MSC3244: Room version capabilities

proposals/3244-room-version-capabilities.md

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+# MSC3244: Room version capabilities
+
+Room version updates can bring new functionalities, for example v7 is introducing `knocking`.
+
+There is a delay before a room version becomes the default version (when supported by enough home servers).
+And when creating rooms clients should not ask for a specific version (or else they will stick to this
+specific version unless they are updated).
+
+So currently clients can't know before creating the room if a feature they need will be supported by the created room.
+
+The goal of this MSC is to give the client a way to have more informations on the supported features of room
+versions available on a given server, to give more feedbacks to users during room creation as well as
+in the room settings section.
+
+## Proposal
+
+The __m.room_versions capability__  [`/_matrix/client/r0/capabilities`]([https://matrix.org/docs/spec/client_server/r0.6.1#m-room-versions-capability])
+endpoint could be decorated to provide more information on room version capabilities.
+
+Actual capabilities response:
+````
+{
+  "capabilities": {
+    "m.room_versions": {
+      "default": "6",
+      "available": {
+        "1": "stable",
+        "2": "stable",
+        "3": "stable",
+        "4": "stable",
+        "5": "stable",
+        "6": "stable",,
+        "org.matrix.msc2176": "unstable",
+        "org.matrix.msc3083": "unstable"
+      }
+    },
+    "m.change_password": {...}
+  }
+}
+````
+
+Proposed modification
+
+````
+{
+  "capabilities": {
+    "m.room_versions": {
+      "default": "6",
+      "available": {
+        "1": "stable",
+        "...,
+        "6": "stable",
+        "7": "stable",
+        "8": "stable",
+        "9": "stable"
+      },
+      "room_capabilities": {
+          "msc2403" : {
+              "best": "7",
+              "support" : ["7"]
+          },
+          "msc3083" : {
+              "best": "9",
+              "support" : ["8", "9"]
+          }
+      }
+    }
+  }
+}
+````
+
+A new object is added under `room_capabilities`, each key is the name of a capability.
+This object provides the list of room versions supporting this capability as well as the preferred version to use.
+
+
+As part of this MSC, two capabilities are defined:
+- msc2403 a.k.a knocking join rule support
+- msc3083 a.k.a restricted join rule support 
+
+## Client recommendations:
+
+When presenting room settings, clients should use capabilities in order to display the correct UI. 
+
+For example if the room support knocking, the client should add this option in the join rule chooser
+(and if not only show `Invite Only` and `Public` for example).
+
+When creating a room, the client could check if the needed feature is supported by the server before creating.
+
+If the feature is not supported, the client could inform the user that this type of room can't be created
+as well as an information message explaining how to contact the home server admin.
+
+If the feature is supported by the default room version, then just continue as usual.
+
+If the feature is supported but by a stable room version that is not the default one, the client should
+then request to use the preferred version (`best`) that supports the feature, in the create room call:
+
+````
+POST /_matrix/client/r0/createRoom HTTP/1.1
+Content-Type: application/json
+
+{
+  "preset": "....",
+  "name": "....",
+  "topic": "....",
+  "creation_content": { ... }
+  "room_version": "8"
+}
+````
+
+If multiple capabilities are needed (e.g mscXX1 and mscXX2), and they have different `best`versions, clients can
+then pick one of the stable version that appears in both `support` array.
+
+It is not recommended to use an unstable room version even if it's the only one supporting a given feature.
+It should be reserved for development.
+
+
+## Unstable prefix
+
+The following mapping will be used for identifiers in this MSC during development:
+
+
+Proposed final identifier       | Purpose | Development identifier
+------------------------------- | ------- | ----
+`room_capabilities` | event type | `org.matrix.msc3244.room_capabilities`
+