SIP ACK

The SIP method sent by the caller to confirm receipt of a final response to an INVITE, completing the three-way handshake that establishes a dialog. ACK is sent for every final response to INVITE — 200 OK and non-2xx alike. For 2xx responses, ACK is sent directly to the contact address; for non-2xx responses, ACK is sent to the proxy. ACK carries no body unless the initial INVITE lacked an SDP offer, in which case ACK contains the answer.