A Brief Introduction To OpenFlow, November 14, 2011 OpenFlow is a specification now managed by the Open Networking Foundation, which defines the functions and protocols used to centrally manage switches via a centralized controller.
Think of defining how network traffic is forwarded much like you think about defining access policies in a firewall. You specify the conditions to match traffic on, such as source and destination addresses, which includes wild cards, and an action to take on a match. In OpenFlow, you define forwarding policies limited by the capabilities of the controller and your own needs, and as your needs change, so can your policy.
In general, we always want traffic to go via the fastest, shortest path. However, when congestion occurs, the fastest, shortest path becomes oversubscribed and we want to prioritize some traffic over others. With an SDN, you can set a policy that prioritizes time-sensitive traffic over bulk traffic. As congestion occurs, you can move some or all of the bulk traffic to a different path, reducing congestion on the shortest path for your time-sensitive traffic.