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MSC3077: Support for multi-stream VoIP

This MSC proposes a method for differentiating WebRTC streams from each other.

MSC2746 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.:

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

{
    "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 ids 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. 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