There are two layers of server-side software: an OpenFlow Controller and OpenFlow software applications built on top of the Controller.

The Controller is a platform that speaks southbound directly with the switches using the OpenFlow protocol. Northbound, the Controller provides a number of functions for the OpenFlow software applications -- these include marshalling the switch resources into a unified view of the network and providing coordination and common libraries to the applications.

At the top layer, the OpenFlow software applications implement the actual control functions for the network, such as switching and routing. The applications are simply software written on top of the unified network view and common libraries provided by the Controller. Thus, those applications can focus on implementing a particular control algorithm and then can leverage the OpenFlow layers below it to instantiate that algorithm in the network.

This three-layer OpenFlow Architecture should feel very familiar to software architects. For example, consider the Web application server architecture: applications sitting on top of a Web application server sitting on top of a database layer. Each of the lower layers presents an abstraction/API upward that simplifies the design of the layers above it.

Today, the term "OpenFlow" is used in two senses -- it can either refer to the "OpenFlow Protocol," the tightly defined "x86 instruction set for the network," or to an "OpenFlow Architecture," with its layers of switches, controllers and applications.