# IP ECN

The IP Explicit Congestion Notification (ECN) field is part of the
[IP TOS](/glossary/i/ip_tos) IP header field.

It is used to signal that the network is, or is about to, experience [congestion](/glossary/c/congestion).

A device can use the ECN field to mark a [packet](/glossary/p/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.  |