synapse/docs/specification.rst

Matrix Specification
====================

TODO(Introduction) : Matthew
 - Similar to intro paragraph from README.
 - Explaining the overall mission, what this spec describes...
 - "What is Matrix?"
 - Draw parallels with email?

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" is an end-user, typically a human using a web application or mobile app. Clients use the
"Client-to-Server" (C-S) API to communicate with their home server. A single Client is usually
responsible for a single user account. A user account is represented by their "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.


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 SHOULD be namespaced according to standard
Java package naming conventions, e.g. ``com.example.myapp.event``. 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 will receive the event. Rooms are uniquely 
identified 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 namespacing room IDs. The room does NOT reside on the
domain specified. Room IDs are not meant to be human readable.

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     |
       +------------------+                          +------------------+
                |       .................................        |
                |______|     Partially 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. Home servers do not need to have completely shared state in order to participate 
in a room. 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

A room alias "points" to a room ID. The room ID the alias is pointing to can be obtained
by visiting the domain specified. Room aliases are designed to be human readable strings
which can be used to publicise rooms. 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.

::

          GET    
   #matrix:domain.com      !aaabaa:matrix.org
           |                    ^
           |                    |
    _______V____________________|____
   |          domain.com            |
   | Mappings:                      |
   | #matrix >> !aaabaa:matrix.org  |
   | #golf >> !wfeiofh:sport.com    |
   | #bike >> !4rguxf:matrix.org    |
   |________________________________|

       
Identity
--------
- Identity in relation to 3PIDs. Discovery of users based on 3PIDs.
- Identity servers; trusted clique of servers which replicate content.
- They govern the mapping of 3PIDs to user IDs and the creation of said mappings.
- Not strictly required in order to communicate.


API Standards
-------------
All communication in Matrix is performed over HTTP[S] using a Content-Type of ``application/json``.
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.

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
client-generated transaction ID which identifies the request. 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, it could be a monotonically
increasing integer, etc). 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
    {
      "key": "This is a post."
    }

    PUT /some/path/here/11
    {
      "key": "This is a put with a txnId of 11."
    }

In contrast, these are invalid requests::

    POST /some/path/here/11
    {
      "key": "This is a post, but it has a txnId."
    }

    PUT /some/path/here
    {
      "key": "This is a put but it is missing a txnId."
    }

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 the client is authorised 
to view will appear in the event stream. When the stream is closed, an ``end`` token is 
returned. This token can be used in the next request to continue where the client left off.

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.

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
    ``public`` 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``

Example::

  {
    "visibility": "public", 
    "room_alias_name": "the pub"
  }

- TODO: This creates a room creation event which serves as the root of the PDU graph for this room.

Modifying aliases
-----------------
- Adding / removing aliases.

Permissions
-----------
- TODO : Room permissions / config / power levels. What they are. How do they work. Examples.

Inviting users
--------------
- API to hit (``$roomid/invite``) with ``user_id`` key. Needs FQ user ID, explain why.
- Outline invite join dance

Joining rooms
-------------
- API to hit (``/join/$alias or id``). Explain how alias joining works (auto-resolving). 
- Outline invite join dance

Leaving rooms
-------------
- API to hit (``$roomid/leave``).
- Is there a dance?

Room events
-----------
- Split into state and non-state data
- Explain what they are, semantics, give examples of clobbering / not, use cases (msgs vs room names).
  Not too much detail on the actual event contents.
- API to hit.
- Extensibility provided by the API for custom events. Examples.
- How this hooks into ``initialSync``.
- See the "Room Events" section for actual spec on each type.

Syncing a room
--------------
- Single room initial sync. API to hit. Why it might be used (lazy loading)

Getting grouped state events
----------------------------
- ``/members`` and ``/messages`` and the events they return.
- ``/state`` and it returns ALL THE THINGS.

Room Events
===========

This specification outlines several standard event types, all of which are
prefixed with ``m.``

State messages
--------------
- m.room.name
- m.room.topic
- m.room.member
- m.room.config
- m.room.invite_join

What are they, when are they used, what do they contain, how should they be used

Non-state messages
------------------
- m.room.message
- m.room.message.feedback (and compressed format)

What are they, when are they used, what do they contain, how should they be used

m.room.message msgtypes
-----------------------
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.

Presence
========

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 ``state`` 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`` : TODO. Behaves as offline, but allows the user to see the client 
    state anyway and generally interact with client features.

This basic ``state`` field applies to the user as a whole, regardless of how many
client devices they have connected. The home server should synchronise this
status choice among multiple devices to ensure the user gets a consistent
experience.

Idle Time
---------
As well as the basic ``state`` field, the presence information can also show a sense
of an "idle timer". This should be maintained individually by the user's
clients, and the home server can take the highest reported time as that to
report. When a user is offline, the home server can still report when the user was last
seen online.

Transmission
------------
- Transmitted as an EDU.
- Presence lists determine who to send to.

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.

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.

Typing notifications
====================

TODO : Leo

Voice over IP
=============

TODO : Dave

Profiles
========

Internally within Matrix users are referred to by their user ID, which is not a
human-friendly string. 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.

- Metadata extensibility
- Bundled with which events? e.g. m.room.member
- Generate own events? What type?

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'.

- TODO Kegan : Make registration like login (just omit the "user" key on the 
  initial request?)

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.

The login process breaks down into the following:
  1. Determine the requirements for logging in.
  2. Submit the login stage credentials.
  3. Get credentials or be told the next stage in the login process and repeat 
     step 2.
     
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::

  The client can login via 3 paths: 1a and 1b, 2a and 2b, or 3. The client should
  select one of these 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>"
      }
    ]
  }

After the login 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 a login has multiple requests, the home server may wish to create a session. If
a home server responds with a 'session' key to a request, clients MUST submit it in 
subsequent requests until the login 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``


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.


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.

Identity
========

TODO : Dave
- 3PIDs and identity server, functions

Federation
==========

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 HTTP 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.

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 HTTP 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
------------
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.

::

 {
  "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, as this is useful for informing peers of other
transaction IDs they should be aware of. This effectively acts as a push
mechanism to encourage peers to continue to replicate content.)

PDUs and EDUs
-------------

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)

[[TODO(paul): Update this structure so that 'pdu_id' is a two-element
[origin,ref] pair like the prev_pdus are]]

::

 {
  "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 causallity
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
  "power_level":TODO
  "prev_state_id":TODO
  "prev_state_origin":TODO}

[[TODO(paul): At this point we should probably have a long description of how
State management works, with descriptions of clobbering rules, power levels, etc
etc... But some of that detail is rather up-in-the-air, on the whiteboard, and
so on. This part needs refining. And writing in its own document as the details
relate to the server/system as a whole, not specifically to server-server
federation.]]

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":...}

Backfilling
-----------
- What it is, when is it used, how is it done

SRV Records
-----------
- Why it is needed

Security
========
- rate limiting
- crypto (s-s auth)
- E2E
- Lawful intercept + Key Escrow

TODO Mark

Policy Servers
==============
TODO

Content repository
==================
- thumbnail paths

Address book repository
=======================
- format


Glossary
========
- domain specific words/acronyms with definitions

User ID:
  An opaque ID which identifies an end-user, which consists of some opaque 
  localpart combined with the domain name of their home server.