Network monitoring is an important task in future networks to mitigate outages, guarantee low latency and high throughput, and quickly adapt to changes of network conditions. Software-Defined Networking (SDN) enables dynamic adaptation to changing network environments through the ability to program network behavior. However, to react properly to changing environments, precise and credible information about performance metrics such as latency is necessary. Recent research activities exploited OpenFlow, an SDN protocol, to measure delays inside the network, without the need for additional hard-or software. In this paper, we propose an improved method to measure one-way delays, which can handle asymmetrical delays between the SDN controller and the switches. We implemented our approach in a hardware testbed to evaluate it in a setup with asymmetrical delays and traffic loads. The results show that we are able to measure asymmetrical delays in the network. Additionally, the proposed solution is capable of discovering the network topology and therefore gives together with the performance metrics the information for efficient routing decisions. In addition, we make the source code of our implementation publicly available.