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Move API endpoint sections up to the top of the proposal

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Andrew Morgan 4 years ago
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proposals/2403-knock.md
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@ -24,164 +24,6 @@ following:
After a knock in a room, a member of the room can invite the knocker, or they
can decide to reject it instead.
## Restrictions
There are restrictions to being able to set this membership, as well as
accepting or denying the knock. A formal description of the changes to the auth rules is given below;
first we summarise the semantics of the proposed changes.
### Current membership
Only users without a current membership or with their current membership
being "leave" can knock on a room. This means that a user that is banned, has
already knocked or is currently in the room cannot knock on it.
### Join Rules
This proposal makes use of the existing "knock" join rule. The value of
`join_rule` in the content of the `m.room.join_rules` state event for a room
must be set to "knock" for a knock to succeed. This means that existing rooms
will need to opt into allowing knocks in their rooms. Other than allowing
knocks, a join rule of "knock" is functionally equivalent to that of
"invite", except that it additionally allows external users to change their
membership to "knock" under certain conditions.
### Auth rules
Each room version defines the auth rules which should be applied in that room version.
This MSC proposes a new room version with the following changes to the [auth
rules from room version 6](https://matrix.org/docs/spec/rooms/v6#authorization-rules-for-events):
* Under "5. If type is `m.room.member`", insert the following after "e. If membership is `ban`":
```
f. If `membership` is `knock`:
i. If the `join_rule` is anything other than `knock`, reject.
ii. If `sender` does not match `state_key`, reject.
iii. If the `sender`'s membership is not `ban`, `invite` or `join`, allow.
iv. Otherwise, reject.
```
Note that:
- Both the `sender` and `state_key` fields are set to the user ID of the knocking
user. This is different to an `invite` membership event, where the `sender` is the inviter and
the `state_key` is the invitee.
- f.ii is justified as one user should not be able to knock on behalf of
another user.
- f.iii is justified as knocks should not be allowed if the knocking user
has been banned from the room, is invited to the room or if they are already
in the room.
- Knocks are not restricted by power level like invites are. The `join_rules`
are already used to enforce whether someone can or cannot knock. However,
power level rules do apply when approving or denying the knock, as discussed
in the Membership Changes section below.
Additionally, note that redactions of knock events are not a concern, as
`membership` keys are excluded from being redacted as defined by all current
room versions.
## Membership changes
Once someone has sent a `knock` membership into the room, the membership for
that user can be transitioned to the following possible states:
- `invite`: In this case, the knock was accepted by someone inside the room
and they are inviting the knocker into the room.
- `leave`: In this case, similar to how kicks are handled, the knock has
been rejected. Alternatively, the knocking user has rescinded their knock.
- `ban`: In this case, the knock was rejected and the user has been prevented
from sending further knocks.
Let's talk about each one of these in more detail.
### Membership change to `invite`
The knock has been accepted by someone in the room.
The user who is accepting the knock must have the power level to perform
invites. The accepting user's homeserver will then send an invite - over federation if
necessary - to the knocking user. The knocking user may then join the room as
if they had been invited normally.
The accept a knock, the client should call [`POST
/_matrix/client/r0/rooms/{roomId}/invite`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-invite)
with the user ID of the knocking user in the JSON body.
If the knocking user is on another homeserver, then the homeserver of the
accepting user will call [`PUT
/_matrix/federation/v2/invite/{roomId}/{eventId}`](https://matrix.org/docs/spec/server_server/r0.1.4#put-matrix-federation-v2-invite-roomid-eventid)
on the knocking homeserver to inform it that the knock has been accepted.
The knocking homeserver should assume an invite to a room it has knocked on means
that its knock has been accepted, even if the invite was not explicitly
related to the knock attempt.
Note that client or homeserver implementations are free to automatically
accept this invite given they're aware that it's the result of a previous
knock.
### Membership change to `leave`
The knock has been rejected by someone in the room, or the knocking user has
rescinded their knock.
To rescind a knock, the knocking user's client must call [`POST
/_matrix/client/r0/rooms/{roomId}/leave`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-leave).
To reject a knock, the rejecting user's client must call [`POST
/_matrix/client/r0/rooms/{roomId}/kick`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-kick)
with the user ID of the knocking user in the JSON body. Rejecting a knock
over federation has a slight catch, however.
When the knocking user is on another homeserver, the homeserver of the
rejecting user needs to send the `leave` event over federation to the
knocking homeserver. However, this is a bit tricky as it is currently very
difficult to have events from a room propagate over federation when the
receiving homeserver is not in the room. This is due to the remote homeserver
being unable to verify that the event being sent is actually from a
homeserver in the room - and that the homeserver in the room had the required
power level to send it. This is a problem that currently affects other,
similar operations, such as disinviting or unbanning a federated user. In
both cases, they won't be notified as their homeserver is not in the room.
While we could send easily send the leave event as part of a generic
transaction to the remote homeserver, that homeserver would have no way to
validate the `prev_events` and `auth_events` that the event references. We
could send those events over as well, but those will also reference other
events that require validation and so on.
A simple thing we could easily do here is to trust the leave event implicitly
if it is sent by the homeserver we originally knocked through. We know this
homeserver is (or at least was) in the room, so they're probably telling the
truth. This is almost an edge case though, as while you'll knock through one
homeserver in the room, there's no guarantee that the admin that denies your
knock will be on the same homeserver you knocked through. Perhaps the
homeserver you knocked through could listen for this and then send the event
back to you - but what if it goes offline in the meantime?
As such, informing remote homeservers about the rejection of knocks over
federation is de-scoped for now, and left to a future MSC which can solve
this class of problem in a suitable way. Rejections should still work for the
homeservers that are in the room, as they can validate the leave event for
they have access to the events it references.
### Membership change to `ban`
The knock has been rejected by someone in the room and the user has been
banned, and is unable to send further knocks.
This one is fairly straightforward. Someone with the appropriate power levels
in the room bans the user. This will have the same effect as rejecting the
knock, and in addition prevent any further knocks by this user from being
allowed into the room.
If the user is unbanned, they will be able to send a new knock which could be
accepted.
To ban the user, the client should call [`POST
/_matrix/client/r0/rooms/{roomId}/ban`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-ban) with the user ID of the knocking user in the JSON body.
Informing the knocking user about the update is the same as rejecting the
knock.
## Client-Server API
A new endpoint is introduced in the Client-Server API: `POST
/_matrix/client/r0/knock/{roomIdOrAlias}`. This allows the client to state
@ -385,9 +227,10 @@ the underlying push rules architecture can support it.
## Server-Server API
Similarly to join and leave over federation, a ping-pong game with two new
endpoints is introduced: `make_knock` and `send_knock`. Both endpoints must
be protected via server ACLs.
Similarly to [join](https://matrix.org/docs/spec/server_server/r0.1.4#joining-rooms)
and [leave](https://matrix.org/docs/spec/server_server/r0.1.4#leaving-rooms-rejecting-invites)
over federation, a ping-pong game with two new endpoints is introduced: `make_knock`
and `send_knock`. Both endpoints must be protected via server ACLs.
### `GET /_matrix/federation/v1/make_knock/{roomId}/{userId}`
@ -555,6 +398,161 @@ request content.
}
```
## Restrictions
There are restrictions to being able to set this membership, as well as
accepting or denying the knock. A formal description of the changes to the auth rules is given below;
first we summarise the semantics of the proposed changes.
### Current membership
Only users without a current membership or with their current membership
being "leave" can knock on a room. This means that a user that is banned, has
already knocked or is currently in the room cannot knock on it.
### Join Rules
This proposal makes use of the existing "knock" join rule. The value of
`join_rule` in the content of the `m.room.join_rules` state event for a room
must be set to "knock" for a knock to succeed. This means that existing rooms
will need to opt into allowing knocks in their rooms. Other than allowing
knocks, a join rule of "knock" is functionally equivalent to that of
"invite", except that it additionally allows external users to change their
membership to "knock" under certain conditions.
### Auth rules
Each room version defines the auth rules which should be applied in that room version.
This MSC proposes a new room version with the following changes to the [auth
rules from room version 6](https://matrix.org/docs/spec/rooms/v6#authorization-rules-for-events):
* Under "5. If type is `m.room.member`", insert the following after "e. If membership is `ban`":
```
f. If `membership` is `knock`:
i. If the `join_rule` is anything other than `knock`, reject.
ii. If `sender` does not match `state_key`, reject.
iii. If the `sender`'s membership is not `ban`, `invite` or `join`, allow.
iv. Otherwise, reject.
```
Note that:
- Both the `sender` and `state_key` fields are set to the user ID of the knocking
user. This is different to an `invite` membership event, where the `sender` is the inviter and
the `state_key` is the invitee.
- f.ii is justified as one user should not be able to knock on behalf of
another user.
- f.iii is justified as knocks should not be allowed if the knocking user
has been banned from the room, is invited to the room or if they are already
in the room.
- Knocks are not restricted by power level like invites are. The `join_rules`
are already used to enforce whether someone can or cannot knock. However,
power level rules do apply when approving or denying the knock, as discussed
in the Membership Changes section below.
Additionally, note that redactions of knock events are not a concern, as
`membership` keys are excluded from being redacted as defined by all current
room versions.
## Membership changes
Once someone has sent a `knock` membership into the room, the membership for
that user can be transitioned to the following possible states:
- `invite`: In this case, the knock was accepted by someone inside the room
and they are inviting the knocker into the room.
- `leave`: In this case, similar to how kicks are handled, the knock has
been rejected. Alternatively, the knocking user has rescinded their knock.
- `ban`: In this case, the knock was rejected and the user has been prevented
from sending further knocks.
Let's talk about each one of these in more detail.
### Membership change to `invite`
The knock has been accepted by someone in the room.
The user who is accepting the knock must have the power level to perform
invites. The accepting user's homeserver will then send an invite - over federation if
necessary - to the knocking user. The knocking user may then join the room as
if they had been invited normally.
To accept a knock, the client should call [`POST
/_matrix/client/r0/rooms/{roomId}/invite`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-invite)
with the user ID of the knocking user in the JSON body.
If the knocking user is on another homeserver, then the homeserver of the
accepting user will call [`PUT
/_matrix/federation/v2/invite/{roomId}/{eventId}`](https://matrix.org/docs/spec/server_server/r0.1.4#put-matrix-federation-v2-invite-roomid-eventid)
on the knocking homeserver to inform it that the knock has been accepted.
The knocking homeserver should assume an invite to a room it has knocked on means
that its knock has been accepted, even if the invite was not explicitly
related to the knock attempt.
Note that client or homeserver implementations are free to automatically
accept this invite given they're aware that it's the result of a previous
knock.
### Membership change to `leave`
The knock has been rejected by someone in the room, or the knocking user has
rescinded their knock.
To rescind a knock, the knocking user's client must call [`POST
/_matrix/client/r0/rooms/{roomId}/leave`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-leave).
To reject a knock, the rejecting user's client must call [`POST
/_matrix/client/r0/rooms/{roomId}/kick`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-kick)
with the user ID of the knocking user in the JSON body. Rejecting a knock
over federation has a slight catch, however.
When the knocking user is on another homeserver, the homeserver of the
rejecting user needs to send the `leave` event over federation to the
knocking homeserver. However, this is a bit tricky as it is currently very
difficult to have events from a room propagate over federation when the
receiving homeserver is not in the room. This is due to the remote homeserver
being unable to verify that the event being sent is actually from a
homeserver in the room - and that the homeserver in the room had the required
power level to send it. This is a problem that currently affects other,
similar operations, such as disinviting or unbanning a federated user. In
both cases, they won't be notified as their homeserver is not in the room.
While we could send easily send the leave event as part of a generic
transaction to the remote homeserver, that homeserver would have no way to
validate the `prev_events` and `auth_events` that the event references. We
could send those events over as well, but those will also reference other
events that require validation and so on.
A simple thing we could easily do here is to trust the leave event implicitly
if it is sent by the homeserver we originally knocked through. We know this
homeserver is (or at least was) in the room, so they're probably telling the
truth. This is almost an edge case though, as while you'll knock through one
homeserver in the room, there's no guarantee that the admin that denies your
knock will be on the same homeserver you knocked through. Perhaps the
homeserver you knocked through could listen for this and then send the event
back to you - but what if it goes offline in the meantime?
As such, informing remote homeservers about the rejection of knocks over
federation is de-scoped for now, and left to a future MSC which can solve
this class of problem in a suitable way. Rejections should still work for the
homeservers that are in the room, as they can validate the leave event for
they have access to the events it references.
### Membership change to `ban`
The knock has been rejected by someone in the room and the user has been
banned, and is unable to send further knocks.
This one is fairly straightforward. Someone with the appropriate power levels
in the room bans the user. This will have the same effect as rejecting the
knock, and in addition prevent any further knocks by this user from being
allowed into the room.
If the user is unbanned, they will be able to send a new knock which could be
accepted.
To ban the user, the client should call [`POST
/_matrix/client/r0/rooms/{roomId}/ban`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-ban) with the user ID of the knocking user in the JSON body.
Informing the knocking user about the update is the same as rejecting the
knock.
# Potential issues
This new feature would allow users to send events into rooms that they don't
partake in. That is why this proposal enables the a `knock` join rule, in

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