SIP INVITE

The SIP method used to initiate a session (phone call, video conference), triggering the offer/answer exchange of session parameters via SDP. The caller sends INVITE with an SDP body describing its media capabilities; the callee responds with its own SDP in the 200 OK. A three-way handshake (INVITE → 200 OK → ACK) establishes the dialog. Re-INVITE within an existing dialog can modify session parameters such as codec or hold state.