What does a "BYE" message signify in SIP?

The "BYE" message in the Session Initiation Protocol (SIP) serves a specific and crucial purpose: it indicates the termination of an existing call. When a participant wishes to end a SIP session—whether due to the conclusion of a conversation or any other reason—they send a BYE message to inform the other party that they are no longer interested in maintaining the session. This message helps both parties clearly understand that the call has been formally terminated and signifies that resources associated with the session can be released.

In the context of SIP, the other options do not accurately describe the function of a BYE message. For example, while a request to join a new session falls under SIP's capabilities, it would be initiated by a different message such as INVITE. Similarly, updating presence information is handled through separate SIP messages like PUBLISH or NOTIFY, which deal specifically with presence status rather than terminating a call. Lastly, acknowledging the completion of media negotiation is typically managed using a 200 OK response to prior INVITE requests rather than through a BYE message, which strictly serves to end an ongoing session.