Re: [MMUSIC] BUNDLE and separate RTP and RTCP ports even when rtcp-mux is used

Stefan Hakansson LK <stefan.lk.hakansson@ericsson.com> Tue, 10 July 2012 12:20 UTC

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Date: Tue, 10 Jul 2012 14:21:02 +0200
From: Stefan Hakansson LK <stefan.lk.hakansson@ericsson.com>
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Subject: Re: [MMUSIC] BUNDLE and separate RTP and RTCP ports even when rtcp-mux is used
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On 07/10/2012 02:18 PM, Stefan Håkansson LK wrote:
> On 2012-07-06 09:00, Magnus Westerlund wrote:
>   > On 2012-07-05 21:14, Muthu Arul Mozhi Perumal (mperumal) wrote:
>   >> Hi Christer,
>   >>
>   >> I think the question boils down to whether the fallback port for RTCP
>   >> described in section 5.1.3 of RFC5761 could be the same as the RTP
>   >> port. This looks fine from a pragmatic point of view. However, the
>   >> following statements from RFC5761 seem to forbid it:
>   >>
>   >> On receipt of the answer, the offerer looks for the presence of the
>   >> "a=rtcp-mux" line for each media where multiplexing was offered.  If
>   >> this is present, then connectivity checks proceed as if only a
>   >> single candidate (for RTP) were offered, and multiplexing is used
>   >> once the session is established.  If the "a=rtcp-mux" line is not
>   >> present, the session proceeds with connectivity checks using both RTP
>   >> and RTCP candidates, eventually leading to a session being
>   >> established with RTP and RTCP on separate ports (as signalled by the
>   >> "a=rtcp:" attribute).
>   >>
>   >> Not sure if the statement "eventually leading to a session being
>   >> established with RTP and RTCP on separate ports" was intentional. If
>   >> not, it needs to be fixed.
>   >>
>   >
>   > As co-author of RFC5761 I think that sentence reflects how we perceived
>   > it would be used. In other words we only thought of the current legacy,
>   > not what could happen when someone made additional proposals that
>   > interacts with the RTP and RTCP port multiplexing. And based on that you
>   > would fall back to having different ports.
>   >
>   > However, I think I have found an issue that prevents one from using the
>   > same port for RTP and RTCP on one side. What I can figure out there
>   > exist nothing in a STUN binding request that are different between a
>   > binding request targeted to the ICE candidate for component ID=0 and
>   > component ID=1. If those two ICE candidates are co-located on the same
>   > port then you don't know which component the binding request is targeted
>   > for. It appears that the only thing keeping those separate is that the
>   > different components would not be present at the same base address+port.
>
> I forgot one thing when writing this. When the offerer gets the SDP
> answer and the candidates it contains it will know the source
> address+port for any candidate that matches the ones the answerer
> provided. Thus most of the answers binding requests will be correctly
> determined and bound to the right component. Unfortunately there exist
> peer-reflexive candidates and those can't be correctly bound at this stage.

So, now I am confused. Are we OK as is, or should the Bundle draft be 
updated, or something else? (To me it sound like the Bundle draft should 
be updated.)

Stefan

>
> Cheers
>
> Magnus Westerlund
>
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