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Software-Defined Networking (SDN) gains much of its value through the use of central controllers with global views of dynamic network state. To support a global view, SDN protocols, such as OpenFlow, expose several counters for each flow-table rule. These counters must be maintained by the data plane, which is typically implemented in hardware as an ASIC. ASIC-based counters are inflexible, and cannot easily be modified to compute novel metrics.
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Mogul et al. (2012) studied this question.