There may be no element more important to a computing environment than the network that holds it all together. What good are the servers, storage systems, load balancers, and links to the rest of the world if they can't communicate with one another quickly?
No question, there's gobs of networking bandwidth available inside most datacenters, but there's also a maddening inflexibility to the complex fabric of switches that provides the bandwidth. For all the talk of virtualized servers, liquid pools of capacity, and elastic computing, the datacenter network remains rigid and difficult to manage.
Enter software-defined networking (SDN), which promises to give datacenter networks the same dynamism that servers and storage have enjoyed for years. Venture money is flowing into SDN-based startups, such as Big Switch, which unveiled a package of new offerings in November. (See: SDN Startup Taps Into Future of the Datacenter.) In addition, leading network vendors such as Cisco Systems, Juniper Networks, IBM, Alcatel-Lucent (this site's sponsor), and Hewlett-Packard are making moves to harness the technology.
Just a few months ago, VMware paid $1.2 billion for the SDN startup Nicira, which should help the virtualization giant fulfill its promise of a "software defined datacenter." IDC has said that the SDN market could grow to $2 billion in the next four years.
One problem with modern, IP-switch-based networks has been that the logic function controlling their pathways and services has been distributed across the switches. Each switch does its own thing, as it were, following the Internet's original, decentralized design philosophy. As a result, managing the network is inordinately difficult.
Changing the network's behavior -- creating a new service from node A to node B, for instance, or grouping together a set of servers -- calls for reprogramming each switch involved, keeping in mind the myriad logical relationships with other switches. The task grows ever more challenging as the network expands, gets divided into subnetworks, and hosts a more varied menu of services.
SDN tackles this problem from a new point of view: from up above. It effectively floats the control plane -- all that control function -- off the data plane, the logic that actually forwards packets from switch to switch. SDN depends on a centralized controller that, in effect, sites above and looks down on the entire network and tells the switches how to create new pathways to handle different streams of packets as needed, moment by moment. This controller maintains a complete and richly detailed model of the network's physical gear and topology and all the services the network is meant to provide.
This model, composed in software, is what gives SDN its name.
A centralized, globally aware controller provides several advantages. One is enormous flexibility, enabling the network to keep up with, say, the changing physical locations of virtual machines. Also, depending on what kinds of traffic need to move and what streams of packets are already in motion, the controller can instantly give each stream the level of service it requires. That translates into better performance without the need to overprovision.
Security is improved, too. Many of today's datacenter nets have grown so complicated that nobody can determine if a link exists between two nodes that are meant to be isolated from each other. But with a centralized controller and its model in charge, all pathways are known.
With the network defined in software, changing and expanding it is also much easier. The network becomes virtualized, essentially, and many routine tasks can be automated.
Finally, and perhaps most importantly, SDN promises to change the economics of networking. Much of the value in today's switches lies in their proprietary control and management software. With SDN, customers will be able to use commodity switching gear, perhaps even from different brands. Therefore (in theory, at least), they can save bundles of money. These potentially disruptive economics are fueling much of the excitement over SDN technology and, in turn, giving incumbents much to consider.
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