Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

■■ Network preemption events for priority calls [14]

■■ Last but not least, presenceas described in the following section.

Presence Event Package

Presence information conveys the ability and willingness of a user to commu-
nicate with the watcher [15]. As mentioned, presence can also be used to com-
municate with applications, though not all application events are useful
presence data and SIPNOTIFYis not indicated as a universal tool.
Presence events are best illustrated with an example, as in Figure 13.5.
The example in Figure 13.5 is client-server-based, though we should keep in
mind that peer-to-peer is also possible, as shown in Figure 13.3.
The authentication and registration of the watcher by the server are not
shown here for simplicity.
An example for the presence message flow is shown in Figure 13.6.
The watcher will first subscribe to the presence information from the server
as shown in Messages 1 and 2. The watcher will also receive the notification of
the presence information and acknowledge it in Messages 3 and 4.
An update of the presence information to the server by the presence UA will
be immediately followed by a new notification shown in Message 5.
Polite blocking is possible for unwanted watchers. A200 OKis generated or
marked pending as a response to the SUBSCRIBErequest, even though the
request has been rejected [16].

Figure 13.6 Example of SIP message flow for presence

Watcher

1 SUBSCRIBE

2 200 OK

3 NOTIFY

4 200 OK

5 NOTIFY

6 200 OK

Update Presence

Server Presence UA

Presence and Instant Messaging 231