3 Software Defined Networking (SDN)
3.3 Centralized Control and Network Programmability

In modern network equipment the control plane gathers information about the status of connections to neighbors and network as a whole that is supplied to routing algorithms. The distributed algorithms provide automatic solution to routing network traffic, but one their shortcomings is possible problem with convergence. Since every device gathers information about the network and makes routing decisions by itself, delay in information propagation about network changes can lead to problems in network operation.

Eliminating control plane from every network node and using centralized control plane not only allows for a complete view of the network without difficulties with convergence inherent to distributed routing algorithms, but they also decrease cost. Additionally, without need for distributed traffic routing, new algorithms can be used, modified and developed. Furthermore, configuring the central control plane makes network management simpler, removing need for configuring every node separately. This reduces chance for misconfiguration and expedites troubleshooting too. The added benefit of software implementation of centralized control plane is ease of modification and development of new features.

However, opponents of SDN and centralized control plane especially point to limits of its usability in large scale networks such as the Internet. Their criticism focuses on controller being the single point of failures for the whole network and insufficient scaling.

Solution of these problems is using logically centralized, but physically distributed control plane – where a number of controllers manage the part of the network and communicate together to reduce the control plane latency, provide high availability and single logical management point for the network administration. New architecture for the inter-domain connection with associated communication protocol for SDN controllers is proposed in [6].