Decoupling of Distributed Consensus, Failure Detection and Agreement in SDN Control Plane

Centralized Software Defined Networking (SDN) controllers and Network Management Systems (NMS) introduce the issue of controller as a single-point of failure (SPOF). The SPOF correspondingly motivated the introduction of distributed controllers, with replicas assigned into clusters of controller instances replicated for purpose of enabling high availability. The replication of the controller state relies on distributed consensus and state synchronization for correct operation. Recent works have, however, demonstrated issues with this approach. False positives in failure detectors deployed in replicas may result in oscillating leadership and control plane unavailability.In this paper, we first elaborate the problematic scenario. We resolve the related issues by decoupling failure detector from the underlying signaling methodology and by introducing event agreement as a necessary component of the proposed design. The effectiveness of the proposed model is validated using an exemplary implementation and demonstration in the problematic scenario. We present an analytic model to describe the worst-case delay required to reliably agree on replica failures. The effectiveness of the analytic formulation is confirmed empirically using varied cluster configurations in an emulated environment. Finally, we discuss the impact of each component of our design on the replica failure- and recovery-detection delay, as well as on the imposed communication overhead.