Abstract Fog computing is a promising paradigm aimed at reducing latency and network traffic between end devices and cloud servers. In fog computing systems, a fog node can be overloaded by an increased number of requests from end devices. In this situation, an efficient offloading mechanism among fog nodes is necessary to meet the requirements of fog computing. Software Defined Networking (SDN) has become the de facto standard for future network infrastructures, including fog computing systems. In this paper, we propose a dynamic offloading service among fog nodes for an SDN-based fog computing system that aims at selecting an optimal offloading node and assisting the offloading path by providing an end-to-end bandwidth guarantee based on SDN technology. The proposed solution outperforms traditional approaches without SDN technology in terms of both throughput and request response time by selecting the appropriate offloading node using current computational and network resource information and providing a reliable end-to-end path.