IP Event Notification and Presence Information to concrete extensions, known as event packages. The name of this package is "presence." This value appears in the event header field present in SUB- SCRIBE and NOTIFY requests. All subscribers and notifiers support the "application/pidf+xml" presence data format described in the next section. The subscribe request contains an accept header field. It includes "application/ pidf+xml." As described in the notify mechanism, the NOTIFY message will contain bodies that describe the state of the subscribed resource. In the presence event package, the body of the notification contains a pres- ence document. This document describes the presence of the presentity that was subscribed to. It will be introduced in next section. The body of the NOTIFY message is sent using the type listed in the accept header field in the most recent SUBSCRIBE request, or using the type "application/pidf+xml" if no accept header field is present. 5.3.4 A Walk-Through Example This message flow illustrates how the presence server can be responsible for sending notifications for a presentity. This flow assumes that the watcher has previously been authorized to subscribe to this resource at the server. In this flow, the presentity informs the server about the updated presence information through some non-SIP means. The [PIDF Document] and [New PIDF Document contain the presence document that will be introduced in next section. When the value of the content-length header field is " means that the value should be whatever the computed length of the body is. (Subscriber) (Notifier) Watcher Presence Server Presentity M1 SUBSCRIBE M2 200 OK M3 NOTIFY M4 200 OK Update Presence M5 NOTIFY M6 200 OK Message Details © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security M1: The watcher subscribe@handbook.com initiates a new subscription to the presence server notify@handbook.com. Expire time is 3600 seconds. SUBSCRIBE sip:resource@handbook.com SIP/2.0 Via: SIP/2.0/UDP example.