我想使用 WebRTC 从 Peer1 向 Peer2 发送两个视频流：我们以前后摄像头为例。
从概念上讲，Peer2 如何知道哪个流来自哪个摄像头？
MediaStream 文档提到所有MediaStream和MediaStreamTrack字段都是只读的，所以我不能直接将任何信息附加到它们。我不能像{"stream1 id": "camera", "stream2 id": "screenshare"}通过信令通道那样发送字典，因为 Peer2 将为每个流和轨道生成自己的 ID。
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.
msid因此，在浏览器支持自定义之前，这似乎无法完成。