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+# MSC 1794 - Federation v2 Invite API
+
+This proposal adds a new `/invite` API to federation that supports different
+room versions.
+
+## Motivation
+
+It is planned for future room versions to be able to change the format of events
+in various ways. To support this, all servers must know the room version
+whenever they need to parse an event.  Currently the `/invite` API does not
+include the room version, so the target server would be unable to parse the event included in the payload.
+
+## Proposal
+
+Add a new version of the invite API under the prefix `/_matrix/federation/v2`,
+which has a payload of:
+
+```
+PUT /_matrix/federation/v2/invite/<room_id>/<event_id>
+
+{
+    "room_version": <room_version>,
+    "event": { ... },
+    "invite_room_state": [ ... ]
+}
+```
+
+The differences between this and `v1` are:
+
+1. The payload in `v1` is the event, while in `v2` the event is instead placed
+   under an `"event"` key. This is for consistency with other APIs, and to allow
+   extra data to be added to the request payload separately from the event.
+2. A required field called `"room_version"` is added that specifies the room
+   version.
+3. The `"invite_room_state"` is moved from the `unsigned` section of the event
+   to a top level key. The key `invite_room_state` being in the `event.unsigned`
+   was a hack due to point 1. above.
+
+
+The response is identical to `v1`, except that:
+
+1. If the receiving server does not support the given room version the
+   appropriate incompatible-room-version error is returned, in the same way
+   as e.g. for `/make_join` APIs.
+2. The response payload is no longer in the format of `[200, { ... }]`, and is
+   instead simply the `{ ... }` portion. This fixes a historical accident to
+   bring the invite API into line with the rest of the federation API.
+
+
+If a call to `v2` `/invite` results in an unrecognised request exception **AND**
+the room version is `1` or `2` then the sending server should retry the request
+with the `v1` API.
+
+
+## Alternatives
+
+
+### Reusing V1 API
+
+One alternative is to add a `room_version` query string parameter to the `v1`
+`/invite` API in a similar way as for the `/make_join` APIs. However, older
+servers would ignore the query string parameter while processing an incoming
+`/invite` request, resulting in the server attempting to parse the event in the
+old `v1` format. This would likely result in either a `400` or `500` response,
+which the sending server could interpret as the receiving server not supporting
+the room version.
+
+This method, however, is fragile and could easily mask legitimate `400` and
+`500` errors that are not due to not supporting the room version.
+
+
+### Using V1 API for V1 room versions
+
+Instead of all servers attempting to use the new API and falling back if the API
+is not found, servers could instead always use the current API for V1 and V2
+rooms.
+
+However, this would not allow us to deprecate the `v1` API.