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+========
+Presence
+========
+
+A description of presence information and visibility between users.
+
+Overview
+========
+
+Each user has the concept of Presence information. This encodes a sense of the
+"availability" of that user, suitable for display on other user's clients.
+
+
+Presence Information
+====================
+
+The basic piece of presence information is an enumeration of a small set of
+state; such as "free to chat", "online", "busy", or "offline". The default state
+unless the user changes it is "online". Lower states suggest some amount of
+decreased availability from normal, which might have some client-side effect
+like muting notification sounds and suggests to other users not to bother them
+unless it is urgent. Equally, the "free to chat" state exists to let the user
+announce their general willingness to receive messages moreso than default.
+
+Home servers should also allow a user to set their state as "hidden" - a state
+which behaves as offline, but allows the user to see the client state anyway and
+generally interact with client features such as reading message history or
+accessing contacts in the address book.
+
+This basic state field applies to the user as a whole, regardless of how many
+client devices they have connected. The home server should synchronise this
+status choice among multiple devices to ensure the user gets a consistent
+experience.
+
+Idle Time
+---------
+
+As well as the basic state field, the presence information can also show a sense
+of an "idle timer". This should be maintained individually by the user's
+clients, and the homeserver can take the highest reported time as that to
+report. Likely this should be presented in fairly coarse granularity; possibly
+being limited to letting the home server automatically switch from a "free to
+chat" or "online" mode into "idle".
+
+When a user is offline, the Home Server can still report when the user was last
+seen online, again perhaps in a somewhat coarse manner.
+
+Device Type
+-----------
+
+Client devices that may limit the user experience somewhat (such as "mobile"
+devices with limited ability to type on a real keyboard or read large amounts of
+text) should report this to the home server, as this is also useful information
+to report as "presence" if the user cannot be expected to provide a good typed
+response to messages.
+
+
+Presence List
+=============
+
+Each user's home server stores a "presence list" for that user. This stores a
+list of other user IDs the user has chosen to add to it (remembering any ACL
+Pointer if appropriate).
+
+To be added to a contact list, the user being added must grant permission. Once
+granted, both user's HS(es) store this information, as it allows the user who
+has added the contact some more abilities; see below. Since such subscriptions
+are likely to be bidirectional, HSes may wish to automatically accept requests
+when a reverse subscription already exists.
+
+As a convenience, presence lists should support the ability to collect users
+into groups, which could allow things like inviting the entire group to a new
+("ad-hoc") chat room, or easy interaction with the profile information ACL
+implementation of the HS.
+
+
+Presence and Permissions
+========================
+
+For a viewing user to be allowed to see the presence information of a target
+user, either
+
+ * The target user has allowed the viewing user to add them to their presence
+   list, or
+
+ * The two users share at least one room in common
+
+In the latter case, this allows for clients to display some minimal sense of
+presence information in a user list for a room.
+
+Home servers can also use the user's choice of presence state as a signal for
+how to handle new private one-to-one chat message requests. For example, it
+might decide:
+
+  "free to chat": accept anything
+  "online": accept from anyone in my addres book list
+  "busy": accept from anyone in this "important people" group in my address
+    book list
+
+
+API Efficiency
+==============
+
+A simple implementation of presence messaging has the ability to cause a large
+amount of Internet traffic relating to presence updates. In order to minimise
+the impact of such a feature, the following observations can be made:
+
+ * There is no point in a Home Server polling status for peers in a user's
+   presence list if the user has no clients connected that care about it.
+
+ * It is highly likely that most presence subscriptions will be symmetric - a
+   given user watching another is likely to in turn be watched by that user.
+
+ * It is likely that most subscription pairings will be between users who share
+   at least one Room in common, and so their Home Servers are actively
+   exchanging message PDUs or transactions relating to that Room.
+
+ * Presence update messages do not need realtime guarantees. It is acceptable to
+   delay delivery of updates for some small amount of time (10 seconds to a
+   minute).
+
+The general model of presence information is that of a HS registering its
+interest in receiving presence status updates from other HSes, which then
+promise to send them when required. Rather than actively polling for the
+currentt state all the time, HSes can rely on their relative stability to only
+push updates when required.
+
+A Home Server should not rely on the longterm validity of this presence
+information, however, as this would not cover such cases as a user's server
+crashing and thus failing to inform their peers that users it used to host are
+no longer available online. Therefore, each promise of future updates should
+carry with a timeout value (whether explicit in the message, or implicit as some
+defined default in the protocol), after which the receiving HS should consider
+the information potentially stale and request it again.
+
+However, because of the likelyhood that two home servers are exchanging messages
+relating to chat traffic in a room common to both of them, the ongoing receipt
+of these messages can be taken by each server as an implicit notification that
+the sending server is still up and running, and therefore that no status changes
+have happened; because if they had the server would have sent them. A second,
+larger timeout should be applied to this implicit inference however, to protect
+against implementation bugs or other reasons that the presence state cache may
+become invalid; eventually the HS should re-enquire the current state of users
+and update them with its own.
+
+The following workflows can therefore be used to handle presence updates:
+
+ 1 When a user first appears online their HS sends a message to each other HS
+   containing at least one user to be watched; each message carrying both a
+   notification of the sender's new online status, and a request to obtain and
+   watch the target users' presence information. This message implicitly
+   promises the sending HS will now push updates to the target HSes.
+
+ 2 The target HSes then respond a single message each, containing the current
+   status of the requested user(s). These messages too implicitly promise the
+   target HSes will themselves push updates to the sending HS.
+
+   As these messages arrive at the sending user's HS they can be pushed to the
+   user's client(s), possibly batched again to ensure not too many small
+   messages which add extra protocol overheads.
+
+At this point, all the user's clients now have the current presence status
+information for this moment in time, and have promised to send each other
+updates in future.
+
+ 3 The HS maintains two watchdog timers per peer HS it is exchanging presence
+   information with. The first timer should have a relatively small expiry
+   (perhaps 1 minute), and the second timer should have a much longer time
+   (perhaps 1 hour).
+
+ 4 Any time any kind of message is received from a peer HS, the short-term
+   presence timer associated with it is reset.
+
+ 5 Whenever either of these timers expires, an HS should push a status reminder
+   to the target HS whose timer has now expired, and request again from that
+   server the status of the subscribed users.
+
+ 6 On receipt of one of these presence status reminders, an HS can reset both
+   of its presence watchdog timers.
+
+To avoid bursts of traffic, implementations should attempt to stagger the expiry
+of the longer-term watchdog timers for different peer HSes.
+
+When individual users actively change their status (either by explicit requests
+from clients, or inferred changes due to idle timers or client timeouts), the HS
+should batch up any status changes for some reasonable amount of time (10
+seconds to a minute). This allows for reduced protocol overheads in the case of
+multiple messages needing to be sent to the same peer HS; as is the likely
+scenario in many cases, such as a given human user having multiple user
+accounts.
+
+
+API Requirements
+================
+
+The data model presented here puts the following requirements on the APIs:
+
+Client-Server
+-------------
+
+Requests that a client can make to its Home Server
+
+ * get/set current presence state
+   Basic enumeration + ability to set a custom piece of text
+
+ * report per-device idle time
+   After some (configurable?) idle time the device should send a single message
+   to set the idle duration. The HS can then infer a "start of idle" instant and
+   use that to keep the device idleness up to date. At some later point the
+   device can cancel this idleness.
+
+ * report per-device type
+   Inform the server that this device is a "mobile" device, or perhaps some
+   other to-be-defined category of reduced capability that could be presented to
+   other users.
+
+ * start/stop presence polling for my presence list
+   It is likely that these messages could be implicitly inferred by other
+   messages, though having explicit control is always useful.
+
+ * get my presence list
+   [implicit poll start?]
+   It is possible that the HS doesn't yet have current presence information when
+   the client requests this. There should be a "don't know" type too.
+
+ * add/remove a user to my presence list
+
+Server-Server
+-------------
+
+Requests that Home Servers make to others
+
+ * request permission to add a user to presence list
+
+ * allow/deny a request to add to a presence list
+
+ * perform a combined presence state push and subscription request
+   For each sending user ID, the message contains their new status.
+   For each receiving user ID, the message should contain an indication on
+   whether the sending server is also interested in receiving status from that
+   user; either as an immediate update response now, or as a promise to send
+   future updates.
+
+Server to Client
+----------------
+
+[[TODO(paul): There also needs to be some way for a user's HS to push status
+updates of the presence list to clients, but the general server-client event
+model currently lacks a space to do that.]]