The Control Plane and the Data Plane
It's hard to move forward in explaining software-defined networking without getting into the control plane and the data plane – what they represent, how they work together and how they might be separated.
The data plane or “user plane” of the network manages the forwarding of packets through the network. In traditional systems, the controls were built into the same avenues as the data forwarding components. Now, that's no longer the case, and all the data plane nodes do is forward packets. By contrast, the control plane is the “routing” plane. The control plane will route packet data and configure how the network moves data.
Some experts refer to the control plane as the “brains” of the network and the data plane as the “muscle.” Certainly, the task of simply forwarding packets is much more of a rote process than routing, and untethering these two would lead to these sorts of comparisons.
Another way to look at this is to explore how control and data planes are used in the industrial process. Components like programmable logic controllers are applied to industrial processes to manage what goes on throughout these physically directed networks. Data plane functions relate to the use of SCADA protocol to, again, direct the forwarding of packets for the flow of information. The control plane is where the “head” resides; it's where data will be used, for example, to alert a human operator if sensors and programs in the distributed line architecture see a high incidence of defects coming through the line, or a need to alter or recalibrate techniques for sealing containers, etc.
That's SDN in a nutshell: by isolating the control plane, it's changing the game in terms of how we view, operate and use network resources. That doesn't tell the story of how each individual instance of SDN is set up – but it tells the story of how we figured out that splitting the control plane from the data plane has big potential in changing network infrastructure.