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authorMatthew Hodgson <matthew@matrix.org>2014-10-09 20:38:00 +0200
committerMatthew Hodgson <matthew@matrix.org>2014-10-09 20:38:00 +0200
commite1170d4edbc3563730028047c23a4169e6420c59 (patch)
tree4664a10735780cca5ef41f08f6abad32f106cfb3 /docs/specification.rst
parentAdd spec-additions.rst with info on recaptcha and common event fields. (diff)
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-Matrix Specification
-====================
-
-WARNING
-=======
-
-.. WARNING::
-  The Matrix specification is still very much evolving: the API is not yet frozen
-  and this document is in places incomplete, stale, and may contain security
-  issues. Needless to say, we have made every effort to highlight the problem
-  areas that we're aware of.
-
-  We're publishing it at this point because it's complete enough to be more than
-  useful and provide a canonical reference to how Matrix is evolving. Our end
-  goal is to mirror WHATWG's `Living Standard <http://wiki.whatwg.org/wiki/FAQ#What_does_.22Living_Standard.22_mean.3F>`_   
-  approach except right now Matrix is more in the process of being born than actually being
-  living!
-
-.. contents:: Table of Contents
-.. sectnum::
-
-Matrix is a new set of open APIs for open-federated Instant Messaging and VoIP
-functionality, designed to create and support a new global real-time
-communication ecosystem on the internet. This specification is the ongoing
-result of standardising the APIs used by the various components of the Matrix
-ecosystem to communicate with one another.
-
-The principles that Matrix attempts to follow are:
-
-- Pragmatic Web-friendly APIs (i.e. JSON over REST)
-- Keep It Simple & Stupid
-
-  + provide a simple architecture with minimal third-party dependencies.
-
-- Fully open:
-
-  + Fully open federation - anyone should be able to participate in the global
-    Matrix network
-  + Fully open standard - publicly documented standard with no IP or patent
-    licensing encumbrances
-  + Fully open source reference implementation - liberally-licensed example
-    implementations with no IP or patent licensing encumbrances
-
-- Empowering the end-user
-
-  + The user should be able to choose the server and clients they use
-  + The user should be control how private their communication is
-  + The user should know precisely where their data is stored
-
-- Fully decentralised - no single points of control over conversations or the
-  network as a whole
-- Learning from history to avoid repeating it
-
-  + Trying to take the best aspects of XMPP, SIP, IRC, SMTP, IMAP and NNTP
-    whilst trying to avoid their failings
-
-The functionality that Matrix provides includes:
-
-- Creation and management of fully distributed chat rooms with no
-  single points of control or failure
-- Eventually-consistent cryptographically secure synchronisation of room
-  state across a global open network of federated servers and services
-- Sending and receiving extensible messages in a room with (optional)
-  end-to-end encryption
-- Extensible user management (inviting, joining, leaving, kicking, banning)
-  mediated by a power-level based user privilege system.
-- Extensible room state management (room naming, aliasing, topics, bans)
-- Extensible user profile management (avatars, displaynames, etc)
-- Managing user accounts (registration, login, logout)
-- Use of 3rd Party IDs (3PIDs) such as email addresses, phone numbers,
-  Facebook accounts to authenticate, identify and discover users on Matrix.
-- Trusted federation of Identity servers for:
-
-  + Publishing user public keys for PKI
-  + Mapping of 3PIDs to Matrix IDs
-
-The end goal of Matrix is to be a ubiquitous messaging layer for synchronising
-arbitrary data between sets of people, devices and services - be that for
-instant messages, VoIP call setups, or any other objects that need to be
-reliably and persistently pushed from A to B in an interoperable and federated
-manner.
-
-Basis
-=====
-
-Architecture
-------------
-
-Clients transmit data to other clients through home servers (HSes). Clients do
-not communicate with each other directly.
-
-::
-
-                         How data flows between clients
-                         ==============================
-
-       { Matrix client A }                             { Matrix client B }
-           ^          |                                    ^          |
-           |  events  |                                    |  events  |
-           |          V                                    |          V
-       +------------------+                            +------------------+
-       |                  |---------( HTTP )---------->|                  |
-       |   Home Server    |                            |   Home Server    |
-       |                  |<--------( HTTP )-----------|                  |
-       +------------------+        Federation          +------------------+
-
-A "Client" typically represents a human using a web application or mobile app.
-Clients use the "Client-to-Server" (C-S) API to communicate with their home
-server, which stores their profile data and their record of the conversations
-in which they participate. Each client is associated with a user account (and
-may optionally support multiple user accounts). A user account is represented
-by a unique "User ID". This ID is namespaced to the home server which allocated
-the account and looks like::
-
-  @localpart:domain
-
-The ``localpart`` of a user ID may be a user name, or an opaque ID identifying
-this user. They are case-insensitive.
-
-.. TODO-spec
-    - Need to specify precise grammar for Matrix IDs
-
-A "Home Server" is a server which provides C-S APIs and has the ability to
-federate with other HSes.  It is typically responsible for multiple clients.
-"Federation" is the term used to describe the sharing of data between two or
-more home servers.
-
-Data in Matrix is encapsulated in an "event". An event is an action within the
-system. Typically each action (e.g. sending a message) correlates with exactly
-one event. Each event has a ``type`` which is used to differentiate different
-kinds of data. ``type`` values MUST be uniquely globally namespaced following
-Java's `package naming conventions
-<http://docs.oracle.com/javase/specs/jls/se5.0/html/packages.html#7.7>`, e.g.
-``com.example.myapp.event``. The special top-level namespace ``m.`` is reserved
-for events defined in the Matrix specification. Events are usually sent in the
-context of a "Room".
-
-Room structure
-~~~~~~~~~~~~~~
-
-A room is a conceptual place where users can send and receive events. Rooms can
-be created, joined and left. Events are sent to a room, and all participants in
-that room with sufficient access will receive the event. Rooms are uniquely
-identified internally via a "Room ID", which look like::
-
-  !opaque_id:domain
-
-There is exactly one room ID for each room. Whilst the room ID does contain a
-domain, it is simply for globally namespacing room IDs. The room does NOT
-reside on the domain specified. Room IDs are not meant to be human readable.
-They ARE case-sensitive.
-
-The following diagram shows an ``m.room.message`` event being sent in the room 
-``!qporfwt:matrix.org``::
-
-       { @alice:matrix.org }                             { @bob:domain.com }
-               |                                                 ^
-               |                                                 |
-      Room ID: !qporfwt:matrix.org                 Room ID: !qporfwt:matrix.org
-      Event type: m.room.message                   Event type: m.room.message
-      Content: { JSON object }                     Content: { JSON object }
-               |                                                 |
-               V                                                 |
-       +------------------+                          +------------------+
-       |   Home Server    |                          |   Home Server    |
-       |   matrix.org     |<-------Federation------->|   domain.com     |
-       +------------------+                          +------------------+
-                |       .................................        |
-                |______|           Shared State          |_______|
-                       | Room ID: !qporfwt:matrix.org    |
-                       | Servers: matrix.org, domain.com |
-                       | Members:                        |
-                       |  - @alice:matrix.org            |
-                       |  - @bob:domain.com              |
-                       |.................................|
-
-Federation maintains shared state between multiple home servers, such that when
-an event is sent to a room, the home server knows where to forward the event on
-to, and how to process the event. State is scoped to a single room, and
-federation ensures that all home servers have the information they need, even
-if that means the home server has to request more information from another home
-server before processing the event.
-
-Room Aliases
-~~~~~~~~~~~~
-
-Each room can also have multiple "Room Aliases", which looks like::
-
-  #room_alias:domain
-
-  .. TODO
-      - Need to specify precise grammar for Room Aliases
-
-A room alias "points" to a room ID and is the human-readable label by which
-rooms are publicised and discovered.  The room ID the alias is pointing to can
-be obtained by visiting the domain specified. They are case-insensitive. Note
-that the mapping from a room alias to a room ID is not fixed, and may change
-over time to point to a different room ID. For this reason, Clients SHOULD
-resolve the room alias to a room ID once and then use that ID on subsequent
-requests.
-
-When resolving a room alias the server will also respond with a list of servers
-that are in the room that can be used to join via.
-
-::
-
-          GET    
-   #matrix:domain.com      !aaabaa:matrix.org
-           |                    ^
-           |                    |
-    _______V____________________|____
-   |          domain.com            |
-   | Mappings:                      |
-   | #matrix >> !aaabaa:matrix.org  |
-   | #golf   >> !wfeiofh:sport.com  |
-   | #bike   >> !4rguxf:matrix.org  |
-   |________________________________|
-       
-Identity
-~~~~~~~~
-
-Users in Matrix are identified via their user ID. However, existing ID
-namespaces can also be used in order to identify Matrix users. A Matrix
-"Identity" describes both the user ID and any other existing IDs from third
-party namespaces *linked* to their account.
-
-Matrix users can *link* third-party IDs (3PIDs) such as email addresses, social
-network accounts and phone numbers to their user ID. Linking 3PIDs creates a
-mapping from a 3PID to a user ID. This mapping can then be used by other Matrix
-users in order to discover other users, according to a strict set of privacy
-permissions.
-
-In order to ensure that the mapping from 3PID to user ID is genuine, a globally
-federated cluster of trusted "Identity Servers" (IS) are used to perform
-authentication of the 3PID.  Identity servers are also used to preserve the
-mapping indefinitely, by replicating the mappings across multiple ISes.
-
-Usage of an IS is not required in order for a client application to be part of
-the Matrix ecosystem. However, without one clients will not be able to look up
-user IDs using 3PIDs.
-
-Presence
-~~~~~~~~
-.. NOTE::
-  This section is a work in progress.
-
-Each user has the concept of presence information. This encodes the
-"availability" of that user, suitable for display on other user's clients. This
-is transmitted as an ``m.presence`` event and is one of the few events which
-are sent *outside the context of a room*. The basic piece of presence
-information is represented by the ``presence`` key, which is an enum of one of
-the following:
-
-  - ``online`` : The default state when the user is connected to an event
-    stream.
-  - ``unavailable`` : The user is not reachable at this time.
-  - ``offline`` : The user is not connected to an event stream.
-  - ``free_for_chat`` : The user is generally willing to receive messages
-    moreso than default.
-  - ``hidden`` : Behaves as offline, but allows the user to see the client
-    state anyway and generally interact with client features. (Not yet
-    implemented in synapse).
-
-This basic ``presence`` 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.
-
-In addition, the server maintains a timestamp of the last time it saw an active
-action from the user; either sending a message to a room, or changing presence
-state from a lower to a higher level of availability (thus: changing state from
-``unavailable`` to ``online`` will count as an action for being active, whereas
-in the other direction will not). This timestamp is presented via a key called
-``last_active_ago``, which gives the relative number of miliseconds since the
-message is generated/emitted, that the user was last seen active.
-
-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_for_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
-
-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. To be added to this
-list, the user being added must receive permission from the list owner. Once
-granted, both user's HS(es) store this information. 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.
-
-Profiles
-~~~~~~~~
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-spec
-  - Metadata extensibility
-
-Internally within Matrix users are referred to by their user ID, which is
-typically a compact unique identifier. Profiles grant users the ability to see
-human-readable names for other users that are in some way meaningful to them.
-Additionally, profiles can publish additional information, such as the user's
-age or location.
-
-A Profile consists of a display name, an avatar picture, and a set of other
-metadata fields that the user may wish to publish (email address, phone
-numbers, website URLs, etc...). This specification puts no requirements on the
-display name other than it being a valid unicode string. Avatar images are not
-stored directly; instead the home server stores an ``http``-scheme URL where
-clients may fetch it from.
-
-API Standards
--------------
-
-The mandatory baseline for communication in Matrix is exchanging JSON objects
-over RESTful HTTP APIs. HTTPS is mandated as the baseline for server-server
-(federation) communication.  HTTPS is recommended for client-server
-communication, although HTTP may be supported as a fallback to support basic
-HTTP clients. More efficient optional transports for client-server
-communication will in future be supported as optional extensions - e.g. a
-packed binary encoding over stream-cipher encrypted TCP socket for
-low-bandwidth/low-roundtrip mobile usage.
-
-.. TODO
-  We need to specify capability negotiation for extensible transports
-
-For the default HTTP transport, all API calls use a Content-Type of
-``application/json``.  In addition, all strings MUST be encoded as UTF-8.
-
-Clients are authenticated using opaque ``access_token`` strings (see
-`Registration and Login`_ for details), passed as a query string parameter on
-all requests.
-
-.. TODO
-  Need to specify any HMAC or access_token lifetime/ratcheting tricks
-
-Any errors which occur on the Matrix API level MUST return a "standard error
-response". This is a JSON object which looks like::
-
-  {
-    "errcode": "<error code>",
-    "error": "<error message>"
-  }
-
-The ``error`` string will be a human-readable error message, usually a sentence
-explaining what went wrong. The ``errcode`` string will be a unique string
-which can be used to handle an error message e.g. ``M_FORBIDDEN``. These error
-codes should have their namespace first in ALL CAPS, followed by a single _.
-For example, if there was a custom namespace ``com.mydomain.here``, and a
-``FORBIDDEN`` code, the error code should look like
-``COM.MYDOMAIN.HERE_FORBIDDEN``. There may be additional keys depending on the
-error, but the keys ``error`` and ``errcode`` MUST always be present. 
-
-Some standard error codes are below:
-
-:``M_FORBIDDEN``:
-  Forbidden access, e.g. joining a room without permission, failed login.
-
-:``M_UNKNOWN_TOKEN``:
-  The access token specified was not recognised.
-
-:``M_BAD_JSON``:
-  Request contained valid JSON, but it was malformed in some way, e.g. missing
-  required keys, invalid values for keys.
-
-:``M_NOT_JSON``:
-  Request did not contain valid JSON.
-
-:``M_NOT_FOUND``:
-  No resource was found for this request.
-
-:``M_LIMIT_EXCEEDED``:
-  Too many requests have been sent in a short period of time. Wait a while then
-  try again.
-
-Some requests have unique error codes:
-
-:``M_USER_IN_USE``:
-  Encountered when trying to register a user ID which has been taken.
-
-:``M_ROOM_IN_USE``:
-  Encountered when trying to create a room which has been taken.
-
-:``M_BAD_PAGINATION``:
-  Encountered when specifying bad pagination query parameters.
-
-:``M_LOGIN_EMAIL_URL_NOT_YET``:
-  Encountered when polling for an email link which has not been clicked yet.
-
-The C-S API typically uses ``HTTP POST`` to submit requests. This means these
-requests are not idempotent. The C-S API also allows ``HTTP PUT`` to make
-requests idempotent. In order to use a ``PUT``, paths should be suffixed with
-``/{txnId}``. ``{txnId}`` is a unique client-generated transaction ID which
-identifies the request, and is scoped to a given Client (identified by that
-client's ``access_token``). Crucially, it **only** serves to identify new
-requests from retransmits. After the request has finished, the ``{txnId}``
-value should be changed (how is not specified; a monotonically increasing
-integer is recommended). It is preferable to use ``HTTP PUT`` to make sure
-requests to send messages do not get sent more than once should clients need to
-retransmit requests.
-
-Valid requests look like::
-
-    POST /some/path/here?access_token=secret
-    {
-      "key": "This is a post."
-    }
-
-    PUT /some/path/here/11?access_token=secret
-    {
-      "key": "This is a put with a txnId of 11."
-    }
-
-In contrast, these are invalid requests::
-
-    POST /some/path/here/11?access_token=secret
-    {
-      "key": "This is a post, but it has a txnId."
-    }
-
-    PUT /some/path/here?access_token=secret
-    {
-      "key": "This is a put but it is missing a txnId."
-    }
-
-Glossary
---------
-.. NOTE::
-  This section is a work in progress.
-
-Backfilling:
-  The process of synchronising historic state from one home server to another,
-  to backfill the event storage so that scrollback can be presented to the
-  client(s). Not to be confused with pagination.
-
-Context:
-  A single human-level entity of interest (currently, a chat room)
-
-EDU (Ephemeral Data Unit):
-  A message that relates directly to a given pair of home servers that are
-  exchanging it. EDUs are short-lived messages that related only to one single
-  pair of servers; they are not persisted for a long time and are not forwarded
-  on to other servers. Because of this, they have no internal ID nor previous
-  EDUs reference chain.
-
-Event:
-  A record of activity that records a single thing that happened on to a context
-  (currently, a chat room). These are the "chat messages" that Synapse makes
-  available.
-
-PDU (Persistent Data Unit):
-  A message that relates to a single context, irrespective of the server that
-  is communicating it. PDUs either encode a single Event, or a single State
-  change. A PDU is referred to by its PDU ID; the pair of its origin server
-  and local reference from that server.
-
-PDU ID:
-  The pair of PDU Origin and PDU Reference, that together globally uniquely
-  refers to a specific PDU.
-
-PDU Origin:
-  The name of the origin server that generated a given PDU. This may not be the
-  server from which it has been received, due to the way they are copied around
-  from server to server. The origin always records the original server that
-  created it.
-
-PDU Reference:
-  A local ID used to refer to a specific PDU from a given origin server. These
-  references are opaque at the protocol level, but may optionally have some
-  structured meaning within a given origin server or implementation.
-
-Presence:
-  The concept of whether a user is currently online, how available they declare
-  they are, and so on. See also: doc/model/presence
-
-Profile:
-  A set of metadata about a user, such as a display name, provided for the
-  benefit of other users. See also: doc/model/profiles
-
-Room ID:
-  An opaque string (of as-yet undecided format) that identifies a particular
-  room and used in PDUs referring to it.
-
-Room Alias:
-  A human-readable string of the form #name:some.domain that users can use as a
-  pointer to identify a room; a Directory Server will map this to its Room ID
-
-State:
-  A set of metadata maintained about a Context, which is replicated among the
-  servers in addition to the history of Events.
-
-User ID:
-  A string of the form @localpart:domain.name that identifies a user for
-  wire-protocol purposes. The localpart is meaningless outside of a particular
-  home server. This takes a human-readable form that end-users can use directly
-  if they so wish, avoiding the 3PIDs.
-
-Transaction:
-  A message which relates to the communication between a given pair of servers.
-  A transaction contains possibly-empty lists of PDUs and EDUs.
-
-.. TODO
-  This glossary contradicts the terms used above - especially on State Events v. "State"
-  and Non-State Events v. "Events".  We need better consistent names.
-
-Events
-======
-
-Receiving live updates on a client
-----------------------------------
-
-Clients can receive new events by long-polling the home server. This will hold
-open the HTTP connection for a short period of time waiting for new events,
-returning early if an event occurs. This is called the `Event Stream`_. All
-events which are visible to the client will appear in the event stream. When
-the request returns, an ``end`` token is included in the response. This token
-can be used in the next request to continue where the client left off.
-
-.. TODO-spec
-  How do we filter the event stream?
-  Do we ever return multiple events in a single request?  Don't we get lots of request
-  setup RTT latency if we only do one event per request? Do we ever support streaming
-  requests? Why not websockets?
-
-When the client first logs in, they will need to initially synchronise with
-their home server. This is achieved via the |initialSync|_ API. This API also
-returns an ``end`` token which can be used with the event stream.
-
-Room Events
------------
-.. NOTE::
-  This section is a work in progress.
-
-This specification outlines several standard event types, all of which are
-prefixed with ``m.``
-
-``m.room.name``
-  Summary:
-    Set the human-readable name for the room.
-  Type: 
-    State event
-  JSON format:
-    ``{ "name" : "string" }``
-  Example:
-    ``{ "name" : "My Room" }``
-  Description:
-    A room has an opaque room ID which is not human-friendly to read. A room
-    alias is human-friendly, but not all rooms have room aliases. The room name
-    is a human-friendly string designed to be displayed to the end-user. The
-    room name is not *unique*, as multiple rooms can have the same room name
-    set. The room name can also be set when creating a room using |createRoom|_
-    with the ``name`` key.
-
-``m.room.topic``
-  Summary:
-    Set a topic for the room.
-  Type: 
-    State event
-  JSON format:
-    ``{ "topic" : "string" }``
-  Example:
-    ``{ "topic" : "Welcome to the real world." }``
-  Description:
-    A topic is a short message detailing what is currently being discussed in
-    the room.  It can also be used as a way to display extra information about
-    the room, which may not be suitable for the room name. The room topic can
-    also be set when creating a room using |createRoom|_ with the ``topic``
-    key.
-
-``m.room.member``
-  Summary:
-    The current membership state of a user in the room.
-  Type: 
-    State event
-  JSON format:
-    ``{ "membership" : "enum[ invite|join|leave|ban ]" }``
-  Example:
-    ``{ "membership" : "join" }``
-  Description:
-    Adjusts the membership state for a user in a room. It is preferable to use
-    the membership APIs (``/rooms/<room id>/invite`` etc) when performing
-    membership actions rather than adjusting the state directly as there are a
-    restricted set of valid transformations. For example, user A cannot force
-    user B to join a room, and trying to force this state change directly will
-    fail. See the `Rooms`_ section for how to use the membership APIs.
-
-``m.room.create``
-  Summary:
-    The first event in the room.
-  Type: 
-    State event
-  JSON format:
-    ``{ "creator": "string"}``
-  Example:
-    ``{ "creator": "@user:example.com" }``
-  Description:
-    This is the first event in a room and cannot be changed. It acts as the 
-    root of all other events.
-
-``m.room.join_rules``
-  Summary:
-    Descripes how/if people are allowed to join.
-  Type: 
-    State event
-  JSON format:
-    ``{ "join_rule": "enum [ public|knock|invite|private ]" }``
-  Example:
-    ``{ "join_rule": "public" }``
-  Description:
-    TODO-doc : Use docs/models/rooms.rst
-   
-``m.room.power_levels``
-  Summary:
-    Defines the power levels of users in the room.
-  Type: 
-    State event
-  JSON format:
-    ``{ "<user_id>": <int>, ..., "default": <int>}``
-  Example:
-    ``{ "@user:example.com": 5, "@user2:example.com": 10, "default": 0 }`` 
-  Description:
-    If a user is in the list, then they have the associated power level. 
-    Otherwise they have the default level. If not ``default`` key is supplied,
-    it is assumed to be 0.
-
-``m.room.add_state_level``
-  Summary:
-    Defines the minimum power level a user needs to add state.
-  Type: 
-    State event
-  JSON format:
-    ``{ "level": <int> }``
-  Example:
-    ``{ "level": 5 }``
-  Description:
-    To add a new piece of state to the room a user must have the given power 
-    level. This does not apply to updating current state, which is goverened
-    by the ``required_power_level`` event key.
-    
-``m.room.send_event_level``
-  Summary:
-    Defines the minimum power level a user needs to send an event.
-  Type: 
-    State event
-  JSON format:
-    ``{ "level": <int> }``
-  Example:
-    ``{ "level": 0 }``
-  Description:
-    To send a new event into the room a user must have at least this power 
-    level. This allows ops to make the room read only by increasing this level,
-    or muting individual users by lowering their power level below this
-    threshold.
-
-``m.room.ops_levels``
-  Summary:
-    Defines the minimum power levels that a user must have before they can 
-    kick and/or ban other users.
-  Type: 
-    State event
-  JSON format:
-    ``{ "ban_level": <int>, "kick_level": <int>, "redact_level": <int> }``
-  Example:
-    ``{ "ban_level": 5, "kick_level": 5 }``
-  Description:
-    This defines who can ban and/or kick people in the room. Most of the time
-    ``ban_level`` will be greater than or equal to ``kick_level`` since 
-    banning is more severe than kicking.
-
-``m.room.aliases``
-  Summary:
-    These state events are used to inform the room about what room aliases it
-    has.
-  Type:
-    State event
-  JSON format:
-    ``{ "aliases": ["string", ...] }``
-  Example:
-    ``{ "aliases": ["#foo:example.com"] }``
-  Description:
-    This event is sent by a homeserver directly to inform of changes to the
-    list of aliases it knows about for that room. As a special-case, the
-    ``state_key`` of the event is the homeserver which owns the room alias.
-    For example, an event might look like::
-
-      {
-        "type": "m.room.aliases",
-        "event_id": "012345678ab",
-        "room_id": "!xAbCdEfG:example.com",
-        "state_key": "example.com",
-        "content": {
-          "aliases": ["#foo:example.com"]
-        }
-      }
-
-    The event contains the full list of aliases now stored by the home server
-    that emitted it; additions or deletions are not explicitly mentioned as
-    being such. The entire set of known aliases for the room is then the union
-    of the individual lists declared by all such keys, one from each home
-    server holding at least one alias.
-
-    Clients `should` check the validity of any room alias given in this list
-    before presenting it to the user as trusted fact. The lists given by this
-    event should be considered simply as advice on which aliases might exist,
-    for which the client can perform the lookup to confirm whether it receives
-    the correct room ID.
-
-``m.room.message``
-  Summary:
-    A message.
-  Type: 
-    Non-state event
-  JSON format:
-    ``{ "msgtype": "string" }``
-  Example:
-    ``{ "msgtype": "m.text", "body": "Testing" }``
-  Description:
-    This event is used when sending messages in a room. Messages are not
-    limited to be text.  The ``msgtype`` key outlines the type of message, e.g.
-    text, audio, image, video, etc.  Whilst not required, the ``body`` key
-    SHOULD be used with every kind of ``msgtype`` as a fallback mechanism when
-    a client cannot render the message. For more information on the types of
-    messages which can be sent, see `m.room.message msgtypes`_.
-
-``m.room.message.feedback``
-  Summary:
-    A receipt for a message.
-  Type: 
-    Non-state event
-  JSON format:
-    ``{ "type": "enum [ delivered|read ]", "target_event_id": "string" }``
-  Example:
-    ``{ "type": "delivered", "target_event_id": "e3b2icys" }``
-  Description:
-    Feedback events are events sent to acknowledge a message in some way. There
-    are two supported acknowledgements: ``delivered`` (sent when the event has
-    been received) and ``read`` (sent when the event has been observed by the
-    end-user). The ``target_event_id`` should reference the ``m.room.message``
-    event being acknowledged. 
-
-``m.room.redaction``
-  Summary:
-    Indicates a previous event has been redacted.
-  Type:
-    Non-state event
-  JSON format:
-    ``{ "reason": "string" }``
-  Description:
-    Events can be redacted by either room or server admins. Redacting an event
-    means that all keys not required by the protocol are stripped off, allowing
-    admins to remove offensive or illegal content that may have been attached
-    to any event. This cannot be undone, allowing server owners to physically
-    delete the offending data.  There is also a concept of a moderator hiding a
-    non-state event, which can be undone, but cannot be applied to state
-    events.
-    The event that has been redacted is specified in the ``redacts`` event
-    level key.
-
-m.room.message msgtypes
-~~~~~~~~~~~~~~~~~~~~~~~
-
-.. TODO-spec
-   How a client should handle unknown message types.
-
-Each ``m.room.message`` MUST have a ``msgtype`` key which identifies the type
-of message being sent. Each type has their own required and optional keys, as
-outlined below:
-
-``m.text``
-  Required keys:
-    - ``body`` : "string" - The body of the message.
-  Optional keys:
-    None.
-  Example:
-    ``{ "msgtype": "m.text", "body": "I am a fish" }``
-
-``m.emote``
-  Required keys:
-    - ``body`` : "string" - The emote action to perform.
-  Optional keys:
-    None.
-  Example:
-    ``{ "msgtype": "m.emote", "body": "tries to come up with a witty explanation" }``
-
-``m.image``
-  Required keys:
-    - ``url`` : "string" - The URL to the image.
-  Optional keys:
-    - ``info`` : "string" - info : JSON object (ImageInfo) - The image info for
-      image referred to in ``url``.
-    - ``thumbnail_url`` : "string" - The URL to the thumbnail.
-    - ``thumbnail_info`` : JSON object (ImageInfo) - The image info for the
-      image referred to in ``thumbnail_url``.
-    - ``body`` : "string" - The alt text of the image, or some kind of content
-      description for accessibility e.g. "image attachment".
-
-  ImageInfo: 
-    Information about an image::
-    
-      { 
-        "size" : integer (size of image in bytes),
-        "w" : integer (width of image in pixels),
-        "h" : integer (height of image in pixels),
-        "mimetype" : "string (e.g. image/jpeg)",
-      }
-
-``m.audio``
-  Required keys:
-    - ``url`` : "string" - The URL to the audio.
-  Optional keys:
-    - ``info`` : JSON object (AudioInfo) - The audio info for the audio
-      referred to in ``url``.
-    - ``body`` : "string" - A description of the audio e.g. "Bee Gees - Stayin'
-      Alive", or some kind of content description for accessibility e.g.
-      "audio attachment".
-  AudioInfo: 
-    Information about a piece of audio::
-
-      {
-        "mimetype" : "string (e.g. audio/aac)",
-        "size" : integer (size of audio in bytes),
-        "duration" : integer (duration of audio in milliseconds),
-      }
-
-``m.video``
-  Required keys:
-    - ``url`` : "string" - The URL to the video.
-  Optional keys:
-    - ``info`` : JSON object (VideoInfo) - The video info for the video
-      referred to in ``url``.
-    - ``body`` : "string" - A description of the video e.g. "Gangnam style", or
-      some kind of content description for accessibility e.g. "video
-      attachment".
-
-  VideoInfo: 
-    Information about a video::
-
-      {
-        "mimetype" : "string (e.g. video/mp4)",
-        "size" : integer (size of video in bytes),
-        "duration" : integer (duration of video in milliseconds),
-        "w" : integer (width of video in pixels),
-        "h" : integer (height of video in pixels),
-        "thumbnail_url" : "string (URL to image)",
-        "thumbanil_info" : JSON object (ImageInfo)
-      }
-
-``m.location``
-  Required keys:
-    - ``geo_uri`` : "string" - The geo URI representing the location.
-  Optional keys:
-    - ``thumbnail_url`` : "string" - The URL to a thumnail of the location
-      being represented.
-    - ``thumbnail_info`` : JSON object (ImageInfo) - The image info for the
-      image referred to in ``thumbnail_url``.
-    - ``body`` : "string" - A description of the location e.g. "Big Ben,
-      London, UK", or some kind of content description for accessibility e.g.
-      "location attachment".
-
-The following keys can be attached to any ``m.room.message``:
-
-  Optional keys:
-    - ``sender_ts`` : integer - A timestamp (ms resolution) representing the
-      wall-clock time when the message was sent from the client.
-
-Events on Change of Profile Information
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Because the profile displayname and avatar information are likely to be used in
-many places of a client's display, changes to these fields cause an automatic
-propagation event to occur, informing likely-interested parties of the new
-values. This change is conveyed using two separate mechanisms:
-
- - a ``m.room.member`` event is sent to every room the user is a member of,
-   to update the ``displayname`` and ``avatar_url``.
- - a presence status update is sent, again containing the new values of the
-   ``displayname`` and ``avatar_url`` keys, in addition to the required
-   ``presence`` key containing the current presence state of the user.
-
-Both of these should be done automatically by the home server when a user
-successfully changes their displayname or avatar URL fields.
-
-Additionally, when home servers emit room membership events for their own
-users, they should include the displayname and avatar URL fields in these
-events so that clients already have these details to hand, and do not have to
-perform extra roundtrips to query it.
-
-Voice over IP
--------------
-Matrix can also be used to set up VoIP calls. This is part of the core
-specification, although is still in a very early stage. Voice (and video) over
-Matrix is based on the WebRTC standards.
-
-Call events are sent to a room, like any other event. This means that clients
-must only send call events to rooms with exactly two participants as currently
-the WebRTC standard is based around two-party communication.
-
-Events
-~~~~~~
-``m.call.invite``
-This event is sent by the caller when they wish to establish a call.
-
-  Required keys:
-    - ``call_id`` : "string" - A unique identifier for the call
-    - ``offer`` : "offer object" - The session description
-    - ``version`` : "integer" - The version of the VoIP specification this
-      message adheres to. This specification is version 0.
-    - ``lifetime`` : "integer" - The time in milliseconds that the invite is
-      valid for. Once the invite age exceeds this value, clients should discard
-      it. They should also no longer show the call as awaiting an answer in the
-      UI.
-      
-  Optional keys:
-    None.
-  Example:
-    ``{ "version" : 0, "call_id": "12345", "offer": { "type" : "offer", "sdp" : "v=0\r\no=- 6584580628695956864 2 IN IP4 127.0.0.1[...]" } }``
-
-``Offer Object``
-  Required keys:
-    - ``type`` : "string" - The type of session description, in this case
-      'offer'
-    - ``sdp`` : "string" - The SDP text of the session description
-
-``m.call.candidates``
-This event is sent by callers after sending an invite and by the callee after
-answering.  Its purpose is to give the other party additional ICE candidates to
-try using to communicate.
-
-  Required keys:
-    - ``call_id`` : "string" - The ID of the call this event relates to
-    - ``version`` : "integer" - The version of the VoIP specification this
-      messages adheres to. his specification is version 0.
-    - ``candidates`` : "array of candidate objects" - Array of object
-      describing the candidates.
-
-``Candidate Object``
-
-  Required Keys:
-    - ``sdpMid`` : "string" - The SDP media type this candidate is intended
-      for.
-    - ``sdpMLineIndex`` : "integer" - The index of the SDP 'm' line this
-      candidate is intended for
-    - ``candidate`` : "string" - The SDP 'a' line of the candidate
-
-``m.call.answer``
-
-  Required keys:
-    - ``call_id`` : "string" - The ID of the call this event relates to
-    - ``version`` : "integer" - The version of the VoIP specification this
-      messages
-    - ``answer`` : "answer object" - Object giving the SDK answer
-
-``Answer Object``
-
-  Required keys:
-    - ``type`` : "string" - The type of session description. 'answer' in this
-      case.
-    - ``sdp`` : "string" - The SDP text of the session description
-
-``m.call.hangup``
-Sent by either party to signal their termination of the call. This can be sent
-either once the call has has been established or before to abort the call.
-
-  Required keys:
-    - ``call_id`` : "string" - The ID of the call this event relates to
-    - ``version`` : "integer" - The version of the VoIP specification this
-      messages
-
-Message Exchange
-~~~~~~~~~~~~~~~~
-A call is set up with messages exchanged as follows:
-
-::
-
-   Caller                   Callee
- m.call.invite ----------->
- m.call.candidate -------->
- [more candidates events]
-                         User answers call
-                  <------ m.call.answer
-               [...]
-                  <------ m.call.hangup
-                  
-Or a rejected call:
-
-::
-
-   Caller                   Callee
- m.call.invite ----------->
- m.call.candidate -------->
- [more candidates events]
-                        User rejects call
-                 <------- m.call.hangup
-
-Calls are negotiated according to the WebRTC specification.
-
-
-Glare
-~~~~~
-This specification aims to address the problem of two users calling each other
-at roughly the same time and their invites crossing on the wire. It is a far
-better experience for the users if their calls are connected if it is clear
-that their intention is to set up a call with one another.
-
-In Matrix, calls are to rooms rather than users (even if those rooms may only
-contain one other user) so we consider calls which are to the same room.
-
-The rules for dealing with such a situation are as follows:
-
- - If an invite to a room is received whilst the client is preparing to send an
-   invite to the same room, the client should cancel its outgoing call and
-   instead automatically accept the incoming call on behalf of the user.
- - If an invite to a room is received after the client has sent an invite to
-   the same room and is waiting for a response, the client should perform a
-   lexicographical comparison of the call IDs of the two calls and use the
-   lesser of the two calls, aborting the greater. If the incoming call is the
-   lesser, the client should accept this call on behalf of the user.
-
-The call setup should appear seamless to the user as if they had simply placed
-a call and the other party had accepted. Thusly, any media stream that had been
-setup for use on a call should be transferred and used for the call that
-replaces it.
-
-Client-Server API
-=================
-
-Registration and Login
-----------------------
-
-Clients must register with a home server in order to use Matrix. After
-registering, the client will be given an access token which must be used in ALL
-requests to that home server as a query parameter 'access_token'.
-
-If the client has already registered, they need to be able to login to their
-account. The home server may provide many different ways of logging in, such as
-user/password auth, login via a social network (OAuth2), login by confirming a
-token sent to their email address, etc. This specification does not define how
-home servers should authorise their users who want to login to their existing
-accounts, but instead defines the standard interface which implementations
-should follow so that ANY client can login to ANY home server. Clients login
-using the |login|_ API. Clients register using the |register|_ API.
-Registration follows the same general procedure as login, but the path requests
-are sent to and the details contained in them are different.
-
-In both registration and login cases, the process takes the form of one or more
-stages, where at each stage the client submits a set of data for a given stage
-type and awaits a response from the server, which will either be a final
-success or a request to perform an additional stage. This exchange continues
-until the final success.
-
-In order to determine up-front what the server's requirements are, the client
-can request from the server a complete description of all of its acceptable
-flows of the registration or login process. It can then inspect the list of
-returned flows looking for one for which it believes it can complete all of the
-required stages, and perform it. As each home server may have different ways of
-logging in, the client needs to know how they should login. All distinct login
-stages MUST have a corresponding ``type``. A ``type`` is a namespaced string
-which details the mechanism for logging in.
-
-A client may be able to login via multiple valid login flows, and should choose
-a single flow when logging in. A flow is a series of login stages. The home
-server MUST respond with all the valid login flows when requested by a simple
-``GET`` request directly to the ``/login`` or ``/register`` paths::
-
-  {
-    "flows": [
-      {
-        "type": "<login type1a>",
-        "stages": [ "<login type 1a>", "<login type 1b>" ]
-      },
-      {
-        "type": "<login type2a>",
-        "stages": [ "<login type 2a>", "<login type 2b>" ]
-      },
-      {
-        "type": "<login type3>"
-      }
-    ]
-  }
-
-The client can now select which flow it wishes to use, and begin making
-``POST`` requests to the ``/login`` or ``/register`` paths with JSON body
-content containing the name of the stage as the ``type`` key, along with
-whatever additional parameters are required for that login or registration type
-(see below). After the flow is completed, the client's fully-qualified user
-ID and a new access token MUST be returned::
-
-  {
-    "user_id": "@user:matrix.org",
-    "access_token": "abcdef0123456789"
-  }
-
-The ``user_id`` key is particularly useful if the home server wishes to support
-localpart entry of usernames (e.g. "user" rather than "@user:matrix.org"), as
-the client may not be able to determine its ``user_id`` in this case.
-
-If the flow has multiple stages to it, the home server may wish to create a
-session to store context between requests. If a home server responds with a
-``session`` key to a request, clients MUST submit it in subsequent requests
-until the flow is completed::
-
-  {
-    "session": "<session id>"
-  }
-
-This specification defines the following login types:
- - ``m.login.password``
- - ``m.login.oauth2``
- - ``m.login.email.code``
- - ``m.login.email.url``
- - ``m.login.email.identity``
-
-Password-based
-~~~~~~~~~~~~~~
-:Type: 
-  ``m.login.password``
-:Description: 
-  Login is supported via a username and password.
-
-To respond to this type, reply with::
-
-  {
-    "type": "m.login.password",
-    "user": "<user_id or user localpart>",
-    "password": "<password>"
-  }
-
-The home server MUST respond with either new credentials, the next stage of the
-login process, or a standard error response.
-
-OAuth2-based
-~~~~~~~~~~~~
-:Type: 
-  ``m.login.oauth2``
-:Description:
-  Login is supported via OAuth2 URLs. This login consists of multiple requests.
-
-To respond to this type, reply with::
-
-  {
-    "type": "m.login.oauth2",
-    "user": "<user_id or user localpart>"
-  }
-
-The server MUST respond with::
-
-  {
-    "uri": <Authorization Request URI OR service selection URI>
-  }
-
-The home server acts as a 'confidential' client for the purposes of OAuth2.  If
-the uri is a ``sevice selection URI``, it MUST point to a webpage which prompts
-the user to choose which service to authorize with. On selection of a service,
-this MUST link through to an ``Authorization Request URI``. If there is only 1
-service which the home server accepts when logging in, this indirection can be
-skipped and the "uri" key can be the ``Authorization Request URI``. 
-
-The client then visits the ``Authorization Request URI``, which then shows the
-OAuth2 Allow/Deny prompt. Hitting 'Allow' returns the ``redirect URI`` with the
-auth code.  Home servers can choose any path for the ``redirect URI``. The
-client should visit the ``redirect URI``, which will then finish the OAuth2
-login process, granting the home server an access token for the chosen service.
-When the home server gets this access token, it verifies that the cilent has
-authorised with the 3rd party, and can now complete the login. The OAuth2
-``redirect URI`` (with auth code) MUST respond with either new credentials, the
-next stage of the login process, or a standard error response.
-    
-For example, if a home server accepts OAuth2 from Google, it would return the 
-Authorization Request URI for Google::
-
-  {
-    "uri": "https://accounts.google.com/o/oauth2/auth?response_type=code&
-    client_id=CLIENT_ID&redirect_uri=REDIRECT_URI&scope=photos"
-  }
-
-The client then visits this URI and authorizes the home server. The client then
-visits the REDIRECT_URI with the auth code= query parameter which returns::
-
-  {
-    "user_id": "@user:matrix.org",
-    "access_token": "0123456789abcdef"
-  }
-
-Email-based (code)
-~~~~~~~~~~~~~~~~~~
-:Type: 
-  ``m.login.email.code``
-:Description:
-  Login is supported by typing in a code which is sent in an email. This login 
-  consists of multiple requests.
-
-To respond to this type, reply with::
-
-  {
-    "type": "m.login.email.code",
-    "user": "<user_id or user localpart>",
-    "email": "<email address>"
-  }
-
-After validating the email address, the home server MUST send an email
-containing an authentication code and return::
-
-  {
-    "type": "m.login.email.code",
-    "session": "<session id>"
-  }
-
-The second request in this login stage involves sending this authentication
-code::
-
-  {
-    "type": "m.login.email.code",
-    "session": "<session id>",
-    "code": "<code in email sent>"
-  }
-
-The home server MUST respond to this with either new credentials, the next
-stage of the login process, or a standard error response.
-
-Email-based (url)
-~~~~~~~~~~~~~~~~~
-:Type: 
-  ``m.login.email.url``
-:Description:
-  Login is supported by clicking on a URL in an email. This login consists of 
-  multiple requests.
-
-To respond to this type, reply with::
-
-  {
-    "type": "m.login.email.url",
-    "user": "<user_id or user localpart>",
-    "email": "<email address>"
-  }
-
-After validating the email address, the home server MUST send an email
-containing an authentication URL and return::
-
-  {
-    "type": "m.login.email.url",
-    "session": "<session id>"
-  }
-
-The email contains a URL which must be clicked. After it has been clicked, the
-client should perform another request::
-
-  {
-    "type": "m.login.email.url",
-    "session": "<session id>"
-  }
-
-The home server MUST respond to this with either new credentials, the next
-stage of the login process, or a standard error response. 
-
-A common client implementation will be to periodically poll until the link is
-clicked.  If the link has not been visited yet, a standard error response with
-an errcode of ``M_LOGIN_EMAIL_URL_NOT_YET`` should be returned.
-
-
-Email-based (identity server)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-:Type:
-  ``m.login.email.identity``
-:Description:
-  Login is supported by authorising an email address with an identity server.
-
-Prior to submitting this, the client should authenticate with an identity
-server.  After authenticating, the session information should be submitted to
-the home server.
-
-To respond to this type, reply with::
-
-  {
-    "type": "m.login.email.identity",
-    "threepidCreds": [
-      {
-        "sid": "<identity server session id>",
-        "clientSecret": "<identity server client secret>",
-        "idServer": "<url of identity server authed with, e.g. 'matrix.org:8090'>"
-      }
-    ]
-  }
-
-
-
-N-Factor Authentication
-~~~~~~~~~~~~~~~~~~~~~~~
-Multiple login stages can be combined to create N-factor authentication during
-login.
-
-This can be achieved by responding with the ``next`` login type on completion
-of a previous login stage::
-
-  {
-    "next": "<next login type>"
-  }
-
-If a home server implements N-factor authentication, it MUST respond with all 
-``stages`` when initially queried for their login requirements::
-
-  {
-    "type": "<1st login type>",
-    "stages": [ <1st login type>, <2nd login type>, ... , <Nth login type> ]
-  }
-
-This can be represented conceptually as::
-
-   _______________________
-  |    Login Stage 1      |
-  | type: "<login type1>" |
-  |  ___________________  |
-  | |_Request_1_________| | <-- Returns "session" key which is used throughout.
-  |  ___________________  |     
-  | |_Request_2_________| | <-- Returns a "next" value of "login type2"
-  |_______________________|
-            |
-            |
-   _________V_____________
-  |    Login Stage 2      |
-  | type: "<login type2>" |
-  |  ___________________  |
-  | |_Request_1_________| |
-  |  ___________________  |
-  | |_Request_2_________| |
-  |  ___________________  |
-  | |_Request_3_________| | <-- Returns a "next" value of "login type3"
-  |_______________________|
-            |
-            |
-   _________V_____________
-  |    Login Stage 3      |
-  | type: "<login type3>" |
-  |  ___________________  |
-  | |_Request_1_________| | <-- Returns user credentials
-  |_______________________|
-
-Fallback
-~~~~~~~~
-Clients cannot be expected to be able to know how to process every single login
-type. If a client determines it does not know how to handle a given login type,
-it should request a login fallback page::
-
-  GET matrix/client/api/v1/login/fallback
-
-This MUST return an HTML page which can perform the entire login process.
-
-
-Rooms
------
-
-Creation
-~~~~~~~~
-To create a room, a client has to use the |createRoom|_ API. There are various
-options which can be set when creating a room:
-
-``visibility``
-  Type: 
-    String
-  Optional: 
-    Yes
-  Value:
-    Either ``public`` or ``private``.
-  Description:
-    A ``public`` visibility indicates that the room will be shown in the public
-    room list. A ``private`` visibility will hide the room from the public room
-    list. Rooms default to ``private`` visibility if this key is not included.
-
-``room_alias_name``
-  Type: 
-    String
-  Optional: 
-    Yes
-  Value:
-    The room alias localpart.
-  Description:
-    If this is included, a room alias will be created and mapped to the newly
-    created room.  The alias will belong on the same home server which created
-    the room, e.g.  ``!qadnasoi:domain.com >>> #room_alias_name:domain.com``
-
-``name``
-  Type: 
-    String
-  Optional: 
-    Yes
-  Value:
-    The ``name`` value for the ``m.room.name`` state event.
-  Description:
-    If this is included, an ``m.room.name`` event will be sent into the room to
-    indicate the name of the room. See `Room Events`_ for more information on
-    ``m.room.name``.
-
-``topic``
-  Type: 
-    String
-  Optional: 
-    Yes
-  Value:
-    The ``topic`` value for the ``m.room.topic`` state event.
-  Description:
-    If this is included, an ``m.room.topic`` event will be sent into the room
-    to indicate the topic for the room. See `Room Events`_ for more information
-    on ``m.room.topic``.
-
-``invite``
-  Type:
-    List
-  Optional:
-    Yes
-  Value:
-    A list of user ids to invite.
-  Description:
-    This will tell the server to invite everyone in the list to the newly
-    created room.
-
-Example::
-
-  {
-    "visibility": "public", 
-    "room_alias_name": "thepub",
-    "name": "The Grand Duke Pub",
-    "topic": "All about happy hour"
-  }
-
-The home server will create a ``m.room.create`` event when the room is created,
-which serves as the root of the PDU graph for this room. This event also has a
-``creator`` key which contains the user ID of the room creator. It will also
-generate several other events in order to manage permissions in this room. This
-includes:
-
- - ``m.room.power_levels`` : Sets the power levels of users.
- - ``m.room.join_rules`` : Whether the room is "invite-only" or not.
- - ``m.room.add_state_level``: The power level required in order to add new
-   state to the room (as opposed to updating exisiting state)
- - ``m.room.send_event_level`` : The power level required in order to send a
-   message in this room.
- - ``m.room.ops_level`` : The power level required in order to kick or ban a
-   user from the room or redact an event in the room.
-
-See `Room Events`_ for more information on these events.
-
-Room aliases
-~~~~~~~~~~~~
-.. NOTE::
-  This section is a work in progress.
-
-Room aliases can be created by sending a ``PUT /directory/room/<room alias>``::
-
-  {
-    "room_id": <room id>
-  }
-
-They can be deleted by sending a ``DELETE /directory/room/<room alias>`` with
-no content. Only some privileged users may be able to delete room aliases, e.g.
-server admins, the creator of the room alias, etc. This specification does not
-outline the privilege level required for deleting room aliases.
-
-As room aliases are scoped to a particular home server domain name, it is
-likely that a home server will reject attempts to maintain aliases on other
-domain names. This specification does not provide a way for home servers to
-send update requests to other servers.
-
-Rooms store a *partial* list of room aliases via the ``m.room.aliases`` state
-event. This alias list is partial because it cannot guarantee that the alias
-list is in any way accurate or up-to-date, as room aliases can point to 
-different room IDs over time. Crucially, the aliases in this event are
-**purely informational** and SHOULD NOT be treated as accurate. They SHOULD
-be checked before they are used or shared with another user. If a room
-appears to have a room alias of ``#alias:example.com``, this SHOULD be checked
-to make sure that the room's ID matches the ``room_id`` returned from the
-request.
-
-Room aliases can be checked in the same way they are resolved; by sending a 
-``GET /directory/room/<room alias>``::
-
-  {
-    "room_id": <room id>,
-    "servers": [ <domain>, <domain2>, <domain3> ]
-  }
-
-Home servers can respond to resolve requests for aliases on other domains than
-their own by using the federation API to ask other domain name home servers.
-
-
-Permissions
-~~~~~~~~~~~
-.. NOTE::
-  This section is a work in progress.
-
-Permissions for rooms are done via the concept of power levels - to do any
-action in a room a user must have a suitable power level. Power levels are
-stored as state events in a given room. 
-
-Power levels for users are defined in ``m.room.power_levels``, where both a
-default and specific users' power levels can be set::
-
-  {
-    "<user id 1>": <power level int>,
-    "<user id 2>": <power level int>,
-    "default": 0
-  }
-
-By default all users have a power level of 0, other than the room creator whose
-power level defaults to 100. Users can grant other users increased power levels
-up to their own power level. For example, user A with a power level of 50 could
-increase the power level of user B to a maximum of level 50. Power levels for 
-users are tracked per-room even if the user is not present in the room.
-
-State events may contain a ``required_power_level`` key, which indicates the
-minimum power a user must have before they can update that state key. The only
-exception to this is when a user leaves a room, which revokes the user's right
-to update state events in that room.
-
-To perform certain actions there are additional power level requirements
-defined in the following state events:
-
-- ``m.room.send_event_level`` defines the minimum ``level`` for sending 
-  non-state events. Defaults to 50.
-- ``m.room.add_state_level`` defines the minimum ``level`` for adding new 
-  state, rather than updating existing state. Defaults to 50.
-- ``m.room.ops_level`` defines the minimum ``ban_level`` and ``kick_level`` to 
-  ban and kick other users respectively. This defaults to a kick and ban levels
-  of 50 each.
-
-
-Joining rooms
-~~~~~~~~~~~~~
-.. TODO-doc What does the home server have to do to join a user to a room?
-   -  See SPEC-30.
-
-Users need to join a room in order to send and receive events in that room. A
-user can join a room by making a request to |/join/<room_alias_or_id>|_ with::
-
-  {}
-
-Alternatively, a user can make a request to |/rooms/<room_id>/join|_ with the
-same request content.  This is only provided for symmetry with the other
-membership APIs: ``/rooms/<room id>/invite`` and ``/rooms/<room id>/leave``. If
-a room alias was specified, it will be automatically resolved to a room ID,
-which will then be joined. The room ID that was joined will be returned in
-response::
-
-  {
-    "room_id": "!roomid:domain"
-  }
-
-The membership state for the joining user can also be modified directly to be
-``join`` by sending the following request to
-``/rooms/<room id>/state/m.room.member/<url encoded user id>``::
-
-  {
-    "membership": "join"
-  }
-
-See the `Room events`_ section for more information on ``m.room.member``.
-
-After the user has joined a room, they will receive subsequent events in that
-room. This room will now appear as an entry in the |initialSync|_ API.
-
-Some rooms enforce that a user is *invited* to a room before they can join that
-room. Other rooms will allow anyone to join the room even if they have not
-received an invite.
-
-Inviting users
-~~~~~~~~~~~~~~
-.. TODO-doc Invite-join dance 
-  - Outline invite join dance. What is it? Why is it required? How does it work?
-  - What does the home server have to do?
-
-The purpose of inviting users to a room is to notify them that the room exists
-so they can choose to become a member of that room. Some rooms require that all
-users who join a room are previously invited to it (an "invite-only" room).
-Whether a given room is an "invite-only" room is determined by the room config
-key ``m.room.join_rules``. It can have one of the following values:
-
-``public``
-  This room is free for anyone to join without an invite.
-
-``invite``
-  This room can only be joined if you were invited.
-
-Only users who have a membership state of ``join`` in a room can invite new
-users to said room. The person being invited must not be in the ``join`` state
-in the room. The fully-qualified user ID must be specified when inviting a
-user, as the user may reside on a different home server. To invite a user, send
-the following request to |/rooms/<room_id>/invite|_, which will manage the
-entire invitation process::
-
-  {
-    "user_id": "<user id to invite>"
-  }
-
-Alternatively, the membership state for this user in this room can be modified 
-directly by sending the following request to 
-``/rooms/<room id>/state/m.room.member/<url encoded user id>``::
-
-  {
-    "membership": "invite"
-  }
-
-See the `Room events`_ section for more information on ``m.room.member``.
-
-Leaving rooms
-~~~~~~~~~~~~~
-.. TODO-spec - HS deleting rooms they are no longer a part of. Not implemented.
-  - This is actually Very Tricky. If all clients a HS is serving leave a room,
-  the HS will no longer get any new events for that room, because the servers
-  who get the events are determined on the *membership list*. There should
-  probably be a way for a HS to lurk on a room even if there are 0 of their
-  members in the room.
-  - Grace period before deletion?
-  - Under what conditions should a room NOT be purged?
-
-
-A user can leave a room to stop receiving events for that room. A user must
-have joined the room before they are eligible to leave the room. If the room is
-an "invite-only" room, they will need to be re-invited before they can re-join
-the room.  To leave a room, a request should be made to
-|/rooms/<room_id>/leave|_ with::
-
-  {}
-
-Alternatively, the membership state for this user in this room can be modified 
-directly by sending the following request to 
-``/rooms/<room id>/state/m.room.member/<url encoded user id>``::
-
-  {
-    "membership": "leave"
-  }
-
-See the `Room events`_ section for more information on ``m.room.member``.
-
-Once a user has left a room, that room will no longer appear on the
-|initialSync|_ API.
-
-If all members in a room leave, that room becomes eligible for deletion. 
-
-Banning users in a room
-~~~~~~~~~~~~~~~~~~~~~~~
-A user may decide to ban another user in a room. 'Banning' forces the target
-user to leave the room and prevents them from re-joining the room. A banned
-user will not be treated as a joined user, and so will not be able to send or
-receive events in the room. In order to ban someone, the user performing the
-ban MUST have the required power level. To ban a user, a request should be made
-to |/rooms/<room_id>/ban|_ with::
-
-  {
-    "user_id": "<user id to ban"
-    "reason": "string: <reason for the ban>"
-  }
-  
-Banning a user adjusts the banned member's membership state to ``ban`` and
-adjusts the power level of this event to a level higher than the banned person.
-Like with other membership changes, a user can directly adjust the target
-member's state, by making a request to
-``/rooms/<room id>/state/m.room.member/<user id>``::
-
-  {
-    "membership": "ban"
-  }
-
-Events in a room
-~~~~~~~~~~~~~~~~
-Room events can be split into two categories:
-
-:State Events:
-  These are events which replace events that came before it, depending on a set
-  of unique keys.  These keys are the event ``type`` and a ``state_key``.
-  Events with the same set of keys will be overwritten. Typically, state events
-  are used to store state, hence their name.
-
-:Non-state events:
-  These are events which cannot be overwritten after sending. The list of
-  events continues to grow as more events are sent. As this list grows, it
-  becomes necessary to provide a mechanism for navigating this list. Pagination
-  APIs are used to view the list of historical non-state events. Typically,
-  non-state events are used to send messages.
-
-This specification outlines several events, all with the event type prefix
-``m.``. However, applications may wish to add their own type of event, and this
-can be achieved using the REST API detailed in the following sections. If new
-events are added, the event ``type`` key SHOULD follow the Java package naming
-convention, e.g. ``com.example.myapp.event``.  This ensures event types are
-suitably namespaced for each application and reduces the risk of clashes.
-
-State events
-~~~~~~~~~~~~
-State events can be sent by ``PUT`` ing to
-|/rooms/<room_id>/state/<event_type>/<state_key>|_.  These events will be
-overwritten if ``<room id>``, ``<event type>`` and ``<state key>`` all match.
-If the state event has no ``state_key``, it can be omitted from the path. These
-requests **cannot use transaction IDs** like other ``PUT`` paths because they
-cannot be differentiated from the ``state_key``. Furthermore, ``POST`` is
-unsupported on state paths. Valid requests look like::
-
-  PUT /rooms/!roomid:domain/state/m.example.event
-  { "key" : "without a state key" }
-
-  PUT /rooms/!roomid:domain/state/m.another.example.event/foo
-  { "key" : "with 'foo' as the state key" }
-
-In contrast, these requests are invalid::
-
-  POST /rooms/!roomid:domain/state/m.example.event/
-  { "key" : "cannot use POST here" }
-
-  PUT /rooms/!roomid:domain/state/m.another.example.event/foo/11
-  { "key" : "txnIds are not supported" }
-
-Care should be taken to avoid setting the wrong ``state key``::
-
-  PUT /rooms/!roomid:domain/state/m.another.example.event/11
-  { "key" : "with '11' as the state key, but was probably intended to be a txnId" }
-
-The ``state_key`` is often used to store state about individual users, by using
-the user ID as the ``state_key`` value. For example::
-
-  PUT /rooms/!roomid:domain/state/m.favorite.animal.event/%40my_user%3Adomain.com
-  { "animal" : "cat", "reason": "fluffy" }
-
-In some cases, there may be no need for a ``state_key``, so it can be omitted::
-
-  PUT /rooms/!roomid:domain/state/m.room.bgd.color
-  { "color": "red", "hex": "#ff0000" }
-
-See `Room Events`_ for the ``m.`` event specification.
-
-Non-state events
-~~~~~~~~~~~~~~~~
-Non-state events can be sent by sending a request to
-|/rooms/<room_id>/send/<event_type>|_.  These requests *can* use transaction
-IDs and ``PUT``/``POST`` methods. Non-state events allow access to historical
-events and pagination, making it best suited for sending messages.  For
-example::
-
-  POST /rooms/!roomid:domain/send/m.custom.example.message
-  { "text": "Hello world!" }
-
-  PUT /rooms/!roomid:domain/send/m.custom.example.message/11
-  { "text": "Goodbye world!" }
-
-See `Room Events`_ for the ``m.`` event specification.
-
-Syncing rooms
-~~~~~~~~~~~~~
-.. NOTE::
-  This section is a work in progress.
-
-When a client logs in, they may have a list of rooms which they have already
-joined. These rooms may also have a list of events associated with them. The
-purpose of 'syncing' is to present the current room and event information in a
-convenient, compact manner. The events returned are not limited to room events;
-presence events will also be returned. A single syncing API is provided:
-
- - |initialSync|_ : A global sync which will present room and event information
-   for all rooms the user has joined.
-
-.. TODO-spec room-scoped initial sync
- - |/rooms/<room_id>/initialSync|_ : A sync scoped to a single room. Presents
-   room and event information for this room only.
- - Room-scoped initial sync is Very Tricky because typically people would
-   want to sync the room then listen for any new content from that point
-   onwards. The event stream cannot do this for a single room currently.
-   As a result, commenting room-scoped initial sync at this time.
-
-The |initialSync|_ API contains the following keys:
-
-``presence``
-  Description:
-    Contains a list of presence information for users the client is interested
-    in.
-  Format:
-    A JSON array of ``m.presence`` events.
-
-``end``
-  Description:
-    Contains an event stream token which can be used with the `Event Stream`_.
-  Format:
-    A string containing the event stream token.
-
-``rooms``
-  Description:
-    Contains a list of room information for all rooms the client has joined,
-    and limited room information on rooms the client has been invited to.
-  Format:
-    A JSON array containing Room Information JSON objects.
-
-Room Information:
-  Description:
-    Contains all state events for the room, along with a limited amount of
-    the most recent non-state events, configured via the ``limit`` query
-    parameter. Also contains additional keys with room metadata, such as the
-    ``room_id`` and the client's ``membership`` to the room.
-  Format:
-    A JSON object with the following keys:
-      ``room_id``
-        A string containing the ID of the room being described.
-      ``membership``
-        A string representing the client's membership status in this room.
-      ``messages``
-        An event stream JSON object containing a ``chunk`` of recent non-state
-        events, along with an ``end`` token. *NB: The name of this key will be
-        changed in a later version.*
-      ``state``
-        A JSON array containing all the current state events for this room.
-
-Getting events for a room
-~~~~~~~~~~~~~~~~~~~~~~~~~
-There are several APIs provided to ``GET`` events for a room:
-
-``/rooms/<room id>/state/<event type>/<state key>``
-  Description:
-    Get the state event identified.
-  Response format:
-    A JSON object representing the state event **content**.
-  Example:
-    ``/rooms/!room:domain.com/state/m.room.name`` returns ``{ "name": "Room name" }``
-
-|/rooms/<room_id>/state|_
-  Description:
-    Get all state events for a room.
-  Response format:
-    ``[ { state event }, { state event }, ... ]``
-  Example:
-    TODO-doc
-
-
-|/rooms/<room_id>/members|_
-  Description:
-    Get all ``m.room.member`` state events.
-  Response format:
-    ``{ "start": "<token>", "end": "<token>", "chunk": [ { m.room.member event }, ... ] }``
-  Example:
-    TODO-doc
-
-|/rooms/<room_id>/messages|_
-  Description:
-    Get all ``m.room.message`` and ``m.room.member`` events. This API supports
-    pagination using ``from`` and ``to`` query parameters, coupled with the
-    ``start`` and ``end`` tokens from an |initialSync|_ API.
-  Response format:
-    ``{ "start": "<token>", "end": "<token>" }``
-  Example:
-    TODO-doc
-    
-|/rooms/<room_id>/initialSync|_
-  Description:
-    Get all relevant events for a room. This includes state events, paginated
-    non-state events and presence events.
-  Response format:
-    `` { TODO-doc } ``
-  Example:
-    TODO-doc
-
-Redactions
-~~~~~~~~~~
-Since events are extensible it is possible for malicious users and/or servers
-to add keys that are, for example offensive or illegal. Since some events
-cannot be simply deleted, e.g. membership events, we instead 'redact' events.
-This involves removing all keys from an event that are not required by the
-protocol. This stripped down event is thereafter returned anytime a client or
-remote server requests it.
-
-Events that have been redacted include a ``redacted_because`` key whose value
-is the event that caused it to be redacted, which may include a reason.
-
-Redacting an event cannot be undone, allowing server owners to delete the
-offending content from the databases.
-
-Currently, only room admins can redact events by sending a ``m.room.redaction``
-event, but server admins also need to be able to redact events by a similar
-mechanism.
-
-Upon receipt of a redaction event, the server should strip off any keys not in
-the following list:
-
- - ``event_id``
- - ``type``
- - ``room_id``
- - ``user_id``
- - ``state_key``
- - ``prev_state``
- - ``content``
-
-The content object should also be stripped of all keys, unless it is one of
-one of the following event types:
-
- - ``m.room.member`` allows key ``membership``
- - ``m.room.create`` allows key ``creator``
- - ``m.room.join_rules`` allows key ``join_rule``
- - ``m.room.power_levels`` allows keys that are user ids or ``default``
- - ``m.room.add_state_level`` allows key ``level``
- - ``m.room.send_event_level`` allows key ``level``
- - ``m.room.ops_levels`` allows keys ``kick_level``, ``ban_level``
-   and ``redact_level``
- - ``m.room.aliases`` allows key ``aliases``
-
-The redaction event should be added under the key ``redacted_because``.
-
-
-When a client receives a redaction event it should change the redacted event
-in the same way a server does.
-
-Presence
-~~~~~~~~
-The client API for presence is on the following set of REST calls.
-
-Fetching basic status::
-
-  GET $PREFIX/presence/:user_id/status
-
-  Returned content: JSON object containing the following keys:
-    presence: "offline"|"unavailable"|"online"|"free_for_chat"
-    status_msg: (optional) string of freeform text
-    last_active_ago: miliseconds since the last activity by the user
-
-Setting basic status::
-
-  PUT $PREFIX/presence/:user_id/status
-
-  Content: JSON object containing the following keys:
-    presence and status_msg: as above
-
-When setting the status, the activity time is updated to reflect that activity;
-the client does not need to specify the ``last_active_ago`` field.
-
-Fetching the presence list::
-
-  GET $PREFIX/presence/list
-
-  Returned content: JSON array containing objects; each object containing the
-    following keys:
-    user_id: observed user ID
-    presence: "offline"|"unavailable"|"online"|"free_for_chat"
-    status_msg: (optional) string of freeform text
-    last_active_ago: miliseconds since the last activity by the user
-
-Maintaining the presence list::
-
-  POST $PREFIX/presence/list
-
-  Content: JSON object containing either or both of the following keys:
-    invite: JSON array of strings giving user IDs to send invites to
-    drop: JSON array of strings giving user IDs to remove from the list
-
-.. TODO-spec
-  - Define how users receive presence invites, and how they accept/decline them
-
-Profiles
-~~~~~~~~
-The client API for profile management consists of the following REST calls.
-
-Fetching a user account displayname::
-
-  GET $PREFIX/profile/:user_id/displayname
-
-  Returned content: JSON object containing the following keys:
-    displayname: string of freeform text
-
-This call may be used to fetch the user's own displayname or to query the name
-of other users; either locally or on remote systems hosted on other home
-servers.
-
-Setting a new displayname::
-
-  PUT $PREFIX/profile/:user_id/displayname
-
-  Content: JSON object containing the following keys:
-    displayname: string of freeform text
-
-Fetching a user account avatar URL::
-
-  GET $PREFIX/profile/:user_id/avatar_url
-
-  Returned content: JSON object containing the following keys:
-    avatar_url: string containing an http-scheme URL
-
-As with displayname, this call may be used to fetch either the user's own, or
-other users' avatar URL.
-
-Setting a new avatar URL::
-
-  PUT $PREFIX/profile/:user_id/avatar_url
-
-  Content: JSON object containing the following keys:
-    avatar_url: string containing an http-scheme URL
-
-Fetching combined account profile information::
-
-  GET $PREFIX/profile/:user_id
-
-  Returned content: JSON object containing the following keys:
-    displayname: string of freeform text
-    avatar_url: string containing an http-scheme URL
-
-At the current time, this API simply returns the displayname and avatar URL
-information, though it is intended to return more fields about the user's
-profile once they are defined. Client implementations should take care not to
-expect that these are the only two keys returned as future versions of this
-specification may yield more keys here.
-
-Security
---------
-
-Rate limiting
-~~~~~~~~~~~~~
-Home servers SHOULD implement rate limiting to reduce the risk of being
-overloaded. If a request is refused due to rate limiting, it should return a
-standard error response of the form::
-
-  {
-    "errcode": "M_LIMIT_EXCEEDED",
-    "error": "string",
-    "retry_after_ms": integer (optional)
-  }
-
-The ``retry_after_ms`` key SHOULD be included to tell the client how long they
-have to wait in milliseconds before they can try again.
-
-.. TODO-spec
-  - Surely we should recommend an algorithm for the rate limiting, rather than letting every
-    homeserver come up with their own idea, causing totally unpredictable performance over
-    federated rooms?
-
-End-to-End Encryption
-~~~~~~~~~~~~~~~~~~~~~
-
-.. TODO-doc
-  - Why is this needed.
-  - Overview of process
-  - Implementation
-
-Content repository
-------------------
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-spec
-  - path to upload
-  - format for thumbnail paths, mention what it is protecting against.
-  - content size limit and associated M_ERROR.
-
-
-Address book repository
------------------------
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-spec
-  - format: POST(?) wodges of json, some possible processing, then return wodges of json on GET.
-  - processing may remove dupes, merge contacts, pepper with extra info (e.g. matrix-ability of
-    contacts), etc.
-  - Standard json format for contacts? Piggy back off vcards?
-
-Federation API
-===============
-
-Federation is the term used to describe how to communicate between Matrix home
-servers. Federation is a mechanism by which two home servers can exchange
-Matrix event messages, both as a real-time push of current events, and as a
-historic fetching mechanism to synchronise past history for clients to view. It
-uses HTTPS connections between each pair of servers involved as the underlying
-transport. Messages are exchanged between servers in real-time by active
-pushing from each server's HTTP client into the server of the other. Queries to
-fetch historic data for the purpose of back-filling scrollback buffers and the
-like can also be performed. Currently routing of messages between homeservers
-is full mesh (like email) - however, fan-out refinements to this design are
-currently under consideration.
-
-There are three main kinds of communication that occur between home servers:
-
-:Queries:
-   These are single request/response interactions between a given pair of
-   servers, initiated by one side sending an HTTPS GET request to obtain some
-   information, and responded by the other. They are not persisted and contain
-   no long-term significant history. They simply request a snapshot state at
-   the instant the query is made.
-
-:Ephemeral Data Units (EDUs):
-   These are notifications of events that are pushed from one home server to
-   another. They are not persisted and contain no long-term significant
-   history, nor does the receiving home server have to reply to them.
-
-:Persisted Data Units (PDUs):
-   These are notifications of events that are broadcast from one home server to
-   any others that are interested in the same "context" (namely, a Room ID).
-   They are persisted to long-term storage and form the record of history for
-   that context.
-
-EDUs and PDUs are further wrapped in an envelope called a Transaction, which is
-transferred from the origin to the destination home server using an HTTP PUT
-request.
-
-
-Transactions
-------------
-.. WARNING::
-  This section may be misleading or inaccurate.
-
-The transfer of EDUs and PDUs between home servers is performed by an exchange
-of Transaction messages, which are encoded as JSON objects, passed over an HTTP
-PUT request. A Transaction is meaningful only to the pair of home servers that
-exchanged it; they are not globally-meaningful.
-
-Each transaction has:
- - An opaque transaction ID.
- - A timestamp (UNIX epoch time in milliseconds) generated by its origin
-   server.
- - An origin and destination server name.
- - A list of "previous IDs".
- - A list of PDUs and EDUs - the actual message payload that the Transaction
-   carries.
- 
-``origin``
-  Type: 
-    String
-  Description:
-    DNS name of homeserver making this transaction.
-    
-``ts``
-  Type: 
-    Integer
-  Description:
-    Timestamp in milliseconds on originating homeserver when this transaction 
-    started.
-    
-``previous_ids``
-  Type:
-    List of strings
-  Description:
-    List of transactions that were sent immediately prior to this transaction.
-    
-``pdus``
-  Type:
-    List of Objects.
-  Description:
-    List of updates contained in this transaction.
-
-::
-
- {
-  "transaction_id":"916d630ea616342b42e98a3be0b74113",
-  "ts":1404835423000,
-  "origin":"red",
-  "destination":"blue",
-  "prev_ids":["e1da392e61898be4d2009b9fecce5325"],
-  "pdus":[...],
-  "edus":[...]
- }
-
-The ``prev_ids`` field contains a list of previous transaction IDs that the
-``origin`` server has sent to this ``destination``. Its purpose is to act as a
-sequence checking mechanism - the destination server can check whether it has
-successfully received that Transaction, or ask for a retransmission if not.
-
-The ``pdus`` field of a transaction is a list, containing zero or more PDUs.[*]
-Each PDU is itself a JSON object containing a number of keys, the exact details
-of which will vary depending on the type of PDU. Similarly, the ``edus`` field
-is another list containing the EDUs. This key may be entirely absent if there
-are no EDUs to transfer.
-
-(* Normally the PDU list will be non-empty, but the server should cope with
-receiving an "empty" transaction.)
-
-PDUs and EDUs
--------------
-.. WARNING::
-  This section may be misleading or inaccurate.
-
-All PDUs have:
- - An ID
- - A context
- - A declaration of their type
- - A list of other PDU IDs that have been seen recently on that context
-   (regardless of which origin sent them)
-
-``context``
-  Type:
-    String
-  Description:
-    Event context identifier
-    
-``origin``
-  Type:
-    String
-  Description:
-    DNS name of homeserver that created this PDU.
-    
-``pdu_id``
-  Type:
-    String
-  Description:
-    Unique identifier for PDU within the context for the originating homeserver
-
-``ts``
-  Type:
-    Integer
-  Description:
-    Timestamp in milliseconds on originating homeserver when this PDU was
-    created.
-
-``pdu_type``
-  Type:
-    String
-  Description:
-    PDU event type.
-
-``prev_pdus``
-  Type:
-    List of pairs of strings
-  Description:
-    The originating homeserver and PDU ids of the most recent PDUs the
-    homeserver was aware of for this context when it made this PDU.
-
-``depth``
-  Type:
-    Integer
-  Description:
-    The maximum depth of the previous PDUs plus one.
-
-
-.. TODO-spec paul
-  - Update this structure so that 'pdu_id' is a two-element [origin,ref] pair
-    like the prev_pdus are
-
-For state updates:
-
-``is_state``
-  Type:
-    Boolean
-  Description:
-    True if this PDU is updating state.
-    
-``state_key``
-  Type:
-    String
-  Description:
-    Optional key identifying the updated state within the context.
-    
-``power_level``
-  Type:
-    Integer
-  Description:
-    The asserted power level of the user performing the update.
-    
-``required_power_level``
-  Type:
-    Integer
-  Description:
-    The required power level needed to replace this update.
-
-``prev_state_id``
-  Type:
-    String
-  Description:
-    PDU event type.
-    
-``prev_state_origin``
-  Type:
-    String
-  Description:
-    The PDU id of the update this replaces.
-    
-``user_id``
-  Type:
-    String
-  Description:
-    The user updating the state.
-
-::
-
- {
-  "pdu_id":"a4ecee13e2accdadf56c1025af232176",
-  "context":"#example.green",
-  "origin":"green",
-  "ts":1404838188000,
-  "pdu_type":"m.text",
-  "prev_pdus":[["blue","99d16afbc857975916f1d73e49e52b65"]],
-  "content":...
-  "is_state":false
- }
-
-In contrast to Transactions, it is important to note that the ``prev_pdus``
-field of a PDU refers to PDUs that any origin server has sent, rather than
-previous IDs that this ``origin`` has sent. This list may refer to other PDUs
-sent by the same origin as the current one, or other origins.
-
-Because of the distributed nature of participants in a Matrix conversation, it
-is impossible to establish a globally-consistent total ordering on the events.
-However, by annotating each outbound PDU at its origin with IDs of other PDUs
-it has received, a partial ordering can be constructed allowing causality
-relationships to be preserved. A client can then display these messages to the
-end-user in some order consistent with their content and ensure that no message
-that is semantically in reply of an earlier one is ever displayed before it.
-
-PDUs fall into two main categories: those that deliver Events, and those that
-synchronise State. For PDUs that relate to State synchronisation, additional
-keys exist to support this:
-
-::
-
- {...,
-  "is_state":true,
-  "state_key":TODO-doc
-  "power_level":TODO-doc
-  "prev_state_id":TODO-doc
-  "prev_state_origin":TODO-doc}
-
-EDUs, by comparison to PDUs, do not have an ID, a context, or a list of
-"previous" IDs. The only mandatory fields for these are the type, origin and
-destination home server names, and the actual nested content.
-
-::
-
- {"edu_type":"m.presence",
-  "origin":"blue",
-  "destination":"orange",
-  "content":...}
-  
-  
-Protocol URLs
--------------
-.. WARNING::
-  This section may be misleading or inaccurate.
-
-All these URLs are namespaced within a prefix of::
-
-  /_matrix/federation/v1/...
-
-For active pushing of messages representing live activity "as it happens"::
-
-  PUT .../send/:transaction_id/
-    Body: JSON encoding of a single Transaction
-    Response: TODO-doc
-
-The transaction_id path argument will override any ID given in the JSON body.
-The destination name will be set to that of the receiving server itself. Each
-embedded PDU in the transaction body will be processed.
-
-
-To fetch a particular PDU::
-
-  GET .../pdu/:origin/:pdu_id/
-    Response: JSON encoding of a single Transaction containing one PDU
-
-Retrieves a given PDU from the server. The response will contain a single new
-Transaction, inside which will be the requested PDU.
-  
-
-To fetch all the state of a given context::
-
-  GET .../state/:context/
-    Response: JSON encoding of a single Transaction containing multiple PDUs
-
-Retrieves a snapshot of the entire current state of the given context. The
-response will contain a single Transaction, inside which will be a list of PDUs
-that encode the state.
-
-To backfill events on a given context::
-
-  GET .../backfill/:context/
-    Query args: v, limit
-    Response: JSON encoding of a single Transaction containing multiple PDUs
-
-Retrieves a sliding-window history of previous PDUs that occurred on the given
-context. Starting from the PDU ID(s) given in the "v" argument, the PDUs that
-preceeded it are retrieved, up to a total number given by the "limit" argument.
-These are then returned in a new Transaction containing all of the PDUs.
-
-
-To stream events all the events::
-
-  GET .../pull/
-    Query args: origin, v
-    Response: JSON encoding of a single Transaction consisting of multiple PDUs
-
-Retrieves all of the transactions later than any version given by the "v"
-arguments.
-
-
-To make a query::
-
-  GET .../query/:query_type
-    Query args: as specified by the individual query types
-    Response: JSON encoding of a response object
-
-Performs a single query request on the receiving home server. The Query Type
-part of the path specifies the kind of query being made, and its query
-arguments have a meaning specific to that kind of query. The response is a
-JSON-encoded object whose meaning also depends on the kind of query.
-
-Backfilling
------------
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-doc
-  - What it is, when is it used, how is it done
-
-SRV Records
------------
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-doc
-  - Why it is needed
-
-State Conflict Resolution
--------------------------
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-doc
-  - How do conflicts arise (diagrams?)
-  - How are they resolved (incl tie breaks)
-  - How does this work with deleting current state
-
-Presence
---------
-The server API for presence is based entirely on exchange of the following
-EDUs. There are no PDUs or Federation Queries involved.
-
-Performing a presence update and poll subscription request::
-
-  EDU type: m.presence
-
-  Content keys:
-    push: (optional): list of push operations.
-      Each should be an object with the following keys:
-        user_id: string containing a User ID
-        presence: "offline"|"unavailable"|"online"|"free_for_chat"
-        status_msg: (optional) string of freeform text
-        last_active_ago: miliseconds since the last activity by the user
-
-    poll: (optional): list of strings giving User IDs
-
-    unpoll: (optional): list of strings giving User IDs
-
-The presence of this combined message is two-fold: it informs the recipient
-server of the current status of one or more users on the sending server (by the
-``push`` key), and it maintains the list of users on the recipient server that
-the sending server is interested in receiving updates for, by adding (by the
-``poll`` key) or removing them (by the ``unpoll`` key). The ``poll`` and
-``unpoll`` lists apply *changes* to the implied list of users; any existing IDs
-that the server sent as ``poll`` operations in a previous message are not
-removed until explicitly requested by a later ``unpoll``.
-
-On receipt of a message containing a non-empty ``poll`` list, the receiving
-server should immediately send the sending server a presence update EDU of its
-own, containing in a ``push`` list the current state of every user that was in
-the orginal EDU's ``poll`` list.
-
-Sending a presence invite::
-
-  EDU type: m.presence_invite
-
-  Content keys:
-    observed_user: string giving the User ID of the user whose presence is
-      requested (i.e. the recipient of the invite)
-    observer_user: string giving the User ID of the user who is requesting to
-      observe the presence (i.e. the sender of the invite)
-
-Accepting a presence invite::
-
-  EDU type: m.presence_accept
-
-  Content keys - as for m.presence_invite
-
-Rejecting a presence invite::
-
-  EDU type: m.presence_deny
-
-  Content keys - as for m.presence_invite
-
-.. TODO-doc
-  - Explain the timing-based roundtrip reduction mechanism for presence
-    messages
-  - Explain the zero-byte presence inference logic
-  See also: docs/client-server/model/presence
-
-Profiles
---------
-The server API for profiles is based entirely on the following Federation
-Queries. There are no additional EDU or PDU types involved, other than the
-implicit ``m.presence`` and ``m.room.member`` events (see section below).
-
-Querying profile information::
-
-  Query type: profile
-
-  Arguments:
-    user_id: the ID of the user whose profile to return
-    field: (optional) string giving a field name
-
-  Returns: JSON object containing the following keys:
-    displayname: string of freeform text
-    avatar_url: string containing an http-scheme URL
-
-If the query contains the optional ``field`` key, it should give the name of a
-result field. If such is present, then the result should contain only a field
-of that name, with no others present. If not, the result should contain as much
-of the user's profile as the home server has available and can make public.
-
-Server-Server Authentication
-----------------------------
-
-.. TODO-doc
-  - Why is this needed.
-  - High level overview of process.
-  - Transaction/PDU signing
-  - How does this work with redactions? (eg hashing required keys only)
-
-
-
-Threat Model
-------------
-
-Denial of Service
-~~~~~~~~~~~~~~~~~
-
-The attacker could attempt to prevent delivery of messages to or from the
-victim in order to:
-
-* Disrupt service or marketing campaign of a commercial competitor.
-* Censor a discussion or censor a participant in a discussion.
-* Perform general vandalism.
-
-Threat: Resource Exhaustion
-+++++++++++++++++++++++++++
-
-An attacker could cause the victims server to exhaust a particular resource
-(e.g. open TCP connections, CPU, memory, disk storage)
-
-Threat: Unrecoverable Consistency Violations
-++++++++++++++++++++++++++++++++++++++++++++
-
-An attacker could send messages which created an unrecoverable "split-brain"
-state in the cluster such that the victim's servers could no longer dervive a
-consistent view of the chatroom state.
-
-Threat: Bad History
-+++++++++++++++++++
-
-An attacker could convince the victim to accept invalid messages which the
-victim would then include in their view of the chatroom history. Other servers
-in the chatroom would reject the invalid messages and potentially reject the
-victims messages as well since they depended on the invalid messages.
-
-.. TODO-spec
-  Track trustworthiness of HS or users based on if they try to pretend they
-  haven't seen recent events, and fake a splitbrain... --M
-
-Threat: Block Network Traffic
-+++++++++++++++++++++++++++++
-
-An attacker could try to firewall traffic between the victim's server and some
-or all of the other servers in the chatroom.
-
-Threat: High Volume of Messages
-+++++++++++++++++++++++++++++++
-
-An attacker could send large volumes of messages to a chatroom with the victim
-making the chatroom unusable.
-
-Threat: Banning users without necessary authorisation
-+++++++++++++++++++++++++++++++++++++++++++++++++++++
-
-An attacker could attempt to ban a user from a chatroom with the necessary
-authorisation.
-
-Spoofing
-~~~~~~~~
-
-An attacker could try to send a message claiming to be from the victim without
-the victim having sent the message in order to:
-
-* Impersonate the victim while performing illict activity.
-* Obtain privileges of the victim.
-
-Threat: Altering Message Contents
-+++++++++++++++++++++++++++++++++
-
-An attacker could try to alter the contents of an existing message from the
-victim.
-
-Threat: Fake Message "origin" Field
-+++++++++++++++++++++++++++++++++++
-
-An attacker could try to send a new message purporting to be from the victim
-with a phony "origin" field.
-
-Spamming
-~~~~~~~~
-
-The attacker could try to send a high volume of solicicted or unsolicted
-messages to the victim in order to:
-
-* Find victims for scams.
-* Market unwanted products.
-
-Threat: Unsoliticted Messages
-+++++++++++++++++++++++++++++
-
-An attacker could try to send messages to victims who do not wish to receive
-them.
-
-Threat: Abusive Messages
-++++++++++++++++++++++++
-
-An attacker could send abusive or threatening messages to the victim
-
-Spying
-~~~~~~
-
-The attacker could try to access message contents or metadata for messages sent
-by the victim or to the victim that were not intended to reach the attacker in
-order to:
-
-* Gain sensitive personal or commercial information.
-* Impersonate the victim using credentials contained in the messages.
-  (e.g. password reset messages)
-* Discover who the victim was talking to and when.
-
-Threat: Disclosure during Transmission
-++++++++++++++++++++++++++++++++++++++
-
-An attacker could try to expose the message contents or metadata during
-transmission between the servers.
-
-Threat: Disclosure to Servers Outside Chatroom
-++++++++++++++++++++++++++++++++++++++++++++++
-
-An attacker could try to convince servers within a chatroom to send messages to
-a server it controls that was not authorised to be within the chatroom.
-
-Threat: Disclosure to Servers Within Chatroom
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-An attacker could take control of a server within a chatroom to expose message
-contents or metadata for messages in that room.
-
-
-Identity Servers
-================
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-doc Dave
-  - 3PIDs and identity server, functions
-
-Lawful Interception
--------------------
-
-Key Escrow Servers
-~~~~~~~~~~~~~~~~~~
-
-Policy Servers
-==============
-.. NOTE::
-  This section is a work in progress.
-
-.. TODO-spec
-  We should mention them in the Architecture section at least: how they fit
-  into the picture.
-
-Enforcing policies
-------------------
-
-
-
-.. Links through the external API docs are below
-.. =============================================
-
-.. |createRoom| replace:: ``/createRoom``
-.. _createRoom: /docs/api/client-server/#!/-rooms/create_room
-
-.. |initialSync| replace:: ``/initialSync``
-.. _initialSync: /docs/api/client-server/#!/-events/initial_sync
-
-.. |/rooms/<room_id>/initialSync| replace:: ``/rooms/<room_id>/initialSync``
-.. _/rooms/<room_id>/initialSync: /docs/api/client-server/#!/-rooms/get_room_sync_data
-
-.. |login| replace:: ``/login``
-.. _login: /docs/api/client-server/#!/-login
-
-.. |register| replace:: ``/register``
-.. _register: /docs/api/client-server/#!/-registration
-
-.. |/rooms/<room_id>/messages| replace:: ``/rooms/<room_id>/messages``
-.. _/rooms/<room_id>/messages: /docs/api/client-server/#!/-rooms/get_messages
-
-.. |/rooms/<room_id>/members| replace:: ``/rooms/<room_id>/members``
-.. _/rooms/<room_id>/members: /docs/api/client-server/#!/-rooms/get_members
-
-.. |/rooms/<room_id>/state| replace:: ``/rooms/<room_id>/state``
-.. _/rooms/<room_id>/state: /docs/api/client-server/#!/-rooms/get_state_events
-
-.. |/rooms/<room_id>/send/<event_type>| replace:: ``/rooms/<room_id>/send/<event_type>``
-.. _/rooms/<room_id>/send/<event_type>: /docs/api/client-server/#!/-rooms/send_non_state_event
-
-.. |/rooms/<room_id>/state/<event_type>/<state_key>| replace:: ``/rooms/<room_id>/state/<event_type>/<state_key>``
-.. _/rooms/<room_id>/state/<event_type>/<state_key>: /docs/api/client-server/#!/-rooms/send_state_event
-
-.. |/rooms/<room_id>/invite| replace:: ``/rooms/<room_id>/invite``
-.. _/rooms/<room_id>/invite: /docs/api/client-server/#!/-rooms/invite
-
-.. |/rooms/<room_id>/join| replace:: ``/rooms/<room_id>/join``
-.. _/rooms/<room_id>/join: /docs/api/client-server/#!/-rooms/join_room
-
-.. |/rooms/<room_id>/leave| replace:: ``/rooms/<room_id>/leave``
-.. _/rooms/<room_id>/leave: /docs/api/client-server/#!/-rooms/leave
-
-.. |/rooms/<room_id>/ban| replace:: ``/rooms/<room_id>/ban``
-.. _/rooms/<room_id>/ban: /docs/api/client-server/#!/-rooms/ban
-
-.. |/join/<room_alias_or_id>| replace:: ``/join/<room_alias_or_id>``
-.. _/join/<room_alias_or_id>: /docs/api/client-server/#!/-rooms/join
-
-.. _`Event Stream`: /docs/api/client-server/#!/-events/get_event_stream
-