Data Flows
----------

Invite
++++++

To invite a remote user to an existing room a server distributes an invitiation
event signed by the remote invitee's server (allowing other servers in the room
to be sure that the invitee's server had seen the invite.)

To get the remote server's signature on the event it is sent in a special
request to the remote server, which then responds with the signed invite (if it
accepted it as valid.) The remote server may respond with an error if the user
does not exist.

Join
++++

If a server is already in the room it can simply emit a join event for the user
joining.

If the server is not currently in the room it needs to join via a remote server
in the room, therefore to join a room a server must have have both the room id
and a list of remote servers that may be in the room.

To join via a remote server the local server first requests a valid join event
for the room from the remote server. The local then fills it out, signs it, and
then sends the final join event to the remote server for distribution. The
remore responds to this second request with the current state of the room at
the join request and the auth chain for that state.


Authorization
-------------

The authorization for an event depends solely on the current state at that
event. If a policy server is configured for the room, then the authorization
for the event is the signature of the policy server.

The state events that affect whether an event is authorized are called
`protocol events`, and are:

  * `m.room.create`
  * `m.room.power_levels`
  * `m.room.member`
  * `m.room.join_rules`

All events *must* list all the protocol events that grant them their
authorization. All origin servers *must* serve up on request the full graph of
protocol events for all events it has sent. The graph of protocol events is the
connected directed acyclic graph with events as nodes and the list of protocol
events their edges.


Join
++++

A user may join a room if:

  * The join rule is "public".
  * The join rule is "invite" and the user has been invited by a user that has
    already joined.
  * The user is in the `may_join` list.


Invite
++++++

A user may invite another user if the join rule is either "public" or "invite"
and the user has joined the room.


Creation
++++++++

A `m.room.create` must be the first event in the room.


Ban, kick and redaction
+++++++++++++++++++++++

To ban or kick another user in the room, or to redact an event, then the user
must have a power level of at least that specificied in the
`m.room.power_level` event for kick, ban and redactions.


Other Events
++++++++++++

A user may send an event if all the following hold true:

  * The user is in the room.
  * If the type of the event is listed in the `m.room.power_levels`, then the
    user must have at least that power level. Otherwise, the user must have a
    power level of at least `events_default` or `state_default`, depending on
    if the event is a message or state event respectively.