Luleå algorithm
technique for storing and searching internet routing tables
The Luleå algorithm of computer science, designed by Degermark et al. (1997), is a technique for storing and searching internet routing tables efficiently. It is named after the Luleå University of Technology, the home institute/university of the technique's authors. The name of the algorithm does not appear in the original paper describing it, but was used in a message from Craig Partridge to the Internet Engineering Task Force describing that paper prior to its publication.
The key task to be performed in internet routing is to match a given IPv4 address (viewed as a sequence of 32 bits) to the longest prefix of the address for which routing information is available. This prefix matching problem may be solved by a trie, but trie structures use a significant amount of space (a node for each bit of each address) and searching them requires traversing a sequence of nodes with length proportional to the number of bits in the address. The Luleå algorithm shortcuts this process by storing only the nodes at three levels of the trie structure, rather than storing the entire trie.
Before building the Luleå trie, the routing table entries need to be preprocessed. Any bigger prefix that overlaps a smaller prefix must be repeatedly split into smaller prefixes, and only the split prefixes which does not overlap the smaller prefix is kept. It is also required that the prefix tree is complete.
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