Consult the kernel documentation for details on this sysctl:
https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
+
+ * The v239 change to turn on "net.ipv4.tcp_ecn" by default has been
+ reverted.
* CPUAccounting=yes no longer enables the CPU controller when using
kernel 4.15+ and the unified cgroup hierarchy, as required accounting