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

Taylor Brandstetter <deadbeef@google.com> Mon, 07 August 2017 17:46 UTC

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From: Taylor Brandstetter <deadbeef@google.com>
Date: Mon, 07 Aug 2017 10:46:10 -0700
Message-ID: <CAK35n0Zph3cWjkmP3Usep6QZLaCxSqe2wof0FsAjrkcx9s5QUg@mail.gmail.com>
To: Iñaki Baz Castillo <ibc@aliax.net>
Cc: Christer Holmberg <christer.holmberg@ericsson.com>, "rtcweb@ietf.org" <rtcweb@ietf.org>
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Subject: Re: [rtcweb] Replacing a=extmap mapping in re-offer, is it legal?
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I don't know if this is stated explicitly anywhere, but my assumption was
that it works like payload types. You can introduce new IDs all you want,
but you can't have one ID refer to multiple things during the same session.
Otherwise, if you receive a packet with that extension ID, you wouldn't be
able to tell which extension it is.

Meaning, if Bob's initial answer was:

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

It would be illegal to have a re-offer of:

a=extmap:1 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
a=extmap:11 urn:ietf:params:rtp-hdrext:toffset

Because if Bob then receives a packet with extension ID 11, Bob doesn't
know if it's a packet sent before Alice received the answer (in which case
it's "abs-send-time"), or after (in which case it's "toffset").

On Mon, Aug 7, 2017 at 7:18 AM, Iñaki Baz Castillo <ibc@aliax.net> wrote:

> On Mon, Aug 7, 2017 at 4:10 PM, Christer Holmberg
> <christer.holmberg@ericsson.com> wrote:
> > 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.
>
> SDP O/A is really error prune. This must end. It cannot happen that,
> for every renegotiation (which may be it just wants to add/remove a
> track) everything regarding ICE, DTLS, codecs. extmap, etc etc must be
> re-inspected.
>
>
> > You said that Chrome keeps the previously negotiated value. Does Chrome
> still accept the new offer?
>
> AFAIR it does not complain, but it's hard to know whether Chrome keeps
> the previously negotiated ext mapping or respects the new one, since
> another "SDP O/A related bug" happens:
>
> https://bugzilla.mozilla.org/show_bug.cgi?id=1247725
>
>
> This is really crazy. I hope we can drop SDP from WebRTC soon. This is not
> sane.
>
> --
> Iñaki Baz Castillo
> <ibc@aliax.net>
>
> _______________________________________________
> rtcweb mailing list
> rtcweb@ietf.org
> https://www.ietf.org/mailman/listinfo/rtcweb
>

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