Introduction to Software Defined Networking

So here we are: this is the first article of this blog. As 6WIND’s products are closely related to what is called Software Defined Networking, it made sense to me to start with some basic notions about this topic. Some following articles should focus on my work on BEBA, a European research project that aims at making programmable switches “faster and smarter”. But you may wonder: what do I mean by “programmable” switches? This is precisely one of the questions to which you will find an answer in this article: here we will see how networking has evolved over the last years to become (even) more complex, with computers or switches no more being delimited by their hardware.

Physical connections: messing up with the wires

Once upon a time… there were computers. At the very beginning, they could not talk with one another. At some point during the 1960s, the few people that would use them figured out that, for reasons that seem quite obvious today, connecting several computers could be a good idea. And so computer networks were created! For some decades, setting up a network would basically consist in plugging cables between pairs of devices; and to connect more than two hosts together on local networks, hubs or switches would be required. To connect several networks between them would be the task of routers.

Traditional network
A traditional wired LAN, with hardware switches

Actually, correctly setting up a network is way more complicated than that. This is why so many protocols have been invented and implemented over the years: making it run, and addressing the requirements in terms of stability, scalability, security, quality of service and so on… it could require quite a lot of skills and knowledge from the administrator. Nevertheless, it would come down to connecting different physical hosts together. Even since wireless networks have been deployed, this has remained a basic principle.

Enters virtualization

Virtualizing hardware is not something new, it was in use back in the 1960s to divide the system resources provided by mainframe computers between different applications. But it has evolved a lot with the years. As hardware resources became more affordable and computers more powerful in the 2000s, it became possible to emulate entire operating systems along with their desktop applications. It soon became a considerable opportunity for the industry, and the use of this technology has been increasing ever since. Deploying virtual applications, either inside virtual machines or in logical containers, has several advantages indeed:

  • It enables faster deployment, with reproducible builds (the emulated environment is easier to reproduce on every instance than it would be on different physical hosts with distinct hardware).
  • It is more scalable, and much more “agile”: if the virtual applications running over a physical machine are too demanding in resources, some of them may be moved onto another physical host. It makes it possible to ensure an optimal consumption of resources for all running physical hosts.
  • Regarding security, it is usually interesting to isolate sensitive applications inside a system that has no access to other sensitive data, and that can be reinitialized easily.

Of course, with the democratisation of Internet and of web services, and the rise of cloud services, those virtual machines would need to be connected between themselves as well as to a physical network. That would be the birth of virtual switches.

Software defined network
Network now also includes “VM to NIC” and “VM to VM” traffic

Virtual switches usually run on top of normal computers and servers instead of dedicated hardware. This also enables them to take advantage of the complex network stacks of the system acting as an hypervisor (e.g. Linux’ or *BDS’s stacks); and because it is usually more complex than the stack available on dedicated hardware, virtual switches were made able to address more and more complex requirements for packet processing operations, to the detriment of speed performance.

And virtualization keeps growing. But simple switches are no longer able to meet the industry’s requirements.

Software Defined Networking (SDN)

Since networks have been increasing a lot in size and in requirements, moving around hardware switches has become a burden. Even manually setting up individual software switches has become a complicated and error-prone task for companies running strongly virtualized environments along with large networks. This is where Software Defined Networking—SDN for short—enters the game, in the early 2010s.

“SDN” is a bit like “cloud computing”: it is a trendy topic in computing, and it is often sold as a “miracle” solution to all network infrastructure problems. But beside marketing, it also corresponds to a new type of network architecture. “Software defined” does not mean that it only uses virtual switches instead of dedicated hardware (even if it mostly does): it refers to the fact that switches can be programmed. Their behavior is defined by a software configuration. Indeed the main feature of SDN is that the switch control plane is decoupled from the data plane. What does this mean?

  • The control plane is where the administration of the network takes place: it corresponds to the setting up of the packet processing rules, and from there to the establishment of the whole network switching policy.
  • Data plane encompasses the application of those rules defined on control planes: this is the actual packet processing. When some packets require some particular, more complex processing, they can be handled to the control plane, where the decision regarding this packet will occur.

As a generic principle, the data plane must be very fast to handle a very high number of packets with simple rules so as to obtain a good bit rate in the network. By contrast, the control plane is slower. For legacy switches, the whole control and data planes would be tightly linked into a same hardware box. They would not be clearly delimited, and the switch would usually present a single setup interface. But virtual switches offer new possibilities: they make it doable to centralize the management of the switches and to remotely program them. The control plane is organized in such a way that switches take orders from a centralized controller in the network. This controller dynamically installs packet processing rules onto the switches, and receives and (slowly) handles exception packets when needed.

SDN stack
SDN architecture concept

If the controller is to be represented at the center of a diagram, it is attached to network-requiring applications—on top—by so-called northbound APIs, while it is linked to the data plane—below—by what are called southbound APIs, used both to set up the switches configuration and to handle the exceptions packets.

So this is what SDN looks like! Or… what it used to be in the first years. In fact, the distinction between control and data plane should be considered as an “implementation choice” of SDN, and not as a definition of SDN by itself. And as technology evolves, so does the architecture. Now the switching equipments often include separated components, thus creating an architecture with three levels:

  • “Control-management” plane is where the administration takes place, this is the controller configuring the remote switches.
  • “Control-plane APIs” of a programmable switch can be used to form a component that both receives setup instructions from the controller, and is able to somewhat perform simple updates of the data plane without always referring to the controller.
  • Data plane, as before, handles fast packet processing.

Some other architectures have also been designed; for example, the Neutron component of the OpenStack suite embeds both the controller and the data plane. They remain logically decoupled, but they run on the same host.

The expected benefits of SDN

This new architecture model provides a way to programmatically configure the switches at runtime, and to manage the network resources in a more efficient way. It enables the network operator to provide bandwidth ”on demand”. As virtualization made application development much more flexible and scalable, the same thing is occurring to networks with SDN.

Another advantage resides in the fact that SDN usually involves vendor-neutral technology: the software and protocols in use are often open-source, and in any case, as there is a move from hardware to software, there is no risk to get locked in on the technical side because of vendor-specific hardware requirements.

Also, since it is less hardware-dependant, and because all the switches need no more embed the control part, this paradigm is expected to lower the barrier to entry of the networking industry for new actors.

Because of those properties, SDN is becoming an essential concept to deploy cloud services, or to incorporate new paradigms such as BYOD (Bring Your Own Device) or IoT into industrial networks. These concepts represent new network usages, and they come with increasing needs for bandwidth: it seems today that some of these needs can only be addressed by Software Defined Networks indeed.


Along with SDN, new challenges have emerged. The basic functionalities of programmable switches have become somewhat independent from the hardware in use, so the software part must provide efficient switching capabilities. New algorithms or protocols (think about the way the controller should configure the switches, for example) had to be designed, both for the control plane and the data plane.

But even with new software tools, the functionalities of the data plane remain at a basic level, so as to gain on processing speed. But this has a cost in terms of available features, and of ease of use: indeed the addition of a new feature (new protocol, modified network topology, …) can require an upgrade of all data planes, thus representing a heavy constraint on production environments. Therefore one of the challenges consists in developing data planes with high performances but presenting a powerful programmable, “updatable” interface. Good software design is of paramount importance!

Hardware is not completely put aside, though. It is mandatory to interface the software side with the hardware cards in an efficient way to obtain good performances. And getting good performances is one of the principal objectives of SDN! Performances for bit rates, but also for resources consumption—the more CPUs remain available to user applications, the better—or even for other subsystems such as storage: higher throughputs mean more data, which in turn must be forwarded to fast and efficient storage backends.

Another huge challenge of SDN is security. The network topology evolves: the basic architecture gives way to a decoupling of control and data planes. This new architecture makes it even more feasible and easy to update the network topology at runtime. This, in turn, makes network components harder to secure and to monitor. In particular, it is essential that commands on the control plane remain protected. And the use of virtualization makes things even worse: when several appliances run on a same physical host, they must share its resources but must not leak their data. There is a lot of ongoing research on this subject—because much remains to be done!

Around SDN

Here are a couple of concepts and technologies that are tightly linked to SDN.

Network Function Virtualization (NFV)

Network Function Virtualization is an alternative to SDN. It is another network services architecture based on virtualization. Instead of focusing on the separation between control and data plane, it places the network functions—routing, firewalling, load balancing, etc.—inside virtual machines that act as some sort of “boxes”. Those blocks can be assembled or chained to compose the whole network architecture. Note that sometimes, we also hear about Virtual Network Function (VNF), but this seems to be just another name for NFV.

SDN and NFV are two “novel designs” for network services. And yet, they are not two concurrent visions, but rather complementary solutions: SDN takes in charge the automation of the network, implementing policy-based forwarding to establish dynamic and resilient traffic rules, while NFV focuses on the services that are to be provided over the network, enabling a good provision and distribution of the virtualized resources.

So NFV and SDN are very close concepts, both relying on virtualization to provide on-demand and scalable network architectures; and concepts or mechanisms that can be used with one of them can often be used for the other as well. Finally, it is worth noting that some articles or presentations (mostly for marketing) will focus only on NFV even though the principles could also apply to SDN—or the other way round.

Specific technologies

The obvious technology involved with SDN are related to virtualization and to networking. About virtualization, the usual hypervisors are used. they are either proprietary software, such as VMWare technology, or open-source, as for Qemu. Technologies in use for the deployment of “cloud” architectures, such as OpenStack, are often closely related to virtualization technologies, and of course to SDN.

But the main point here is about networking technologies. Besides encapsulation protocols such as VXLAN, GRE or MPLS, that are often used in data planes to deploy flexible architectures, there is an essential protocol which is used to link control to data plane: this is OpenFlow®. OpenFlow, managed by the Open Networking Foundation (ONF) and currently in its 1.5.1 version (published in March 2015), is used between the controller and the switch. The controller uses OpenFlow messages to configure or to monitor the software switches. It is an implementation of the aforementioned southbound API. This protocol is considered as a historical basis for SDN, and as of today, it is part of most deployed SDN architectures.

As for the virtual switches themselves, it seems that the main software in use is Open vSwitch, an OpenFlow-compliant software switch with additional extensions from Nicira (VMWare).

On the controller side, many solutions exist. There are large frameworks such as OpenDaylight platform, or simpler controllers such as Ryu (Python)… and many others.


There are a couple of organisms in charge of developing and managing standards for the SDN universe.

The Open Networking Foundation (ONF), as stated above, handles the specification of OpenFlow protocol.

The European Telecommunications Standards Institute (ETSI) work and develop industrial standards on a variety of topics related to telecommunications and networks, including SDN. Also, after intensive work sessions, they sometimes have “beer” events with excellent vodka, or so I heard; but I am afraid this might bring us off-topic, so let’s focus on the standardization part…

The Open Platform for NFV Project (OPNFV) is a platform supported by the Linux Foundation and develops material for NFV, so as to provide improved Virtualized Network Functions (VNF) and Management and Network Orchestration (MANO) components.

All in all…

Software Defined Networking, along with Network Function Virtualization, are two new paradigms that heavily rely on virtualization to move networking services and architecture from hardware to software. For wide architectures, in datacenters and networks of telecommunication or cloud-based services operators, baremetal switches are being abandoned for virtual ones. Thanks to automation of policy-based forwarding and to dynamic resources allocation, they scale much better and provide a better use of resources, while making deployment, configuration and monitoring of the services much easier. They are mostly based, for the network components, on open-source technologies, keeping away the constraints associated to locked-in proprietary solutions.

There has been a lot of activity on SDN ever since it appeared in early 2010s, and there is still much to work on… Providing by the way some matter for more blog articles!

Additional references

  • SDxCentral is a website gathering information and news about SDN. It is often related to events or marketing campaigns from companies working in this field (including 6WIND), but they also have some technical resources about SDN or NFV.
  • RFC 7426 is an informational document from the IETF, and defines Software-Defined Networking (SDN): Layers and Architecture Terminology. It may not be the easiest introduction to SDN, but it contains precise definitions of the terms in use in the domain.
  • The Open Networking Foundation (ONF) has some resources as well, including the specification of OpenFlow protocol.
  • Wikipedia, as usual, has a number of articles related to SDN technologies. Some parts of the article about SDN itself are strangely similar to what is available on the ONF website.
  • … aaaand as I work at 6WIND, of course I encourage you to have a look at the resources available on our website! There are both technical and commercial information about SDN, NFV or technical solutions. Also, if you love SDN already and would like to push packets much faster into the pipe, check our job offers!