Historically, a network consisted of mostly hardware with just a modicum of software involvement. Changing the topology of it involved installing and configuring new switches or blades in a chassis or, at the very least, re-patching some cables. Now, the scenario has changed, and the complex infrastructures built to cater for multi-tenant environments such as cloud hosting, or microservice-based deployments, require a network that is more agile and flexible. This has led to the emergence of Software Defined Networking (SDN), an approach that centralizes the network configuration (where historically it was configured on a per-device basis) and results in network topology being defined as a whole, rather than as a series of component parts. It is, if you like, an abstraction layer for the network itself and thus implies that just like infrastructure as a service...