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 [off-list ref] 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 [off-list ref] instead of you. So I sent this mail.

Thanks,

Zhibin Liu | Software Engineer | zhibinliu@google.com | +86 21 6123 2345