The internet began as a system for applying addresses to servers. Now it’s a means for giving names to services, and distributing those services across the planet. SDN gave rise to a new way of implementing computing on the broadest scale of infrastructure, and has become, for better or worse, the whole point of networking.
The phrase software-defined networking (SDN) was coined when it was necessary to distinguish the concept from the hardware-based variety. Since that time, “SDN” has come to mean the type of dynamic configuration that takes place whenever software-based services in a data center network are made accessible through an Internet Protocol (IP) address. More to the point, SDN is networking now.
How SDN works
SDN encompasses several types of technologies, including functional separation, network virtualization and automation through programmability.
Originally, SDN technology focused solely on separation of the network control plane from the data plane. While the control plane makes decisions about how packets should flow through the network, the data plane actually moves packets from place to place.
In a classic SDN scenario, a packet arrives at a network switch, and rules built into the switch’s proprietary firmware tell the switch where to forward the packet. These packet-handling rules are sent to the switch from the centralized controller.
The switch — also known as a data plane device — queries the controller for guidance as needed, and it provides the controller with information about traffic it handles. The switch sends every packet going to the same destination along the same path and treats all the packets the exact same way