A newbie exploring the mythical lands of SDN might decide to start at the ONF definition of SDN, which currently (November 2015) starts with a battle cry:
The physical separation of the network control plane from the forwarding plane, and where a control plane controls several devices.
The rest of that same page is what I’d call the marketing definition of SDN: directly programmable, agile, centrally managed, programmatically configured, open standards based and vendor-neutral.
If you’ve been tracking the state of SDN products, you probably know that at least some of those claims aren’t worth the cost of the bits transporting them.
Next level: SDN whitepaper. One of the beauties I found in there: current network technologies cannot scale (page 6). I wonder who built the global Internet? Last time I checked, it wasn’t running on an OpenFlow controller using centralized control plane… or maybe the mastermind behind that “discovery” claims that OpenFlow scales to interstellar distances, because we already have the first nodes in the inter-planetary Internet.
Going deeper into the technical details, you’ll find SDN Architecture document, which is so long it needs an SDN Architecture Overview companion. These documents already talk about the “logically centralized control planes” and “control exercised within data plane systems”.
Why is there such a huge discrepancy between the high-level marketing fluff and the technical documents? Because the reality tends to be complex and ugly regardless of what the marketers want us to believe, or in the immortal words of Colonel Jessep: “YOU CAN’T HANDLE THE TRUTH!”
Or maybe it’s because after promising to revolutionize the networking, doing badly-needed gradual improvements on the stuff we’ve been using for decades just isn’t sexy enough.
Finally, if you feel you CAN handle the truth (and I know you can, or you wouldn’t be reading this blog ;), start with the free Introduction to SDN, explore real-life SDN resources on ipSpace.net, and continue your journey with Advanced SDN training (and I'm really sorry you missed last week's SDN retreat - we had fantastic sessions and great fun).