This will switch the default TCP congestion control algorithm from
new Reno to CUBIC. CUBIC is the default since Linux kernel 2.6.19
(see 597811ec167fa) and most (all?) distributions keep this default
(e.g. Debian and Ubuntu). On NixOS the default was still new Reno
because generate-config.pl changes TCP_CONG_CUBIC from y to m (since we
try to build everything as a module by default).
To check the active and available algorithms:
$ sysctl net.ipv4.tcp_congestion_control
net.ipv4.tcp_congestion_control = cubic
$ sysctl net.ipv4.tcp_available_congestion_control
net.ipv4.tcp_available_congestion_control = cubic reno
Note: E.g. x86_64_defconfig sets TCP_CONG_CUBIC=y indirectly via
CONFIG_TCP_CONG_ADVANCED=y (but CUBIC is also the default if set to no,
see net/ipv4/Kconfig).
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