on Initiation Protocol when a subscription exists between the subscriber and the notifier. However, it is possible for a subscription to be established using non-SIP means (no SUBSCRIBE is sent) and may also be implicit in another SIP request type (for example, a REFER establishes an implicit subscription). Since it is sent within a dialog, the NOTIFY will contain a To tag, From tag, and existing Call-ID. A basic call flow showing NOTIFY is shown in Figure 4.8. A NOTIFY request normally receives a 200 OK response to indicate that it has been received. If a 481 Dialog/Transaction Does Not Exist response is received, the subscription is automatically terminated and no more NOTIFYs are sent. NOTIFY requests contain an Event header field indicating the package and a Subscription-State header field indicating the current state of the subscription. The Event header field (see Section 6.2.6) will contain the pack- age name used in the subscription. Currently defined packages are listed in Table 4.8 The Subscription-State header field (see Section 6.2.27) will either be active, pending, or terminated. A NOTIFY is always sent at the start of a subscription and at the termina- tion of a subscription. If a NOTIFY contains incremental (delta) state informa- tion, the message body will contain a state version number that will incremented by 1 for each NOTIFY sent. This way, the receiver of the NOTIFY can tell if information is missing our received out of sequence. An example NOTIFY request is shown here: NOTIFY sip:tyoung@parlour.elasticity.co.uk SIP/2.0 Via SIP/2.0/UDP artouche.rosettastone.org 5060 branch=z9hG4bK3841323 Max-Forwards: 70 To: Thomas Young <sip:tyoung@elasticity.co.uk>;tag=1814 From: <sip:ptolemy@rosettastone.org>;tag=5363956k Call-ID: 452k59252058dkfj349241k34 CSeq: 3 NOTIFY Contact: <sip:Ptolemy@cartouche.rosettastone.org> Event: dialog Subscription-State active Allow-Events: dialog Content-Type: application/xml+dialog Content-Length: (XML Message body not shown Table 4.10 lists the mandatory header fields i