L4S is an enhanced version of ECN (Explicit Congestion Notification). It spans layer 3 and 4 in the OSI model. ECN works by having a field in the IP header part of the packet that says if the packet experienced congestion on its way to the destination. A ECN-enabled router along the way changes the packet in transit, marking it as congested instead of dropping it. L4S is an advancement upon that, using a bigger field and special handling in routers, which allows for finer grained control over the congestion notification and more advanced algorithms to improve latency.