All traditional networking devices like router and switches uses distributed control plane. But newer model of networking i.e., Software-defined Networking (SDN) uses centralized control plane. Distributed control plane means that control plane of all networking devices lies within the device itself.
Each device have their own control plane to control data plane. In Centralized control plane system, there is a device which contains control plane of all devices. This device control the activities of data plane of all networking devices simultaneously. This device is called Controller or SDN controller.
The following figure shows a model of controller based networking.
- Southbound Interface :
In SDN, all networking devices must be connected to controller so that it can regulate data planes of all devices. When drawing architecture of network, usually the network architect places networking devices below controller. Now according to map conventions, interface between controller and networking devices lies to south of controller. Hence, these interfaces are called Southbound Interface.
Southbound interface is an interface between a program on controller and a program on networking device. Note that these interfaces we are discussing are software interface not physical one.
- Northbound Interface :
Controller need to know many information regarding network so that it can control data plane of networking devices All these information are provided by Network Programmer. Network Programmer provide essential information to controller through various software or script about what functions it has to do. Again these softwares/scripts are placed above controller in network architecture. This placement of software/script makes interfaces between controller and software in north direction, according to map conventions. Hence, Interfaces between controller and softwares are called Northbound Interface. These interfaces enable programmability of network.
All interfaces we discussed above are program based interfaces. These interfaces in a broader sense are called Application Program Interface (API). An API is an interface through which two program can exchange data between them.