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

Table 2.4 Examples of Rich Presence Information

On the phone

Away

Appointment

Holiday

Meal

Meeting

Driving

In transit

Travel

Vacation

Busy

Permanent absence

Rich presence information as described in Chapter 13 is not only very valu-

able for communications between humans but can also support the under-

standing of presence by machines.

Presence requires the messages SUBSCRIBEand NOTIFYbetween the par-

ties that display the presence (the “presentity”) and the party that is interested

in this information (the “watcher”) SIP presence can also be explained in a nut-

shell with the help of the message flow shown in Figure 2.1 by noting the

analogies between the SIP and presence messages shown in Table 2.5.

The SUBSCRIBEand NOTIFYmessages can be extended from human users

to any type of applications, and this can support the integration of communi-

cations and applications, as we will be described in Chapter 13, “Presence and

Instant Messaging.”

Table 2.5 SIP Messages for Presence and IM vs. Voice and Video

PRESENCE AND IM VOICE/VIDEO

SUBSCRIBE INVITE

200 OK 200 OK

NOTIFY ACK

MESSAGE RTP Media

22 Chapter 2