Re: [rtcweb] draft-ietf-rtcweb-jsep-05 - Subsequent Offers

Justin Uberti <juberti@google.com> Mon, 04 November 2013 06:55 UTC

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From: Justin Uberti <juberti@google.com>
Date: Sun, 3 Nov 2013 22:55:35 -0800
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To: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [rtcweb] draft-ietf-rtcweb-jsep-05 - Subsequent Offers
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On Fri, Nov 1, 2013 at 2:45 PM, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:

> Section 5.2.2 (Subsequent Offers) says:
>
>    o  The m= line and corresponding "a=rtpmap" and "a=fmtp" lines MUST
>       only include codecs present in the remote description.
>
>    o  The RTP header extensions MUST only include those that are present
>       in the remote description.
>
>    o  The RTCP feedback extensions MUST only include those that are
>       present in the remote description.
>
>    o  The "a=rtcp-mux" line MUST only be added if present in the remote
>       description.
>
>    o  The "a=rtcp-rsize" line MUST only be added if present in the
>       remote description.
>
> Including only codecs that were present in the prior answer is a bad idea.
> It doesn't play well with SIP. There is no harm in continuing to include
> all the codecs you support. And it keeps options open for changing codecs
> later.
>
> The same is true for most other things that are being restricted based on
> what was in the last answer. (But I won't say that with certainty without
> checking the semantics of each one.)
>
> For further info, see RFC 6337, especially section 5.1.
>

According to section 5.2.5, it seems that this is only the cases for offers
triggered by offerless reINVITEs. I don't think we want to be re-allocating
and negotiating codecs every time we want to make a change to the session
descriptions in use.

Generally I think we should consider that the capabilities of the remote
side will not change unexpectedly during the call, and so full
renegotiation is not needed with each reINVITE. If full renegotiation is
needed, use an INVITE-with-replaces, as Cullen has suggested. This avoids
complicated cases like having to unBUNDLE in mid-call, or re-reserve codecs
that are unlikely to be used.



>
>         Thanks,
>         Paul
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>