The rate of network packets encapsulating requests from clients can significantly affect the utilization, and thus performance and sleep states of processors in servers deploying a power management policy. To improve energy efficiency, servers may adopt an aggressive power management policy that frequently transitions a processor to a low-performance or sleep state at a low utilization. However, such servers may not respond to a sudden increase in the rate of requests from clients early enough due to a considerable performance penalty of transitioning a processor from a sleep or low-performance state to a high-performance state. This in turn entails violations of a service level agreement (SLA), discourages server operators from deploying an aggressive power management policy, and thus wastes energy during low-utilization periods. For both fast response time and high energy-efficiency, we propose NCAP, Network-driven, packet Context-Aware Power management for client-server architecture. NCAP enhances a network interface card (NIC) and its driver such that it can examine received and transmitted network packets, determine the rate of network packets containing latency-critical requests, and proactively transition a processor to an appropriate performance or sleep state. To demonstrate the efficacy, we evaluate on-line data-intensive (OLDI) applications and show that a server deploying NCAP consumes 37~61% lower processor energy than a baseline server while satisfying a given SLA at various load levels.