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.



 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."


 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.



 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.

 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.


 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.



 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.