What is software-defined networking?
Software-defined networking (SDN) is a new approach to the network infrastructure and protocols that make networks more agile, add simplicity to the networking protocols and allow bandwidth to scale exceptionally high.
The defining characteristics of SDN are two types of protocols—underlay and overlay—that work together as a powerful team. Simply put, an underlay is a protocol that runs lower on the OSI model, typically right above Ethernet. An overlay is a protocol that runs higher in the OSI model, analogous to an application running on UDP.