We present a transparent and scalable flow-based monitoring solution that is fully compatible with current off-the-shelf OpenFlow switches. In order to reduce the overhead, we implement three different flow sampling methods depending on the OpenFlow features available in the switch. As in NetFlow/IPFIX, we aggregate packets into flows directly in the switches and asynchronously send traffic reports to an external collector. We developed our solution within OpenDaylight and evaluated its accuracy in a testbed with Open vSwitch. Our experimental results using real-world traffic traces show that the proposed sampling methods are accurate and can effectively reduce the resource requirements of flow measurements in OpenFlow.