Hi Matt,
On 08.07.21 at 00:38 Matt Mathis wrote:
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.
So yes, BBR follows option b), but I guess that you are referring to
BBRv1 here.
We have shown in [1, Sec.III] that BBRv1 flows will *always* run
(conceptually) toward their above quoted inflight-cap of
2*minRTT*maxBW, if more than one BBR flow is present at the bottleneck.
So strictly speaking " which *normally only* comes
into play during load spikes and at very short RTTs" isn't true for
multiple BBRv1 flows.
It seems that in BBRv2 there are many more mechanisms present
that try to control the amount of inflight data more tightly and the new
"cap"
is at 1.25 BDP.
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.
Best regards,
Roland
[1] https://ieeexplore.ieee.org/document/8117540
On Wed, Jul 7, 2021 at 3:19 PM Bless, Roland (TM)
<roland.bl...@kit.edu <mailto:roland.bl...@kit.edu>> wrote:
Hi Matt,
[sorry for the late reply, overlooked this one]
please, see comments inline.
On 02.07.21 at 21:46 Matt Mathis via Bloat wrote:
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.
I'd like to separate the functions here a bit:
1) "automatic pacing" by ACK clocking
2) congestion-window-based operation
I agree that the automatic pacing generated by the ACK clock
(function 1) is increasingly
distorted these days and may consequently cause micro bursts.
This can be mitigated by using paced sending, which I consider
very useful.
However, I consider abandoning the (congestion) window-based
approaches
with ACK feedback (function 2) as harmful:
a congestion window has an automatic self-stabilizing property
since the ACK feedback reflects
also the queuing delay and the congestion window limits the amount
of inflight data.
In contrast, rate-based senders risk instability: two senders in
an M/D/1 setting, each sender sending with 50%
bottleneck rate in average, both using paced sending at 120% of
the average rate, suffice to cause
instability (queue grows unlimited).
IMHO, two approaches seem to be useful:
a) congestion-window-based operation with paced sending
b) rate-based/paced sending with limiting the amount of inflight data
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.
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.
See the slides I gave at the Stanford Buffer Sizing workshop
december 2019: Buffer Sizing: Position Paper
<https://docs.google.com/presentation/d/1VyBlYQJqWvPuGnQpxW4S46asHMmiA-OeMbewxo_r3Cc/edit#slide=id.g791555f04c_0_5>
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:
in case of an non application-limited sender, a very small buffer
suffices to let the ACK clock
run steady. The large buffers were mainly required for loss-based
CCs to let the standing queue
build up that keeps the bottleneck busy during CWnd reduction
after packet loss, thereby
keeping the (bottleneck link) utilization high.
Regards,
Roland
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.
Thanks,
--MM--
The best way to predict the future is to create it. - Alan Kay
We must not tolerate intolerance;
however our response must be carefully measured:
too strong would be hypocritical and risks spiraling
out of control;
too weak risks being mistaken for tacit approval.
On Fri, Jul 2, 2021 at 9:59 AM Stephen Hemminger
<step...@networkplumber.org <mailto:step...@networkplumber.org>>
wrote:
On Fri, 2 Jul 2021 09:42:24 -0700
Dave Taht <dave.t...@gmail.com <mailto:dave.t...@gmail.com>>
wrote:
> "Debunking Bechtolsheim credibly would get a lot of
attention to the
> bufferbloat cause, I suspect." - dpreed
>
> "Why Big Data Needs Big Buffer Switches" -
>
http://www.arista.com/assets/data/pdf/Whitepapers/BigDataBigBuffers-WP.pdf
<http://www.arista.com/assets/data/pdf/Whitepapers/BigDataBigBuffers-WP.pdf>
>
Also, a lot depends on the TCP congestion control algorithm
being used.
They are using NewReno which only researchers use in real life.
Even TCP Cubic has gone through several revisions. In my
experience, the
NS-2 models don't correlate well to real world behavior.
In real world tests, TCP Cubic will consume any buffer it
sees at a
congested link. Maybe that is what they mean by capture effect.
There is also a weird oscillation effect with multiple
streams, where one
flow will take the buffer, then see a packet loss and back
off, the
other flow will take over the buffer until it sees loss.
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