Original text:
If present, "a=msid" attributes MUST be parsed as specified in I- D.ietf-mmusic-msid, Section 3.2, and their values stored, ignoring <del> any "appdata" field. </del>  Any "a=imageattr" attributes MUST be parsed as specified in RFC6236, Section 3, and their values stored.

Reviewer comments:
Fixed
also tagging NAME (who might be unhappy with 'default' which matches Chrome behaviour but not Firefox behaviour (it generates a uuid)
ping NAME NAME
NAME NAME PTAL - Barring any objections this one ought to get merged.
Folks, I think this one is pretty uncontroversial - planning to land this next week if there are no objections.
Take this simplified SDP offer from a remote endpoint (in parts taken from : A single audio and video stream. No a=mid (there is quite some text taking care of that) but no a=msid line either. URL shows a live example. Both Chrome (without unified-plan) and Firefox will fire the ontrack event (or legacy onaddstream) with a single mediastream containing an audio and video track. This has been like that since... 2012-ish? Unified plan support in Chrome changes this behaviour: URL ontrack is fired with an empty set of media streams. onaddstream is not fired. This is quite a change in behaviour. And unexpected, I didn't expect any issues for this trivial case of a single audio/video track. As NAME webrtc-pc assumes/requires a=msid to be present and . In particular this says which means firing with no streams is technically correct. JSEP seems to assume it is present as well, but has this text about a=msid in Note the "If present". What if it is not present? The closest I could find in terms of guidance was this from the and its section about handling a=msid is supposed to be present in initial offer so we are talking about dealing with non-webrtc entities send an offer. Given that there is plenty of support for entities that do not support a=mid (an RFC published in 2010) in JSEP do we need backward compability for the case of a single audio/video track each but no msid? Interop between browsers is probably not affected, due to Chrome still interpreting and sending a=ssrc:123 msid:...
note that a proper webrtc client will include "no streams" as (maybe with a track) which allows differentiating between "no streams because I do not use any" and "msid???" Thanks NAME for another
aside: the interesting case you have trouble distinguishing between is "no msid because I haven't started sending a track yet" and "no msid because I don't support msid". It's one case where we still have trouble if RTP packets arrive ahead of signalling. But - yes, I think we are better off sticking with the 2012 behavior, default stream and all. We can handle the case above by changing the stream associations of the receiver if a new SDP (with msid) arrives after the RTP packets.
there used to be a a=msid-semantic line that would allow determining whether a party wants its offer to be treated assuming msid support. Browsers still send it :-)
How are non-signaled tracks supposed to be handled anyway? There's JSEP language about creating a track for it, but what about webrtc-pc? (If a track exists in the forest, and nobody is around to surface it to JavaScript, does it still exist?) "Run the following steps for each media description in description" means no ontrack will fire until we do have something signaled. If the track (and/or stream) only exists in some inaccessible way they're almost conceptual until the signaling does arrive. And when it does arrive, we know whether or not MediaStreams were signaled. So we should know what to do, same things as if we didn't receive early RTP packets? Does the distinction matter? I love that we can tell the difference between "no streams because I do not use any" and "no msid signaled" whether or not "a=msid:-" was present. I had not thought about that! Unless I'm confused about something, I suggest: If is present, do what Chrome does: don't create any stream. If the line is completely missing, do what Firefox (or Chrome Plan B) does: create a MediaStream with a random id. Does that work for people?
By the way I thought this was a webrtc-pc issue when I replied above, so I was thinking about what to do in JavaScript rather than talking about any JSEP change.
I suppose we need some JSEP language to handle the lack of any a=msid but i agree with what you propose above. We do have have a=msid:- and we need to handle that in webrtc-pc too. Will file an issue there.
All webrtc-pc says is "let msids be a list of the MSIDs that the media description indicates transceiver.[[Receiver]].[[ReceiverTrack]] is to be associated with. Otherwise, let msids be an empty list" And says "The special value "-" indicates "no MediaStream"." So that already takes care of the case correctly, I claim. I agree. We need to solve what the media description (JSEP) says about this. Right now : "If present, "a=msid" attributes MUST be parsed as specified in [I-D.ietf-mmusic-msid], Section 3.2, and their values stored." I think it needs to say what happens when it is not present. Something like: "If no a=msid is present, then in place of "msid-id", create a single "id" unique to this end-point, if one hasn't already been created for this connection."
"if one hasn't already been created for this connection"; so future tracks will share the same stream? There is always at most a single "default stream"?
NAME says he observed this is how Firefox and Chrome work. URL
See URL
LGTMreason I asked is that chrome generates a MediaStream with id "default" -- see URL while Firefox does not. (I do not see any reason to change the legacy behaviour of chrome even though it seems a bit odd that the RNG always emits the right bytes for "default")

Revised text:
If present, "a=msid" attributes MUST be parsed as specified in I- D.ietf-mmusic-msid, Section 3.2, and their values stored, ignoring  <ins> any "appdata" field.  If no "a=msid" attributes are present, a random msid-id value is generated for a "default" MediaStream for the session, if not already present, and this value is stored. </ins> Any "a=imageattr" attributes MUST be parsed as specified in RFC6236, Section 3, and their values stored.