Bitmaps & Bitmasks: Efficient Tools to Compress Deterministic Automata

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Shiva Shankar Subramanian
PinXing Lin
Andreas Herkersdorf
Thomas Wild


Deep Packet Inspection, Network Security, Regular Expressions, Finite Automata, Transition Compression


Accelerating the signature matching function is essential to perform Deep Packet Inspection (DPI) at line rates. The conversion of the signatures into the Deterministic Finite Automaton (DFA) enables performance of this function at linear time. However, since the DFA is extremely storage inefficient, it is compressed before being stored in the memory. Although state-of-the-art bitmap-based compression algorithms can perform line rate signature matching, they only achieve transition compression of ~90-95%. Addressing the storage inefficiency, two bitmap-based transition compression algorithms were proposed by Subramanian et al. in 2016 to achieve transition compression of over 98%. A theoretical relationship is established in this article between the achievable signature matching throughput and the number of pipeline stages required to perform the decompression through the hardware accelerator based on the proposed techniques. Additional optimizations are proposed and evaluated to improve the per-stream signature matching throughput through the proposed decompression engines. The experimental evaluation of the optimizations shows that the per-stream signature matching throughput can be improved by a factor of 1.2–1.4x. A software model of the proposed decompression engines was designed and evaluated across a multitude of payload byte streams to verify the functional correctness of the proposed compression methods.

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