WebRTC - how to differentiate between two MediaStreamTracks sent over the same connection?

Question

I want to send two video streams from Peer1 to Peer2 using WebRTC: let's use front and rear camera as an example.

Conceptually, how does Peer2 know which stream comes from which camera?

MediaStream documentation mentions that all MediaStream and MediaStreamTrack fields are read-only, so I can't attach any information directly to them. I cannot send a dictionary like {"stream1 id": "camera", "stream2 id": "screenshare"} over a signalling channel, because Peer2 will generate its own id for each stream and track.

Answer 1

https://datatracker.ietf.org/doc/html/draft-ietf-mmusic-msid-11

In the RTP specification, media streams are identified using the SSRC
field.  Streams are grouped into RTP Sessions, and also carry a
CNAME.  Neither CNAME nor RTP session correspond to a MediaStream.
Therefore, the association of an RTP media stream to MediaStreams
need to be explicitly signaled.

WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where
one SDP media description is used to describe each MediaStreamTrack,
and the BUNDLE mechanism [I-D.ietf-mmusic-sdp-bundle-negotiation] is
used to group MediaStreamTracks into RTP sessions.  Therefore, the
need is to specify the ID of a MediaStreamTrack and its associated
MediaStream for each media description, which can be accomplished
with a media-level SDP attribute.

This document defines a new SDP [RFC4566] media-level "msid"
attribute.  This new attribute allows endpoints to associate RTP
media streams that are described in different media descriptions with
the same MediaStreams as defined in [W3C.WD-webrtc-20150210]., and to
carry an identifier for each MediaStreamTrack in its "appdata" field.

So it seems that this cannot be done until custom msid is supported by browsers.