matrix-spec-proposals/proposals/2403-knock.md

# MSC2403: Add "knock" feature
Many people are in invite-only rooms. Sometimes, someone wants to join such a
room and can't, as they aren't invited. This proposal adds a feature for a
user to indicate that they want to join a room.

# Proposal
This proposal implements the reserved "knock" membership type for the
`m.room.member` state event. This state event indicates that when a user
knocks on a room, they are asking for permission to join. It contains an
optional "reason" parameter to specify the reason you want to join. Like
other membership types, the parameters "displayname" and "avatar_url" are
optional. This membership can be set from users who aren't currently in said
room. An example for the membership would look like the following:
```json
{
  "membership": "knock",
  "displayname": "Alice",
  "avatar_url": "mxc://example.org/avatar",
  "reason": "I want to join this room as I really love foxes!"
}
```

After a knock in a room, a member of the room can invite the knocker.

## Restrictions
There are restrictions to being able to set this membership.

### 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 introduces a new possible value for `join_rule` in
`m.room.join_rules`: "knock". The value of `join_rule` in `m.room.join_rules`
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, "knock" is no different from the "invite" join rule.

As we're updating the join rules, and thus the auth rules, this proposal will
need to be introduced as part of a new room version.

## 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 Alternatively, the knocking user has rescinded their knock.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.
This user must have the power level to perform invites. The 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 its 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.

### Membership change to `leave`

The knock has been rejected by someone in the room.


XXX: There is also an open question here about who should be able to reject a
knock. When revoking an invite for a user, perhaps counter-intuitively, you
need to have a high enough power level to kick users, rather than invite
them. You also need to have a higher power level than them. Should the same
be done for knocking, assuming the knocking user has the default power level?
Or should it be the same power level that's required to accept the knock?

To reject a knock, the client should 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.

At this point, if the knocking user is on another homeserver, then 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
if 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 dumb 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, this feature working over federation is de-scoped for now, and left
to a future MSC which can solve this problem across the board for all
affected features 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.

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, then knocks will be accepted again.

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.


## Client-Server API
Two new endpoints are introduced in the Client-Server API (similarly to
join): `POST /_matrix/client/r0/rooms/{roomId}/knock` and `POST
/_matrix/client/r0/knock/{roomIdOrAlias}`. These allow the client to state
their intent to knock on a room.

Additionally, extensions to the `GET /_matrix/client/r0/sync` endpoint are
introduced. These allow a client to receive information about the status of
their knock attempt.

### `POST /_matrix/client/r0/rooms/{roomId}/knock`
The path parameter (`roomId`) is the room on which you want to knock. It is
required. The post body accepts an optional string parameter, `reason`, which
is the reason you want to join the room. A request could look as follows:

```json
POST /_matrix/client/r0/rooms/%21d41d8cd%3Amatrix.org/knock  HTTP/1.1
Content-Type: application/json

{
  "reason": "I want to join this room as I really love foxes!"
}
```

#### Responses:
##### Status code 200:
The user knocked successfully. Example reply:
```json
{}
```

##### Status code 400:
This request was invalid, e.g. bad JSON. Example reply:
```json
{
  "errcode": "M_UNKNOWN",
  "error": "An unknown error occurred"
}
```

##### Status code 403:
The user wasn't allowed to knock (e.g. they are banned). Error reply:
```json
{
  "errcode": "M_FORBIDDEN",
  "error": "The user isn't allowed to knock in this room."
}
```

##### Status code 429:
This request was rate-limited. Example reply:
```json
{
  "errcode": "M_LIMIT_EXCEEDED",
  "error": "Too many requests",
  "retry_after_ms": 2000
}
```

### `POST /_matrix/client/r0/knock/{roomIdOrAlias}`
The path parameter (`roomIdOrAlias`) is either the room ID or the alias of
the room you want to knock on. Additionally several `server_name` parameters
can be specified via the query parameters. The post body accepts an optional
string parameter, `reason`, which is the reason you want to join the room. A
request could look as follows:

```json
POST /_matrix/client/r0/knock/%23foxes%3Amatrix.org?server_name=matrix.org&server_name=elsewhere.ca  HTTP/1.1
Content-Type: application/json

{
  "reason": "I want to join this room as I really love foxes!"
}
```

#### Responses:
The possible responses are the same as for the `POST
/_matrix/client/r0/rooms/{roomId}/knock` endpoint.

### Extensions to `GET /_matrix/client/r0/sync`

In [the response to
`/sync`](https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-client-r0-sync)
is a `rooms` field. This is a dictionary which currently contains keys
`join`, `invite` and `leave`, which each provide information to the client on
various membership states regarding the user.

It is proposed to add a fourth possible key to `rooms`, called `knock`. Its
value is a mapping from room ID to room information. The room information is
a mapping from a key `knock_state` to another mapping with key `events` being
a list of `StrippedStateEvent`. `StrippedStateEvent`s are defined as state
events that only contain the `sender`, `type`, `state_key` and `content`
keys.

These stripped state events contain information about the room, most notably
the room's name and avatar. A client will need this information to show a
nice representation of pending knocked rooms. The recommended events to
include are the join rules, canonical alias, avatar, and name of the room,
rather than all room state. This behaviour matches the information sent to
remote homeservers when invited their users to a room.

This prevents unneeded state from the room leaking out, and also speeds
things up (think not sending over hundreds of membership events from big
rooms).

The following is an example of knock state coming down `/sync`.

Request:
```
GET /_matrix/client/r0/sync HTTP/1.1
Content-Type: application/json
```

Response:
```json
{
  ...
  "rooms": {
    "knock": {
      "!abcdefghijklmo:example.com": {
        "knock_state": {
          "events": [
            {
              "content": {
                "join_rule": "knock"
              },
              "sender": "@room_admin:example.com",
              "state_key": "",
              "type": "m.room.join_rules"
            },
            {
              "content": {
                "name": "Some cool room"
              },
              "sender": "@room_admin:example.com",
              "state_key": "",
              "type": "m.room.name"
            },
            {
              "content": {
                "url": "mxc://example.com/xyz54321"
              },
              "sender": "@room_admin:example.com",
              "state_key": "",
              "type": "m.room.avatar"
            },
            {
              "content": {
                "avatar_url": "mxc://example.org/abc1234",
                "displayname": "Knocking User",
                "membership": "knock"
              },
              "sender": "@knocking_user:example.org",
              "state_key": "@knocking_user:example.org",
              "type": "m.room.member",
            }
          ]
        }
      }
    }
  },
  ...
}
```

## 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.

### `GET /_matrix/federation/v1/make_knock/{roomId}/{userId}`

Asks the receiving server to return information that the sending server will
need to prepare a knock event.

Request format:

| Parameter | Type | Description |
|-----------|------|-------------|
| Path parameters:
| roomId | string | Required. The room ID that should receive the knock.
| userId | string | Required. The user ID the knock event will be for.
| Query Parameters:
| ver | [string] | The room versions the sending server has support for. Defaults to `[1]`.

Response Format:

| Parameter | Type | Description |
|-----------|------|-------------|
| room_version | string | The version of the room where the server is trying to knock.
| event | Event Template | An unsigned template event. May differ between room versions.

#### Responses
##### Status code 200:
Returns a template to be used to knock on rooms. May depend on room version.
```json
{
  "room_version": "2",
  "event": {
    "type": "m.room.member",
    "room_id": "!somewhere:example.org",
    "content": {
      "membership": "knock"
    },
    "state_key": "@someone:example.org",
    "origin": "example.org",
    "origin_server_ts": 1549041175876,
    "sender": "@someone:example.org"
  }
}
```

##### Status code 400:
This request was invalid, e.g. bad JSON. Example reply:
```json
{
  "errcode": "M_INCOMPATIBLE_ROOM_VERSION",
  "error": "Your homeserver does not support the features required to join this room",
  "room_version": "3"
}
```

### `PUT /_matrix/federation/v1/send_knock/{roomId}/{eventId}`
Submits a signed knock event to the resident homeserver for it to accept into
the room's graph. Note that event format may differ between room versions.

Request format:

| Parameter | Type | Description |
|-----------|------|-------------|
| Path parameters:
| roomId | string | Required. The room ID that should receive the knock.
| eventId | string | Required. The event ID for the knock event.

The JSON body is expected to be the full event.

Response Format:

| Parameter | Type | Description |
|-----------|------|-------------|
| `knock_room_state` | [StrippedStateEvent] | Required. State events providing public room metadata

A request could look as follows:
```json
PUT /_matrix/federation/v1/send_knock/%21abc123%3Amatrix.org/%24abc123%3Aexample.org HTTP/1.1
Content-Type: application/json

{
  "sender": "@someone:example.org",
  "origin": "matrix.org",
  "origin_server_ts": 1234567890,
  "type": "m.room.member",
  "state_key": "@someone:example.org",
  "content": {
    "membership": "knock",
    "displayname": "Alice",
    "avatar_url": "mxc://example.org/avatar",
    "reason": "I want to join this room as I really love foxes!"
  }
}
```

#### Response:
##### Status code 200:
The event was successfully accepted into the graph by the homeserver that
received the knock. It must then send this knock into the room, before
responding to the knocking homeserver, indicating the knock succeeded.

The response contains `StrippedStateEvent`s with room metadata (room name,
avatar ...) that the knocking homeserver can pass to the client. The event
types that can be sent here should match those of the `/sync` extensions
mentioned above.

This is loosely based on the
[federated invite](https://matrix.org/docs/spec/server_server/r0.1.4#put-matrix-federation-v2-invite-roomid-eventid)
request content.
```json
{
  "knock_room_state": [
    {
      "content": {
        "join_rule": "knock"
      },
      "sender": "@room_admin:example.com",
      "state_key": "",
      "type": "m.room.join_rules"
    },
    {
      "content": {
        "name": "Some cool room"
      },
      "sender": "@room_admin:example.com",
      "state_key": "",
      "type": "m.room.name"
    },
    {
      "content": {
        "url": "mxc://example.com/xyz54321"
      },
      "sender": "@room_admin:example.com",
      "state_key": "",
      "type": "m.room.avatar"
    },
    {
      "content": {
        "avatar_url": "mxc://example.org/abc1234",
        "displayname": "Knocking User",
        "membership": "knock"
      },
      "sender": "@knocking_user:example.org",
      "state_key": "@knocking_user:example.org",
      "type": "m.room.member",
    }
  ]
}
```

# Potential issues
This new feature would allow users to send events into rooms that they don't
partake in. That is why this proposal adds a new join rule, in order to allow
room admins to opt in to this behaviour.

# Alternatives
The two endpoints for the Client-Server API seem redundant, this MSC followed
how JOIN is working currently: One "proper" endpoint (`/rooms/{roomId}/join`)
and one to work properly over federation (`/join/{roomIdOrAlias}`). They
could both be merged into one, however, as that would also affect the join
endpoint it seems out-of-scope for this MSC.

# Client UX recommendations
After a knock is received in a room, it is expected to be displayed in the
timeline, similar to other membership changes. Clients can optionally add a way
for users of a room to review all current knocks.

# Security considerations
Clients must take care when implementing this feature in order to prevent
simple abuse vectors that can be accomplished by individual users. For
instance, when a knock occurs, clients are advised to hide the reason until
the user interacts with the client in some way (e.g. clicking on a "show
reason" button). The user should reveal the reason only if they choose to.

It is recommended to not display the reason by default as else this would
essentially allow outsiders to send messages into the room.

It is still theoretically possible for a homeserver admin to create many users
with different user IDs or display names, all spelling out an abusive
message, and then having each of them knock in order. 

Another abuse vector is allowed by the ability for users to rescind knocks.
This is to help users in case they knocked on a room accidentally, or simply
no longer want to join a room they've knocked on. While this is a useful
feature, it also allows users to spam a room by knocking and rescinding their
knocks over and over.

In both cases, room admins should employ typical abuse mitigation tools, such
as user bans and server ACLs. Clients are encouraged to ease employing these
tools easy even if the offensive user or server is present not in the room.

# Unstable prefix

An unstable feature flag is added to the `unstable_features` dict of
`/_matrix/client/versions` with the key `xyz.amorgan.knock` and value `true`.
If this key is present, this is a signal to clients that the homeserver has
experimental support for room knocking.

The new endpoints should contain an unstable prefix during experimental
implementation. The unstable counterpart for each endpoint is:

* `POST /_matrix/client/r0/rooms/{roomId}/knock`
* `POST /_matrix/client/unstable/xyz.amorgan/rooms/{roomId}/knock`

* `POST /_matrix/client/knock/{roomIdOrAlias}`
* `POST /_matrix/client/unstable/xyz.amorgan/knock/{roomIdOrAlias}`

* `GET /_matrix/federation/v1/make_knock/{roomId}/{userId}`
* `GET /_matrix/federation/unstable/xyz.amorgan/make_knock/{roomId}/{userId}`

* `PUT /_matrix/federation/v1/send_knock/{roomId}/{eventId}`
* `PUT /_matrix/federation/unstable/xyz.amorgan/send_knock/{roomId}/{eventId}`

And finally, an unstable prefix is added to the key that comes down `/sync`,
the new join rule for rooms and the `content.membership` key of the member
event sent into rooms when a user has knocked successfully. Instead of
`knock`, experimental implementations should use `xyz.amorgan.knock`.