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Network Intrusion Detection: Evasion,
Traffic Normalization, and End-to-End Protocol Semantics

Mark Handley and Vern Paxson
AT&T Center for Internet Research at ICSI (ACIRI)
International Computer Science Institute
Berkeley, CA 94704     USA

Christian Kreibich
Institut für Informatik
Technische Universität München
80290 München, Germany



A fundamental problem for network intrusion detection systems is the ability of a skilled attacker to evade detection by exploiting ambiguities in the traffic stream as seen by the monitor. We discuss the viability of addressing this problem by introducing a new network forwarding element called a traffic normalizer. The normalizer sits directly in the path of traffic into a site and patches up the packet stream to eliminate potential ambiguities before the traffic is seen by the monitor, removing evasion opportunities. We examine a number of tradeoffs in designing a normalizer, emphasizing the important question of the degree to which normalizations undermine end-to-end protocol semantics. We discuss the key practical issues of ``cold start'' and attacks on the normalizer, and develop a methodology for systematically examining the ambiguities present in a protocol based on walking the protocol's header. We then present norm, a publicly available user-level implementation of a normalizer that can normalize a TCP traffic stream at 100,000 pkts/sec in memory-to-memory copies, suggesting that a kernel implementation using PC hardware could keep pace with a bidirectional 100 Mbps link with sufficient headroom to weather a high-speed flooding attack of small packets.

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Vern Paxson