SDN Architecture is used synonymously with OpenFlow, a new standard communications protocol between the control and forwarding layers of a software-driven network. 

SDN architecture
Fig - SDN Architecture

The Open Networking Foundation (ONF) is the organization to promotes and adoption of Software-Defined Networking (SDN) through open standards development. ONF defines Software-Defined Networking as "an emerging network architecture where network control is decoupled from forwarding and is directly programmable. This migration of control, formerly tightly bound in individual network devices, into accessible computing devices enables the underlying infrastructure to be abstracted for applications and network services, which can treat the network as a logical or virtual entity." 

SDN architecture - SDN layers
Fig - SDN layers

SDNS is a way of architecting networks in such a way that network automation is a central concept. One way to achieve this is to move away from hardware and give it to a software application called controller, When a packet arrives at a switch in a conventional network, the rules are integrated into the firmware owner of the switch where to transfer the packet. The switch sends cach packets to the same destination on the same path - and treats all packets in exactly the same way. In the enterprise, smart switches designed with application-specific integrated circuits (ASICS) are sophisticated enough to recognize different types of packets and treat them differently, but these switches can be quite costly. In a software-defined network, a network administrator can shape traffic from a centralized control console without having to touch individual switches, The administrator can change any rule of network switch when necessary - giving or removing priority, or even blocking specific types of packets with a very detailed level of control. The SDN is sometimes referred to as the "Cisco killer" because it allows network engineers to support a company switching through multiple vendors' hardware and application-specific integrated circuits. Currently, the most popular specification for creating a software-defined network is an open standard called OpenFlow. OpenFlow allows network administrators to remotely control routing tables. In an SDN environment, customers can see all of their devices and TCP flows, which means they can slice up the network from the data or management plane to support a variety of applications and configurations.