netdev - Re: [Linux Kernel Network Bug Report] tcp_rmem[1] is not resulting in correct TCP window size since 3.4

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Date:   Wed, 10 Feb 2021 16:37:13 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Zhibin Liu <zhibinliu@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>,
        Tianpeng Jin <tianpengjin@...gle.com>,
        Chunlei Zhu <chunleiz@...gle.com>,
        Weiming Liu <weimingliu@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [Linux Kernel Network Bug Report] tcp_rmem[1] is not resulting in
 correct TCP window size since 3.4

On Wed, Feb 10, 2021 at 4:14 PM Zhibin Liu <zhibinliu@...gle.com> wrote:
>
> Hi, Linux Kernel Networking TCP maintainers,
>
> I sent this bug report mail following the reporting bug guideline.
>
> Here is the details of the bug:
> tcp_rmem[1] has value 87380, which will result in a window of 65535 when tcp_adv_win_scale=2. (87380 - 87380/2^2 = 65535)
>
> But since 3.4, commit b49960a05e32121d29316cfdf653894b88ac9190, tcp_adv_win_scale default value is changed from 2 to 1.
>
> The change causes tcp_rmem[1] with value 87380 will result in a window of 43690, instead of expected 65535.


win of 65535 was used in SYN and SYNACK way after commit
b49960a05e32121d (this commit is 8 years old : Wed May 2 02:28:41
2012)

Change was done by Yuchung in 2018
in commit a337531b942bd8a03e7052444d7e36972aac2d92 tcp: up initial
rmem to 128KB and SYN rwin to around 64KB



>
> Now the doc Documentation/networking/ip-sysctl.rst is still mentioning 87380 tcp_rmem[1] will result in 65535 window size with default tcp_adv_win_scale.
>
> tcp_rmem[1] should be changed from 87380 to 131070.


Do you have a packetdrill test demonstrating the issue, on recent
upstream kernels ?

I suspect you use an old kernel with missing backports.

>
> At same time, tcp_rmem[2] should be changed from "between 87380B and 6MB" to "between 131070B and 6MB".
>
> I've fired a bug in Kernel Bugzilla, too. The bug is assigned to Stephen Hemminger <stephen@...workplumber.org> instead of you. So I sent this mail.
>
> Thanks,
>
> Zhibin Liu | Software Engineer | zhibinliu@...gle.com | +86 21 6123 2345