Now, if we look at the benefits we can attempt to infer what the problems SDN is trying to solve:

These are certainly problems for IT organizations of a variety of sizes and composition. The question then is, how does SDN uniquely solve those problems?

The answer is that as a concrete solution (i.e. components, software, and architectures)  it does not uniquely solve the problem. As a concept, however, it does.

Someone's no doubt quite upset at the moment at that statement. Let's explain it before someone's head explodes.

CONCEPT versus CONCRETE

The concept of separating data and control plane enables programmability. Without that separation, we have what we have today – static, inflexible networking components. But the concept of separating data and control planes isn't unique to solutions labeled specifically SDN. ADN is a good example of this (you saw that coming, didn't you?)

A network component can – and this may surprise some people – internally decouple control and data planes. Yeah, I know, right? And doing so enables a platform that looks a whole lot like SDN diagrams, doesn't it – with plug-ins and programmability. This occurs in full-proxy architectures when there exist dual stacks – one on the client side, one on the server side. Where traffic transitions from one stack to the other exists an opportunity to inspect, to manipulate, to modify, the traffic. Because the architecture requires acting as an endpoint to clients (and conversely as the point of origin for the server side), protocols can even be implemented in this "no man's land" between client and server. That enables protocol transitioning, such as enabling SPDY on the outside while still speaking HTTP on the inside or IPv4 to servers while supporting IPv6 on the client (and vice-versa).

Where the separation occurs is not necessarily as important as the fact that it exists – unless you're focused on concrete, SDN-labeled solutions as being the only solutions that can provide the flexibility that programmability offers.

Automation occurs by exposing the management plane through an API (or implementing a specific API, such as OpenFlow) such that operational tasks and configuration can be achieved through tools instead of time.

Between automation and programmability, you realize network control.

Now, this is not SDN, at least not in terms of protocol support and concrete architecture. But it is software-defined, and it is networking, so does it count?

I guess it depends. ADN has always approached layers 4-7 with an eye toward extensibility, programmability and control that enables agility in the network. We didn't call it SDN and I don't see the industry deciding to "SDN-wash" existing ADN solutions as SDN just because a new term came along and became the TLA du jour.

What I do see is that ADN and specifically full-proxy based ADC (application delivery controllers) already offer the same benefits using the same concepts as SDN. Consider again the core characteristics of SDN:

1. Control and data planes are decoupled

2. Intelligence and state are logically centralized

3. Underlying network infrastructure abstracted from applications

All of these characteristics are present in an ADN. The ability to leverage network-side scripting on the control plane side of the equation enables extensibility, rapid innovation, and ability to adapt to support new protocols, new applications, new business requirements – all without involving the vendor. Which is exactly one of the benefits cited for SDN solutions and specifically OpenFlow-enabled architectures.

So the question really is, does it matter if a solution to the problem of "agility in the network" is a concrete or conceptual SDN solution if it ultimately solves the same set of problems?