The ONF has specified the OpenFlow protocol as the open standard way controllers communicate with and control OpenFlow-compliant network devices.
"OpenFlow allows, for the first time, an external control plane to abstract the entire underlying network fabric so that fabric is universally addressable and all topology and state information is commonly managed," says Jason Matlof, vice president of marketing at Big Switch, a poster child of the SDN movement. "Today every device has its own control plane, manages it own state, has its own policy definitions, its own configuration and has to be managed through its own CLI. Once you have all that information centrally managed in an SDN controller, it enables you to build apps to program the network as a universal element. So this eliminates the complexity and static nature of traditional networks."
Given the early stage of SDN development, there are, of course, different takes on the definition of SDN. Metzler writes in Network World: "The definition of SDN that is currently emerging focuses somewhat less on decoupling and more on providing programmatic interfaces into network equipment, whether or not there is a separation of the control and forwarding planes. A minor reason for this shift in focus is because Cisco recently announced that as part of its SDN offerings, it will provide APIs into multiple platforms they provide."
"SDN is not equivalent to OpenFlow," says Lee Doyle, principal analyst at Doyle Research. "SDN is much broader than that. As long as you have APIs and can program the devices, it can be SDN, but it might be proprietary."
Regardless of definition, suffice it to say the broad aim is the same — to simplify networking and to catch up to the pace of change made possible by the adoption of technologies such as cloud computing and virtual servers.
Regarding the latter, the oft-cited problem today is you can spin up 1,000 virtual machines in minutes but it will take you two more weeks to bend the network into shape to support that new environment. SDN is designed to remove the network as the bottleneck in this fluid new world.
Which gives rise to the question about the difference between software defined networking and virtual networks, terms that are used interchangeably by some and referenced as distinct notions by others.
Matlof views virtual networks as a data center thing, a way to set up virtual tunnels between virtual server elements in the data center, and views virtual networks as just one application for SDNs. In fact, he calls virtual networks the first killer app for SDN, and Big Switch has rolled out a product to address the need. But ultimately he says a true SDN should be able to do that as well as control physical devices that support the OpenFlow protocol.
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