Re: Draft new version: draft-ietf-mmusic-mux-exclusive-11

On Fri, Feb 17, 2017 at 12:04 PM, Paul Kyzivat <[email protected]>
wrote:

> On 2/17/17 3:46 AM, Christer Holmberg wrote:
>
>> Hi,
>>
>> Based on the comments from Ekr, I've submitted a new version of
>> draft-mux-exclusive.
>>
>> The SDP 'rtcp-mux-only' attribute is now only allowed in SDP offers.
>>
>
> With the new text the offerer never knows if the answerer supports
> rtcp-mux-only. He only knows that the answerer supports rtcp-mux.
>
> I don't think this matters for that O/A, but it might matter for
> subsequent O/As.
>
> At the least, I think section 4.5 is now confusing. For m-lines previously
> negotiated with rtcp-mux-only, must the side that is sending a new offer
> include the attribute again, while the side that is answering must *not*
> include it? (Note the implications when the new offer is in the opposite
> direction from the original one.) I think this will be somewhat of a pain -
> perhaps more pain than simply always including it in all offers and answers.
>

Based comments from Ekr, the offerer never cares if the answerer supports
rtcp-mux-only. It only cares that answerer supports rtcp-mux. The whole
purpose of rtcp-mux-only attribute is to promise to all the parties on the
signaling path, such as SBC, that offerer will not accept an answer without
rtcp-mux. Based on this promise SBC does not need to allocate an RTCP port
for the duration of offer transaction. There is no difference here between
the initial offer and offers in session updates -- in either case without
rtcp-mux-only attribute SBC would need to allocate an extra port during the
session update transaction. So, it does not matter what was negotiated
before in the session, every offer in every direction must include
rctp-mux-only if offerer is not willing to accept the answer without
rtcp-mux, and should not include rtcp-mux-only if offerer allocated the
RTCP port and is willing to accept an answer without mux. It makes no sense
to include rtcp-mux-only in the answer, since if it includes rtcp-mux
attribute, it is enough of the indication that RTP and RTCP are muxed and
no extra port is needed.

Regards,
_____________
Roman Shpount

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