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The controller, as its name indicates, is designed to control the data plane and receives from the application layer the necessary elements to determine the type of control that needs to be exerted. This position is shown in Figure 2.11. Thus, the controller must receive policy rules to be respected. On the basis of the description of the applications to be taken into account, it deduces the actions needed on the networking equipment. The actions can be carried out on routers, switches, firewalls, load balancers, virtual VPNs and on any other hardware that is necessary to the good functioning of the network.

A very great many open source controllers have been developed. OpenDaylight represents a good example; this open source software was developed by the Linux foundation. A good 40 companies devoted experienced developers to the project. The controller as such has numerous decision-making modules and various northbound and southbound interfaces. We will describe the latest release of OpenDaylight hereinafter.

Figure 2.11. The control layer and its interfaces

The controller contains modules that handle different functions necessary for the proper operation of the network. Of these functions, one of the most important is that of a “load balancer”. In fact, this term denotes algorithms that determine the best paths to follow on the data plane. This module must decide, on the basis of the statistics received, which nodes in the network infrastructure the packets should be sent through. This decision should optimize the user demands or, more specifically, the user applications. Load balancing is essentially valid during peak hours. During other times, the load balancer must determine the most appropriate paths, on the basis of the users’ requirements. Load balancing becomes load unbalancing: unlike at peak times, we need to channel as many streams as possible through common paths so as to be able to shut down the maximum possible number of intermediary nodes. This function is represented diagrammatically in Figure 2.12.

Figure 2.12. The load balancing protocol