XOR-Based Schemes for Fast Parallel IP Lookups

Abstract

An IP router must forward packets at gigabit speed in order to guarantee a good QoS. Two important factors make this task a challenging problem: (i) for each packet, the longest matching preffix in the forwarding table must be computed; (ii) the routing tables contain several thousands of entries and their size grows significantly every year. Because of this, parallel routers have been developed which use several processors to forward packets. In this work, we present a novel algorithmic technique which, for the first time, exploits the parallelism of the router to also reduce the size of the routing table. Our method is scalable and requires onlya minimal additional hardware. Indeed, we prove that any IP routing table T can be split into two subtables T 1 and T 2 such that: (a) |T 1| can be any positive integer k ≤ |T| and |T 2| ≤ |T| ∑ k; (b) the two routing tables can be used separately by two processors so that the IP lookup on T is obtained by simply XOR-ing the IP lookup on the two tables. Our method is independent on the data structure used to implement the lookup search and it allows for a better use of the processors L2 cache. For real routers routing tables, we also show how to achieve simultaneously: (a) |T 1| is roughly 7% of the original table T; (b) the lookup on table T 2 does not require the best matching preffix computation.