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Providing resilient network control is a critical concern for deployingSoftware-Defined Networking (SDN) into Wide-Area Networks (WANs). Forperformance reasons, a Software-Defined WAN is divided into multiple domainscontrolled by multiple controllers with a logically centralized view. Undercontroller failures, we need to remap the control of offline switches fromfailed controllers to other active controllers. Existing solutions could eitheroverload active controllers to interrupt their normal operations or degradenetwork performance because of increasing the controller-switch communicationoverhead. In this paper, we propose RetroFlow to achieve low communicationoverhead without interrupting the normal processing of active controllersduring controller failures. By intelligently configuring a set of selectedoffline switches working under the legacy routing mode, RetroFlow relieves theactive controllers from controlling the selected offline switches whilemaintaining the flow programmability (e.g., the ability to change paths offlows) of SDN. RetroFlow also smartly transfers the control of offline switcheswith the SDN routing mode to active controllers to minimize the communicationoverhead from these offline switches to the active controllers. Simulationresults show that compared with the baseline algorithm, RetroFlow can reducethe communication overhead up to 52.6% during a moderate controller failure byrecovering 100% flows from offline switches and can reduce the communicationoverhead up to 61.2% during a serious controller failure by setting to recover90% of flows from offline switches.

RetroFlow: Maintaining Control Resiliency and Flow Programmability for Software-Defined WANs