The IETF Model for Presence and IM
Presence and IM are made possible by the packet nature of the Internet and may
merit dedicated books on their own. We will attempt to give some basic notions
that help in understanding the new SIP-based IM services and their potential.
The IETF model for a presence service is shown in Figure 13.1.
The model is meant to help the understanding of the basics and is not a stan-
dard by itself. Note the distinction in the model for presence between the user
agents (UA) of the principals, shown here as people, the Presence UA and the
Watcher UA, and Presentity and Watcher functions that may be network-
based elements or co-located with the respective UAs. The presentity is an
application that manages the presence information and presents it to the pres-
ence service.
The Presence service is an abstraction that communicates with the presen-
tity and watcher using the Presence Protocol, the SIMPLE protocol in our case.
The presentity information is displayed to the watcher on the UA as a
“buddy” or “contact.” We will use here the term “buddy” so as to avoid con-
fusion with the “contact” as in the Contact address. A watcher usually has a
buddy list and one can see their status at a glance.Figure 13.1 Model for presence servicePresence ServicePresence
ProtocolPresentity WatcherPresence UA Watcher UAPrincipal Principal226 Chapter 13