Let's start with an honest statement: OpenFlow is not SDN, and SDN is not OpenFlow. People often interchanged the two terms in the early days, but they are certainly not the same. OpenFlow is a very important building block of SDN, and many credit it to be the origin of the SDN movement. OpenFlow originally started at Stanford University as a research project, which eventually jump-started the startups of Nicira and Big Switch Networks. Nicira was acquired by VMWare and is now part of the company's network-virtualization product line. Big Switch Networks leads a number of OpenFlow open source projects such as Floodlight Controller, Switch Light OS for bar-metal Ethernet switches, and Switch Light vSwitch for virtual switches.

The original version of OpenFlow 1.1 was released in February 2011, and soon after the release, the OpenFlow effort was overseen by the Open Network Foundation, which retains control for the 1.2 release and beyond. In 2012, Google made a big wave by describing how the company's internal network had been redesigned to run under OpenFlow at the Open Networking Summit. The current version of OpenFlow is 1.4, with most switches supporting version 1.3.

In most cases, the controller is a software-based package that supports a range of OpenFlow versions from 1.1 and up. The switches, however, vary in their support for the version of OpenFlow specification. To help with the adaptation and increase confidence, the Open Network Foundation provides certification labs for switches (https://www.opennetworking.org/openflow-conformance-certification) and publishes the list of certified switches. In addition, the controller manufacturer, such as Ryu, might also perform their own compatibility tests (https://osrg.github.io/ryu/certification.html) and publish the results.

Let's look at the basic OpenFlow operations in the next section.