<div dir="ltr">Actually BBR does have a window based backup, which normally only comes into play during load spikes and at very short RTTs. It defaults to 2*minRTT*maxBW, which is twice the steady state window in it's normal paced mode.<div><br></div><div>This is too large for short queue routers in the Internet core, but it helps a lot with cross traffic on large queue edge routers.<br><div><div><br></div><div>Thanks,<div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div dir="ltr"><div dir="ltr">--MM--<br>The best way to predict the future is to create it. - Alan Kay<br><br>We must not tolerate intolerance;</div><div dir="ltr"> however our response must be carefully measured: </div><div> too strong would be hypocritical and risks spiraling out of control;</div><div> too weak risks being mistaken for tacit approval.</div></div></div></div></div><br></div></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Jul 7, 2021 at 3:19 PM Bless, Roland (TM) <<a href="mailto:roland.bless@kit.edu">roland.bless@kit.edu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<div>Hi Matt,<br>
<br>
[sorry for the late reply, overlooked this one]</div>
<div><br>
</div>
<div>please, see comments inline.<br>
</div>
<div><br>
</div>
<div>On 02.07.21 at 21:46 Matt Mathis via
Bloat wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">The argument is absolutely correct for Reno, CUBIC
and all other self-clocked protocols. One of the core
assumptions in Jacobson88, was that the clock for the entire
system comes from packets draining through the bottleneck
queue. In this world, the clock is intrinsically brittle if the
buffers are too small. The drain time needs to be a substantial
fraction of the RTT.</div>
</blockquote>
I'd like to separate the functions here a bit:<br>
<p>1) "automatic pacing" by ACK clocking</p>
<p>2) congestion-window-based operation</p>
<p>I agree that the automatic pacing generated by the ACK clock
(function 1) is increasingly <br>
distorted these days and may consequently cause micro bursts.<br>
This can be mitigated by using paced sending, which I consider
very useful. <br>
However, I consider abandoning the (congestion) window-based
approaches <br>
with ACK feedback (function 2) as harmful:<br>
a congestion window has an automatic self-stabilizing property
since the ACK feedback reflects<br>
also the queuing delay and the congestion window limits the amount
of inflight data.<br>
In contrast, rate-based senders risk instability: two senders in
an M/D/1 setting, each sender sending with 50%<br>
bottleneck rate in average, both using paced sending at 120% of
the average rate, suffice to cause<br>
instability (queue grows unlimited).<br>
<br>
IMHO, two approaches seem to be useful:<br>
a) congestion-window-based operation with paced sending<br>
b) rate-based/paced sending with limiting the amount of inflight
data<br>
</p>
<blockquote type="cite">
<div dir="ltr">
<div><br>
</div>
<div>However, we have reached the point where we need to discard
that requirement. One of the side points of BBR is that in
many environments it is cheaper to burn serving CPU to pace
into short queue networks than it is to "right size" the
network queues.</div>
<div><br>
</div>
<div>The fundamental problem with the old way is that in some
contexts the buffer memory has to beat Moore's law, because to
maintain constant drain time the memory size and BW both have
to scale with the link (laser) BW.</div>
<div><br>
</div>
<div>See the slides I gave at the Stanford Buffer Sizing
workshop december 2019: <a href="https://docs.google.com/presentation/d/1VyBlYQJqWvPuGnQpxW4S46asHMmiA-OeMbewxo_r3Cc/edit#slide=id.g791555f04c_0_5" target="_blank">Buffer Sizing: Position Paper</a> </div>
<div><br>
</div>
</div>
</blockquote>
<p>Thanks for the pointer. I don't quite get the point that the
buffer must have a certain size to keep the ACK clock stable:<br>
in case of an non application-limited sender, a very small buffer
suffices to let the ACK clock <br>
run steady. The large buffers were mainly required for loss-based
CCs to let the standing queue <br>
build up that keeps the bottleneck busy during CWnd reduction
after packet loss, thereby <br>
keeping the (bottleneck link) utilization high.<br>
</p>
<p>Regards,</p>
<p> Roland<br>
</p>
<p><br>
</p>
<blockquote type="cite">
<div dir="ltr">
<div>Note that we are talking about DC and Internet core. At
the edge, BW is low enough where memory is relatively cheap.
In some sense BB came about because memory is too cheap in
these environments.</div>
<div><br>
</div>
<div>
<div>
<div dir="ltr">
<div dir="ltr">
<div>
<div dir="ltr">
<div>
<div dir="ltr">
<div>Thanks,</div>
--MM--<br>
The best way to predict the future is to create
it. - Alan Kay<br>
<br>
We must not tolerate intolerance;</div>
<div dir="ltr"> however our response must be
carefully measured: </div>
<div> too strong would be hypocritical
and risks spiraling out of control;</div>
<div> too weak risks being mistaken for
tacit approval.</div>
</div>
</div>
</div>
</div>
</div>
</div>
<br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, Jul 2, 2021 at 9:59 AM
Stephen Hemminger <<a href="mailto:stephen@networkplumber.org" target="_blank">stephen@networkplumber.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On
Fri, 2 Jul 2021 09:42:24 -0700<br>
Dave Taht <<a href="mailto:dave.taht@gmail.com" target="_blank">dave.taht@gmail.com</a>>
wrote:<br>
<br>
> "Debunking Bechtolsheim credibly would get a lot of
attention to the<br>
> bufferbloat cause, I suspect." - dpreed<br>
> <br>
> "Why Big Data Needs Big Buffer Switches" -<br>
> <a href="http://www.arista.com/assets/data/pdf/Whitepapers/BigDataBigBuffers-WP.pdf" rel="noreferrer" target="_blank">http://www.arista.com/assets/data/pdf/Whitepapers/BigDataBigBuffers-WP.pdf</a><br>
> <br>
<br>
Also, a lot depends on the TCP congestion control algorithm
being used.<br>
They are using NewReno which only researchers use in real
life.<br>
<br>
Even TCP Cubic has gone through several revisions. In my
experience, the<br>
NS-2 models don't correlate well to real world behavior.<br>
<br>
In real world tests, TCP Cubic will consume any buffer it sees
at a<br>
congested link. Maybe that is what they mean by capture
effect.<br>
<br>
There is also a weird oscillation effect with multiple
streams, where one<br>
flow will take the buffer, then see a packet loss and back
off, the<br>
other flow will take over the buffer until it sees loss.<br>
<br>
_______________________________________________</blockquote>
</div>
<pre>_______________________________________________
</pre>
</blockquote>
<br>
</div>
</blockquote></div>