Greg Ferro recently blogged about how attempts to define software-defined networking (SDN) are a waste of time. He wrote: "You can’t define 'Software Defined Network' because it's not a thing. It's not a single thing or even a few things. It's combination of many things including intangibles. Stop trying to define it. Just deploy it."

To a great extent I agree with him. It’s hard to define SDN as one thing, given that it is applied to so many different areas of networking: Data centers, enterprise campus, the WAN, radio access networks, etc. And each vendor that introduces an SDN product to the market is working from a definition that fits into its own strategy. Cisco’s is hardware-centric. VMware’s is software-centric, and so on.

So, yes. Just deploy it. But … what do those people who deploy SDN have to say?

EMA did offer a definition of SDN in its recently published research report Managing Tomorrow’s Networks: The Impacts of SDN and Network Virtualization on Network Management. The research is based on a survey of 150 enterprises that have deployed SDN in production or have plans to do so within 12 months. The report explores the benefits and challenges of SDN. Much of the research explores the readiness of incumbent network management tools to support SDN infrastructure and it identifies new functional requirements for these management tools.

(Side note: We also surveyed 76 communications service providers on the same topics, but I’m limiting this blog discussion to enterprise networking).

Since we were surveying people who were actually implementing SDN, we thought it would be valuable to get their take on what SDN actually is. We asked them the following question: When thinking about the definition of SDN, what characteristics of an SDN solution are important to you? Here are the top three defining characteristics of SDN for early enterprise adopters:

■ Centralized controller (39% of respondents)

■ Fluid network architecture (27%)

■ Low-cost hardware (25%)

A decoupled control plane and data plane (13%) was tied with intent-based networking as the least important defining aspect of SDN solutions.

These top three responses from early adopters of SDN present a pretty simple definition of the technology. And when you think about it, these terms align what we’re seeing in the market place. Nearly every SDN solution has a centralized controller, or at least a centrally accessible, distributed controller. This controller serves as a single point of control, access, programmability and data collection for the network. Most solutions also offer low-cost hardware, or — in the case of overlays — require no new hardware.

Fluid network architecture, I would argue, gets to the heart of what SDN is all about. It enables networks that are flexible and responsive to changes in infrastructure conditions and business requirements. This contrasts sharply with static, highly manual legacy networks, where any change to network connectivity in a data center or a remote site can require days, weeks or even months to implement. SDN’s promise is a network that can respond to change quickly and fluidly, thanks to increased programmability, for instance.

Therefore, I defer to the wisdom of early adopters when trying to come with up a definition. SDN is characterized by a fluid network architecture that is enabled by a centralized controller and low-cost hardware.

One final point on the subject of defining SDN. We asked early adopters of software-defined WAN (SD-WAN) a similar but distinct question on the defining characteristics of SD-WAN, which EMA considers sufficiently different from other varieties of SDN to warrant its own definition. In the case of SD-WAN, cloud-based network and security services were the number one defining aspect of such solutions. Centralized control was the number two priority, followed by hybrid WAN connectivity.

Shamus McGillicuddy is Senior Analyst, Network Management at Enterprise Management Associates (EMA).