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.