TCP receiving window size higher than net.core.rmem_max

Question

I am running iperf measurements between two servers, connected through 10Gbit link. I am trying to correlate the maximum window size that I observe with the sys

Answer 1

net.ipv4.tcp_rmem takes precedence net.core.rmem_max according to https://serverfault.com/questions/734920/difference-between-net-core-rmem-max-and-net-ipv4-tcp-rmem:

It seems that the tcp-setting will take precendence over the common max setting

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But I agree with what you say, this seems to conflict with what's written in man tcp, and I can reproduce your findings. Maybe the documentation is wrong? Please find out and comment!

Answer 2

A quick search turned up:

https://github.com/torvalds/linux/blob/4e5448a31d73d0e944b7adb9049438a09bc332cb/net/ipv4/tcp_output.c

in void tcp_select_initial_window()

if (wscale_ok) {
    /* Set window scaling on max possible window
     * See RFC1323 for an explanation of the limit to 14
     */
    space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
    space = min_t(u32, space, *window_clamp);
    while (space > 65535 && (*rcv_wscale) < 14) {
        space >>= 1;
        (*rcv_wscale)++;
    }
}

max_t takes the higher value of the arguments. So the bigger value takes precedence here.

One other reference to sysctl_rmem_max is made where it is used to limit the argument to SO_RCVBUF (in net/core/sock.c).

All other tcp code refers to sysctl_tcp_rmem only.

So without looking deeper into the code you can conclude that a bigger net.ipv4.tcp_rmem will override net.core.rmem_max in all cases except when setting SO_RCVBUF (whose check can be bypassed using SO_RCVBUFFORCE)