The networking applications are built and deployed on the top of the application layer and these applications communicate with Network Operating System (NOS) through Application Programming Interfaces (APIs) which are called “Northbound APIs”. NOS communicates and implements the routing rules and policies on forwarding devices via “Southbound APIs”. The end devices are connected to these forwarding devices for executing the task of communication through packets.
Interfaces of the controller
The SDN controller can interact with three layers through some standard open interfaces which are explained below.
Southbound interface: Southbound interface creates a channel to interact with the controller and underlying forwarding devices. OpenFlow is the standard-ized protocol supported by ONF, is the widely used southbound interface , which establishes a secured link between the controller and forwarding devices.
Northbound interface: The north bound APIs are interfaces between the controller and the applications application layer.This interface helps the application developers to manage the network through the program.
East-west Bridge: In practice, a large scale enterprise network is partitioned into sub-networks, where each sub-network is handled by a different controller. In this multi-controller based architecture, each network has the global network view to communicate with the other domain and
exchange topology among the sub-domains. Direct intra domain communication is not possible without the help of a proper interface. East-west protocols solve this problem by exchanging network view among the controllers.