Let us start with a formal definition of SDN, The following definition cited comes from the Open Networking Foundation (ONF) (www.opennetworking.org),which is a user-driven nonprofit organization dedicated to the promotion and adoption of SDN through open standards development.
SDN is an emerging architecture that is dynamic, manageable, cost-effective, and adaptable, making it ideal for the high-bandwidth, dynamic nature of today's applications. This architecture decouples network control and forwarding functions, enabling network control to become directly programmable and the underlying infrastructure to be abstracted for applications and network services. The OpenFlow® protocol is a foundational element for building SDN solutions.
The key themes of the ONF definition are:
Separation of the control from the forwarding plane
Software programmability for network elements
Centralized network control and management
What do we mean by the decoupling of forwarding...