IPStash: a set-associative memory approach for efficient IP-lookup

IP-lookup is a challenging problem because of the increasing routing table sizes, increased traffic and higher speed links. These characteristics lead to the prevalence of hardware solutions such as TCAMs (ternary content addressable memories), despite their high power consumption, low update rate and increased board area requirements. We propose a memory architecture called IPStash to act as a TCAM replacement, offering at the same time, high update rate, higher performance and significant power savings. The premise of our work is that full associativity is not necessary for IP-lookup. Rather, we show that the required associativity is simply a function of the routing table size, Thus, we propose a memory architecture similar to set-associative caches but enhanced with mechanisms to facilitate IP-lookup and in particular longest prefix match (LPM). To reach a minimum level of required associativity we introduce an iterative method to perform LPM in a small number of iterations. This allows us to insert route prefixes of different lengths in IPStash very efficiently, selecting the most appropriate index in each case. Orthogonal to this, we use skewed associativity to increase the effective capacity of our devices. We thoroughly examine different choices in partitioning routing tables for the iterative LPM and the design space for the IPStash devices. The proposed architecture is also easily expandable. Using the Cacti 3.2 access time and power consumption simulation tool we explore the design space for IPStash devices and we compare them with the best blocked commercial TCAMs.