What does the "ACK" method signify in SIP?

The "ACK" method in SIP (Session Initiation Protocol) plays a crucial role in the signaling process, particularly in the context of establishing a communication session. When a client sends an INVITE request to initiate a session, it expects to receive a response back from the server or the other party. This response can indicate whether the INVITE was successful, rejected, or needs further processing.

The ACK method is specifically designed to confirm the successful receipt of that response, particularly with a 200 OK response to an INVITE. This confirmation completes the session establishment phase in SIP, indicating that both parties are synchronized and ready to proceed with the media transmission. The use of ACK is essential in the reliability and correctness of the SIP protocol, ensuring that the initial communication state is acknowledged by the client sending the INVITE.

In contrast, other options relate to different aspects of SIP. Initiating a media session is handled through INVITE requests, ending a session involves methods like BYE, and requesting information is typically done using OPTIONS. Each of these methods serves distinct purposes, underscoring why the ACK method's function in confirming receipt of an INVITE's response is central to the establishment of the session.