For the past year, software-defined networking (SDN) has been the buzz of the networking world. But in many ways, networking has always been defined by software. Software is pervasive within all of the technology that impacts our lives and networking is no different. However, networks have been constrained by the way software has been configured, delivered and managed—literally within a box, updated monolithically, managed through command lines that are throw-back to the days of mini-computers and DOS in the 1980’s.
http://www.juniper.net/us/en/local/pdf/whitepapers/sdn-whitepaper.pdf