[R-C] LEDBAT vs RTCWeb

[Matt Mathis]

Jim,
For the record, my earlier "General thoughts" messages reflects my bet that
better AQM will not be sufficient by itself, because the best that you can
do with AQM will not in general be good enough for teleconferencing.   Two
reasons:
- The optimal AQM setpoint for throughput maximization will consume too
much of the RTCWEB end-to-end delay budget.
- AQM is designed to allow transient long queues for TCP slowstart, etc.

I do not disagree that AQM helps a lot.  Without AQM the situation
is abysmal.  But this is really a different problem than bufferbloat, and
good solutions to bufferbloat are not likely to automatically solve the
RTCWEB problem because RTCWEB's target delay is well below the target
delays for nearly all other applications.

The exception might be the gaming community.  Just last week somebody was
telling me that all serious gaming hackers roll QoS on their home LAN, and
that many of the large ISPs honor enough of the bits to make a difference.

Thanks,
--MM--
The best way to predict the future is to create it.  - Alan Kay



On Tue, Apr 10, 2012 at 12:14 PM, Jim Gettys <jg at freedesktop.org> wrote:

> On 04/10/2012 02:58 PM, Randell Jesup wrote:
> >
> > 100ms is just bad, bad, bad for VoIP on the same links.  The only case
> > where I'd say it's ok is where it knows it's competing with
> > significant TCP flows.  If it reverted to 0 queuing delay or close
> > when the channel is not saturated by TCP, then we might be ok (not
> > sure).  But I don't think it does that.
> >
> You aren't going to see delay under saturating load under 100ms unless
> the bottleneck link is running a working AQM; that's the property of
> tail drop, and the "rule of thumb" for sizing buffers has been of order
> 100ms.  This is to ensure maximum bandwidth over continental paths of a
> single TCP flow.
>
> Unfortunately, the bloat in the broadband edge is often/usually much,
> much higher than this, being best measured in seconds :-(.
> http://gettys.files.wordpress.com/2010/12/uplink_buffer_all.png
> http://gettys.files.wordpress.com/2010/12/downlink_buffer_all.png
> (thanks to the Netalyzr folks).
>
> Worse yet, the broadband edge is typically a single queue today (even in
> technologies that may support multiple classifications.  So your VOIP
> and other traffic is likely stuck behind other traffic.  ISP's telephony
> services are typically bypassing these queues.
>
> If there is AQM, then you'll get packet marking going on (drop or ECN),
> and decent latencies.
>
> There is hope here for AQM algorithms that are self tuning: I now know
> of two of such beasts, though they are a long way from "running code"
> state at the moment.
>
> So the direction I'm going to to get AQM that works..... (along with
> classification...).  But the high order bit is AQM, to keep the end
> point's TCP's behaving, which you can't do solely by classification.
>                                 - Jim
>
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