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In Software-Defined Networks (SDNs) the role of the centralized controller is crucial, and thus it becomes a single point of failure. In this work, a distributed controller architecture is explored as a possible solution to improve fault tolerance. A network partitioning strategy, with small subnetworks, each with its own Master controller, is combined with the use of Slave controllers for recovery aims. A novel formula is proposed to calculate the reliability rate of each subnetwork, based on the load and considering the number and degree of the nodes as well as the loss rate of the links. The reliability rates are shared among the controllers through a newly-designed East/West bound interface, to select the coordinator for the whole network. This proposed method is called “Reliable Distributed SDN (RDSDN).” In RDSDN, the failure of controllers is detected by the coordinator that may undertake a fast recovery scheme to …