[R-C] LEDBAT vs RTCWeb

[Jim Gettys]

On 04/11/2012 02:16 AM, Harald Alvestrand wrote:
> On 04/10/2012 09:14 PM, Jim Gettys 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).
> the encouraging thing in those (depressing) charts is that the fiber
> stuff (green subcloud) seems to be less broken than the DSL. So the
> future may actually be less depressing than the past.
Get out your anti-depressants.  The ICSI data *understates* the severity
of the problem.

The ICSI data tops out at 20Mbps due to a limitation in their server
systems, so we don't really know how good/bad fiber is (since most fiber
tiers of service start around 20Mbps and so won't show up where it
should on that plot).

Secondly, the home router situation is even worse than broadband.  As
soon as the bandwidth is higher in the broadband hop than the wireless
hop (and 802.11g tops out at about 20-22Mbps), the bottleneck shifts to
the wireless hop, and you have the problem on either side of the
wireless hop (our OS's and home routers).  This is why I spend my time
on home routers and Linux.  Home routers/our operating systems have yet
more bloat than broadband, typically.

We have a disaster on our hands.

Sorry to be the bearer of such horrifying news.
                            - Jim