SDN – Ask why, not what!

Nathan Pearce of F5, the provider of application delivery networking technology, takes a look at the emerging trend to adopt software defined networks (SDN) and concludes that only when adopted in the right circumstances and when coupled with the intelligence of application properties and behaviour will SDN really make an impact.

A lot of vendors have an SDN strategy. Unfortunately, our customers tell us that these strategies tend to vary in coherency. SDN does have a place but there is a danger of it becoming a solution in search of a problem. Consequently, starting an SDN conversation with the business case first can highlight whether the investment now, as an early adopter, is worth the risk.

Why do anything?

§SDN is about programmability in the network. But why would we want this? Adding programmability to the network is the first step in moving away from static connectivity and towards the ability to rapidly react to requirements. But how did the need for it come about in the first place? An excellent example of this comes from the telecoms industry.

How did we get from staffed telephone exchanges – ‘operator, how can I connect your call?’ – to the cellular telephony we use today? Present technology allows us to maintain a call even while in transit, maybe on a train or in a car (while using hands-free technology, of course). This made possible due to the service itself, the telephone call, not being reliant upon every physical device it traverses.

Learn from cellular handoff

In cellular telecommunications there is a process called handoff. One example of when handoff may occur is when the phone is moving away from the area covered by one cell tower and entering the area covered by another cell tower. With handoff, the call is transferred to the second cell tower in order to avoid call termination when the phone gets outside the range of the first cell tower.

The ability to do this within the data centre would be hugely beneficial. However, this is not something we have achieved in the past without great effort. A rigid network slows a businesses ability to react to increased demand or to roll out a new service.

The rest of the solution

Handoff is fine for cellular services because of the nature of the services. Once a call is made, the properties of that call do not change throughout its duration: a two way street of voice data runs until the call is terminated. This two way street can move cells so long as the flow is not interrupted. The new cell tower, post-handoff, requires no more information than the source and destination of the voice data.

Applications and services running in the data centre exist in changeable states and with varying demands throughout the length of interaction. For example, an HTTP application delivers different media types to the consumer even for a simple web page. At a minimum there are both graphical and text-based data types. More common today is the addition of rich media content (e.g. HTML5) and dynamic scripting technologies that must track state and interaction at all times.

With the added complexity of an application, the process of handoff becomes increasingly difficult.

SDN’s purpose is for the underlying plumbing of the network to take instruction and invoke change. However, it is important that these instructions take into consideration the complexities of the applications and services on which they run. Simply implementing a low level, cell-handoff style solution would likely break most data centre services.

Ask why

If the promise of SDN’s programmability is intriguing, consider the following checklist:

1) Application Support

Will all of my applications and services support change? Many organisations are running legacy apps and services that cannot cope with change. They may have been developed under the assumption that the network would always remain static and consistent. This alone could prevent any SDN adoption plans.

2) Business agility

We all like a shiny new gadget but ask yourself, is it really necessary for my business? Many organisations don’t have aggressive rollout or upgrade cycles in their data centre. If your data centre operations are relatively static then maybe early adoption of network programmability is not for you.

3) In-house skills

Do you have the in-house skills to run such a technology? With SDN comes new concepts like SDN Controllers. While the running of your network should become simpler with the maturity of SDN controllers, is the significant change worth the future gains. They may well be but only after evaluating 1) and 2) first.


SDN is an important step towards greater data centre agility and the reduction of delays to business. However, only when adopted in the right circumstances and when coupled with the intelligence of application properties and behaviour will it really make an impact.

What is SDN?

Software-Defined Networking (SDN) is an emerging architecture that is dynamic, manageable, cost-effective, and adaptable, making it ideal for the high-bandwidth, dynamic nature of today’s applications. This architecture decouples the network control and forwarding functions enabling the network control to become directly programmable and the underlying infrastructure to be abstracted for applications and network services. The OpenFlow protocol is a foundational element for building SDN solutions. (The Open Networking Foundation)


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David Dungay

Editor - Comms Business Magazine
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