INTERNET-DRAFT                                               Mark Day
Expires December 27, 1999                                       Lotus

                                                   Jonathan Rosenberg
                                                            Bell Labs

                A Model for Presence and Instant Messaging

                        draft-ietf-impp-model-01.txt

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

. 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.

. 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. A number of requirements and
protocols have been proposed to support it [WHODP, ENVY, RVP-ADDR,
RPIM, SIP-PIP, PIPR, RVP].  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.

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, 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.

Sugano has submitted a set of diagrams to help explain this model
(draft-sugano-diagrams-00.txt).  We expect that similar diagrams will
be incorporated into a future version of this document.

. 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.

3.1 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.

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.

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.

3.2 INSTANT MESSAGE SERVICE

The INSTANT MESSAGE SERVICE also has two distinct sets of "clients":
INSTANT MESSAGE USER AGENTS and INSTANT INBOXES. An INSTANT MESSAGE
USER AGENT provides INSTANT MESSAGES to the INSTANT MESSAGE SERVICE
for delivery. Each INSTANT MESSAGE is addressed to a particular
INSTANT INBOX ADDRESS, and the INSTANT MESSAGE SERVICE either delivers
the message to a corresponding INSTANT INBOX or informs the sender
that delivery failed.

3.3 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, INSTANT MESSAGE USER AGENTS, 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.

3.4 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.

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. 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.

3.5 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.

3.6 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; INSTANT MESSAGE 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; INSTANT MESSAGE SERVICE; PRESENTITY; WATCHER).

3.7 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 INSTANT MESSAGE 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.

. 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. This value means
    that the associated INSTANT INBOX ADDRESS, if any, corresponds to an
    INSTANT INBOX that is unable to accept an INSTANT MESSAGE.  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.

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 known size, to be
    sent to an INSTANT INBOX.

INSTANT MESSAGE PROTOCOL: The messages that can be exchanged between
    an INSTANT MESSAGE 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 INSTANT MESSAGE 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.

    -- 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.

INSTANT MESSAGE USER AGENT: means for a PRINCIPAL to send an INSTANT MESSAGE
    to an INSTANT INBOX.

    Motivation: Note that sending a message does not require a long-term
    relationship between the INSTANT MESSAGE SERVICE and the sending
    PRINCIPAL, so there is no entity analogous to PRESENTITY, WATCHER, or
    INSTANT INBOX for the purpose of sending. Instead, we need only
    distinguish between the PRINCIPAL, the program used by the sender (the
    INSTANT MESSAGE USER AGENT), and the INSTANT MESSAGE SERVICE that
    accepts and delivers the message.

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. This value means
    that the associated INSTANT INBOX ADDRESS, if any, corresponds to an
    INSTANT INBOX that is ready to accept an INSTANT MESSAGE.  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.

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, and/or
    INSTANT INBOXES.

    -- 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.

    -- 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.

    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.

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.

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.
    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. 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: somehow requests PRESENCE INFORMATION about a PRESENTITY or
    WATCHER INFORMATION about a WATCHER. 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.

WATCHER USER AGENT: means for a PRINCIPAL to manipulate zero or
    more WATCHERS controlled by that PRINCIPAL.

    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.

. 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.

. 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.

. Acknowledgements

This document has been improved by comments from Jesse Vincent and
Colin Benson, and by the participants in the Cambridge, MA meeting on
June 11, 1999.  The authors gratefully acknowledge their assistance.

. References

[ENVY] M. Day. ''HTTP Envy'' and Presence Information Protocols.
Internet-Draft draft-day-envy-00.txt

[PIPR] M. Calsyn, L. Dusseault. Presence Information Protocol
Requirements. Internet-Draft draft-dusseault-pipr-00.txt

[RPIM] M. Day. Requirements for Presence and Instant Messaging.
Internet-Draft draft-day-rpim-00.txt

[RVP-ADDR] L. Dusseault, G. Mohr. Addressing and Location for
RVP. Internet-Draft draft-dusseault-rvp-addr-00.txt

[RVP] M. Calsyn, L. Dusseault, G. Mohr. RVP: A Presence Notification
Protocol. Internet-Draft draft-calsyn-rvp-01.txt

[SIP-PIP] J. Rosenberg, H.Schulzrinne. SIP For Presence.
Internet-Draft draft-rosenberg-sip-pip-00.txt

[WHODP] G. Mohr. WhoDP: Widely Hosted Object Data Protocol.
Internet-Draft draft-mohr-whodp-00.txt

. Authors' Addresses

Mark Day
Lotus Development Corporation
 Cambridge Parkway
Cambridge, MA 02142
email: Mark_Day@lotus.com

Jonathan Rosenberg
Lucent Technologies, Bell Laboratories
 Crawfords Corner Rd.
Holmdel, NJ 07733
Rm. 4C-526
email: jdrosen@bell-labs.com