Which statement correctly describes DSCP and ECN in IPv4?

• A. DSCP marks packets for QoS; ECN signals congestion by marking packets instead of dropping, where supported.
• B. ECN marks packets for QoS; DSCP signals congestion by dropping packets.
• C. DSCP and ECN both encrypt header content for privacy.
• D. DSCP marks packets for routing decisions; ECN modifies packet size.

Answer: (A)

DSCP and ECN in IPv4 work together to improve how networks handle traffic without always resorting to packet loss. DSCP marks packets to indicate the level of service they should receive, guiding routers to apply different forwarding treatments for quality of service (QoS). ECN uses two bits in the IP header to signal congestion by marking packets when the network is congested, rather than dropping them, provided the path and endpoints support ECN. This combination means DSCP handles priority, while ECN provides a way to react to congestion without immediate packet loss. In IPv4, the DSCP portion uses the top six bits of the differentiated services field, and the ECN portion uses the remaining two bits. The described statement is best because it correctly assigns DSCP to QoS marking and ECN to congestion signaling by marking, where supported. The other choices misstate these roles or describe unrelated concepts like encryption or routing decisions.