Re: [MMUSIC] Sending a=rtcp-mux-only w/o a=rtcp-mux

Eric Rescorla <ekr@rtfm.com> Fri, 03 February 2017 22:08 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 3 Feb 2017 14:07:21 -0800
Message-ID: <CABcZeBOktubY-dF11Cnn7cmMbaSYQrKuSs-GkhhDiTf8iyneGQ@mail.gmail.com>
To: Roman Shpount <roman@telurix.com>
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Subject: Re: [MMUSIC] Sending a=rtcp-mux-only w/o a=rtcp-mux
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On Fri, Feb 3, 2017 at 10:05 AM, Roman Shpount <roman@telurix.com> wrote:

> In 3, offerer sends rtcp-mux and rtcp-mux-only together. If it gets an
> answer without either rtcp-mux or rtcp-mux-only, offerer must tear down the
> connection.
>
> In 4, offerer sends rtcp-mux-only. If it gets an answer without
> rtcp-mux-only, offer must tear down the connection.
>
> In general 3 is redundant and was intended to be used during transition
> while rtcp-mux-only is implemented.
>

You can only do 4 once there is no chance that there will ever be a peer
which would do mux but will not recognize rtcp-mux-only. That seems
unlikely to ever occur.

-Ekr


> Regards,
>
> _____________
> Roman Shpount
>
> On Fri, Feb 3, 2017 at 12:41 PM, Eric Rescorla <ekr@rtfm.com> wrote:
>
>> Following up to myself, I don't think it's sensible for answers to
>> contain a=rtcp-mux-only, because either you accepted mux, in which case all
>> is good, or you rejected it, in which case it was rejected.
>>
>> -Ekr
>>
>>
>> On Fri, Feb 3, 2017 at 9:38 AM, Eric Rescorla <ekr@rtfm.com> wrote:
>>
>>> I have been reading the mux-exclusive document and I'm not sure it says
>>> quite what we want. Specifically, S 4.2 says:
>>>
>>>    When an offerer sends the initial offer, if the offerer wants to
>>>    indicate exclusive RTP/RTCP multiplexing for RTP-based media, the
>>>    offerer MUST associate an SDP 'rtcp-mux-only' attribute with the
>>>    associated SDP media description ("m=" line).
>>>
>>>    In addition, if the offerer associates an SDP 'rtcp-mux-only'
>>>    attribute with an SDP media description ("m=" line), the offerer MAY
>>>    also associate an SDP 'rtcp-mux' attribute with the same SDP media
>>>    description ("m=" line), following the procedures in [RFC5761].
>>>
>>> As I understand this text, the offerer may say the following things:
>>>
>>>  1. No a=rtcp-mux: No muxing.
>>>  2. a=rtcp-mux: I am offering RTCP mux
>>>  3. a=rtcp-mux-only + a=rtcp-mux: I will only do RTCP mux
>>>  4. a=rtcp-mux-only: I will only do RTCP mux (same as #3).
>>>
>>> I don't think the last of these is sensible. No current implementation
>>> will know what to do with a=rtcp-mux-only w/o a=rtcp-mux, so this will
>>> result in interop failures. Thus the MAY in the second graf needs to be
>>> a MUST.
>>>
>>> -Ekr
>>>
>>>
>>
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>