 and activate and that it will expire in 1 hour unless refreshed by Chebychev (using another SUBSCRIBE request). The Common Presence and Instant Message Presence Information Data Format (CPIM PIDF) [6] XML message body contains the status information that Poisson is currently off-line (closed). Chebychev sends a 200 OK response to the NOTIFY to confirm that it has been received: SIP/2.0 200 OK Via SIP/2.0/TCP dist.probablilty.org 5060 ;branch=z9hG4bk4321;received=24.32 To: P. L. Chebychev<sip:chebychev@academy.ru>;tag=21171 From: M. Poisson <sip:poisson@probability.org>;tag=25140 Call-ID: 58dkfj349241k34452k592520 CSeq: 1026 NOTIFY Content-Length: 0 Later, when Poisson does sign in, this information is provided in a second NOTIFY containing the change in status: NOTIFY sip:pafnuty@lecturehall21.academy.ru SIP/2.0 Via SIP/2.0/TCP dist.probablilty.org:5060 branch=z9hG4bK334241 Max-Forwards 70 To: P. L. Chebychev <sip:chebychev@academy.ru>;tag=21171 From: M. Poisson <sip:poisson@probability.org>;tag=25140 Call-ID: 58dkfj349241k34452k592520 CSeq: 1027 NOTIFY Allow: ACK, INVITE, CANCEL, BYE, NOTIFY, SUBSCRIBE, MESSAGE Allow-Events: presence Contact: <sip:s.possion@dist.probability.org;transport=tcp> Subscription-State: active;expires=1800 Event: presence Content-Type: application/cpim-pidf+xml Content-Length: 325 <?xml version="1.09 encoding="UTF-8"?> <presence emlns="urn:ietf:params:xml:ns:cpim-pidf" entity="sip:poisson@probability.org"> <tuple id="452426775"> <status> <basic>open</basic> </status> <contact>sip:s.possion@dist.probability.org;transport=tcp </contact> Introduction to SIP </tuple> </presence> The expiration time indicated in the Subscription-State header field indicates that 30 minutes have passed since the subscription was estab- lished. The CPIM PIDF XML message body now indicates that Poisson is on-line (open) and can be reached via the URI sip:s.pos- sion@dist.probability.org;transport=tcp. Chebychev confirms receipt of the NOTIFY with a 200 OK response: SIP/2.0 200 OK Via SIP/2.0/TCP dist.probablil