ECN is an extension to the Internet Protocol which allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature, and is only used when both endpoints signal that they want to use it.
Traditionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a bit in the IP header instead of dropping a packet in order to signal the beginning of congestion. The receiver of the packet echoes the congestion indication to the sender, which must react as though a packet drop were detected.
ECN uses two bits in the Differentiated Services field in the IP header, in the IPv4 TOS Byte or the IPv6 Traffic Class Octet. These two bits can be used to encode one of the values ECN-unaware transport, ECN-aware transport or congestion experienced.
Some outdated or buggy network equipment drops packets with the ECN bit set, rather than ignoring the bit.
Reference:
http://en.wikipedia.org/wiki/Explicit_congestion_notification
