SIP protocol basics for telecom

SIP — The Language of IMS

Session Initiation Protocol (SIP) is the signaling protocol used by IMS for all session management. SIP uses text-based messages similar to HTTP. The key methods are REGISTER (bind UE address to S-CSCF), INVITE (initiate a call), ACK (confirm session establishment after a three-way handshake), BYE (terminate an active session), CANCEL (abort a pending INVITE), and UPDATE or re-INVITE (modify an active session mid-call). Each SIP transaction follows a request-response model where the initiator sends a method and receives numeric response codes indicating the result.