Traffic volume and diversity can have a significant impact on the ability of network intrusion detection systems (NIDS) to report malicious activity accurately. Based on the observation that a great deal of traffic is, in fact, not important to accurate attack identification, we investigate connection filtering as a method for improving the performance of NIDS. We describe three different classes of connection filters that were developed to explore the design space and trade off's in load

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... ion versus alarm rates. We implement instances of each filter class on a network processor that can be used with any NIDS that runs on commodity hardware, and evaluate the impact of each filter in a series of laboratory-based tests. First, we establish an idealized maximum performance by using static connection filters for all benign traffic. Next, we show that volume sensitive random connection filters can improve performance significantly with respect to alarm rates under heavy traffic load. Finally, we show that dynamic connection filters that attempt to infer benign traffic can improve performance almost to the level of idealized static filters. These results underscore the potential for hardware-based connection filtering as an effective means for improving the performance of NIDS.