Tree bitmap: hardware/software IP lookups with incremental updates

• Authors: Will Eatherton;George Varghese;Zubin Dittia
• Venue: ACM SIGCOMM Computer Communication Review
• Year: 2004

Even with the significant focus on IP address lookup in the published literature as well as focus on this market by commercial semiconductor vendors, there is still a challenge for router architects to find solutions that simultaneously meet 3 criteria: scaling in terms of lookup speeds as well as table sizes, the ability to perform high speed updates, and the ability to fit into the overall memory architecture of an Level 3 forwarding engine or packet processor with low systems cost overhead. In this paper, we describe a scheme that meets all three criteria. By contrast, published and commercial semiconductor solutions meet some but not all of these three criteria.For example, many approaches that provide dense tables have poor update times; others require large amounts of expensive high speed memory dedicated to this application. Many IP address lookup approaches do not take into account the flexibility of ASICs or the structure of modern high speed memory technologies such as RLDRAM[1] and FCRAM[2]. In this paper, we present a family of IP lookup schemes using a data structure that compactly encodes large prefix tables in order to address the criteria listed above. We also present a series of optimizations to the core algorithm that allows the memory access width of the algorithm to be reduced at the cost of memory references or allocated memory. Such flexibility in performance versus density is an important feature for the lookup engine of routers that may be deployed in different networks with varying requirements on address lookup length and table density (e.g. global IPv4 networks, global v6, VPN based v4 networks, MPLS, and IP tunneling encapsulation points).