f the resource. NOTIFY sip:user@handbook.com SIP/2.0 Via: SIP/2.0/UDP 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: 1 NOTIFY Max-Forwards: 70 Event: XXX Subscription-State: active; expires=3599 Contact: sip:server.handbook.com Content-Type: XXXXXX Content-Length: [the state information of the resource] M4: The subscriber confirms receipt of the NOTIFY request. SIP/2.0 200 OK Via: SIP/2.0/UDP erver.handbook.com;branch=z9hG4bk8sdf2; received=168.0. To: <sip:subscribe@handbook.com>;tag=12341234 From: <sip:notify@handbook.com>;tag=abcd1234 Call-ID: 10275342@handbook.com CSeq: 1 NOTIFY Content-Length: 0 M5: The notifier detects that a reportable change has been made to the resource's information, and sends a new notification to the watcher. NOTIFY sip:user@handbook.com SIP/2.0 Via: SIP/2.0/UDPserver.handbook.com;branch=z9hG4bk4cd42a To: sip:subscribe@handbook.com>;tag=12341234 From: <sip:notify@handbook.com>;tag=abcd1234 Call-ID: 10275342@handbook.com CSeq: 2 NOTIFY Max-Forwards: 70 C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Event: XXX Subscription-State: active; expires=3400 Contact: sip:server.handbook.com Content-Type: XXXXXX Content-Length: [New state information of the resource] M6: The subscriber confirms receipt of the NOTIFY request. SIP/2.0 200 OK Via: SIP/2.0/UDP server.handbook.com:branch=z9hG4bk4cd42a1 received=168.0.2.6 To: sip:subscribe@handbook.com>;tag=12341234 From: <sip:notify@handbook.com>;tag=abcd1234 Call-ID: 10275342@handbook.com CSeq: 2 NOTIFY Content-Length: 0 M7: The subscriber subscribe@handbook.com initiates a refresh subscrip- tion to the notifier notify@handbook.com. SUBSCRIBE sip:resource@handbook.com SIP/2.0 Via: SIP/2.0/UDP example.com;branch=z9hG4bknashds7 To: <sip:notify@handbook.com>;tag=abcd1234 From: k<sip:subscribe@handbook.com>;tag=12341234 Call-ID: 10275342@handbook.com CSeq: 2 SUBSCRIBE Max-Forwards: 70 Expires: 