What is the length of the 'ECN' field in an IPv4 header?

The 'ECN' field in an IPv4 header is 2 bits long. This field is part of the Differentiated Services Field (DS Field) in the IPv4 header, which is used to indicate whether the packet can be marked for Explicit Congestion Notification. The ECN field allows for the indication of network congestion without dropping packets, thereby supporting better performance for applications that require reliable data transmission.

Understanding that the ECN is only 2 bits is important in the context of the overall header structure, which consists of various fields that each have their specific lengths. The rest of the DS Field, which consists of 6 bits, is reserved for Differentiated Services Code Point (DSCP) marking. This configuration allows network devices to manage traffic flows based on priority.

The other options represent lengths that do not accurately correspond to the size of the ECN field within the IPv4 header. This provides further clarity on the specific role and size of the ECN field in relation to the overall IPv4 header structure.