Over the last couple of years, two major philosophies for SDN have evolved which I will call the overlay model, and the flow programmability model. Overlay networks are the notion of building multiple virtual networks in parallel on top of a physical network fabric, using some means of separating the virtual networks — typically an encapsulation method like VXLAN or NVGRE. Then we have the “flow programmability” model, based on the idea of programming SDN behaviors on a flow-by-flow basis into your existing (or new) physical and virtual network switches using a protocol like OpenFlow.
Herding Packets 