Today's wireless networks consist of a multitude of Radio Access Technologies (RATs), each being controlled individually, leading to suboptimal utilization of network resources. However, the unprecedented growth of data traffic is creating the need for an efficient inter-working of various RATs to circumvent the problem of suboptimal utilization of resources. Application of Software Defined Networking (SDN) principles enables the control and management of various RATs in a unified way. In this paper, we specifically focus on the inter-working between Long Term Evolution (LTE) and Wireless Fidelity (WiFi). We propose an SDN based architecture for a network comprising LTE Base Stations (BSs) and WiFi Access Points (APs). Users can be offloaded from one RAT to another based on different criteria, viz., user priority and channel state of users. We consider the problem of optimal RAT selection to maximize the total system throughput subject to constraints on the blocking probability of high priority users and the offloading probability of high priority users and formulate it as a Constrained Markov Decision Process (CMDP). We propose a low-complexity RAT selection algorithm which does not require the knowledge of the statistics of system dynamics. To conduct experiments, we develop a Network Simulator-3 (ns-3) based evaluation platform in accordance with the SDN principles. Experimental results demonstrate that the proposed algorithm provides a near-optimal performance.