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

between two user agents, a MESSAGErequest can be sent at any time. SIP sup-
port for presence and instant messaging includes SIP messages as in this
example. The other methods in SIP to support instant communications are
event subscription and notification for presence.

Event Subscription and Notification

The ability to request and receive notification when a certain event occurs is
supported in SIP by the SUBSCRIBEand NOTIFYrequest types [26], [27]. For
example, the automatic callback feature in telephony can be used when the
called party is busy (off hook) and the caller wishes to be notified as soon as the
called party is available [29]. In Figure 6.11, user A sends an INVITErequest
and receives a 486 Busy Hereresponse from user B’s user agent. User A then
sends a SUBSCRIBErequest to user B requesting notification when user B is
available to establish a session. When user B sends a NOTIFYrequest indicating
that the user is now available, User A immediately establishes the session.

Figure 6.11 Automatic callback feature example using SUBSCRIBEand NOTIFY

SIP User Agent

1 INVITE

Called party sends

notification

The called party is

busy so call fails.

Caller requests

notification when

called party is no

longer busy.

Caller calls back

and successfully

establishes session.

SIP User Agent

5 200 OK

6 NOTIFY

9 200 OK

10 ACK

7 200 OK

8 INVITE

2 486 Busy Here

3 ACK

4 SUBSCRIBE

Media Session

SIP Overview 127