To meet the deadlines of interactive applications, congestion-agnostic transport protocols like UDP are increasingly used side-by-side with congestion-responsive TCP. As bandwidth is not totally virtualized in data centers, service outage may occur (for some applications) when such diverse traffics contend for the small buffers in the switches. In this paper we present SDN-GCC, a simple and practical software-based congestion control mechanism that puts monitoring and control decisions in a centralized controller and traffic control enforcement in the hypervisors on the servers. SDN-GCC builds a congestion control loop between the SDN controller and hypervisors without assuming any cooperation from tenants applications (or transport protocol), ultimately making it deployable in existing data centers without any service disruption or hardware upgrade. SDN-GCC is implemented and evaluated via simulation in ns2 as well as real-life small-scale test-bed deployment and experiments.