In modern switches, a packet can go through a number of processing steps to determine, for example, if the packet has to be discarded due to security policies, if it needs to be marked for quality of service or to determine the next hop for the packet. Most of those steps can be modeled as a matching of some of the packet fields with a set of rules that are stored in the switch. This has been generalized with the adoption of Software Defined Networks, using for example, the Openflow protocol, on which the processing steps are programmable as table matching operations and defined dynamically by a controller. Implementing this flexible packet matching in a switch is challenging, as we need to support multiple matching tables, each having different key size, and the size of the tables should also be programmable. The main options to support multiple tables are to use different memories for each table or to have several tables share the same memories. In the first approach, each table would have to match the size and width of the memories to achieve an efficient memory usage. This is a severe limitation when flexible table size and entry width need to be supported. In the second approach, all the tables can dynamically share the memories, providing better flexibility. The problem is that the width of the memories needs to be dimensioned to support the largest entry size. This leads to significant memory waste for smaller entries. Hash based techniques like cuckoo hashing can be used to efficiently implement exact matching using standard SRAM memories, and are widely used in modern switches. However, current implementations only support entries of one size. This article presents the Single Double cuckoo hash, which can support elements of two sizes. Its main benefit is to improve memory utilization when multiple tables with entries of different sizes share the same memories. This is achieved at the cost of a small increase in circuit complexity.
