Fast packet classification for two-dimensional conflict-free filters

Routers can use packet classification to support advanced functions. Routers with packet classification capability can forward packets based on multiple header fields, such as source address, protocol type, or application port numbers. The destination-based forwarding can be thought of as one-dimensional packet classification. While several efficient solutions are known for the one-dimensional IP lookup problem, the multi-dimensional packet classification has proved to be far more difficult. While an O(log w) time scheme is known for the IP lookup, Srinivisan et al. (1999) show a lower bound of Ω(ω^k-1) for k-dimensional filter lookup, where ω is the number of bits in a header field. In particular, this lower bound precludes the possibility of a binary search like scheme even for 2-dimensional filters. In this paper, we examine this lower bound more closely, and discover that the lower bound depends crucially on conflicts in the filter database. We then show that for two-dimensional conflict-free filters, a binary search scheme does work! Our lookup scheme requires O(log² ω) hashes in the worst-case, and uses O(n log² ω) memory. Alternatively, our algorithm can be viewed as making O (log ω) calls to a prefix lookup scheme. It has been observed in practice that filter databases have very few conflicts, and these conflicts can be removed by adding additional filters (one per conflict). Thus, our scheme may also be quite practical. Our simulation and experimental results show that the proposed scheme also performs as good as or better than existing schemes