ledge subscription NOTIFY Return current state information NOTIFY Return current state information SUBSCRIBE Subscriptions are expired and refreshed NOTIFY Return current state information UNSUBSCRIBE Unsubscribe the subscription NOTIFY Return terminate notify When a subscriber wishes to subscribe to a particular state for a resource, it forms a SUBSCRIBE message. This SUBSCRIBE request will be confirmed with a final response. A 200-class responses indicate that the subscription has been accepted, and that a NOTIFY message will be sent immediately to communicate the current resource state to the subscriber. As in SUBSCRIBE requests, NOTIFY "Event" headers will contain a single event package name for which a notification is being generated. The "Expires" header in a 200- class response to SUBSCRIBE indicates the actual duration for which the subscription will remain active. The body of the NOTIFY request provides the event package information. When a change in the subscribed state occurs, the notifier immediately con- structs and sends a NOTIFY request to inform subscribers of changes in the state to which the subscriber has a subscription. At any time before a sub- scription expires, the subscriber may refresh the timer on such a subscription by sending another SUBSCRIBE request. When a notifier receives a subscrip- tion refresh, the notifier updates the expiration time for the subscription and sends a NOTIFY message with current state information to the subscriber immediately. A SUBSCRIBE with an "Expires" of 0 constitutes a request to unsubscribe from an event. A subscription is destroyed when a notifier sends a NOTIFY request with a "Subscription-State" of "terminated." So a sub- scriber sends a subscribe request with an "Expires" of 0 in order to trigger the sending of such a "terminated" NOTIFY request. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security 5.2.3 A Walk-Through Example This section shows an example of using the NOTIFY mechanism method for subscribing a state