Shape shifting tries for faster IP route lookup

Some of the fastest practical algorithms for IP route lookup are based on space-efficient encodings of multi-bit tries (M. Degermark, et al., 1997, W. Eatherton, 1999). Unfortunately, the time required by these algorithms grows in proportion to the address length, making them less attractive for IPv6. This paper describes and evaluates a new data structure called a shape-shifting trie, in which the data structure nodes correspond to arbitrarily shaped subtrees of the underlying binary trie for a given set of address prefixes. The ability to adapt the node shape to the trie reduces the number of nodes that must be accessed to perform a lookup, especially for tries with large sparse regions. We give a fast algorithm for optimally dividing a trie into nodes so as to minimize the maximum lookup depth. We show that seven data structure accesses are sufficient for route tables with more than 150,000 IPv6 prefixes. This makes it possible to achieve wire-speed processing for OC192 link using a single QDRII SRAM chip.