XOR-based schemes for fast parallel IP lookups

• Authors: Giancarlo Bongiovanni;Paolo Penna
• Venue: CIAC'03 Proceedings of the 5th Italian conference on Algorithms and complexity
• Year: 2003

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 prefix 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 only a minimal additional hardware. Indeed, we prove that any IP routing table T can be split into two subtables T1 and T2 such that: (a) |T1| can be any positive integer k ≤ |T| and |T2| ≤ |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) |T1| is roughly 7% of the original table T; (b) the lookup on table T2 does not require the best matching prefix computation.