OpenFlow is the first standard communications interface defined between
the control and forwarding layers of an SDN architecture. OpenFlow allows
direct access to and manipulation of the forwarding plane of network devices
such as switches and routers, both physical and virtual (hypervisor-based).
It is the absence of an open interface to the forwarding plane that has led
to the characterization of today’s networking devices as monolithic, closed,
and mainframe-like. No other standard protocol does what OpenFlow does,
and a protocol like OpenFlow is needed to move network control out of the
networking switches to logically centralized control software.