Reconfigurable content-addressable memory (CAM) on FPGAs: A tutorial and survey

Abstract

Content-addressable memory (CAM) is a massively parallel searching device that returns the address of a given search input in one clock cycle. Field-programmable gate array (FPGA)-based CAMs are becoming popular due to their applications in the latest networking systems, e.g., software-defined networks (SDNs) leading to upcoming 5G networks. Ternary CAM (TCAM) implements a routing table in a network router to classify and forward data packets where don’t care bits (X-bits) correspond to multiple addresses. FPGAs do not have a hard-core CAM, although it is a prime element in networking applications. This paper serves as a comprehensive survey on FPGA-based CAM/TCAMs implemented using block random-access memory (BRAM), lookup table RAM (LUTRAM), and flip-flops (FFs). BRAM-based TCAM suffers from the pre-processing of mapping data, requires the data to be in a specific order in some cases, and has a large SRAM/TCAM bit ratio. LUTRAM-based CAM/TCAM suffers from wide bit-wise ANDing, high routing complexity, but has a small SRAM/TCAM bit ratio of 14 compared to 16 in the case of BRAM-based TCAM. Shallow and wide RAM blocks are required to implement large-size RAM-based TCAMs (BRAM-based and LUTRAM-based TCAMs). FF-based TCAMs use FFs as their memory elements and have reduced hardware costs per TCAM bit. However, due to the routing complexity, it suffers from scalability and a large amount of power consumption. The update latency of BRAM-based TCAM and LUTRAM-based TCAM is proportional to the depth of BRAM and LUTRAM, respectively. However, FF-based CAM updates in 1 or 2 clock cycles depending on the availability of input/output pins on target FPGA.