HEXA: Compact Data Structures

Data structures representing directed graphs with edges labeled by symbols from a finite alphabet are used to implement packet processing algorithms used in a variety of network applications. In this paper we present a novel approach to represent such data structures, which significantly reduces the amount of memory required. This approach called History-based Encoding, eXecution and Addressing (HEXA) challenges the conventional assumption that graph data structures must store pointers of Flog2nl bits to identify successor nodes. We show how the data structures can be organized so that implicit information can be used to locate successors, significantly reducing the amount of information that must be stored explicitly. We demonstrate that the binary tries used for IP route lookup can be implemented using just two bytes per stored prefix (roughly half the space required by Eatherton's tree bitmap data structure) and that string matching can be implemented using 20-30% of the space required by conventional data representations. Compact representations are useful, because they allow the performance-critical part of packet processing algorithms to be implemented using fast, on-chip memory, eliminating the need to retrieve information from much slower off-chip memory. This can yield both substantially higher performance and lower power utilization. While enabling a compact representation, HEXA does not add significant complexity to the graph traversal and update, thus maintaining a high performance.