Software Defined Network: Definition & Purpose

Instructor: Muhammad Wannous

Muhammad has been teaching Computer Sci. and Eng. and has a Ph.D. degree in Computer Sci. and Electrical Eng.

This lesson covers Software Defined Networking (SDN), the paradigm that enables making changes to complex networks which are typically challenging to adjust. You will study what SDN does and how.

SDN for Easing Network Tasks

Teresa is an IT professional specialized in computer networks. She is currently coaching Connie, the fresh IT engineer who recently joined the company. The two ladies are now at one of the company's data-centers examining its infrastructure.

Teresa: When the company was just a startup the network was simple and easy to manage, but now it has expanded both horizontally and vertically. Currently, it covers some data centers and contains hundreds of devices from many vendors.

Connie: I think that adjusting such a complex network to cope up with the ability to bring applications online with a click of a button, as in a cloud computing environment, is one of the difficult tasks you have to carry, right?

Teresa: You have brought up an excellent example that will help me explain about the technologies we adopt in our network. As you mentioned, the cloud computing environment increases the dynamics of the network traffic. A complex conventional network would not adapt as quickly as required, but another one adopting Software Defined Networking (SDN) architecture, would.

Connie has studied SDN but never had the chance to see it in action, but she thinks that it is complicated and not readily applicable.

Connie: But don't you think that SDN adds complexity to the network?

What is SDN?

Teresa: SDN is a networking scheme that divides the conventional architecture into separate layers. Take a look at Figure-1 which shows the new arrangement.


Figure-1: SDN Architecture
SDN Architecture


In the middle layer resides an SDN controller that maintains a unified image of the whole network. It simplifies and centralizes all administration tasks. SDN has made the network programmable or in other words, enabled the automation of many administrative tasks.

How does SDN work?

Connie: But how does the SDN controller communicate with the underlying network? Does it use the protocol specific to every device in the network it is managing?

Teresa: The SDN controller communicates with the underlying network devices over a secure channel using a standard communication interface called the southbound interface, and one of the broadly accepted protocols for this interface is OpenFlow. This protocol uses the concept of a flow to identify network traffic. The flow is a concept abstract from the underlying network structure. The SDN controller uses the flow to determine the real path of traffic and configure the devices on this path. OpenFlow-compatible network switches maintain flow tables that are used for forwarding the traffic. Some flows can be grouped to follow one rule when forwarding and this rule is saved in a group table. The switch performance metrics related to a particular flow are collected in a metric table. You can imagine such a switch as the one in Figure-2.


Figure-2: OpenFlow-compliant Switch
OpenFlow-compliant Switch


The SDN controller uses a programming interface to communicate with the physical switches over OpenFlow protocol and give instructions to manage its configurations.

Connie: Amazing. However, what happens when a change in the network configuration is necessary? For example, a case when the resources assigned to a customer in a cloud need to be migrated into a new server/location! Don't you have to make some changes to the physical connections?

Teresa: Not at all. The event of resource migration or any change in the traffic pattern triggers the SDN controller to update the flow tables inside the physical devices after determining the new real path of the flow. The existing connections need not be changed.

Connie: Wow. It is like a new layer above the routing layer.

Teresa: You can say that.

Connie: But I can see in Figure-1 that the applications also communicate with the control layer.

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