3 Software Defined Networking (SDN)
3.2 Separation of Network Control and Data Forwarding Plane

Before we describe the concept of SDN, it is necessary to define what the Control plane and Data Forwarding plane are.

In most of routers (or any networking equipment for that matter) there is specialized hardware for fast switching of data between interfaces - Data Forwarding plane. The forwarding is managed by rules created by processor running operating system, routing algorithms, address translation, and other higher functions - this is the Control plane.

In traditional networking, both control plane and data forwarding plane are implemented in every network node. This enables every device to be totally autonomous and make all high level decisions, such as packet routing independently.

This stems from origin of the Internet – ARPANET, initially designed by and for the military with high resilience and survivability as primary concern. Flexibility, straightforward modification or adoption of new features was a secondary goal at best.

The fundamental principle of SDN is separation of control and data forwarding plane in network as depicted in Fig. 3. By the implementing of the separated control plane by the software for general purpose computer from forwarding plane on network equipment, it is possible to centralize routing and switching decisions as well as configuration of all network devices.

The centralized control plane implemented in software executed on general purpose processors can bring many advantages to networking - especially speeding up innovation, new network features development and deployment.

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Fig. 3 - Basic SDN architecture