Which IPv6 header field replaces the IPv4 ToS concept for traffic prioritization?

• A. Flow Label
• B. Payload Length
• C. Hop Limit
• D. Traffic Class

Answer: (D)

In IPv6, the mechanism used to signal packet priority and quality of service is carried by the Traffic Class field. This 8-bit field is designed to carry DSCP (Differentiated Services Code Point) and ECN (Explicit Congestion Notification) values, similar to how the IPv4 ToS field was used for prioritization. By marking packets with a Traffic Class value, routers can classify and treat them with different QoS policies as they traverse the network.

The Flow Label is there to help identify packets that belong to the same traffic flow, which can assist with per-flow handling and can support QoS in some scenarios, but its primary purpose isn’t to indicate priority for individual packets. Payload Length and Hop Limit serve other roles—Payload Length specifies how much data is in the packet, and Hop Limit (the IPv6 version of TTL) prevents infinite looping. So the field that replaces the ToS concept for traffic prioritization is the Traffic Class field.