Waiting until a queue is completely full makes congestion feedback arrive late and can produce long delays. Active queue management (AQM) lets a network device signal growing congestion before persistent buffer overflow.

One option is to drop selected packets early. With Explicit Congestion Notification (ECN), compatible IP and transport endpoints can instead mark packets to indicate congestion without necessarily discarding them.

The receiver reports the mark, and the sender responds by reducing its sending rate much as it would after a congestion loss.

AQM must be tuned carefully: signaling too aggressively wastes capacity, while signaling too late allows queues and latency to grow.

The reusable idea is feedback placement. Congestion-control algorithms run mainly at endpoints, but routers can provide earlier information about queue pressure so endpoints do not need to infer every congestion event from packet loss.