RFC 2778 (rfc2778) - Page 3 of 17
A Model for Presence and Instant Messaging
Alternative Format: Original Text Document
RFC 2778 A Model for Presence and Instant Messaging February 2000
+---------------------------+
| PRESENCE SERVICE |
| |
+---------------------------+
^ |
| |
| v
+------------+ +------------+
| PRESENTITY | | WATCHER |
+------------+ +------------+
Fig. 1: Overview of Presence Service
There are two kinds of WATCHERS, called FETCHERS and SUBSCRIBERS. A
FETCHER simply requests the current value of some PRESENTITY's
PRESENCE INFORMATION from the PRESENCE SERVICE. In contrast, a
SUBSCRIBER requests notification from the PRESENCE SERVICE of
(future) changes in some PRESENTITY's PRESENCE INFORMATION. A
special kind of FETCHER is one that fetches information on a regular
basis. This is called a POLLER.
+----------------WATCHER---------------+
| |
| +----FETCHER---+ +--SUBSCRIBER--+ |
| | | | | |
| | +--POLLER--+ | | | |
| | | | | | | |
| | +----------+ | | | |
| +--------------+ +--------------+ |
+--------------------------------------+
Fig. 2: Varieties of WATCHER
The PRESENCE SERVICE also has WATCHER INFORMATION about WATCHERS and
their activities in terms of fetching or subscribing to PRESENCE
INFORMATION. The PRESENCE SERVICE may also distribute WATCHER
INFORMATION to some WATCHERS, using the same mechanisms that are
available for distributing PRESENCE INFORMATION.
Changes to PRESENCE INFORMATION are distributed to SUBSCRIBERS via
NOTIFICATIONS. Figures 3a through 3c show the flow of information as
a piece of PRESENCE INFORMATION is changed from P1 to P2.
Day, et al. Informational
