MSC2403: Add "knock" feature

proposals/2403-knock.md

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+# 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 this user to indicate that they want to join
+said room.
+
+# Proposal
+This proposal implements the reserved "knock" membership type for the `m.room.member` state event.
+This state event indicates that a user knocks a room, that is asking for permission to join. It
+contains an optional "reason" parameter to specify the reason you want to join. This membership can
+be set from users who aren't currently in said room. An example for the membership would look as
+follows:
+```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 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. After a knock in a room a member of the room can invite the knocker.
+
+To be able to implement this properly two new endpoints need to be added, one in the client-server
+API and one in the server-server API.
+
+## 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 a
+room. This means that a user that is banned or currently in the room can't knock on it.
+
+### Join Rules
+The `join_rule` of `m.room.join_rules` must be set to "invite". This means that people can't knock
+in public rooms. Additionaly the new join rule "private" is introduced. This is so that people can,
+when creating a new room, prevent anyone from knocking.
+
+### Power levels
+The default power level for "knock" is 0. If a user has a too low power level to knock they aren't
+allowed to do this. As power levels can be set for users not currently in the room this can be used
+as a way to limit who can knock and who can't.
+
+#### Example:
+`@alice:example.org` CAN knock, but `@bob:example.org` can't: The (incomplete) content of
+`m.room.power_levels` is as follows:
+```json
+{
+  "users": {
+    "@alice:example.org": 1
+  },
+  "users_default": 0,
+  "knock": 1
+}
+```
+
+## Client-Server API
+The new endpoint for the client-server API is `POST /_matrix/client/r0/rooms/{roomId}/knock`.
+The path parameter (`roomId`) is the room you want to knock. It is required. The post body accepts
+an optional parameter, `reason`, which is the reason you want to join the room. A request could look
+as follows:
+
+```
+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. Empty reply:
+```json
+{}
+```
+
+#### 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."
+}
+```
+
+## Server-Server API
+The new endpoint for the server-server API is `PUT /_matrix/federation/v2/knock/{roomId}/{eventId}`.
+The path parameters are the room id you want to knock and the event id of the knock event. The post
+body consists of an `event` parameter, which is the knock event. A request could look as follows:
+
+```
+PUT /_matrix/federation/v2/knock/%21abc123%3Amatrix.org/%24abc123%3Aexample.org  HTTP/1.1
+Content-Type: application/json
+
+{
+  "event": {
+    "sender": "@alice:example.org",
+    "origin": "example.org",
+    "origin_server_ts": 1234567890,
+    "type: "m.room.member",
+    "state_key": "@alice:example.org",
+    "content": {
+      "membership": "knock",
+      "reason": "I want to join this room as I really love foxes!"
+    }
+  }
+}
+```
+
+### Responses
+#### Status code 200:
+The knock was performed successfully. The knock event is sent back with the "event" key.
+```json
+{
+  "event": {
+    "sender": "@alice:example.org",
+    "origin": "example.org",
+    "origin_server_ts": 1234567890,
+    "type": "m.room.member",
+    "state_key": "@alice:example.org",
+    "content": {
+      "membership": "knock",
+      "reason": "I want to join this room as I really love foxes!"
+    }
+  }
+}
+```
+
+#### Status code 403:
+The user wasn't allowed to knock. Error reply:
+```json
+{
+  "errcode": "M_FORBIDDEN",
+  "error": "The user isn't allowed to knock in this room."
+}
+```
+
+# Potential issues
+This new feature would allow users to spam rooms that they don't partake in. That is why this proposal
+adds both the new join rule and the new power level, in order to allow room admins to mitigate such
+potential spam.
+
+# Alternatives
+As for the join rule "invite", instead the join rule "knock" could be introduced, meaning the room
+is like "invite" only that people can also knock. The difference is for existing rooms: With this
+proposal people can knock in existing "invite" rooms, with the alternative suggestion they can't.
+
+# Security considerations
+None. This doesn't allow users access to a room in any way.