d presence information. We can find that the protocols introduced in this chapter are broadly used in third-generation wireless networks. Because of their popularity and usefulness, these protocols are extended in many ways to make them fit the requirements of the new services and applications in recent years. 5.2 Session Initiation Protocol (SIP)-Specific Event Notification This section discusses the event notification mechanism, which provides a SIP- specific framework for event notification. The discussion is mainly based on [2]. 5.2.1 Definitions In order to understand the operation of notification mechanism, we first define the following terminologies: Definition 1 (Event Package) An event package is an additional specifi- cation that defines a set of state information to be reported by a notifier to a subscriber. Definition 2 (Notifier) A notifier is a user agent that generates NOTIFY requests for the purpose of notifying subscribers of the state of a resource. Notifiers typically also accept SUBSCRIBE requests to create subscriptions. Definition 3 (Notification) Notification is the act of a notifier sending a NOTIFY message to a subscriber to inform the subscriber of the state of a resource. Definition 4 (Subscriber) A subscriber is a user agent that receives NOTIFY requests from notifiers; these NOTIFY requests contain information about the state of a resource in which the subscriber is interested. Subscribers typically also generate SUBSCRIBE requests and send them to notifiers to create subscriptions. Definition 5 (Subscription) A subscription is a set of application states associated with a dialog. This application state includes a pointer to the © 2009 by Taylor & Francis Group, LLC SIP Event Notification and Presence Information associated dialog, the event package name, and possibly an identification token. Event packages will define additional subscription state information. 5.2.2 Operation Overview A typical flow of messages would be: Subscriber Notifier SUBSCRIBE Request state subscription Acknow