[Galene] Re: using up more ports in ipv6 for better congestion control

[T H Panton <tim@pi.pe>]

> On 10 Jul 2021, at 17:15, Dave Taht <dave.taht@gmail.com> wrote:
> 
> tim panton wrote me just now on linked in:
> 
> "It is still possible. Just set bundle-policy to max-compat and you'll
> get one stream for audio and one for video. Turn off rtcp-mux and
> you'll get 4 ports (voice, video, voice-RTCP, video-RTCP) - But your
> ability to connect will drop significantly (according to Google's
> data) and your connection setup time will increase. Even with port
> muxing congestion control is still possible, it just works
> _differently_ - which arguably it should, because realtime video has
> quite different needs from streaming or file transfer. Happy to have a
> chat about this...."
> 
> (so... chatting via email is preferred for me)
> 
> I am also under the impression that the congestion control
> notifications in rtcp are essentially obsolete in the rfc, mandating a
> 500ms interval instead of something sane, like a frame?

Oh, no, the congestion control is _very_ much alive in webRTC but under guise of 
bandwidth estimation - The ‘simplest' is google’s REMB RTCP message which basically
looks at the arrival time of packets and uses any _lengthening_ in the tof to deduce the onset of
additional buffering in the path. Transport CC tries to expand this to apply to all the streams muxed over a port
by adding an RTP header extension with an accurate (NTP) clock in it.

The thing that drives design this is that losing a packet is catastrophic for realtime video,  
one dropped packet makes seconds (aka megabytes) worth of data un-renderable.
At 50 fps the frame interval is shorter than the roundtrip time on the path (for VDSL users anyway - perhaps not fiber)
so a NAK/resend will fix it too late.

So the strategy is to dynamically estimate the capacity of the path and try to surf _just_ under that, ensuring minimal packet loss.

I realise this is anathema  to TCP folks, it certainly came as a shock to me….

T.

> 
> My other fantasy is to somehow start using udplite for more things.
> 
> The context for this of course is my never ending quest to have an IP
> based video and audio streaming system good enough to have a band
> playing with each other across town.
> 
> 
> -- 
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> 
> Dave Täht CTO, TekLibre, LLC