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rfc:rfc2778

Network Working Group M. Day Request for Comments: 2778 Lotus Category: Informational J. Rosenberg

                                                        dynamicsoft
                                                          H. Sugano
                                                            Fujitsu
                                                      February 2000
             A Model for Presence and Instant Messaging

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 This document defines an abstract model for a presence and instant
 messaging system. It defines the various entities involved, defines
 terminology, and outlines the services provided by the system. The
 goal is to provide a common vocabulary for further work on
 requirements for protocols and markup for presence and instant
 messaging.

1. Introduction

 A presence and instant messaging system allows users to subscribe to
 each other and be notified of changes in state, and for users to send
 each other short instant messages. To facilitate development of a
 suite of protocols to provide this service, we believe that it is
 valuable to first develop a model for the system. The model consists
 of the various entities involved, descriptions of the basic functions
 they provide, and most importantly, definition of a vocabulary which
 can be used to facilitate discussion.
 We note that the purpose of this model is to be descriptive and
 universal: we want the model to map reasonably onto all of the
 systems that are informally described as presence or instant
 messaging systems. The model is not intended to be prescriptive or
 achieve interoperability: an element that appears in the model will
 not necessarily be an element of an interoperable protocol, and may
 not even be a good idea.

Day, et al. Informational [Page 1] RFC 2778 A Model for Presence and Instant Messaging February 2000

 In this document, each element of the model appears in upper case
 (e.g., PRESENCE SERVICE). No term in lower case or mixed case is
 intended to be a term of the model.
 The first part of this document is intended as an overview of the
 model.  The overview includes diagrams, and terms are presented in an
 order that is intended to help the reader understand the relationship
 between elements. The second part of the document is the actual
 definition of the model, with terms presented in alphabetical order
 for ease of reference.
 The overview is intended to be helpful but is not definitive; it may
 contain inadvertent differences from the definitions in the model.
 For any such difference, the definition(s) in the model are taken to
 be correct, rather than the explanation(s) in the overview.

2. Overview

 The model is intended to provide a means for understanding,
 comparing, and describing systems that support the services typically
 referred to as presence and instant messaging. It consists of a
 number of named entities that appear, in some form, in existing
 systems. No actual implementation is likely to have every entity of
 the model as a distinct part. Instead, there will almost always be
 parts of the implementation that embody two or more entities of the
 model. However, different implementations may combine entities in
 different ways.
 The model defines two services: a PRESENCE SERVICE and an INSTANT
 MESSAGE SERVICE. The PRESENCE SERVICE serves to accept information,
 store it, and distribute it.  The information stored is
 (unsurprisingly) PRESENCE INFORMATION. The INSTANT MESSAGE SERVICE
 serves to accept and deliver INSTANT MESSAGES to INSTANT INBOXES.

2.1 PRESENCE SERVICE

 The PRESENCE SERVICE has two distinct sets of "clients" (remember,
 these may be combined in an implementation, but treated separately in
 the model).  One set of clients, called PRESENTITIES, provides
 PRESENCE INFORMATION to be stored and distributed.  The other set of
 clients, called WATCHERS, receives PRESENCE INFORMATION from the
 service.

Day, et al. Informational [Page 2] 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 [Page 3] RFC 2778 A Model for Presence and Instant Messaging February 2000

                 +---------------------------+
                 |     PRESENCE SERVICE      |
                 |            P1             |
                 +---------------------------+
              +------------+       +------------+
              |   P1->P2   |       |     P1     |
              | PRESENTITY |       | SUBSCRIBER |
              +------------+       +------------+
                 Fig. 3a: NOTIFICATION (Step 1)
                 +---------------------------+
                 |     PRESENCE SERVICE      |
                 |          P1->P2           |
                 +---------------------------+
                     ^
                     |P2
              +------------+       +------------+
              |     P2     |       |    P1      |
              | PRESENTITY |       | SUBSCRIBER |
              +------------+       +------------+
                 Fig. 3b: NOTIFICATION (Step 2)
                 +---------------------------+
                 |     PRESENCE SERVICE      |
                 |            P2             |
                 +---------------------------+
                                         |P2
                                         v
              +------------+       +------------+
              |     P2     |       |   P1->P2   |
              | PRESENTITY |       | SUBSCRIBER |
              +------------+       +------------+
                 Fig. 3c: NOTIFICATION (Step 3)

2.2 INSTANT MESSAGE SERVICE

 The INSTANT MESSAGE SERVICE also has two distinct sets of "clients":
 SENDERS and INSTANT INBOXES. A SENDER provides INSTANT MESSAGES to
 the INSTANT MESSAGE SERVICE for delivery. Each INSTANT MESSAGE is

Day, et al. Informational [Page 4] RFC 2778 A Model for Presence and Instant Messaging February 2000

 addressed to a particular INSTANT INBOX ADDRESS, and the INSTANT
 MESSAGE SERVICE attempts to deliver the message to a corresponding
 INSTANT INBOX.
               +---------------------------+
               |  INSTANT MESSAGE SERVICE  |
               |                           |
               +---------------------------+
                   ^                 |
                   |                 |
                   |                 v
            +------------+       +---------------+
            |   SENDER   |       | INSTANT INBOX |
            +------------+       +---------------+
          Fig. 4: Overview of Instant Message Service

2.3 Protocols

 A PRESENCE PROTOCOL defines the interaction between PRESENCE SERVICE,
 PRESENTITIES, and WATCHERS. PRESENCE INFORMATION is carried by the
 PRESENCE PROTOCOL.
 An INSTANT MESSAGE PROTOCOL defines the interaction between INSTANT
 MESSAGE SERVICE, SENDERS, and INSTANT INBOXES. INSTANT MESSAGES are
 carried by the INSTANT MESSAGE PROTOCOL.
 In terms of this model, we believe that the IMPP working group is
 planning to develop detailed requirements and specifications for the
 structure and formats of the PRESENCE PROTOCOL, PRESENCE INFORMATION,
 INSTANT MESSAGE PROTOCOL, and INSTANT MESSAGES.

2.4 Formats

 The model defines the PRESENCE INFORMATION to consist of an arbitrary
 number of elements, called PRESENCE TUPLES. Each such element
 consists of a STATUS marker (which might convey information such as
 online/offline/busy/away/do not disturb), an optional COMMUNICATION
 ADDRESS, and optional OTHER PRESENCE MARKUP.  A COMMUNICATION ADDRESS
 includes a COMMUNICATION MEANS and a CONTACT ADDRESS. One type of
 COMMUNICATION MEANS, and the only one defined by this model, is
 INSTANT MESSAGE SERVICE.  One type of CONTACT ADDRESS, and the only
 one defined by this model, is INSTANT INBOX ADDRESS. However, other
 possibilities exist: a COMMUNICATION MEANS might indicate some form
 of telephony, for example, with the corresponding CONTACT ADDRESS
 containing a telephone number.

Day, et al. Informational [Page 5] RFC 2778 A Model for Presence and Instant Messaging February 2000

    +------------------------------------+
    | PRESENCE INFORMATION               |
    +------------------------------------+
     | +-------------------------------+
     =>| PRESENCE TUPLE                |
     | +-------------------------------+
     |   | +-------------------------+
     |   =>| STATUS                  |
     |   | +-------------------------+
     |   | +-------------------------+
     |   =>| COMMUNICATION ADDRESS   |
     |   | +-------------------------+
     |   |     | +-----------------+
     |   |     =>| CONTACT MEANS   |
     |   |     | +-----------------+
     |   |     | +-----------------+
     |   |     =>| CONTACT ADDRESS |
     |   |       +-----------------+
     |   | +-------------------------+
     |   =>| OTHER MARKUP            |
     |     +-------------------------+
     | +-------------------------------+
     =>| PRESENCE TUPLE                |
     | +-------------------------------+
     |   | +-------------------------+
     |   =>| STATUS                  |
     |   | +-------------------------+
     |   | +-------------------------+
     |   =>| COMMUNICATION ADDRESS   |
     |   | +-------------------------+
     |   |     | +-----------------+
     |   |     =>| CONTACT MEANS   |
     |   |     | +-----------------+
     |   |     | +-----------------+
     |   |     =>| CONTACT ADDRESS |
     |   |       +-----------------+
     |   | +-------------------------+
     |   =>| OTHER MARKUP            |
     |     +-------------------------+
     | +-------------------------------+
     =>| PRESENCE TUPLE                |
     | +-------------------------------+
     |    ...
      Fig. 5: The structure of PRESENCE INFORMATION

Day, et al. Informational [Page 6] RFC 2778 A Model for Presence and Instant Messaging February 2000

 STATUS is further defined by the model to have at least two states
 that interact with INSTANT MESSAGE delivery -- OPEN, in which INSTANT
 MESSAGES will be accepted, and CLOSED, in which INSTANT MESSAGES will
 not be accepted. OPEN and CLOSED may also be applicable to other
 COMMUNICATION MEANS -- OPEN mapping to some state meaning "available"
 or "open for business" while CLOSED means "unavailable" or "closed to
 business." The model allows STATUS to include other values, which may
 be interpretable by programs or only by persons.  The model also
 allows STATUS to consist of single or multiple values.

2.5 Presence and its effect on Instant Messages

 An INSTANT INBOX is a receptacle for INSTANT MESSAGES. Its INSTANT
 INBOX ADDRESS is the information that can be included in PRESENCE
 INFORMATION to define how an INSTANT MESSAGE should be delivered to
 that INSTANT INBOX. As noted above, certain values of the STATUS
 marker indicate whether INSTANT MESSAGES will be accepted at the
 INSTANT INBOX.  The model does not otherwise constrain the delivery
 mechanism or format for instant messages. Reasonable people can
 disagree about whether this omission is a strength or a weakness of
 this model.

2.6 PRINCIPALS and their agents

 This model includes other elements that are useful in characterizing
 how the protocol and markup work. PRINCIPALS are the people, groups,
 and/or software in the "real world" outside the system that use the
 system as a means of coordination and communication. It is entirely
 outside the model how the real world maps onto PRINCIPALS -- the
 system of model entities knows only that two distinct PRINCIPALS are
 distinct, and two identical PRINCIPALS are identical.
 A PRINCIPAL interacts with the system via one of several user agents
 (INBOX USER AGENT; SENDER USER AGENT; PRESENCE USER AGENT; WATCHER
 USER AGENT). As usual, the different kinds of user agents are split
 apart in this model even though most implementations will combine at
 least some of them. A user agent is purely coupling between a
 PRINCIPAL and some core entity of the system (respectively, INSTANT
 INBOX; SENDER; PRESENTITY; WATCHER).

Day, et al. Informational [Page 7] RFC 2778 A Model for Presence and Instant Messaging February 2000

                 +---------------------------+
                 |     PRESENCE SERVICE      |
                 +---------------------------+
                     ^                   |
                     | PRESENCE PROTOCOL |
                     |                   v
              +------------+       +------------+
              | PRESENTITY |       |  WATCHER   |
              +------------+       +------------+
                    ^                   ^
                    |                   |
                    |                   |
      o      +--------------+      +-------------+      o
     /|\  -->| PRESENCE UA  |      | WATCHER UA  |<--  /|\
      X      +--------------+      +-------------+      X
 (PRINCIPAL)                                        (PRINCIPAL)
                  Fig. 6: A presence system
                +---------------------------+
                |  INSTANT MESSAGE SERVICE  |
                +---------------------------+
                    ^                    |
                  IM|   INSTANT MESSAGE  |IM
                    |       PROTOCOL     v
             +------------+        +---------------+
             |   SENDER   |        | INSTANT INBOX |
             +------------+        +---------------+
                   ^                      ^
                   |                      |
                   |                      |
     o      +-------------+       +------------------+      o
    /|\  -->|  SENDER UA  |       |  INBOX UA        |<--  /|\
     X      +-------------+       +------------------+      X
 (PRINCIPAL)                                           (PRINCIPAL)
              Fig. 7: An instant messaging system

Day, et al. Informational [Page 8] RFC 2778 A Model for Presence and Instant Messaging February 2000

2.7 Examples

 A simple example of applying the model is to describe a generic
 "buddy list" application. These applications typically expose the
 user's presence to others, and make it possible to see the presence
 of others. So we could describe a buddy list as the combination of a
 PRESENCE USER AGENT and WATCHER USER AGENT for a single PRINCIPAL,
 using a single PRESENTITY and a single SUBSCRIBER.
 We could then extend our example to instant messaging and describe a
 generic "instant messenger" as essentially a buddy list with
 additional capabilities for sending and receiving instant messages.
 So an instant messenger would be the combination of a PRESENCE USER
 AGENT, WATCHER USER AGENT, INBOX USER AGENT, and SENDER USER AGENT
 for a single PRINCIPAL, using a single PRESENTITY, single SUBSCRIBER,
 and single INSTANT INBOX, with the PRESENTITY's PRESENCE INFORMATION
 including an INSTANT INBOX ADDRESS that leads to the INSTANT INBOX.

3. Model

 ACCESS RULES: constraints on how a PRESENCE SERVICE makes PRESENCE
    INFORMATION available to WATCHERS. For each PRESENTITY's PRESENCE
    INFORMATION, the applicable ACCESS RULES are manipulated by the
    PRESENCE USER AGENT of a PRINCIPAL that controls the PRESENTITY.
    Motivation: We need some way of talking about hiding presence
    information from people.
 CLOSED: a distinguished value of the STATUS marker. In the context of
    INSTANT MESSAGES, this value means that the associated INSTANT
    INBOX ADDRESS, if any, corresponds to an INSTANT INBOX that is
    unable to accept an INSTANT MESSAGE.  This value may have an
    analogous meaning for other COMMUNICATION MEANS, but any such
    meaning is not defined by this model. Contrast with OPEN.
 COMMUNICATION ADDRESS: consists of COMMUNICATION MEANS and CONTACT
    ADDRESS.
 COMMUNICATION MEANS: indicates a method whereby communication can
    take place. INSTANT MESSAGE SERVICE is one example of a
    COMMUNICATION MEANS.
 CONTACT ADDRESS: a specific point of contact via some COMMUNICATION
    MEANS. When using an INSTANT MESSAGE SERVICE, the CONTACT ADDRESS
    is an INSTANT INBOX ADDRESS.

Day, et al. Informational [Page 9] RFC 2778 A Model for Presence and Instant Messaging February 2000

 DELIVERY RULES: constraints on how an INSTANT MESSAGE SERVICE
    delivers received INSTANT MESSAGES to INSTANT INBOXES. For each
    INSTANT INBOX, the applicable DELIVERY RULES are manipulated by
    the INBOX USER AGENT of a PRINCIPAL that controls the INSTANT
    INBOX.
    Motivation: We need a way of talking about filtering instant
    messages.
 FETCHER: a form of WATCHER that has asked the PRESENCE SERVICE to for
    the PRESENCE INFORMATION of one or more PRESENTITIES, but has not
    asked for a SUBSCRIPTION to be created.
 INBOX USER AGENT: means for a PRINCIPAL to manipulate zero or more
    INSTANT INBOXES controlled by that PRINCIPAL.
    Motivation: This is intended to isolate the core functionality of
    an INSTANT INBOX from how it might appear to be manipulated by a
    product. This manipulation includes fetching messages, deleting
    messages, and setting DELIVERY RULES. We deliberately take no
    position on whether the INBOX USER AGENT, INSTANT INBOX, and
    INSTANT MESSAGE SERVICE are colocated or distributed across
    machines.
 INSTANT INBOX: receptacle for INSTANT MESSAGES intended to be read by
    the INSTANT INBOX's PRINCIPAL.
 INSTANT INBOX ADDRESS: indicates whether and how the PRESENTITY's
    PRINCIPAL can receive an INSTANT MESSAGE in an INSTANT INBOX. The
    STATUS and INSTANT INBOX ADDRESS information are sufficient to
    determine whether the PRINCIPAL appears ready to accept the
    INSTANT MESSAGE.
    Motivation: The definition is pretty loose about exactly how any
    of this works, even leaving open the possibility of reusing parts
    of the email infrastructure for instant messaging.
 INSTANT MESSAGE: an identifiable unit of data, of small size, to be
    sent to an INSTANT INBOX.
    Motivation: We do not define "small" but we seek in this
    definition to avoid the possibility of transporting an arbitrary-
    length stream labelled as an "instant message."

Day, et al. Informational [Page 10] RFC 2778 A Model for Presence and Instant Messaging February 2000

 INSTANT MESSAGE PROTOCOL: The messages that can be exchanged between
    a SENDER USER AGENT and an INSTANT MESSAGE SERVICE, or between an
    INSTANT MESSAGE SERVICE and an INSTANT INBOX.
 INSTANT MESSAGE SERVICE: accepts and delivers INSTANT MESSAGES.
  1. - May require authentication of SENDER USER AGENTS and/or INSTANT

INBOXES.

  1. - May have different authentication requirements for different

INSTANT INBOXES, and may also have different authentication

       requirements for different INSTANT INBOXES controlled by a
       single PRINCIPAL.
  1. - May have an internal structure involving multiple SERVERS

and/or PROXIES. There may be complex patterns of redirection

       and/or proxying while retaining logical connectivity to a
       single INSTANT MESSAGE SERVICE. Note that an INSTANT MESSAGE
       SERVICE does not require having a distinct SERVER -- the
       service may be implemented as direct communication between
       SENDER and INSTANT INBOX.
  1. - May have an internal structure involving other INSTANT MESSAGE

SERVICES, which may be independently accessible in their own

       right as well as being reachable through the initial INSTANT
       MESSAGE SERVICE.
 NOTIFICATION: a message sent from the PRESENCE SERVICE to a
       SUBSCRIBER when there is a change in the PRESENCE INFORMATION
       of some PRESENTITY of interest, as recorded in one or more
       SUBSCRIPTIONS.
       Motivation: We deliberately take no position on what part of
       the changed information is included in a NOTIFICATION.
 OPEN: a distinguished value of the STATUS marker. In the context of
    INSTANT MESSAGES, this value means that the associated INSTANT
    INBOX ADDRESS, if any, corresponds to an INSTANT INBOX that is
    ready to accept an INSTANT MESSAGE.  This value may have an
    analogous meaning for other COMMUNICATION MEANS, but any such
    meaning is not defined by this model. Contrast with CLOSED.
 OTHER PRESENCE MARKUP: any additional information included in the
    PRESENCE INFORMATION of a PRESENTITY. The model does not define
    this further.
 POLLER: a FETCHER that requests PRESENCE INFORMATION on a regular
    basis.

Day, et al. Informational [Page 11] RFC 2778 A Model for Presence and Instant Messaging February 2000

 PRESENCE INFORMATION: consists of one or more PRESENCE TUPLES.
 PRESENCE PROTOCOL: The messages that can be exchanged between a
    PRESENTITY and a PRESENCE SERVICE, or a WATCHER and a PRESENCE
    SERVICE.
 PRESENCE SERVICE: accepts, stores, and distributes PRESENCE
    INFORMATION.
  1. - May require authentication of PRESENTITIES, and/or WATCHERS.
  1. - May have different authentication requirements for different

PRESENTITIES.

  1. - May have different authentication requirements for different

WATCHERS, and may also have different authentication

       requirements for different PRESENTITIES being watched by a
       single WATCHER.
  1. - May have an internal structure involving multiple SERVERS

and/or PROXIES. There may be complex patterns of redirection

       and/or proxying while retaining logical connectivity to a
       single PRESENCE SERVICE. Note that a PRESENCE SERVICE does not
       require having a distinct SERVER -- the service may be
       implemented as direct communication among PRESENTITY and
       WATCHERS.
  1. - May have an internal structure involving other PRESENCE

SERVICES, which may be independently accessible in their own

       right as well as being reachable through the initial PRESENCE
       SERVICE.
 PRESENCE TUPLE: consists of a STATUS, an optional COMMUNICATION
    ADDRESS, and optional OTHER PRESENCE MARKUP.
 PRESENCE USER AGENT: means for a PRINCIPAL to manipulate zero or more
    PRESENTITIES.
    Motivation: This is essentially a "model/view" distinction: the
    PRESENTITY is the model of the presence being exposed, and is
    independent of its manifestation in any user interface. In
    addition, we deliberately take no position on how the PRESENCE
    USER AGENT, PRESENTITY, and PRESENCE SERVICE are colocated or
    distributed across machines.
 PRESENTITY (presence entity): provides PRESENCE INFORMATION to a
    PRESENCE SERVICE.

Day, et al. Informational [Page 12] RFC 2778 A Model for Presence and Instant Messaging February 2000

    Motivation: We don't like to coin new words, but "presentity"
    seemed worthwhile so as to have an unambiguous term for the entity
    of interest to a presence service. Note that the presentity is not
    (usually) located in the presence service: the presence service
    only has a recent version of the presentity's presence
    information.  The presentity initiates changes in the presence
    information to be distributed by the presence service.
 PRINCIPAL: human, program, or collection of humans and/or programs
    that chooses to appear to the PRESENCE SERVICE as a single actor,
    distinct from all other PRINCIPALS.
    Motivation: We need a clear notion of the actors outside the
    system. "Principal" seems as good a term as any.
 PROXY: a SERVER that communicates PRESENCE INFORMATION, INSTANT
    MESSAGES, SUBSCRIPTIONS and/or NOTIFICATIONS to another SERVER.
    Sometimes a PROXY acts on behalf of a PRESENTITY, WATCHER, or
    INSTANT INBOX.
 SENDER: source of INSTANT MESSAGES to be delivered by the INSTANT
    MESSAGE SERVICE.
 SENDER USER AGENT: means for a PRINCIPAL to manipulate zero or more
    SENDERS.
 SERVER: an indivisible unit of a PRESENCE SERVICE or INSTANT MESSAGE
    SERVICE.
 SPAM: unwanted INSTANT MESSAGES.
 SPOOFING: a PRINCIPAL improperly imitating another PRINCIPAL.
 STALKING: using PRESENCE INFORMATION to infer the whereabouts of a
    PRINCIPAL, especially for malicious or illegal purposes.
 STATUS: a distinguished part of the PRESENCE INFORMATION of a
    PRESENTITY. STATUS has at least the mutually-exclusive values OPEN
    and CLOSED, which have meaning for the acceptance of INSTANT
    MESSAGES, and may have meaning for other COMMUNICATION MEANS.
    There may be other values of STATUS that do not imply anything
    about INSTANT MESSAGE acceptance. These other values of STATUS may
    be combined with OPEN and CLOSED or they may be mutually-exclusive
    with those values.

Day, et al. Informational [Page 13] RFC 2778 A Model for Presence and Instant Messaging February 2000

    Some implementations may combine STATUS with other entities. For
    example, an implementation might make an INSTANT INBOX ADDRESS
    visible only when the INSTANT INBOX can accept an INSTANT MESSAGE.
    Then, the existence of an INSTANT INBOX ADDRESS implies OPEN,
    while its absence implies CLOSED.
 SUBSCRIBER: a form of WATCHER that has asked the PRESENCE SERVICE to
    notify it immediately of changes in the PRESENCE INFORMATION of
    one or more PRESENTITIES.
 SUBSCRIPTION: the information kept by the PRESENCE SERVICE about a
    SUBSCRIBER's request to be notified of changes in the PRESENCE
    INFORMATION of one or more PRESENTITIES.
 VISIBILITY RULES: constraints on how a PRESENCE SERVICE makes WATCHER
    INFORMATION available to WATCHERS. For each WATCHER's WATCHER
    INFORMATION, the applicable VISIBILITY RULES are manipulated by
    the WATCHER USER AGENT of a PRINCIPAL that controls the WATCHER.
    Motivation: We need a way of talking about hiding watcher
    information from people.
 WATCHER: requests PRESENCE INFORMATION about a PRESENTITY, or WATCHER
    INFORMATION about a WATCHER, from the PRESENCE SERVICE. Special
    types of WATCHER are FETCHER, POLLER, and SUBSCRIBER.
 WATCHER INFORMATION: information about WATCHERS that have received
    PRESENCE INFORMATION about a particular PRESENTITY within a
    particular recent span of time. WATCHER INFORMATION is maintained
    by the PRESENCE SERVICE, which may choose to present it in the
    same form as PRESENCE INFORMATION; that is, the service may choose
    to make WATCHERS look like a special form of PRESENTITY.
    Motivation: If a PRESENTITY wants to know who knows about it, it
    is not enough to examine only information about SUBSCRIPTIONS. A
    WATCHER might repeatedly fetch information without ever
    subscribing. Alternately, a WATCHER might repeatedly subscribe,
    then cancel the SUBSCRIPTION.  Such WATCHERS should be visible to
    the PRESENTITY if the PRESENCE SERVICE offers WATCHER INFORMATION,
    but will not be appropriately visible if the WATCHER INFORMATION
    includes only SUBSCRIPTIONS.
 WATCHER USER AGENT: means for a PRINCIPAL to manipulate zero or more
    WATCHERS controlled by that PRINCIPAL.

Day, et al. Informational [Page 14] RFC 2778 A Model for Presence and Instant Messaging February 2000

    Motivation: As with PRESENCE USER AGENT and PRESENTITY, the
    distinction here is intended to isolate the core functionality of
    a WATCHER from how it might appear to be manipulated by a product.
    As previously, we deliberately take no position on whether the
    WATCHER USER AGENT, WATCHER, and PRESENCE SERVICE are colocated or
    distributed across machines.

4. Security Considerations

 This document provides a model and vocabulary for systems with
 certain intrinsic security issues. In particular, presence and
 instant messaging systems must deal with "the three S's": STALKING,
 SPOOFING, and SPAM. ACCESS RULES, VISIBILITY RULES, and WATCHER
 INFORMATION are intended to deal with STALKING.  The several kinds of
 authentication mentioned for INSTANT MESSAGE SERVICE and PRESENCE
 SERVICE are intended to deal with SPOOFING. DELIVERY RULES are
 intended to deal with SPAM.

5. Conclusion

 This document has provided a model for a presence and instant
 messaging system. The purpose of the model is to provide a common
 vocabulary for the further work of defining and implementing
 interoperable presence and instant messaging protocols.

6. Acknowledgements

 This document has been improved by comments from Jesse Vincent and
 Colin Benson, by the participants in the Cambridge, MA meeting on
 June 11, 1999, and by Roy Salisbury, who contributed the original
 version of Figure 5. The authors gratefully acknowledge their
 assistance.

Day, et al. Informational [Page 15] RFC 2778 A Model for Presence and Instant Messaging February 2000

7. Authors' Addresses

 Mark Day
 SightPath, Inc.
 135 Beaver Street
 Waltham, MA 02452
 USA
 EMail: mday@alum.mit.edu
 (Formerly Mark_Day@lotus.com)
 Jonathan Rosenberg
 dynamicsoft
 200 Executive Drive
 Suite 120
 West Orange, NJ 07046
 Email: jdrosen@dynamicsoft.com
 Hiroyasu Sugano
 Fujitsu Laboratories Ltd.
 64 Nishiwaki, Ohkubo-cho
 Akashi 674-8555
 Japan
 EMail: suga@flab.fujitsu.co.jp

Day, et al. Informational [Page 16] RFC 2778 A Model for Presence and Instant Messaging February 2000

8. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
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 This document and the information contained herein is provided on an
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Day, et al. Informational [Page 17]

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