Which field in an IPv4 header is responsible for end-to-end notification of network congestion?

The field in an IPv4 header responsible for end-to-end notification of network congestion is the Explicit Congestion Notification (ECN) field. ECN is part of the IPv4 header that allows routers to signal congestion to endpoints without dropping packets. When congestion is detected, routers can set specific bits in this field, which informs the sending host to reduce its transmission rate, thus helping to alleviate congestion in the network.

The Identification field, while important for uniquely identifying fragments of a datagram during reassembly, does not relate to congestion notification. The Total Length field indicates the overall size of the IP packet, including both header and data, and is not associated with congestion indication. The Flags field is used to control and identify fragments but does not serve the purpose of signaling congestion. Therefore, the ECN field is specifically designed to handle situations of network congestion, allowing mechanisms like Traffic Control Protocols to maintain efficient data flow and enhance overall network performance.