The IP Explicit Congestion Notification (ECN) field is part of the IP TOS IP header field.

It is used to signal that the network is, or is about to, experience congestion.

A device can use the ECN field to mark a packet instead of dropping it. The receiver of the packet repeats the ECN back to the sender, which can reduce the transmission rate.

ECN uses the two last bits of the TOS field encode four different codepoints:

Binary Value Description
00 Non-ECT Non ECN-Capable Transport.
10 ECT(0) ECN Capable Transport.
01 ECT(1) ECN Capable Transport.
11 CE Congestion Encountered.