Discussion of explicit congestion notification's impact on the Internet
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From: Neal Cardwell <ncardwell@google.com>
To: Dave Taht <dave.taht@gmail.com>
Cc: ECN-Sane <ecn-sane@lists.bufferbloat.net>,
	 BBR Development <bbr-dev@googlegroups.com>,
	iccrg IRTF list <iccrg@irtf.org>
Subject: Re: [Ecn-sane] [bbr-dev] cwnd 4 in BBRv2?
Date: Fri, 29 Mar 2019 01:03:05 +0100	[thread overview]
Message-ID: <CADVnQy=M8ckdTo9ZxErW=DSN7h-mcF22k-YJFeEP=zGjT9ixag@mail.gmail.com> (raw)
In-Reply-To: <CAA93jw7ME=9E=6Vb++7F1BoEPC_qSsvgNVAzfr8HCOFLS=zQrg@mail.gmail.com>

On Thu, Mar 28, 2019 at 6:10 PM Dave Taht <dave.taht@gmail.com> wrote:
>
> I just finished watching the sessions in ietf iccrg...
>
> and although I have many questions about BBRv2, I am curious if the
> cwnd in it is capped to a minimum of 4, not 2, still? BBRv1 is...
>
> https://github.com/torvalds/linux/blob/master/net/ipv4/tcp_bbr.c#L385

So far that part of the algorithm has not changed in BBRv2. So, except
for certain corner cases in loss recovery (e.g. cwnd=1 on RTO), the
min cwnd is generally 4.

But keep in mind that BBR generally tries to be pacing-limited, rather
than cwnd-limited. And below a cwnd of 4 delayed ACKs can inhibit
pipelining, causing stop-and-wait-type problems that significantly
hurt throughput.

best,
neal

      reply	other threads:[~2019-03-29  0:03 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-28 17:09 [Ecn-sane] " Dave Taht
2019-03-29  0:03 ` Neal Cardwell [this message]

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