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proposals/3780-action-join-knock.md

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+# MSC3780: Knock on `action=join`
+
+[Matrix URIs][matrix-uri] have an `action` query component,
+of which `=join` is taken as a hint that the user joins a particular room as described within the URI.
+
+Knocking was introduced in [room version 7](https://spec.matrix.org/v1.2/rooms/v7/#authorization-rules),
+which makes it possible to "knock" (i.e. notify) a room, announcing an intention to join.
+
+However, the [current specification for `=join`][matrix-uri] does not detail knocks as an action a client should present,
+when join rules prohibit public joining.
+
+## Proposal
+
+The specification should be updated to state that `=join` will prompt a user with a knocking prompt,
+if it cannot join the room via an invite, or a normal public join.
+
+## Potential issues
+
+As the spec currently stands, this would have to be done on a "fallback" basis; The client has to try
+joining via a public join, and then try to infer if its a knockable-only room, and present the user a
+knock prompt.
+
+As far as this author knows, there exists no specced error code/string that tells the client (when
+failing to join) that a room is only knockable.
+
+[MSC3266] - Room Summaries - maybe be
+required to allow the client to easily and deterministically infer if a room is (only) knockable.
+
+## Alternatives
+
+`action=knock`, this would allow the client to instantly know which action (out of joining or knocking)
+to take when it is given this URI. However, once/if a room join rules change, it might not be possible
+to "blindly knock" a room, and this author believes that encoding "current states" of the room into the
+URI is not a good practice because of this.
+
+In comparison, `action=join` encompassing knocks would allow for more generic, unified and resilient logic when it
+comes to joining rooms.
+
+## Security considerations
+
+None identified.
+
+## Unstable prefix
+
+None required.
+
+## Dependencies
+
+For optimal performance, this MSC heavily leans on [MSC3266].
+
+[matrix-uri]: https://spec.matrix.org/v1.2/appendices/#matrix-uri-scheme
+[MSC3266]: https://github.com/matrix-org/matrix-spec-proposals/pull/3266