  The emerging Software Defined Networking (SDN) paradigm separates the data
plane from the control plane and centralizes network control in an SDN
controller. Applications interact with controllers to implement network
services, such as network transport with Quality of Service (QoS). SDN
facilitates the virtualization of network functions so that multiple virtual
networks can operate over a given installed physical network infrastructure.
Due to the specific characteristics of optical (photonic) communication
components and the high optical transmission capacities, SDN based optical
networking poses particular challenges, but holds also great potential. In this
article, we comprehensively survey studies that examine the SDN paradigm in
optical networks; in brief, we survey the area of Software Defined Optical
Networks (SDONs). We mainly organize the SDON studies into studies focused on
the infrastructure layer, the control layer, and the application layer.
Moreover, we cover SDON studies focused on network virtualization, as well as
SDON studies focused on the orchestration of multilayer and multidomain
networking. Based on the survey, we identify open challenges for SDONs and
outline future directions.
