How Did We Get Into This Mess? Isolating Fault- Inducing Inputs to SDN Control Software

Software bugs are inevitable in software-defined networking (SDN) control planes, and troubleshooting is a tedious, time-consuming task. In this paper we discuss how one might improve SDN network troubleshooting by presenting a technique, retrospective causal inference, for automatically identifying a minimal sequence of inputs responsible for triggering a given bug in the control software. Retrospective causal inference works by iteratively pruning inputs from the history of the execution, and coping with divergent histories by reasoning about the functional equivalence of events. We apply retrospective causal inference to three open source SDN control platforms---Floodlight, POX, and NOX---and illustrate how our technique found minimal causal sequences for the bugs we encountered.