Congestion Without Loss
Traditionally TCP infers congestion only by packet loss: a queue overflows, a packet is dropped, and the sender slows down. Loss is a blunt and costly signal because the data must be retransmitted.
Marking Instead Of Dropping
Explicit Congestion Notification, or ECN, lets a router warn endpoints before it has to drop anything. When a queue starts filling, a router sets a flag in the IP header rather than discarding the packet.
- The sender marks packets as ECN capable.
- A congested router sets the congestion experienced codepoint.
- The receiver echoes this back to the sender in the TCP header.
- The sender reduces its rate as if a loss occurred, but no data was lost.
Why It Helps
- It avoids the retransmission and delay that a dropped packet causes.
- It lets senders react to congestion earlier, smoothing queues.
- It is especially valuable for latency sensitive and interactive traffic.
The Requirements
ECN must be supported and enabled on both endpoints and honored by routers along the path. Misconfigured middleboxes historically interfered with it, though modern deployments are far more reliable.
Key idea
Explicit Congestion Notification lets routers mark packets to signal a filling queue, so senders slow down early without the cost of a dropped and retransmitted packet.