/iː-siː-ɛn/
n. “A mechanism in TCP/IP networks for signaling congestion without dropping packets.”
ECN, short for Explicit Congestion Notification, is a feature of modern IP networks that allows routers and endpoints to signal network congestion to senders proactively. Instead of relying solely on packet loss to indicate congestion, ECN marks packets to alert the sender to slow down, improving network efficiency and reducing latency.
ECN works in conjunction with TCP by using two flags in the TCP header: ECE and CWR. Routers capable of ECN can mark packets rather than dropping them when queues become full. The receiving TCP endpoint echoes this signal to the sender, which then reduces its transmission rate.
Key characteristics of ECN include:
- Congestion Signaling: Notifies senders about network congestion without packet loss.
- TCP Integration: Uses ECE and CWR flags to communicate congestion feedback.
- Improves Efficiency: Reduces retransmissions and packet drops.
- Optional Feature: Requires both sender and receiver to support ECN.
- QoS Enhancement: Helps maintain low latency in sensitive applications like VoIP and streaming.
Conceptual example of ECN usage:
// ECN workflow
Router detects potential congestion and marks packets
Receiver receives ECN-marked packet and sets ECE flag
Sender receives ECE and reduces transmission rate
Transmission continues smoothly without dropped packetsConceptually, ECN is like a “yellow light” in networking: instead of crashing into congestion (packet loss), it warns the sender to slow down, maintaining smoother traffic flow across the network.