or user failure
  • 5xx server failure
  • 6xx global failure or rejection

First questions

Did the request leave? Did a response return? Did SDP negotiate a reachable media address? Did RTP flow both ways? See [SIP methods]( /sip-methods-call-flow/) and SIP response codes.

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 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 response codes: a practical troubleshooting guide

Understand common SIP status codes, what they mean, and what to check when registration or call setup fails.