Two planes
A controller can see more than one device.
Traditional routing distributes control decisions across routers. SDN places a controller or management plane above forwarding devices. The controller can build a broader topology view and install rules, while switches keep moving packets at line speed.
Build the model
Four SDN roles
Controller
Maintains a network view and computes policy-aware paths.
Control plane
Decides where traffic should go.
Data plane
Applies forwarding rules to packets.
Southbound API
Connects the controller to network devices.
Controller to device
The southbound API turns a decision into device rules.
A southbound API is the communication boundary between an SDN controller and forwarding devices. The controller can ask a switch for state, receive events, and install, change, or remove rules in its flow table.
The API carries control information such as match conditions and actions: for example, which packets to recognise and which port, queue, or next hop to use. OpenFlow is one example of a southbound protocol, but the important idea is the direction of the relationship.
Mini experiment
Compare the control model
Select who makes the forwarding decision.
A controller sees several devices and installs forwarding rules according to a global policy.
A broad view improves coordination, but the controller and control channel must remain available.
Knowledge check
Test your SDN model
Answer five questions.