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Re: [rtcweb] Does RID require the same ext id for all the m= sections?

"Mo Zanaty (mzanaty)" <mzanaty@cisco.com> Mon, 11 September 2017 17:39 UTC

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From: "Mo Zanaty (mzanaty)" <mzanaty@cisco.com>
To: Iñaki Baz Castillo <ibc@aliax.net>
CC: "rtcweb@ietf.org" <rtcweb@ietf.org>
Thread-Topic: [rtcweb] Does RID require the same ext id for all the m= sections?
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Date: Mon, 11 Sep 2017 17:39:00 +0000
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Subject: Re: [rtcweb] Does RID require the same ext id for all the m= sections?
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I found a restriction against aliases in 5285bis:
https://tools.ietf.org/html/draft-ietf-avtcore-rfc5285-bis-14#section-7


"... If an RTP header extension, i.e. a particular extension
URI and configuration using <extensionattributes>, is offered in
multiple m-lines that are part of the same bundle group it MUST use
the same ID in all of these m-lines."

Remap ID to different URN is disallowed when bundled:
m=...
a=extmap:x urn:foo
m=...
a=extmap:x urn:bar

Alias ID for same URN is disallowed when bundled:
m=...
a=extmap:x urn:foo
m=...
a=extamp:y urn:foo



The remap restriction is clearly necessary. I think the alias restriction
is unnecessary, but I'm fine with letting it proceed to publication if it
simplifies things for RTP stack implementors. (It complicates things for
middle box implementors that want to avoid rewriting RTP packets, which
aliases can help avoid.)

Mo


-----Original Message-----
From: Iñaki Baz Castillo <ibc@aliax.net>
Date: Monday, September 11, 2017 at 11:52 AM
To: mzanaty <mzanaty@cisco.com>
Cc: "rtcweb@ietf.org" <rtcweb@ietf.org>
Subject: Re: [rtcweb] Does RID require the same ext id for all the m=
sections?

On Mon, Sep 11, 2017 at 5:02 PM, Mo Zanaty (mzanaty) <mzanaty@cisco.com>
wrote:
> Bundle defines MID, and section 13 restricts all extmap IDs (including
>MID,
> RID, etc.) to be consistent across all bundled m= sections, just like
>PTs. I
> see no restriction against aliases, nor any reason to impose one.

The RTP MID extension value is used (should be used when someone
implements it) to lookup the associated m=section, so the extmap ID
MUST be unique across all the m=sections in the SDP. Otherwise the
lookup operation would be impossible.

This has been discussed in this mailing list not so long ago.


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

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