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Why all the fuss about SDN?

By Saar Gillai, VP and CTO of HP Networking


sg.jpgSoftware Defined Networking (SDN) is a very popular subject these days. Beyond the basic concepts, there are a number of misunderstandings on the subject that I’ll be clearing up and explaining with this blog post.


First off, what is SDN?  Well, I define SDNs as requiring the following three elements:


  1. A method for modifying packet forwarding rules and/or applying policy to packets
  2. A method for doing element 1 across multiple devices, i.e. applying packet forwarding rules and polices across multiple devices (not just one device) in a dynamic and coordinated fashion
  3. The ability to perform elements 1 and 2 in a programmable fashion


Switches, Routers and other devices already apply lots of packet forwarding rules, so what’s the big deal about SDN?  Well that is true, but you have to keep in mind is this is only one element of SDNs. So you ask, what about element 2: “applying these rules across multiple devices?” This can be done with a good network management system – the HP Intelligent Management Center comes to mind.  So what’s the big deal?  Well, this is where element 3 – “programmable”- comes into play. 


Here’s where SDN gets really good


Switches forward packets and apply policy, and network management systems can configure switches with these forwarding rules, but what if you wanted to program an elaborate packet forwarding and policy behavior?  Today, we have an answer for that too – and this is where SDN gets really good.


There are lots of specialized devices that can program packet forwarding and policy behavior such as load balancers, application delivery controllers and network access control appliances . However, what if you could program such “specialized” packet forwarding and policy control on your “regular” switches and routers? Furthermore, what if you could coordinate it across many networking elements all at once in a dynamic fashion?  There you have it – the true value of SDN.   


Switches can forward packets and apply policy with access control list entries – and they can do it quite well. But what if you wanted something more sophisticated? What if you wanted to dynamically change the access policy and QoS specific to streams from users. You could base it on a multitude of dynamic elements such as their identity, location, time of day, resources being accessed and concurrent behavior of other users? This would require:


  • Visibility to all the user streams
  • Applying granular per-connection policy in real time across the entire network

 Can this be done with specialized appliances at the edge of the network? Probably! With SDN, you could actually have your existing OpenFlow-capable switches do most of the heavy lifting. Have an application running on a controller track existing traffic behavior, and then, as new connections are added, calculate the appropriate metrics and business logic. You then apply the relevant forwarding rules and policy in real time on existing connections traversing the switches in the network.    


Correcting SDN misconceptions: concept vs. applications


This brings us to another key misconception around SDN: the difference between SDN as a concept and SDN applications. SDN is just the ability to do the three elements I listed above. An SDN application is an algorithm or set of business logic rules/policies that are applied using these elements to create a specific networking solution.  At times, these algorithms might be combined with additional non-SDN specific capabilities to enable a full solution. A great example of this is “Network Virtualization.” SDN is not network virtualization. Network virtualization is an application that can be created using SDN principles.  However, as demonstrated above, this is just one application. There are many more applications coming and we have only just started.


The value of SDN is in the ability to program unique packet forwarding and apply policy to packets across multiple devices in a coordinated programmable fashion. Using these 3 key elements, you can now implement unique network applications without the need for specialized hardware. Our belief is that this flexibility will unleash a new wave of creativity in networking, greatly benefiting the industry and its customers. We hope you are as excited about it as we are!


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