From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-x42f.google.com (mail-wr1-x42f.google.com [IPv6:2a00:1450:4864:20::42f]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id 8C6053B29E for ; Wed, 7 Jul 2021 18:39:01 -0400 (EDT) Received: by mail-wr1-x42f.google.com with SMTP id d2so4966635wrn.0 for ; Wed, 07 Jul 2021 15:39:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=OAONE92Ma8cIQwvfvtdmLQvt6kzcZNtuQyoseo444Sg=; b=GaTP/G2+JOyX1DSLmpJF/3NcjNzBVTWFEc3gYiEjtiPd/hc9i43Nrh9ix0z/J78xM+ Q5GMUVywxUcnBEOGn3Ee6KSB+Z9X1NJV1c503HVHxOJfHRcdcZ2mtepmx3YqPeHiJK2/ LXr5fBm1gkx3mQrEza8Vm3q+UkVii+UOqzK0f/lHUadJH8b6gTki0lknxnw2ruJiPRYb zfOA3bBrxjMIPHX+1//0mDSnNLkECWqM88US+VYEAx+bGLKpgWEz6C99GeWzBVOinChw 8lR2aH7ZiyDONKzN4MzHEluQY/2I7z4zzY4eUWk3QVgwlLqIznZWaLLowMXnfkH7O58C LPAg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=OAONE92Ma8cIQwvfvtdmLQvt6kzcZNtuQyoseo444Sg=; b=JucTFsKROPqA598zvOVcjCK8ev4kt/GsL+RbdlJSUwJE75PoFHN9G5+vSbdbTFE4YI B/Aipylxe8xlsXw9E/jg/oPDCJuB7jEekrFpiCO8VNefwERtuk/O6NK9y1hyY+cbai9W /6nwmBzs/6FgkfMZhmowde/mHfh5kEdtK1tAOb1YRdqQyWmO+ajP6EHvsu5Rn/4dbRtt pL9zvuAa111HJF04DuJJj++J45+aivPMw6BAVRkrLG37eZt15+mUzlC8v4FedIKD+Ak1 dCfoIoNqCLxYS9xv8QVFXvMHiFgLsHq6O8sxiy5s1d90Ruh97YMlCIXBYhpDhug8KQzs VU2Q== X-Gm-Message-State: AOAM530QVCjYKTcd9zoUYbCD1aD8EbNicg2vNvZN0/nBpauZukAjEETl NUdnyZrV6z4/e9pubQUh9gKQRWhfG2CKClFJEhsmTw== X-Google-Smtp-Source: ABdhPJwdDy1GbdqWvW+bnVqGdCwiOdMvmaeSSH2EFjoYZfIvhO/nOA5twraJ6TjqnklPKrXpGn4iJZ4M4n10XOuw7uY= X-Received: by 2002:a5d:560c:: with SMTP id l12mr31155731wrv.310.1625697540314; Wed, 07 Jul 2021 15:39:00 -0700 (PDT) MIME-Version: 1.0 References: <55fdf513-9c54-bea9-1f53-fe2c5229d7ba@eggo.org> <871t4as1h9.fsf@toke.dk> <3D32F19B-5DEA-48AD-97E7-D043C4EAEC51@gmail.com> <1465267957.902610235@apps.rackspace.com> <20210702095924.0427b579@hermes.local> <1bab95a0-7904-2807-02fe-62674c19948f@kit.edu> In-Reply-To: <1bab95a0-7904-2807-02fe-62674c19948f@kit.edu> From: Matt Mathis Date: Wed, 7 Jul 2021 15:38:48 -0700 Message-ID: To: "Bless, Roland (TM)" Cc: Dave Taht , "cerowrt-devel@lists.bufferbloat.net" , bloat Content-Type: multipart/alternative; boundary="000000000000679dcd05c69034ce" Subject: Re: [Bloat] Abandoning Window-based CC Considered Harmful (was Re: Bechtolschiem) X-BeenThere: bloat@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: General list for discussing Bufferbloat List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 07 Jul 2021 22:39:01 -0000 --000000000000679dcd05c69034ce Content-Type: text/plain; charset="UTF-8" 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. 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. 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 Wed, Jul 7, 2021 at 3:19 PM Bless, Roland (TM) 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 > > > > 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@networkplumber.org> wrote: > >> On Fri, 2 Jul 2021 09:42:24 -0700 >> Dave Taht 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 >> > >> >> 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. >> >> _______________________________________________ > > _______________________________________________ > > > --000000000000679dcd05c69034ce Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Actually BBR does have a window based backup, which normal= ly only comes into play during load spikes and at very short RTTs.=C2=A0 = =C2=A0It defaults to 2*minRTT*maxBW, which is twice the steady state window= in it's normal paced mode.

This is too large for sh= ort queue routers in the Internet core, but it helps a lot with cross traff= ic on large queue edge routers.

Thanks,
=
--MM--
The best way t= o predict the future is to create it. =C2=A0- Alan Kay

We must not t= olerate intolerance;
=C2=A0 =C2=A0 =C2=A0 =C2=A0howev= er our response must be carefully measured:=C2=A0
=C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 too strong would be hypocritical and risks spir= aling out of control;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 t= oo weak risks being mistaken for tacit approval.


On Wed, Jul 7, 2021 at 3:19 PM Bless, Roland (TM) = <roland.bless@kit.edu> wr= ote:
=20 =20 =20
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:
=20
The argument is absolutely correct for Reno, CUBIC and all other=C2=A0self-clocked protocols.=C2=A0 One of the core assumptions in Jacobson88, was that the clock=C2=A0for the entire system comes from packets draining through the bottleneck queue.=C2=A0 In this world, the clock is intrinsically brittle if t= he buffers=C2=A0are too small.=C2=A0 The drain time needs to be a subs= tantial 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=C2=A0we need to disca= rd that requirement.=C2=A0 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" th= e network=C2=A0queues.

The fundamental problem with the old=C2=A0way is that in some contexts the buffer memory has to beat Moore's law, because t= o maintain constant drain time the memory=C2=A0size and BW both hav= e to scale with the link (laser) BW.

See the slides I gave at the=C2=A0Stanford Buffer Sizing workshop december 2019:=C2=A0Buffer Sizing: Position Paper=C2=A0<= /div>

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,

=C2=A0Roland


Note that we are talking about DC and Internet core.=C2=A0 At the edge, BW is low enough where memory is relatively cheap.=C2= =A0 =C2=A0In 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. =C2=A0- Alan Kay

We must not tolerate intolerance;
=C2=A0 =C2=A0 =C2=A0 =C2=A0however o= ur response must be carefully measured:=C2=A0
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 too st= rong would be hypocritical and risks spiraling out of control;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 too we= ak risks being mistaken for tacit approval.


On Fri, Jul 2, 2021 at 9:59 A= M Stephen Hemminger <stephen@networkplumber.org> wrote:
On Fri, 2 Jul 2021 09:42:24 -0700
Dave Taht <dave.taht@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
>

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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