Broadly speaking, SDN refers to the abstraction of the control plane from the data plane. The implications for this abstraction — the benefits as well as the challenges — are profound, and represent the next iteration of what data center and transit networks will look like. Indeed, the vast majority of large networks today are built on and operated by SDN. CDNs like Akamai and hyperscalers like AWS literally could not function without it. But before we get into those details, it’s helpful to take a step back to understand what the ‘control' and ‘data’ planes are, and what it means to pull the two apart.