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

Hi,

>> Taylor's explanation makes sense. But, in this case:
>> 
>> 1) Re-using the same value with a new mapping should be an explicit 
>> MUST NOT
>[Roni Even] This is specified in RFC5285 section 6 
>
>Identifiers values in the valid range MUST NOT  be altered (remapped).

Correct. Taylor indicated this earlier.

What if a previously negotiated mapping is not included at all in a subsequent offer? Is it considered an error? If not, does the previous mapping apply? Or, is the previous mapping removed?

Regards,

Christer

> From: Iñaki Baz Castillo [mailto:ibc@aliax.net]
> Sent: 07 August 2017 19:56
> To: Taylor Brandstetter <deadbeef@google.com>
> Cc: Christer Holmberg <christer.holmberg@ericsson.com>; 
> rtcweb@ietf.org
> Subject: Re: [rtcweb] Replacing a=extmap mapping in re-offer, is it legal?
> 
> On Mon, Aug 7, 2017 at 7:46 PM, Taylor Brandstetter 
> <deadbeef@google.com> wrote:
> > 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").
> 
> Thanks, your rationale clearly makes sense. However:
> 
> 1) I don't think Firefox cares about overriding a previous extmap ID 
> during a reoffer.
> 
> 2) Should we really accept so complex and error prune things just 
> because SDP allows everything?
> 
> 
> --
> Iñaki Baz Castillo
> <ibc@aliax.net>
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