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Re: [rtcweb] Replacing a=extmap mapping in re-offer, is it legal?

Christer Holmberg <christer.holmberg@ericsson.com> Mon, 07 August 2017 14:10 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Iñaki Baz Castillo <ibc@aliax.net>, "rtcweb@ietf.org" <rtcweb@ietf.org>
Thread-Topic: [rtcweb] Replacing a=extmap mapping in re-offer, is it legal?
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Date: Mon, 07 Aug 2017 14:10:24 +0000
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Subject: Re: [rtcweb] Replacing a=extmap mapping in re-offer, is it legal?
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Hi,

My assumption is that it by default is always allowed to re-negotiate SDP attribute values. And, if it's not, it should be explicitly indicated.

You said that Chrome keeps the previously negotiated value. Does Chrome still accept the new offer?

Regards,

Christer

-----Original Message-----
From: rtcweb [mailto:rtcweb-bounces@ietf.org] On Behalf Of Iñaki Baz Castillo
Sent: 07 August 2017 16:06
To: rtcweb@ietf.org
Subject: [rtcweb] Replacing a=extmap mapping in re-offer, is it legal?

Hi,

Let's assume that Alice sends a offer to Bob with a single m=video section that contains:

  a=extmap:11 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time

Bob replies with the same extmap.

Later Bob generates a re-offer and it looks like this:

  a=extmap:1 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
  a=extmap:2 urn:ietf:params:rtp-hdrext:toffset

Is this legal? Couldn't find nothing about renegotiations in https://tools.ietf.org/html/rfc5285, however it looks to me that the world would be better if the already negotiated extension mapping is kept.

Said that, Chrome does keep the previously negotiated ext mapping, but Firefox does not, behaving exactly as shown above.


Bug report in Firefox tracker:
https://bugzilla.mozilla.org/show_bug.cgi?id=1384064

JSFiddle reproducing it (open the devtool console):
https://jsfiddle.net/ibcaliax/uan2fove/


--
Iñaki Baz Castillo
<ibc@aliax.net>

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