Software-defined networking enables more flexible network management and operation by creating a virtual network that is abstracted above the physical network.
Instead of the physical network equipment -- which is programmed at the hardware level -- driving network control, software-defined networking (SDN) introduces a software-driven controller that handles these tasks and enables the changes to happen on the fly, without complicated and potentially error-prone reprogramming.
Architecturally, an SDN controller sits between the higher-level network applications -- like security, load balancing or firewalls -- and the lower-level physical network devices, like switches or routers. By sitting in the middle, the controller can manage the network more efficiently using SDN protocols. It sends information about the physical network below up to the applications and pushes the application control data down to the individual network devices.
Because it's located in the middle, the SDN controller communicates up to the network applications via northbound APIs -- like REST -- and down to the network devices through southbound APIs -- like OpenFlow.
How the SD-WAN controller communicates
Users often assume software-defined WAN (SD-WAN) operates like SDN. SD-WAN also abstracts the control and management of network functions from the hardware and brings them to a software-based controller. This controller acts as an overlay of the traditional WAN connection.
While it has the same abstraction, an SD-WAN controller does not rely on the same northbound and southbound protocols that an SDN controller uses. This is because the WAN connection begins at the edge of the network and reaches down further to a remote branch site.
SD-WAN starts with a network device at the headquarters -- such as an edge router or a WAN appliance -- and then connects to the remote site via an MPLS, broadband internet, or even a 3G or 4G connection. All of the SDN protocols occur upstream of this edge router or SD-WAN appliance, as the WAN connection is primarily focused on securing and transporting network traffic between two points via IP and Open Shortest Path First.
An edge router at headquarters for SD-WAN could be managed via the southbound SDN APIs just as other network devices like switches and routers are managed. That SDN protocol would not continue beyond the edge router or WAN appliance, however.