What are the layers of an SDN architecture, and which APIs connect them?

Applications on top, the controller in the middle and the network devices (data plane) at the bottom. Northbound APIs connect applications to the controller, southbound APIs connect the controller to the devices, and east/westbound APIs connect controllers to each other.

* Applications talk to the controller northbound, the controller programs devices southbound, and controllers coordinate east/west. *

From top to bottom:

1. Applications: firewall, access control, IDS/IPS, monitoring, load balancing.
2. Northbound API: for example a REST API (OpenDaylight REST and others). Applications use it to tell the controller what they need.
3. Controller: the network operating system with its internal services (path computation, security), network APIs and drivers for the devices.
4. Southbound API: for example OpenFlow, ForCES or NETCONF. The controller uses it to program the devices.
5. Data plane: the physical and virtual switches, routers and access points that forward packets.

To the sides, east/westbound APIs let several controllers coordinate with each other, for redundancy or across domains.

Tip: Picture a map. North is up toward the applications, south is down toward the hardware.

Go deeper:

• Software-defined networking — Wikipedia — architecture, history and the main controllers.
• OpenFlow — Wikipedia — the classic southbound protocol for programming switches.