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

Paul Kyzivat <> Fri, 01 November 2013 21:45 UTC

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

    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.