An Fpga Overlay Architecture For Cost Effective Regular Expression Search

Snort and Bro are Deep Packet Inspection systems which express complex rules with regular expressions. Before performing a regular expression search, these applications apply a filter to select which regular expressions must be searched. One way to search a regular expression is through a Nondeterministic Finite Automaton (NFA). Traversing an NFA is very time consuming on a sequential machine like a CPU. One solution so is to implement the NFA into hardware. Since FPGAs are reconfigurable and are massively parallel they are a good solution. Moreover, with the advent of platforms combining FPGAs and CPUs, implementing accelerators into FPGA becomes very interesting. Even though FPGAs are reconfigurable, the reconfiguration time can be too long in some cases. This paper thus proposes an overlay architecture that can efficiently find matches for regular expressions. The architecture contains multiple contexts that allow fast reconfiguration. Based on the results of a string filter, a context is selected and regular expression search is performed. The proposed design can support all rules from a set such as Snort while significantly reducing compute resources and allowing fast context updates. An example architecture was implemented on a Xilinx® xc7a200 Artix-7. It achieves a throughput of 100 million characters per second, requires 20 ns for a context switch, and occupies 9% of the slices and 85% of the BRAM resources of the FPGA.