An Acronym for Explicit Congestion Notification.

From the linux configuration help -

Explicit Congestion Notification (ECN) allows routers to notify clients about network congestion, resulting in fewer dropped packets and increased network performance. This option adds ECN support to the Linux kernel, as well as a sysctl (/proc/sys/net/ipv4/tcp_ecn) which allows ECN support to be disabled at runtime.

Note that, on the Internet, there are many broken firewalls which refuse connections from ECN-enabled machines, and it may be a while before these firewalls are fixed. Until then, to access a site behind such a firewall (some of which are major sites, at the time of this writing) you will have to disable this option, either by saying N now or by using the sysctl.

Apparently this is caused by Cisco PIX? routers dropping IP packets with ECN set in them.

If you encounter this problem, you can disable ECN at run time

1. echo 0 > /proc/sys/net/ipv4/tcp_ecn