Which statement about the Differentiated Services Field (DS Field) is true?

• A. It carries IPv6 flow labels.
• B. It carries ECN for congestion notification only.
• C. It carries DSCP for class-based forwarding and ECN for congestion notification; used for QoS.
• D. It carries IP routing options.

Answer: (C)

The Differentiated Services Field in IPv4 is an 8-bit field used to mark packets for quality of service. It splits into two parts: the high 6 bits carry the DSCP value, which routers use to make class-based forwarding decisions and apply different levels of service; the low 2 bits carry ECN information, which signals congestion when both endpoints and the network support ECN.

So, the statement that it carries DSCP for class-based forwarding and ECN for congestion notification, used for QoS, is the accurate description. It’s not about IPv6 flow labels (those are in a different IPv6 field) and it doesn’t carry IP routing options.