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Clarification regarding msid, label, mslabel and cname SDP attributes

From: Iñaki Baz Castillo <ibc@aliax.net>
Date: Mon, 15 Sep 2014 22:53:32 +0200
Message-ID: <CALiegfn18fE-iS_k=k_vw-JSRbVoUCXw5ZvG64Eu3W-S=f+w5w@mail.gmail.com>
To: "public-webrtc@w3.org" <public-webrtc@w3.org>

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

m=video [...]
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
Received on Monday, 15 September 2014 20:54:25 UTC

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