From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <ncardwell@google.com>
Received: from mail-ot1-x32a.google.com (mail-ot1-x32a.google.com
 [IPv6:2607:f8b0:4864:20::32a])
 (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 CAEE73B2A4
 for <ecn-sane@lists.bufferbloat.net>; Thu, 28 Mar 2019 20:03:22 -0400 (EDT)
Received: by mail-ot1-x32a.google.com with SMTP id e5so399223otk.12
 for <ecn-sane@lists.bufferbloat.net>; Thu, 28 Mar 2019 17:03:22 -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=gUVzlttkv+fi2IzCgO7e78vlSNqPq24WKzSD1PxSYPo=;
 b=eIE8Pp8OTqChV78uhUi2S6mACFiR5n3p+UZ/a1+m1oV7NXE68oTfbFtgKem9On5GH/
 g5h7EUfHbzjiBEee+uevWqCMVS8CeNlvAyoMU/kGV0pxA4fcbfMza7W8cAT4nNa/W+/2
 4htQl2O0Vs0bf8Rr7/p3pYWMVS7S9oeBLrvsktL34O3Ysi6J2JrnsZQFvVc5AIaxycrt
 5YaASErvwIlZfPyYPerp6UagzzmXlZhNHo7bqVlpItbhfO+wo4ijuINlx7HB8dog+6mC
 kgt3hVuVjjccUHb7qYhsf9zCACkIRCcbCUAmwbBtHnZ+UmE0VV5vQZO7UJT+x0MU3hrB
 YVww==
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=gUVzlttkv+fi2IzCgO7e78vlSNqPq24WKzSD1PxSYPo=;
 b=Y5DLYRXciIBBfPPJrSNaxIeZ621NeXjnhUymsEcBNaNeBQlsEq+Up/iMjFPsGG+nlk
 MHVW4Wf3lWYqfx8m6335UhZUc3CCJ5rfYIDz6T5xAvSdTAYAgf8pEJ4U2rgclBueJ5Do
 TGotE7RDz//hUT1y296mkLl6mJocBwwjkRUIQ1QSJowYfV1kubtmWBOAljC1QcXEZ6Mt
 Zuw5w+UAHdZXKpAYMpKTi3A3XWU5fLSvnoN50wvfzX9LC0CoFl6eCfbh2zQzhapjM8U1
 1iVxV8U0Qax8VRoyCwZWQCxvY/mnEvGGcvrrR3rN9iDuLiCfwbUKnXTVYsso6BIXdDqa
 h8Qg==
X-Gm-Message-State: APjAAAWNDiTW6Y0+WzEd1yeN1DVTkwn1KU/PQSCIh99m+DKDirYY7eNA
 sGNnjGi4wBcSes+Ef1C9rTfoPNdw5RnbLFpoPJBtXA==
X-Google-Smtp-Source: APXvYqzqaCHbDtoHpfD56t8wjCimWFGyomONm5gF1gCaAtsKzyDJI9qYxQvfsD/sjGNGzo1nwVyaDANTWSuCyJIeLq0=
X-Received: by 2002:a05:6830:160e:: with SMTP id
 g14mr15603437otr.370.1553817802053; 
 Thu, 28 Mar 2019 17:03:22 -0700 (PDT)
MIME-Version: 1.0
References: <CAA93jw7ME=9E=6Vb++7F1BoEPC_qSsvgNVAzfr8HCOFLS=zQrg@mail.gmail.com>
In-Reply-To: <CAA93jw7ME=9E=6Vb++7F1BoEPC_qSsvgNVAzfr8HCOFLS=zQrg@mail.gmail.com>
From: Neal Cardwell <ncardwell@google.com>
Date: Fri, 29 Mar 2019 01:03:05 +0100
Message-ID: <CADVnQy=M8ckdTo9ZxErW=DSN7h-mcF22k-YJFeEP=zGjT9ixag@mail.gmail.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>
Content-Type: text/plain; charset="UTF-8"
X-Mailman-Approved-At: Tue, 02 Apr 2019 08:07:03 -0400
Subject: Re: [Ecn-sane] [bbr-dev] cwnd 4 in BBRv2?
X-BeenThere: ecn-sane@lists.bufferbloat.net
X-Mailman-Version: 2.1.20
Precedence: list
List-Id: Discussion of explicit congestion notification's impact on the
 Internet <ecn-sane.lists.bufferbloat.net>
List-Unsubscribe: <https://lists.bufferbloat.net/options/ecn-sane>,
 <mailto:ecn-sane-request@lists.bufferbloat.net?subject=unsubscribe>
List-Archive: <https://lists.bufferbloat.net/pipermail/ecn-sane>
List-Post: <mailto:ecn-sane@lists.bufferbloat.net>
List-Help: <mailto:ecn-sane-request@lists.bufferbloat.net?subject=help>
List-Subscribe: <https://lists.bufferbloat.net/listinfo/ecn-sane>,
 <mailto:ecn-sane-request@lists.bufferbloat.net?subject=subscribe>
X-List-Received-Date: Fri, 29 Mar 2019 00:03:22 -0000

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