RFC 2778 (rfc2778) - Page 3 of 17

A Model for Presence and Instant Messaging

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