📄 rfc2778.txt
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| | =>| 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
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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.
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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."
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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.
-- 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.
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.
-- 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
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