# A Systematic Literature Review on Military Software Defined Networks
## 1 Introduction
The second part of this literature review discusses SDN related technologies such as Network Function Virtualization and Software Defined Radio that we refer to as additional contributions, as the principles of separating data and control planes have also been adopted by Software Defined Radios. Compared to traditional SDN such as OpenFlow, SDR differs because it works on layer 1 and 2 and focuses more on virtualized software. Correspondingly, other SDN related technologies use similar principles by virtualizing both networks and software, raising the question on which technologies are defined as SDN and what SDN really is. This literature review does not aim to review SDN or define SDN, but aims to review the types of SDN technologies that have been adopted to military networks. Hence, the second part of this review classifies SDN related technologies in a military context. Historically the first SDN research from Stanford in 2008 aimed to provide a centralized controller capable of making global forwarding decisions based on packet header attributes. The main idea was to reduce the need of expensive network devices and advanced routing protocols, with a centralized controller making all forwarding decisions. However, in modern networks, a hybrid SDN approach is used to face the challenges of legacy management and legacy control. Hence, modern routers can have multiple instances of SDN domains where we face new challenges of inter domain communication between network controllers. Not only control commands to network devises are important, but orchestration of controllers and control messages between the controllers. The multiple levels of command in military networks indicate a network topology associated with multiple SDN domains, while the results of this SLR also indicate that securing such topologies is a significant future research challenge. This study is a systematic literature review on military SDN and not a comparative survey of SDN technologies at large, as comprehensive studies with this focus already exist (see Section 2). Therefore, the scope of our study covers only research articles that include SDN in a military context, seeking to establish and examine the current state of obtained knowledge on military SDN. This implies that papers which include SDN technologies but not within a military context are omitted by this review. Section 3 includes a detailed description of the method followed for conducting this SLR. This work seeks to evaluate the current state of the art in terms of research tracks, publications, methods, trends, and most active research areas, with focus on military SDN. The remainder of this article is structured as follows: Section 2 presents related work, while Section 3 presents the utilized research method for this systematic literature review. Section 4 contains a comprehensive discussion over identifiedsignificant research contributions, while Section 5 discuss additional contributions. Finally, Section 6 provides a statistical analysis of our findings and the corresponding metrics, while Section 7 analyses directions of future work, and Section 8 includes our conclusions.