What does an 'ACK' message in the SIP protocol confirm?

The 'ACK' message in the SIP protocol plays a crucial role in call establishment and confirms that an INVITE response has been successfully received. When a SIP client sends an INVITE request to initiate a call, it waits for a response to this request. Once it receives a final response—like a 200 OK indicating that the call setup was successful—it then sends an ACK message. This ACK effectively acknowledges that the client has received the final response, thereby completing the three-way handshake required to establish the session.

Understanding the function of the ACK message is essential in the context of how SIP establishes and maintains communication sessions. It is important to note that the ACK does not initiate a call or terminate a session; its primary role is to confirm the receipt of the INVITE response, indicating that the client can now start sending and receiving media traffic associated with that call.