MED: The Monitor-Emulator-Debugger for Software-Defined Networks

Software-Defined Networks (SDN) greatly improves programmability but brings in extra challenges for debugging. We propose an SDN debugging framework, Monitor-Emulator-Debugger (MED). It closely monitors the physical network, and automatically creates an emulator that can be set to the network state at any given point of time. In the emulator, MED synchronously constructs a virtual SDN that is identical to the physical SDN and replays real packet samples. The emulator also handles the non-determinism due to packet reordering. On top of the emulator, we provide fast and efficient debugging tools including loop and reachability detector, race condition detector and forwarding table checker. All the tools run on the emulator without adding any additional overhead to the physical SDN. We implement MED for an OpenFlow-based SDN in a data center network employing 20 switches. Using a combination of micro-benchmarks and real debugging case studies, we show that MED is both fast and useful in SDN debugging. During the evaluation, we reveal two physical switch bugs that have been confirmed by the vendor.