Figure 2.20 BYE message flow (with and without record routing)
The lefthand message flow of Figure 2.20 shows how a BYE (request within dialogue established by INVITE) is sent directly to the other user agent when there is no Record-Route header field in the message.The righthand message flow shows how the situation changes when the proxy puts a Record-Route header field into the message.
2.2.2.6.5 Event subscription and notification
The SIP specification has been extended to support a general mechanism allowing subscription to events. Such evens can include, SIP Proxy statistics changes to, presence information, session changes and so on.
The mechanism is used mainly to convey information on presence (the willingness to communicate) of users. Figure 2.21 shows the basic message flow.
User Agent
Server
SUBSCRIBE
200 OK
NOTIFY
200 OK
Event
NOTIFY