ot indicate whether the subscription has been authorized-it merely means it has been understood by the server. The basic call flow is shown in Figure 4.8. The client sends a SUBSCRIBE, which is successful, and receives NOTIFYS as the requested events occur at the server. Before the expiration of the subscription time, the client re-SUBSCRIBEs to extend the subscription and hence receives more notifications. Note that a client must be prepared to receive a NOTIFY before receiving a 200 OK response to the SUBSCRIBE. Also, due to forking, a client must be prepared to receive NOTIFYs from multiple servers (the NOTIFYs will have Watcher Proxy Presence agent SUBSCRIBE SUBSCRIBE 202 Accepted 202 Accepted NOTIFY 200 OK SUBSCRIBE 200 OK Figure 4.8 Example SUBSCRIBE and NOTIFY call flow. SIP: Understanding the Session Initiation Protocol different To tags and hence will establish separate dialogs), although only one 200 OK response to the SUBSCRIBE may be received. An example SUBSCRIBE request is shown below: SUBSCRIBE sip:ptolemy@rosettastone.org SIP/2.0 Via SIP/2.0/UDP proxy.elasticity.co.uk: 5060 branch=z9hG4bK348471123 Via SIP/2.0/UDP parlour.elasticity.co.uk : 5060 :branch=z9hG4bKABDA received=192.0.3.4 Max-Forwards: 69 To: <sip: Ptolemy@rosettastone.org> From: Thomas Young sip:tyoung@elasticity.co.uk> Call-ID: 452k59252058dkfj349241k34 CSeq: 3412 SUBSCRIBE Allow-Events: dialog Contact: sip:tyoung@parlour.elasticity.co.uk> Event: dialog Content-Length: 0 The type of event subscription is indicated by the required Event header field (see Section 6.4.7) in the SUBSCRIBE request. Each application of the SIP Events framework [5] defines a package with a unique event tag. Each pack- age defines the following things: Default subscription expiration interval; Expected SUBSCRIBE message bodies; What events cause a NOTIFY to be sent, and what message body is expected in the NOTIFY; Whether the NOTIFY contains complete state or increments (deltas); Maximum notification rate. A protocol called PSTN and Internet Interworking (PIN