Clarification regarding msid, label, mslabel and cname SDP attributes

Hi,

Those are the per SSRC attributes Chrome currently adds (I set custom values):

m=video [...]
a=mid=videoX
a=ssrc:66621 cname:some_cname_value
a=ssrc:66621 msid:1234 video666
a=ssrc:66621 mslabel:1234
a=ssrc:66621 label:video666;


AFAIU the only standard SSRC related attributes defined for WebRTC are:

- msid: which includes the MediaStream.id and MediaStreamTrack.id, and
  it is defined in https://tools.ietf.org/html/draft-ietf-mmusic-msid.

- cname: which is the SDES for this SSRC.

By reading the archive I understand that mslabel (which seems to point
to the MediaStream.id) and label (which seems to point to the
MediaStreamTrack.id) are deprecated.

Am I right?


Said that, I also understand that msid values depend on the
application, which in the case of WebRTC requires the usage of a
global "a=msid-semantic: WMS 1234" in order to announce that "1234" is
the id of a JavaScript MediaStream.

And I also understand that the a=mid attribute is not intended for the
application but just for the SDP parser (so it can indicate stuff like
which tracks share the same transport / RTPsession by using Bundle).


Question: must two tracks within the same MediaStream (let's say audio
and video) share the same cname attribute?


Thanks a lot.


-- 
Iñaki Baz Castillo
<ibc@aliax.net>