Network Working Group M. Day

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.

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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.

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+---------------------------+
| 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.

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+---------------------------+
| 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

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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.

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+------------------------------------+
| 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

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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).

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+---------------------------+
| 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

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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.

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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."

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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.

-- May require authentication of SENDER USER AGENTS and/or INSTANT
INBOXES.

-- 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.

-- 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.

-- 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.

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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.

-- May require authentication of PRESENTITIES, and/or WATCHERS.

-- May have different authentication requirements for different
PRESENTITIES.

-- May have different authentication requirements for different
WATCHERS, and may also have different authentication
requirements for different PRESENTITIES being watched by a
single WATCHER.

-- 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.

-- 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.

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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.

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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.

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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.

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7. Authors' Addresses

Mark Day
SightPath, Inc.
135 Beaver Street
Waltham, MA 02452
USA

EMail: [email protected]
(Formerly [email protected])

Jonathan Rosenberg
dynamicsoft
200 Executive Drive
Suite 120
West Orange, NJ 07046

Email: [email protected]

Hiroyasu Sugano
Fujitsu Laboratories Ltd.
64 Nishiwaki, Ohkubo-cho
Akashi 674-8555
Japan

EMail: [email protected]

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8. Full Copyright Statement

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

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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

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

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