Figure 2.21 Event subscription and notification
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[IP Telephony Cookbook] / Technological Background
A user agent interested in an event notification sends a SUBSCRIBE message to a SIP server.The SUBSCRIBE message establishes a dialogue and is immediately replied to by the server using a 200 OK response. At this point, the dialogue is established.The server sends a NOTIFY request to the user every time the event to which the user subscribed changes. NOTIFY messages are sent within the dialogue established by the SUBSCRIBE.
Note that the first NOTIFY message in Figure 2.21 is sent regardless of any event that triggers notifications.
Subscriptions, as well as registrations, have a limited life span and therefore must be periodically refreshed.
2.2.2.6.6 Instant messages
Instant messages are sent using a MESSAGE request. MESSAGE requests do not establish a dialogue and therefore they will always traverse the same set of proxies.This is the simplest form of sending instant messages.The text of the instant message is transported in the body of the SIP
request.
User Agent
Proxy
User Agent
MESSAGE
MESSAGE
200 OK
200 OK
MESSAGE
MESSAGE
200 OK