Scalable Best Matching Prefix Lookups

Abstract

All global routing protocols use hierarchies to allow scaling to a world wide community while keeping the routing database size manageable. Databases of variable length prefixes are a powerful tool for providing this in a flexible manner, but require a Longest Prefix Matching algorithm. In this paper, we report a fundamentally new solution that is both algorithmically interesting and practical.<br /><br />Our scheme is based on doing binary search on hash tables organized by prefix lengths, and scales very well as address and routing table sizes increase: independent of the table size, it requires a worst case time of log2(address bits) hash lookups. With the current Internet Protocol, which uses 32 bit addresses, at most 5 hash lookups are needed; for the upcoming 128 bit addresses of the next generation Internet Protocol (IPv6), 7 lookups suffice. Several refinements, including specializing the Binary Search with every match, considerably reduce the average number of hash search steps to less than 2.