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Software-defined networking (SDN) is a newly emerging computer networking architecture. Its main distinguishing factor is the separation of the data plane from the control plane in routers and switches. In other words, the control is decoupled from hardware and implemented in software. Under this architecture, the implementation of the control plane is via software within the servers and is separate from networking equipment, while the data plane is implemented within networking hardware or equipment. The best example of this architecture is OpenFlow.
To understand software-defined networking better, first we must look at how traditional networking architecture treats data packets. When a data packet arrives at a switch or router, the firmware tells the hardware where to forward the packet and sends all packets to that destination via the same path. In other words, it treats all packets in the same fashion. More advanced smart switches equipped with application-specific integrated circuits (ASICs) can recognize different types of packets and treat them differently based on the ASICs are programmed. The problem is that these solutions are quite expensive.
SDN however, decouples the control from the networking hardware’s firmware and puts it in the hands of the network administrator. He or she can "shape" network traffic from a central control console without changing the settings of individual switches. This means the administrator can change network rules, prioritization as necessary and even block certain packets with great control. SDN is therefore very important for cloud computing (which has a multi-tenant architecture) because it allows for a more efficient and flexible control of traffic loads.
SDN is a cheaper alternative to previous types of networking because it allows for the use of cheaper commodity switches yet provides better control of traffic than before. Network engineers and administrators can support switching fabrics across hardware from different vendors and models as well as integrate switches with ASICs and those without. OpenFlow is currently the most popular specification for SDN and allows remote control of routing tables.