ECNEcho

ECNEcho refers to the ECN Echo mechanism within the TCP/IP family, associated with Explicit Congestion Notification (ECN). In this usage, ECNEcho denotes the use of the TCP header flag known as ECN Echo (ECE), which a receiver sets in a TCP segment to inform the sender that congestion has been observed in the network path. The concept is part of the broader ECN framework that aims to signal congestion without dropping packets.

How it works in practice: When a path supports ECN, IP packets can carry ECN marks (ECN-Capable Transport or CE marks). If a receiver encounters a CE-marked packet, it signals the sender by setting the ECE flag in the corresponding TCP segment, effectively “echoing” the congestion notification. The sender responds by reducing its congestion window and, after adjusting, may set the Congestion Window Reduced (CWR) bit to acknowledge the change. This interaction allows congestion to be managed with fewer packet losses and potentially smoother throughput.

Standards and implementation: ECNEcho is grounded in RFC 3168, which defines ECN operation and the ECE/CWR signaling in TCP. For ECN to function, both endpoints must support ECN and negotiate capability during connection setup (using ECT values in the IP header). Real-world deployment depends on network devices along the path honoring ECN marks; some middleboxes may modify or strip ECN fields, reducing effectiveness. When fully supported, ECNEcho contributes to more efficient congestion control and improved performance in congested networks.

See also: ECN, TCP congestion control, ECE, CWR, CE.