Break-out rooms MSC

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

proposals/3985-break-out.md

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+# MSC3985: Break-out rooms
+
+VoIP applications such as Zoom or MS Teams support break-out rooms - a mechanism
+for splitting users into multiple calls. This proposal suggests a mechanism for
+break-out rooms in Matrix which would not be limited to VoIP use-cases.
+
+## Proposal
+
+When a user wants to break-out the "parent" room's users into multiple "child"
+rooms, their client should first create those rooms and then send an
+`m.breakout` event:
+
+```json5
+{
+    "type": "m.breakout",
+    "state_key": "cvsiu2813",
+    "content": {
+        "m.breakout": {
+            "!roomId1": {
+                "via": ["example.org", "other.example.org"],
+                "users": ["userId1", "userId2"]
+            },
+            "!roomId2": {
+                "via": ["example.org", "other.example.org"]
+            }
+        }
+    }
+}
+```
+
+This event contains a map of the "child" rooms along with their `via`s. The
+event may also include information about the users who are suggested to join a
+given "child" room.
+
+When the event is received by the other clients in the room, they should give
+their users the option to join the given rooms. If the user is suggested to join
+a specific room, the UI should reflect that.
+
+If a user is participating in a call when the event is sent, they should either
+start a call or join an existing call in the "child" room, if there is one.
+
+The "child" rooms may have `join_rules` of the user's, which has initiated the
+break-out, or their client's choosing.
+
+## Potential issues
+
+None that I can think of.
+
+## Alternatives
+
+An alternative for the VoIP use-case would be having multiple calls in one room
+along with multiple threads for chat. While this works alright for some
+use-cases, it has its limitations.
+
+## Security considerations
+
+None that I can think of.
+
+## Unstable prefix
+
+While this MSC is unstable, `org.matrix.msc3985.breakout` should be used instead
+of `m.breakout`.
+
+## Dependencies
+
+None that I can think of.