Improving the accuracy of network intrusion detection systems under load using selective packet discarding

Under conditions of heavy traffic load or sudden traffic bursts, the peak processing throughput of network intrusion detection systems (NIDS) may not be sufficient for inspecting all monitored traffic, and the packet capturing subsystem inevitably drops excess arriving packets before delivering them to the NIDS. This impedes the detection ability of the system and leads to missed attacks. In this work we present selective packet discarding, a best effort approach that enables the NIDS to anticipate overload conditions and minimize their impact on attack detection. Instead of letting the packet capturing subsystem randomly drop arriving packets, the NIDS proactively discards packets that are less likely to affect its detection accuracy, and focuses on the traffic at the early stages of each network flow. We present the design of selective packet discarding and its implementation in Snort NIDS. Our experiments show that selective packet discarding significantly improves the detection accuracy of Snort under increased traffic load, allowing it to detect attacks that would have otherwise been missed.