Which statement about the Differentiated Services Field in IPv4 is true?

• A. It carries TTL and fragmentation flags
• B. It carries source and destination addresses
• C. It carries DSCP for class-based forwarding and ECN for congestion notification; used for QoS.
• D. It carries the protocol number

Answer: (C)

The Differentiated Services Field in IPv4 is an internal 8-bit header field designed to signal how routers should treat a packet for quality of service. It carries DSCP, which defines class-based forwarding decisions, and ECN, which allows congestion notification without dropping packets. This combination enables QoS by mapping packets to different forwarding behaviors and letting endpoints know about congestion. TTL and fragmentation flags, source and destination addresses, and the protocol number are located in other parts of the IPv4 header, not in this field. So the statement that this field carries DSCP for class-based forwarding and ECN for congestion notification, used for QoS, is the accurate description.