ld does not yet contain a tag. Poisson accepts the subscription request by sending a 202 Accepted response back to Chebychev: SIP/2.0 202 Accepted Via SIP/2.0/TCP lecturehall21.academy.ru:! 5060 ;branch=z9hG4bK348471123;received=19.34.3. To: M. Poisson <sip:poisson@probability.org>;tag=25140 From: P. L. Chebychev <sip:chebychev@academy.ru>;tag=21171 Call-ID: 58dkfj349241k34452k592520 CSeq: 3412 SUBSCRIBE Allow-Events: presence Allow: ACK, INVITE, CANCEL, BYE, NOTIFY, SUBSCRIBE, MESSAGE Contact: sip:s.possion@dist.probability.org;transport=tcp> Event: presence Expires: 3600 Content-Length: 0 In this example, there are no proxy servers between Chebychev's watcher and Poisson's presence server, although there could be any number. The Expires header field indicates that the subscription expires in 1 hour. The actual subscription is begun by Poisson sending the first NOTIFY back to Chebychev: NOTIFY sip :pafnuty@lecturehall21.academy.ru SIP/2.0 Via SIP/2.0/TCP dist.probablilty.org:5060 ; bbranch=z9hG4bK4321 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: 1026 NOTIFY Allow: ACK, INVITE, CANCEL, BYE, NOTIFY, SUBSCRIBE, MESSAGE Allow-Events: dialog Contact: <sip:s.possion@dist.probability.org;transport=tcp> Subscription-Stat active;expires=3600 Event: presence Content-Type: application/cpim-pidf+xml Content-Length: 244 <?xml version="1.0" encoding="UTF-8"?> <presence xmlns="urn:ietf:params:xml:ns:cpim-pidf" entity="sip:poisson@probability.org"> <tuple id="452426775"> <status> <basic>closed</basic> </status> </tuple> </presence> SIP: Understanding the Session Initiation Protocol Note that this NOTIFY is sent within the dialog established with the SUBSCRIBE-it uses the same dialog identifier (Call-ID, local and remote tags)-and the request is sent to the Contact URI provided by Chebychev in the subscription request. The Subscription-State header field indi- cates that the subscription has been authorized