A SDN can have some level of logical decentralization, with multiple logical controllers. An interesting type of proxy controller, called FlowVisor, can be utilized to add a level of network virtualization to OpenFlow networks and allow multiple controllers to simultaneously control overlapping sets of physical switches. Initially developed to allow experimental research to be conducted on deployed networks alongside production traffic, it also facilitates and demonstrates the ease of deploying new services in SDN environments. FlowVisor is a special purpose OpenFlow controller that acts as a transparent proxy between OpenFlow switches and multiple OpenFlow controllers, as shown in the following figure:

FlowVisor creates rich slices of network resources and delegates control of each slice to a different controller and promotes isolation between slices. FlowVisor, originally developed at Stanford University, was widely used in experimental research and education networks to support slicing where multiple experimenters get their own isolated slice of the infrastructure and control it using their own network OS and a set of control and management applications.

FlowVisor enables you to conduct network research in real production environments and using real network traffic. As an open source proxy controller, you can customize the code to adapt to your needs, with a configuration and monitoring interface in JSON for users and a Java programming language for developers. Everyone has the ability to customize by opting into different services. You can freely and quickly experiment with SDN, with all the foundational SDN functions that enable you to learn about network virtualization and test new methods for deploying services rapidly. Since it is based on open standards that can run on a multi-vendor infrastructure, it supports multiple vendors (for example, NEC, HP, Pronto, OVS, and so on) as well as multiple guest network OSes (that is, OpenFlow controllers).