netdev - [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 23:34:01 +0800
From:   Zhibin Liu <zhibinliu@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Cc:     Chunlei Zhu <chunleiz@...gle.com>,
        Tianpeng Jin <tianpengjin@...gle.com>,
        Weiming Liu <weimingliu@...gle.com>, stephen@...workplumber.org
Subject: [Linux Kernel Network Bug Report] tcp_rmem[1] is not resulting in
 correct TCP window size since 3.4

Hi, Linux Kernel Networking TCP maintainers,

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.

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.
At same time, tcp_rmem[2] should be changed from "between 87380B and
6MB" to "between 131070B and 6MB".

I've fired a bug (id 211677) 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