  Providing resilient network control is a critical concern for deploying
Software-Defined Networking (SDN) into Wide-Area Networks (WANs). For
performance reasons, a Software-Defined WAN is divided into multiple domains
controlled by multiple controllers with a logically centralized view. Under
controller failures, we need to remap the control of offline switches from
failed controllers to other active controllers. Existing solutions could either
overload active controllers to interrupt their normal operations or degrade
network performance because of increasing the controller-switch communication
overhead. In this paper, we propose RetroFlow to achieve low communication
overhead without interrupting the normal processing of active controllers
during controller failures. By intelligently configuring a set of selected
offline switches working under the legacy routing mode, RetroFlow relieves the
active controllers from controlling the selected offline switches while
maintaining the flow programmability (e.g., the ability to change paths of
flows) of SDN. RetroFlow also smartly transfers the control of offline switches
with the SDN routing mode to active controllers to minimize the communication
overhead from these offline switches to the active controllers. Simulation
results show that compared with the baseline algorithm, RetroFlow can reduce
the communication overhead up to 52.6% during a moderate controller failure by
recovering 100% flows from offline switches and can reduce the communication
overhead up to 61.2% during a serious controller failure by setting to recover
90% of flows from offline switches.
