Memory efficient deep packet inspection using transition functions

Regular expression matching is the state of the art in signature based intrusion detection systems. A regular expression matching algorithm used in intrusion detection systems is expected to process data at a speed linear in size of the incoming data and also to be able to run on network devices with limited memory. Traditional DFA and NFA based algorithms fail to meet either of these two requirements. The existing techniques try to either modify a DFA or a NFA, or combine both these to find a trade off between speed and memory requirements. The idea we propose in this paper is orthogonal to existing techniques. We propose a new approach to store a finite automaton in memory which otherwise is stored as a transition table. Our approach can be used by existing algorithms to further reduce the memory requirements with a minimal increase in the processing speed.