Fast Firewall Implementations for Software-based Routers

Routers must perform packet classi cation at high speeds to e ciently implement functions such as rewalls. The classi cation can be based on an arbitrary number of pre x and range elds in the packet header. The classi cation required for rewalls is beyond the capabilities o ered by standard Operating System classi ers such as BPF [12], DPF [7], PathFinder [1] and others. In fact, there are theoretical results that show the general rewall classi cation problem has poor worst case cost: for searching over N arbitrary lters using k packet elds, either the worst-case search time is ((logN) ) or the worstcase storage is O(N). In this paper, we re-examine two basic mechanisms that have been dismissed in the literature as being too ine cient: backtracking search and set pruning trees. We nd using real databases that the time for backtracking search is much better than the worst case bound; instead of ((logN) ), the search time is only roughly twice the optimal search time. Similarly, we nd that set pruning trees (using a DAG optimization) have much better storage costs than the worst case bound; it has memory requirements similar to the RFC scheme of Gupta and McKeown [10]. We also propose several new techniques to further improve the two basic mechanisms. Our major ideas are a novel compression algorithm, the ability to trade smoothly between backtracking and set pruning, and algorithms to e ectively make use of hardware if hardware is available. We quantify the performance gain of each technique using real databases. We show that on real rewall databases our schemes, with the accompanying optimizations, are close to optimal in time and storage.