Explicit Congestion Notification(ECN)

Linux

tcp_ecn - INTEGER
Control use of Explicit Congestion Notification (ECN) by TCP.
ECN is used only when both ends of the TCP connection indicate
support for it. This feature is useful in avoiding losses due
to congestion by allowing supporting routers to signal
congestion before having to drop packets.
Possible values are:
0 Disable ECN. Neither initiate nor accept ECN.
1 Enable ECN when requested by incoming connections and
also request ECN on outgoing connection attempts.
2 Enable ECN when requested by incoming connections
but do not request ECN on outgoing connections.
Default: 2

$ echo 1 > /proc/sys/net/ipv4/tcp_ecn

Mac OS X

$ sudo sysctl net.inet.tcp.ecn_negotiate_in
net.inet.tcp.ecn_negotiate_in: 0
$ sudo sysctl net.inet.tcp.ecn_negotiate_out
net.inet.tcp.ecn_initiate_out: 0