 <sip:notify@handbook.com>;tag=abcd1234 From: <sip:subscribe@handbook.com>;tag=12341234 © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Call-ID: 10275342@handbook.com CSeq: 3 SUBSCRIBE Contact: sip:server.handbook.com Expires: 0 Content-Length 0 M13: In order to complete the process, the Notifier sends the subscriber terminated NOTIFY to destroy the subscription. NOTIFY sip:user@handbook.com SIP/2.0 Via: SIP/2.0/UDR server.handbook.com;branch=z9hG4bk8sdf2 To: <sip:subscribe@handbook.com>;tag=12341234 From:<sip:notify@handbook.com>;tag=abcd1234 Call-ID: 10275342@handbook.com CSeq: 4 NOTIFY Max-Forwards: 70 Event: XXX Subscription-State: terminated; expires=0 Reason:timeout Contact: sip:server.handbook.com Content-Type: XXXXXX Content-Length: [empty] M14: The subscriber confirms receipt of the NOTIFY request. SIP/2.0 200 OK Via: SIP/2.0/UDP server.handbook.com;branch=z9hG4bk8sdf2 received=168.0.2. To: sip:subscribe@handbook.com>;tag=12341234 From: <sip:notify@handbook.com>;tag=abcd1234 Call-ID: 10275342@handbook.com CSeq: 4 NOTIFY Content-Length: 0 5.3 Simple Presence Information 5.3.1 Overview Presence, also known as presence information, conveys the ability and willing- ness of a user to communicate across a set of devices [3]. It makes use of the general event notification framework and the SUBSCRIBE and NOTIFY methods mentioned before. When an entity, the subscriber, wishes to learn about presence information from some user, it creates a SUBSCRIBE request. It eventually arrives at a presence server, which generates a response to the C 2009 by Taylor & Francis Group, LLC SIP Event Notification and Presence Information request and sends a notify request including the presence information to the subscriber. 5.3.2 Definition A presence service is a system that accepts, stores, and distributes pres- ence information to interested parties called watchers [4]. The PRESENCE SERVICE has two distinct sets of "clients" One set of clients is PRESENTI- TIES, the other set is WATCHERS