[Cerowrt-devel] Abandoning Window-based CC Considered Harmful (was Re: [Bloat] Bechtolschiem)

Bless, Roland (TM) roland.bless at kit.edu
Thu Jul 8 10:05:09 EDT 2021

Hi Matt,

On 08.07.21 at 15:29 Matt Mathis wrote:
> I think there is something missing from your model.    I just scanned 
> your paper and noticed that you made no mention of rounding errors, 
> nor some details around the drain phase timing,   The 
> implementation guarantees that the actual average rate across the 
> combined BW probe and drain is strictly less than the measured maxBW 
> and that the flight size comes back down to minRTT*maxBW before 
> returning to unity pacing gain.  In some sense these checks are 
> redundant, but If you don't do them, it is absolutely true that you 
> are at risk of seeing divergent behaviors.
Sure, most models abstract things away and so does our model leave out
some details, but it describes quite accurately what happens if multiple
BBRv1 flows are present. So the model was not only confirmed by our
own measurements, but also by many others who did BBRv1 experiments.
> That said, it is also true that multi-stream BBR behavior is quite 
> complicated and needs more queue space than single stream.   This
Yes, mostly between 1bdp and 1.5bdp of queue space.
> complicates the story around the traditional workaround of using 
> multiple streams to compensate for Reno & CUBIC lameness at larger 
> scales (ordinary scales today). Multi-stream does not help BBR 
> throughput and raises the queue occupancy, to the detriment of other 
> users.
> And yes, in my presentation, I described the core BBR algorithms as a 
> framework, which might be extended to incorporate many additional 
> algorithms if they provide optimal control in some settings.  And yes, 
> several are present in BBRv2.

Ok, thanks for clarification.


> 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 Thu, Jul 8, 2021 at 4:24 AM Bless, Roland (TM) 
> <roland.bless at kit.edu <mailto:roland.bless at kit.edu>> wrote:
>     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
>     <https://ieeexplore.ieee.org/document/8117540>
>>     On Wed, Jul 7, 2021 at 3:19 PM Bless, Roland (TM)
>>     <roland.bless at kit.edu <mailto:roland.bless at 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
>>>         <stephen at networkplumber.org
>>>         <mailto:stephen at networkplumber.org>> wrote:
>>>             On Fri, 2 Jul 2021 09:42:24 -0700
>>>             Dave Taht <dave.taht at gmail.com
>>>             <mailto:dave.taht at 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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