What does the SIP method "REGISTER" do?

The SIP method "REGISTER" is crucial in the Session Initiation Protocol as it allows a client to inform a SIP server of its current location, specifically its IP address and other contact information. This is essential for enabling proper routing of SIP requests to the correct client.

When a SIP user agent (like a VoIP phone or an application) initiates a REGISTER request with a SIP server, it essentially tells the server, "Here I am, and I am available at this address." This functionality is vital for establishing communication in a dynamic network environment where IP addresses can change frequently, ensuring that incoming calls and messages can be directed accurately to the user's current location.

Other choices do not accurately describe the function of the REGISTER method. Establishing a call, ending a session, or switching accounts involves different SIP methods that serve distinct purposes, such as INVITE for initiating calls and BYE for ending them. Thus, recognizing that the REGISTER method is explicitly for registering user locations with a SIP server helps clarify its role in the SIP framework.