ECN (explicit congestion notification)
Beginner
A mark a busy switch puts on passing data to say “I’m getting crowded.” The sender then slows down before anything is lost.
Novice
Explicit congestion notification: switches set a bit in the IP header of packets that pass through a queue above a threshold. The receiver reports the marks back, and the sender lowers its rate.
Expert
Two IP-header bits (RFC 3168). Data-center schemes mark on instantaneous queue depth (RED with Kmin/Kmax, or a single threshold); DCQCN relays marks as CNPs, and Ultra Ethernet asks switches to mark at dequeue rather than enqueue for a fresher signal.