Explicit Congestion Notification (ECN)

Weighted Random Early Detection (WRED) drops packets based on the average length exceeding a specific threshold value to indicate congestion. Explicit Congestion Notification (ECN) is an extension to WRED that marks the drop-eligible packets, instead of dropping, using the same criteria of minimum threshold, maximum threshold, and drop probability.

The current WRED implementation prevents tail dropping of packets that are queued in the output queue by selectively dropping packets early. This avoids dropping of all packets at once when congestion occurs. With ECN functionality, routers are no longer limited to packet drops to indicate congestion. Routers can instead set the CE (Congestion Experienced) codepoint in the IP header of packets from ECN-capable transport entities. The transport endpoints use the ECE and CWR bits of the TCP header to mitigate congestion.