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ReStructuredText
.. Copyright 2016 OpenMarket Ltd
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..
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.. Licensed under the Apache License, Version 2.0 (the "License");
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.. you may not use this file except in compliance with the License.
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.. You may obtain a copy of the License at
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..
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.. http://www.apache.org/licenses/LICENSE-2.0
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..
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.. Unless required by applicable law or agreed to in writing, software
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.. distributed under the License is distributed on an "AS IS" BASIS,
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.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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.. See the License for the specific language governing permissions and
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.. limitations under the License.
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.. contents:: Table of Contents
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.. sectnum::
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.. Note that this file is specifically unversioned because we don't want to
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.. have to add Yet Another version number, and the commentary on what specs we
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.. have should hopefully not get complex enough that we need to worry about
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.. versioning it.
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Introduction
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------------
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.. WARNING::
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The Matrix specification is still evolving: the APIs are not yet frozen
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and this document is in places a work in progress or stale. We have made every
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effort to clearly flag areas which are still being finalised.
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We're publishing it at this point because it's complete enough to be more than
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useful and provide a canonical reference to how Matrix is evolving. Our end
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goal is to mirror WHATWG's `Living Standard
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<https://whatwg.org/faq?#living-standard>`_.
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Matrix is a set of open APIs for open-federated Instant Messaging (IM), Voice
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over IP (VoIP) and Internet of Things (IoT) communication, designed to create
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and support a new global real-time communication ecosystem. The intention is to
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provide an open decentralised pubsub layer for the internet for securely
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persisting and publishing/subscribing JSON objects. This specification is the
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ongoing result of standardising the APIs used by the various components of the
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Matrix ecosystem to communicate with one another.
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The principles that Matrix attempts to follow are:
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- Pragmatic Web-friendly APIs (i.e. JSON over REST)
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- Keep It Simple & Stupid
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+ provide a simple architecture with minimal third-party dependencies.
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- Fully open:
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+ Fully open federation - anyone should be able to participate in the global
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Matrix network
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+ Fully open standard - publicly documented standard with no IP or patent
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licensing encumbrances
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+ Fully open source reference implementation - liberally-licensed example
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implementations with no IP or patent licensing encumbrances
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- Empowering the end-user
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+ The user should be able to choose the server and clients they use
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+ The user should be control how private their communication is
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+ The user should know precisely where their data is stored
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- Fully decentralised - no single points of control over conversations or the
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network as a whole
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- Learning from history to avoid repeating it
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+ Trying to take the best aspects of XMPP, SIP, IRC, SMTP, IMAP and NNTP
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whilst trying to avoid their failings
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The functionality that Matrix provides includes:
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- Creation and management of fully distributed chat rooms with no
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single points of control or failure
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- Eventually-consistent cryptographically secure synchronisation of room
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state across a global open network of federated servers and services
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- Sending and receiving extensible messages in a room with (optional)
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end-to-end encryption
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- Extensible user management (inviting, joining, leaving, kicking, banning)
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mediated by a power-level based user privilege system.
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- Extensible room state management (room naming, aliasing, topics, bans)
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- Extensible user profile management (avatars, display names, etc)
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- Managing user accounts (registration, login, logout)
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- Use of 3rd Party IDs (3PIDs) such as email addresses, phone numbers,
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Facebook accounts to authenticate, identify and discover users on Matrix.
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- Trusted federation of Identity servers for:
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+ Publishing user public keys for PKI
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+ Mapping of 3PIDs to Matrix IDs
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The end goal of Matrix is to be a ubiquitous messaging layer for synchronising
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arbitrary data between sets of people, devices and services - be that for
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instant messages, VoIP call setups, or any other objects that need to be
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reliably and persistently pushed from A to B in an interoperable and federated
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manner.
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Architecture
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------------
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Matrix defines APIs for synchronising extensible JSON objects known as
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"events" between compatible clients, servers and services. Clients are
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typically messaging/VoIP applications or IoT devices/hubs and communicate by
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synchronising communication history with their "homeserver" using the
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"Client-Server API". Each homeserver stores the communication history and
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account information for all of its clients, and shares data with the wider
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Matrix ecosystem by synchronising communication history with other homeservers
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and their clients.
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Clients typically communicate with each other by emitting events in the
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context of a virtual "room". Room data is replicated across *all of the
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homeservers* whose users are participating in a given room. As such, *no
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single homeserver has control or ownership over a given room*. Homeservers
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model communication history as a partially ordered graph of events known as
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the room's "event graph", which is synchronised with eventual consistency
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between the participating servers using the "Server-Server API". This process
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of synchronising shared conversation history between homeservers run by
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different parties is called "Federation". Matrix optimises for the the
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Availability and Partitioned properties of CAP theorem at
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the expense of Consistency.
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For example, for client A to send a message to client B, client A performs an
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HTTP PUT of the required JSON event on its homeserver (HS) using the
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client-server API. A's HS appends this event to its copy of the room's event
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graph, signing the message in the context of the graph for integrity. A's HS
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then replicates the message to B's HS by performing an HTTP PUT using the
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server-server API. B's HS authenticates the request, validates the event's
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signature, authorises the event's contents and then adds it to its copy of the
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room's event graph. Client B then receives the message from his homeserver via
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a long-lived GET request.
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::
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How data flows between clients
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==============================
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{ Matrix client A } { Matrix client B }
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^ | ^ |
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| events | Client-Server API | events |
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| V | V
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+------------------+ +------------------+
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| |---------( HTTPS )--------->| |
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| homeserver | | homeserver |
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| |<--------( HTTPS )----------| |
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+------------------+ Server-Server API +------------------+
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History Synchronisation
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(Federation)
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Users
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~~~~~
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Each client is associated with a user account, which is identified in Matrix
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using a unique "user ID". This ID is namespaced to the homeserver which
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allocated the account and has the form::
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@localpart:domain
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See the `Identifier Grammar`_ section for full details of the structure of
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user IDs.
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Devices
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~~~~~~~
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The Matrix specification has a particular meaning for the term "device". As a
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user, I might have several devices: a desktop client, some web browsers, an
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Android device, an iPhone, etc. They broadly relate to a real device in the
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physical world, but you might have several browsers on a physical device, or
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several Matrix client applications on a mobile device, each of which would be
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its own device.
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Devices are used primarily to manage the keys used for end-to-end encryption
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(each device gets its own copy of the decryption keys), but they also help
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users manage their access - for instance, by revoking access to particular
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devices.
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When a user first uses a client, it registers itself as a new device. The
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longevity of devices might depend on the type of client. A web client will
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probably drop all of its state on logout, and create a new device every time
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you log in, to ensure that cryptography keys are not leaked to a new user. In
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a mobile client, it might be acceptable to reuse the device if a login session
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expires, provided the user is the same.
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Devices are identified by a ``device_id``, which is unique within the scope of
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a given user.
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A user may assign a human-readable display name to a device, to help them
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manage their devices.
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Events
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~~~~~~
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All data exchanged over Matrix is expressed as an "event". Typically each client
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action (e.g. sending a message) correlates with exactly one event. Each event
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has a ``type`` which is used to differentiate different kinds of data. ``type``
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values MUST be uniquely globally namespaced following Java's `package naming
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conventions`_, e.g.
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``com.example.myapp.event``. The special top-level namespace ``m.`` is reserved
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for events defined in the Matrix specification - for instance ``m.room.message``
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is the event type for instant messages. Events are usually sent in the context
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of a "Room".
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.. _package naming conventions: https://en.wikipedia.org/wiki/Java_package#Package_naming_conventions
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Event Graphs
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~~~~~~~~~~~~
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.. _sect:event-graph:
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Events exchanged in the context of a room are stored in a directed acyclic graph
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(DAG) called an "event graph". The partial ordering of this graph gives the
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chronological ordering of events within the room. Each event in the graph has a
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list of zero or more "parent" events, which refer to any preceding events
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which have no chronological successor from the perspective of the homeserver
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which created the event.
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Typically an event has a single parent: the most recent message in the room at
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the point it was sent. However, homeservers may legitimately race with each
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other when sending messages, resulting in a single event having multiple
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successors. The next event added to the graph thus will have multiple parents.
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Every event graph has a single root event with no parent.
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To order and ease chronological comparison between the events within the graph,
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homeservers maintain a ``depth`` metadata field on each event. An event's
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``depth`` is a positive integer that is strictly greater than the depths of any
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of its parents. The root event should have a depth of 1. Thus if one event is
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before another, then it must have a strictly smaller depth.
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Room structure
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~~~~~~~~~~~~~~
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A room is a conceptual place where users can send and receive events. Events are
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sent to a room, and all participants in that room with sufficient access will
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receive the event. Rooms are uniquely identified internally via "Room IDs",
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which have the form::
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!opaque_id:domain
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There is exactly one room ID for each room. Whilst the room ID does contain a
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domain, it is simply for globally namespacing room IDs. The room does NOT
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reside on the domain specified.
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See the `Identifier Grammar`_ section for full details of the structure of
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a room ID.
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The following conceptual diagram shows an
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``m.room.message`` event being sent to the room ``!qporfwt:matrix.org``::
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{ @alice:matrix.org } { @bob:domain.com }
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| ^
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[HTTP POST] [HTTP GET]
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Room ID: !qporfwt:matrix.org Room ID: !qporfwt:matrix.org
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Event type: m.room.message Event type: m.room.message
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Content: { JSON object } Content: { JSON object }
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V |
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+------------------+ +------------------+
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| homeserver | | homeserver |
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| matrix.org | | domain.com |
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+------------------+ +------------------+
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| ^
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| [HTTP PUT] |
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| Room ID: !qporfwt:matrix.org |
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| Event type: m.room.message |
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| Content: { JSON object } |
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`-------> Pointer to the preceding message ------`
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PKI signature from matrix.org
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Transaction-layer metadata
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PKI Authorization header
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...................................
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| Shared Data |
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| State: |
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| Room ID: !qporfwt:matrix.org |
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| Servers: matrix.org, domain.com |
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| Members: |
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| - @alice:matrix.org |
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| - @bob:domain.com |
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| Messages: |
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| - @alice:matrix.org |
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| Content: { JSON object } |
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|...................................|
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Federation maintains *shared data structures* per-room between multiple home
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servers. The data is split into ``message events`` and ``state events``.
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Message events:
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These describe transient 'once-off' activity in a room such as an
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instant messages, VoIP call setups, file transfers, etc. They generally
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describe communication activity.
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State events:
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These describe updates to a given piece of persistent information
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('state') related to a room, such as the room's name, topic, membership,
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participating servers, etc. State is modelled as a lookup table of key/value
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pairs per room, with each key being a tuple of ``state_key`` and ``event type``.
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Each state event updates the value of a given key.
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The state of the room at a given point is calculated by considering all events
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preceding and including a given event in the graph. Where events describe the
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same state, a merge conflict algorithm is applied. The state resolution
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algorithm is transitive and does not depend on server state, as it must
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consistently select the same event irrespective of the server or the order the
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events were received in. Events are signed by the originating server (the
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signature includes the parent relations, type, depth and payload hash) and are
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pushed over federation to the participating servers in a room, currently using
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full mesh topology. Servers may also request backfill of events over federation
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from the other servers participating in a room.
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Room Aliases
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++++++++++++
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Each room can also have multiple "Room Aliases", which look like::
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#room_alias:domain
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See the `Identifier Grammar`_ section for full details of the structure of
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a room alias.
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A room alias "points" to a room ID and is the human-readable label by which
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rooms are publicised and discovered. The room ID the alias is pointing to can
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be obtained by visiting the domain specified. Note that the mapping from a room
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alias to a room ID is not fixed, and may change over time to point to a
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different room ID. For this reason, Clients SHOULD resolve the room alias to a
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room ID once and then use that ID on subsequent requests.
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When resolving a room alias the server will also respond with a list of servers
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that are in the room that can be used to join via.
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::
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HTTP GET
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#matrix:domain.com !aaabaa:matrix.org
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| ^
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_______V____________________|____
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| domain.com |
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| Mappings: |
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| #matrix >> !aaabaa:matrix.org |
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| #golf >> !wfeiofh:sport.com |
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| #bike >> !4rguxf:matrix.org |
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|________________________________|
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Identity
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~~~~~~~~
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Users in Matrix are identified via their Matrix user ID. However,
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existing 3rd party ID namespaces can also be used in order to identify Matrix
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users. A Matrix "Identity" describes both the user ID and any other existing IDs
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from third party namespaces *linked* to their account.
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Matrix users can *link* third-party IDs (3PIDs) such as email addresses, social
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network accounts and phone numbers to their user ID. Linking 3PIDs creates a
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mapping from a 3PID to a user ID. This mapping can then be used by Matrix
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users in order to discover the user IDs of their contacts.
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In order to ensure that the mapping from 3PID to user ID is genuine, a globally
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federated cluster of trusted "Identity Servers" (IS) are used to verify the 3PID
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and persist and replicate the mappings.
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Usage of an IS is not required in order for a client application to be part of
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the Matrix ecosystem. However, without one clients will not be able to look up
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user IDs using 3PIDs.
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Profiles
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~~~~~~~~
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Users may publish arbitrary key/value data associated with their account - such
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as a human readable display name, a profile photo URL, contact information
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(email address, phone numbers, website URLs etc).
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.. TODO
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Actually specify the different types of data - e.g. what format are display
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names allowed to be?
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Private User Data
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~~~~~~~~~~~~~~~~~
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Users may also store arbitrary private key/value data in their account - such as
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client preferences, or server configuration settings which lack any other
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dedicated API. The API is symmetrical to managing Profile data.
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.. TODO
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Would it really be overengineered to use the same API for both profile &
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private user data, but with different ACLs?
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License
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-------
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The Matrix specification is licensed under the `Apache License, Version 2.0
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<http://www.apache.org/licenses/LICENSE-2.0>`_.
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