The SIP specification has been extended over time to support a general mechanism allowing for subscription to asynchronous events. Such events can include SIP proxy statistics changes, presence information, session changes and so on.
A user agent interested in event notification sends a SUBSCRIBE message to an SIP server. The SUBSCRIBE message establishes a dialog and is immediately followed by the server replying with 200 OK response. At this point the dialog is established. The server sends a NOTIFY request to the user every time the event to which the user subscribed changes. NOTIFY messages are sent within the dialog established by the SUBSCRIBE.