 information of the resource from a subscriber user agent to a notifier user agent. The subscriber in this example is subscribing to the state information of the resource. In the message, "xxx" means some event type. "xxxxxx" means some content type. When the value of the content- length header field is "...", , this means that the value should be whatever the computed length of the body is. Subscriber Notifier SUBSCRIBE M1: Request state subscription M2: Acknowledge subscription NOTIFY M3: Return current state information M4: Acknowledge notification NOTIFY M5: Return current state information M6: Acknowledge notification SUBSCRIBE M7: Subscriptions are expired and refreshed M8: Acknowledge subscription NOTIFY M9: Return current state information M10: Acknowledge notification UNSUBSCRIBE M11: Unsubscribe the subscription M12: Acknowledge subscription NOTIFY M13: Return terminate notify M14: Acknowledge terminate notification Message flow: M1: The subscriber subscribe@handbook.com initiates a new subscription to the notifier notify@handbook.com. Expire time is 3600 sec. SUBSCRIBE sip:resource@handbook.com SIP/2.0 Via: SIP/2.0/UDP example.com;branch=z9hG4bknashds7 To: ksip:notify@handbook.com> From: sip:subscribe@handbook.com>;tag=12341234 Call-ID: 10275342@handbook.com CSeq: 1 SUBSCRIBE Max-Forwards: 70 Expires: 3600 Event: XXX Contact: sip:user@handbook.com Content-Length: 0 M2: The notifier for notify@handbook.com processes the subscription requ- est and creates a new subscription. A 200 (OK) response is sent to confirm the subscription. SIP/2.0 200 OK Via: SIP/2.0/UDP example.com;branch=z9hG4bkKnashds7; © 2009 by Taylor & Francis Group, LLC SIP Event Notification and Presence Information received=168.0.2.5 To: ssip:notify@handbook.com>;tag=abcd1234 From: sip:subscribe@handbook.com>;tag=12341234 Call-ID: 10275342@handbook.com CSeq: 1 SUBSCRIBE Contact: sip:server.handbook.com Expires: 3600 Content-Length: 0 M3: In order to complete the process, the notifier sends the subscriber a NOTIFY with the current state o