What exactly is SDN?

A recently undertaken survey by Exponential-e, polling its wider audience of UK IT and business leaders revealed that 86% of those surveyed felt they did not really know what SDN was. Is it any wonder when there are so many conflicting opinions from the various tech companies all vying for our IT dollars?

Some would have you believe it’s a replacement for your network control system. To others it’s a software layer that runs on top of your network. For the costliest it’s a totally new type of network built on new equipment (which of course you need to buy). The reality is both all of these things, and none of them.

The Evolution of the Network

So how did we get to this place where managing a network is such a problem? As it turns out, networks today are infinitely more complex than only a few years ago. Where we used to have just a few connected bits of computer equipment, today’s networks are a web of wired and wireless connections with traffic moving in every conceivable direction and changing constantly.

The founding principal of the Internet was always that the complexity belonged at the edge, in the end devices themselves, not in the network. Unfortunately, security concerns, manageability and the ever changing nature of how we expect networks to work has led to a creeping complexity in our networks cores. It’s this complexity that we seek to eliminate through SDN.

Networking in Layers

The good thing about all of our problems with complex networks, is that these complex networks are built on a fantastic and brilliant piece of work called the OSI model or “Open Systems Interconnection Model”. What this model provides, at least in principal, is the ability for end-to-end communications to happen at each of seven layers without really understanding what’s happening at the layers below. We see this already working today in many respects. I can for example have a Skype or Facetime phone call across the Internet, with some reasonable assurance of privacy, even though I do not own or control hardly any of the networks between my PC and the PC of the person I’m speaking with.

What this means for SDN is that it is at least possible for me to build and control a “network” while allowing other people to deliver some of the underlying elements; such as the fibre buried in the streets and the large service provider switches and routers that make up the Internet. In some very simple terms, we do this today with VPNs or “Virtual Private Networks”.

A VPN already allows me to connect together my local networks in two locations, across someone else’s network yet having it appear as though I’m in total control and the network is directly happening from end-to-end, LAN to LAN. Of course VPNs are very complex, require costly performance robbing encryption, and are generally difficult to maintain and nearly impossible to change quickly or without disruption. Again we have solid reason why we would want to move to SDN… no more need of VPNs!

So back to the question – what is SDN?

Software Defined Networking, or SDN as we’ll call it, is not one thing, it’s actually four separate shifts in network technology coming together for a common purpose:

  • Centralised Control: Instead of each network device deciding how to forward traffic, use a central “brain” to make those decisions with some knowledge of the end-to-end goal.
  • Open Networking: Instead of proprietary remote control technologies, use a standard agreed open framework to allow for network device control through software instead of people.
  • Resource Virtualisation: Stop our networks being defined by how the cables are connected and just like virtual servers, be able to create networks as we imagine them with a click of a button.
  • Function Virtualisation: While we’re creating our virtual network we need to create virtual routers, firewalls or load-balancers and not be limited by what’s physically available in the network.

By combining together centralised control, with open control technology (software), the ability to create virtual networks and then to populate those virtual networks with virtual network functions, we can begin to realise our dream of defining our networks in software. The key is for the underlying technology to support at least some of these four tenants for us to really get the most out of SDN.

It will take time to transition, and it will require new skills, new software and new equipment. In the end it will virtualise the final frontier of our connected lives, meshing together with virtual servers, datacentres, desktops and applications to allow us all to create our IT landscape through software, from the comfort of our desk – wherever that may be.

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