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I was reading about how TCP congestion variants are implemented in Linux. Each variant is implemented as a separate module.

When I had read this question: Add TCP congestion control variant to Linux Ubuntu

I understood that the variant can be loaded using modprobe.

Does that mean when lsmod is running, TCP cubic must show as default-loaded? When I run lsmod, I cannot find the loaded module associated with congestion control. Are there specific types of kernel modules are listed under lsmod?

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There's the question to address and a little clarification for this specific case (congestion).

A kernel module is a part of the kernel that is optional and doesn't have to be present at kernel initial start nor later if not needed. It's a kind of plug-in. This allows to have most functionalities available on-demand, without using memory if never used. Most modules can be compiled as modules or built-in. If built-in, it implies you won't find a .ko module file because it's already in the initial kernel.

You can check what was done:

  • either by knowing the configuration option (here CONFIG_TCP_CONG_CUBIC), having access to the configuration used, usually as a file in /boot/config-$(uname -r), and verifying what option was used:

    grep CONFIG_TCP_CONG_CUBIC /boot/config-$(uname -r)
    
  • or by knowing the module name and checking if the module is in the list of built-in modules:

    fgrep tcp_cubic.ko /lib/modules/$(uname -r)/modules.builtin
    

Examples of likely results for your case since you didn't find the module, but with cubic the likely default it probably means it was built-in:

$ grep _CUBIC /boot/config-$(uname -r)
CONFIG_TCP_CONG_CUBIC=y
CONFIG_DEFAULT_CUBIC=y
$ fgrep tcp_cubic.ko /lib/modules/$(uname -r)/modules.builtin
kernel/net/ipv4/tcp_cubic.ko

So this answers the question: consider you have the module always loaded, because it's built-in. It makes sense to have the default be built-in (it's often required).

The list of built-in module files you might expect to find somewhere in /lib/modules/$(uname -r) and be displayed with lsmod but that you won't, can be displayed with cat /lib/modules/$(uname -r)/modules.builtin. They are not special, but were chosen so (by your Linux distribution) often but not always because a default had to be chosen among a list of choices, to be built-in, including all related module dependencies.

Now about the little clarification: of course a module has to be loaded (or built-in) to have its functionality available. But the Q/A you linked to didn't say you only had to load the tcp congestion module to have it activated. Quote:

To try one of these you need to install it using modprobe -a tcp_westwood or whatever you want. You can then test it using this

echo "westwood" > /proc/sys/net/ipv4/tcp_congestion_control

Since this pseudo-file is read/write, you can just query it to know what congestion is in use with probably this result:

$ cat /proc/sys/net/ipv4/tcp_congestion_control
cubic

Now to change the algorithm:

# lsmod | grep tcp_westwood
# echo westwood > /proc/sys/net/ipv4/tcp_congestion_control
# lsmod | grep tcp_westwood
tcp_westwood           16384  1

The module was auto-loaded and is now in use (some systems might not auto-load).

Anyway the adequate information for this specific case is in /proc/sys/net/ipv4/tcp_congestion_control, not lsmod's output.

  • Thanks for the clarification. It makes more sense now. Also, does that mean multiple tcp variants can me simultaneously loaded, but the one used will be based on the tcp_congestion_control file? – Gautam Ramakrishnan Sep 16 '18 at 13:15
  • Yes. Now I don't know if there are options to use multiple ones at once (probably doesn't make much sense), but I can tell it's a per socket use: I now temporarily have sockets using cubic and others using westwood (until they close). – A.B Sep 16 '18 at 13:18

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