What is the purpose of DSCP in Quality of Service, and which DSCP value is commonly used for Expedited Forwarding (EF) traffic?

• A. DSCP marks packets to indicate per-hop treatment; EF (Expedited Forwarding) commonly uses DSCP value 46.
• B. DSCP is used to identify the source address of a packet.
• C. DSCP values are assigned by the destination.
• D. DSCP is only used for IPv6 traffic.

Answer: (A)

DSCP marks packets to indicate how they should be treated at each hop, enabling routers to apply different forwarding priorities as part of Quality of Service. Expedited Forwarding is a high-priority class that aims for minimal delay and jitter, often used for voice and other time-sensitive traffic. The DSCP value commonly used for Expedited Forwarding is 46, which in binary is 101110 and sits in the 6 most-significant bits of the IP header’s differentiated services field (the remaining two bits are for ECN). This marking tells intermediate routers to place EF traffic in a fast, low-latency forwarding path. DSCP isn’t about identifying the source address, it isn’t assigned only by the destination, and it applies to both IPv4 and IPv6, not just IPv6.