SI-DFA: Sub-expression integrated deterministic finite automata for deep packet inspection

Finite automata is widely used for Deep Packet Inspection (DPI) of network traffic. Two types of automata employed for this purpose are Non-deterministic Finite Automata (NFA) and Deterministic Finite Automata (DFA). An NFA suffers from a large memory bandwidth per character due to multiple active states. A DFA, in comparison, ensures a linear processing time of O(1) for memory based architectures. However, the DFA state explosion conditions commonly occurring in today's NIDS rule-sets, render the automata with practically infeasible memory space requirements. To avoid state blowup we propose a semi-deterministic automata, Sub-expression Integrated DFA (SI-DFA), that ensures processing time of a single standard DFA. Rules are broken into sub-expressions at blowup conditions and compiled into a single DFA along with an association table, to correctly encapsulate equivalent automata. We list the rare cases in regular expressions for which sub-expression Integration is incorrect and present methodology to detect their occurrences. We evaluate SI-DFA on real-world rule-sets like Bro, Snort and Linux filters and compare their performance with the state-of-the-art hybrid automata solutions. SI-DFA renders a 66-97% reduction in processing bandwidth, up to 68% lower space requirement and an improvement trend with increasing rule complexity when compared to the traditional solutions.