The prevailing wisdom is that a software-defined network (SDN) operates under the premise that the logically centralized control plane has an accurate representation of the actual data plane state. Unfortunately, bugs, misconfigurations, faults or attacks can introduce inconsistencies between the network control and the data plane that can undermine the correct operation at runtime. Through our experiments, we realize that P4 SDNs are no exception, and are prone to similar problems. With the aim to verify the control-data plane inconsistency, we present the design and implementation of P4CONSIST, a system to detect the inconsistency between control and data plane in P4 SDNs. P4CONSIST generates active probe-based traffic continuously or periodically as an input to the P4 SDNs to check whether the actual behavior on the data plane corresponds to the expected control plane behavior. In P4CONSIST, the control plane and the data plane generate independent reports which are later, compared to verify the control-data plane consistency. The previous works in the field of monitoring and verification mostly aim to test the P4 programs through static analysis and thus, are insufficient to verify the network consistency at runtime. Experiments with our prototype implementation of P4CONSIST are promising and show that P4CONSIST can verify the controldata plane consistency in the complex datacenter 4-ary fat-tree (20 switches) and multipath grid (4, 9 and 16 switches) topologies with 60k rules per switch within a minimum time of 4 minutes. At the same time, P4CONSIST scales to multiple source-destination pairs to detect control-data plane inconsistency.

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