ECN
Explicit Congestion Notification — routers mark IP packets at incipient congestion instead of dropping them. Endpoints (TCP) see the marking and slow down. RFC 3168 (2001). Later extension: L4S (Low Latency, Low Loss).
2 bits in the IP header: ECT(0)/ECT(1) says "I can do ECN", CE = Congestion Experienced. The TCP receiver echoes back via the ECE flag. Earlier problem: ISP middleboxes stripped ECN bits ("RFC zeroing"), causing fail-to-fallback issues. Modern adoption: ~90% of flows between large cloud providers; Apple enables ECN by default on iOS/macOS. DCTCP uses ECN aggressively inside datacenters. Lower latency than packet-drop feedback because timeouts are avoided.