Which statement about the Differentiated Services Byte is true?

• A. Used to mark packet identifiers
• B. Used for error detection
• C. Used for congestion notification
• D. Used to carry sequence numbers

Answer: (C)

The Differentiated Services Byte is about classifying packets to influence how they are treated in the network. Routers examine this 8-bit field to decide the level of service, queuing, and forwarding behavior for each packet. A key aspect is signaling congestion through the ECN portion of the field, which lets routers mark or echo congestion without dropping packets, enabling endpoints to adjust sending rates. That’s why this statement is true: it’s used for congestion notification.

This field isn’t about marking packet identifiers, nor is it for error detection or carrying sequence numbers. Packet identifiers and error detection are handled by other parts of the IP and transport headers, and sequence numbers belong to the transport layer protocols like TCP, not the Differentiated Services Byte.