 transferring status information, and the delivery of that information. The actual information transferred, and how that information is presented to the user is application dependent. In terms of the SIP protocol, SUBSCRIBE is used to request status or presence updates from the presence server (or "presentity"), and NOTIFY is used to deliver that infor- mation to the requestor or presence "watcher." SIP presence uses the SIP Events extensions [4] and instant message extensions [5]. In this example, Chebychev wishes to communicate with Poisson. The message flow is shown in Figure 2.4. To find out the status of Poisson, Cheby- chev subscribes to Poisson's presence information by sending a SUBSCRIBE message to Poisson. The request looks like: SUBSCRIBE sip:poisson@probability.org SIP/2.0 Via SIP/2.0/TCP lecturehall21.academy ru:5060 ; branch=z9hG4bK348471123 Max-Forwards: 70 To: M. Poisson <sip:poisson@probability.org> From: P. L. Chebychev chebychev@academy.ru> tag=21171 Call-ID: 58dkfj349241k34452k592520 CSeq: 3412 SUBSCRIBE Allow-Events: presence SIP: Understanding the Session Initiation Protocol Chebychev Poisson SUBSCRIBE 200 OK NOTIFY 200 OK NOTIFY 200 OK MESSAGE 200 OK MESSAGE 200 OK Figure 2.4 SIP presence and instant message example. Allow: ACK, INVITE, CANCEL, BYE, NOTIFY, SUBSCRIBE, MESSAGE Contact : <sip:pafnuty@lecturehall21.academy. ru; transport=tcp> Event: presence Content-Length: 0 In this example, TCP is used as the transport for the SIP messages as indi- cated in the Via header field and in the transport=tcp parameter in the Contact URI. This request also contains Allow and Allow-Events header fields, which are used to advertise capabilities. In this example, Cheby- chev is indicating support for receiving seven methods listed in the Allow header field, and also presence subscriptions in the Allow-Event header field. As this SUBSCRIBE is creating a dialog (in an analogous way that an Introduction to SIP INVITE created a dialog in the earlier examples), the From contains a tag but the To header fie