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+# MSC3077: Support for multi-stream VoIP
+
+This MSC proposes a method for differentiating WebRTC streams from each other.
+
+[MSC2746](https://github.com/matrix-org/matrix-doc/pull/2746) has improved VoIP
+immeasurably. Yet, there is still no clear way to handle things such as
+screen-sharing.
+
+Simple VoIP calls only ever feature one stream, though often clients will want
+to send multiple - usermedia, screensharing and possibly more. In a situation
+with more streams, it can be very helpful to provide the other side with
+metadata about the content of the streams.
+
+## Proposal
+
+This MSC proposes adding an `sdp_stream_metadata` field to the events containing
+a session description i.e.:
+
++ [`m.call.invite`](https://spec.matrix.org/v1.7/client-server-api/#mcallinvite)
++ [`m.call.answer`](https://spec.matrix.org/v1.7/client-server-api/#mcallanswer)
++ [`m.call.negotiate`](https://spec.matrix.org/v1.7/client-server-api/#mcallnegotiate)
+
+The `sdp_stream_metadata` field is an object in which each key is one stream
+`id` in the session description. The values are objects with the
+following fields:
+
++ `purpose` - a string indicating the purpose of the stream. For compatibility
+  between client the following values are defined:
+  + `m.usermedia` - stream that contains the webcam and/or microphone tracks
+  + `m.screenshare` - stream with the screen-sharing tracks
+
+### Example
+
+```JSON
+{
+    "type": "m.call.invite",
+    "room_id": "!roomId",
+    "content": {
+        "call_id": "1414213562373095",
+        "invitee": "@bob:matrix.org",
+        "party_id": "1732050807568877",
+        "lifetime": "60000",
+        "capabilities": {
+            "m.call.transferee": true,
+        },
+        "offer": {
+            "sdp": "...",
+            "type": "offer",
+        },
+        "sdp_stream_metadata": {
+            "271828182845": {
+                "purpose": "m.screenshare",
+            },
+            "314159265358": {
+                "purpose": "m.usermedia",
+            },
+        },
+        "version": "1",
+    },
+}
+```
+
+### Edge cases
+
++ If an incoming stream is not described in `sdp_stream_metadata` and
+  `sdp_stream_metadata` is present, the stream should be ignored.
++ If a stream has a `purpose` of an unknown type (i.e. not `m.usermedia` or
+  `m.screenshare`), it should be ignored.
+
+### Backwards compatibility
+
+During the initial invite and answer exchange clients find out if the field
+`sdp_stream_metadata` is missing. If it is not present in the event sent by the
+opponent, the client should ignore any new incoming streams (i.e. it should use
+the first one) and it shouldn't send more than one stream (i.e. clients cannot send a video feed and a screenshare at the same time, as is the case in current clients).
+
+## Alternatives
+
+Setting the stream `id`s to custom values had been considered. Though this is
+possible on some platforms, it is not in browsers. That is because the `id`
+property of `MediaStream` is _read-only_ as the [MDN Documentation
+states](https://developer.mozilla.org/en-US/docs/Web/API/MediaStream/id).
+Similar is true for SDP attributes.
+
+This proposal is also more practical for cases where more complex metadata is
+needed. For conferencing, a `user_id` field could be added to
+the objects in `sdp_stream_metadata`; for differentiating between the front and rear camera of a
+phone, a `camera_type` field could be added.
+
+Previously, it has been thought that the `purpose` field has to be unique (or
+another unique field has to be added), though this could only ever be important
+if we wanted to replace a stream with another one in-place. It was deemed as a
+rather uncommon thing for which there doesn't seem to be any use-case, so
+uniqueness is not required.
+
+## Unstable prefix
+
+During development, the following fields should be used:
+
+|Release                     |Development                                    |
+|----------------------------|-----------------------------------------------|
+|`sdp_stream_metadata`       |`org.matrix.msc3077.sdp_stream_metadata`       |
+|`m.call.sdp_stream_metadata`|`org.matrix.msc3077.call.sdp_stream_metadata`  |
+
+## Potential issues
+
+None that I can think of.
+
+## Security considerations
+
+None that I can think of.