file: /proc/sys/net/ipv4/tcp_ecn_fallback
variable: net.ipv4.tcp_ecn_fallback
Official reference

If the kernel detects that ECN connection misbehaves, enable fall back to non-ECN. Currently, this knob implements the fallback from RFC3168, section, but we reserve that in future, additional detection mechanisms could be implemented under this knob. The value is not used, if tcp_ecn or per route (or congestion control) ECN settings are disabled. Default: 1 (fallback enabled)