Network virtualization is a big concern for SDN researchers. Their goal is to create multiple, potentially overlapping, virtual networks on the top of software-defined networks. This would allow different, isolated usages of the same physical infrastructure, in order to optimize the usage of physical resources. As an instance, the same physical infrastructure may be used in the same time for both a content delivery network and as a research plateform, without interferences between the two. These virtual networks, on top of SDNs, are called overlay networks.

Research on SDN virtualization began in 2009 with the project FlowVisor, which first aimed at dividing a network into several virtual SDNs. Virtual SDNs are also called slices, or tenants. Projects of SDN virtualization then quickly expanded into new projects, now aiming at integrating directly these virtualization functionalities into common-used SDN controllers (VTN for OpenDayLight, ONF Trellis for ONOS, etc).

• FlowVisor
• OpenVirteX
• Virtual Tenant Network

• http://www.ieice.org/~nv/nvs2015/nvs2015-3-chen.pdf