The Software-Defined Networking (SDN) goal is to create a mechanism that allows the administrators to answer to the changes in the connection requirements in a quick way, decoupling the control planes (with which the paths and circuits are established) of the forwarding plane. This way, it is possible to administrate resources abstracting the low level functionalities in every device.
SDN is a step towards the networking world in the era of Cloud-Computing. This evolution in the networks promises more elastic, agile and dynamic environments to accompany the demands on the current businesses. The new architectures of cloud type and the dynamic resources use are evidence of the need for a new administration scheme for networks. SDN creates a new layer in which the network operators can create, design and specify different services over a network, without having to assign them and configure them in specific interfaces. This allows flexibility, allowing the entities to change their physical interface without having to change their identity or specifications of the service. The SDN scheme allows to simplify the operations, separating the identity control and the logic control of traffic flows in the physical devices of their functions (switchng and routing). In SDN, an administrator can create traffic paths from a central control, without having to configure every device in particular. The operator can change the policies and configurations of the devices when necessary from a high level control through the SDN layer (prioritize traffic, block traffic, etc.). This allows the administrators to use regular simple switches (reducing the CAPEX) and they have more control over the traffic of their network.
Currently, the most popular specification used for the application of SDN is OpenFlow, a standard that allows the deployment of different innovative protocols in production networks.
More information at: http://archive.openflow.org/wp/learnmore