Traditionally, packet congestion in TCP/IP networks is indicated by packet loss. However, Explicit Congestion Notification (ECN) is a mechanism that allows two endpoints in a TCP/IP network to signal congestion without dropping packets. The two endpoints are an ECN-enabled sender and an ECN-enabled receiver. ECN must be enabled on both endpoints and on all of the intermediate devices between the endpoints for ECN to work properly. Any device in the transmission path that does not support ECN breaks the end-to-end ECN functionality.

With ECN enabled, routers and switches that detect congestion can mark packets in the IP header instead of discarding them. An ECN field is used in the IP header with two bits (‘10’ and ‘01’) to indicate that the endpoints are ECN capable transport. The receiving endpoint then echoes this congestion notification back to the sender, prompting it to reduce the transmission rate just as if a packet loss had occurred.

ECN operation requires support on both endpoints as well as on the network devices in the forwarding path. If any device in the transmission path does not support ECN, the end-to-end ECN capability is disrupted. When successfully negotiated, ECN improves efficiency by reducing packet loss, lowering retransmissions, and enabling faster recovery during congestion events.

OcNOS supports following ECN Markings: