When debugging an SDN, it is sometimes necessary to explain the absence of an event: why a certain rule was not installed, or why a certain packet did not arrive. Existing SDN debuggers offer some support for explaining the presence of events, usually by providing the equivalent of a "backtrace" in conventional debuggers, but they are not very good at answering "Why not?" questions: there is simply no starting point for a possible backtrace. In this paper, we show that the concept of negative

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... ovenance can be used to explain the absence of events in SDNs. Negative provenance relies on counterfactual reasoning to identify the conditions under which the missing event could have occurred. We outline a simple technique that can track negative provenance in SDNs, and we present a case study to illustrate how our technique can be used to answer concrete "Why not?" questions. Using our approach, it should be possible to build SDN debuggers that can explain both the presence and the absence of events.