# A Systematic Literature Review on Military Software Defined Networks
## 1 Introduction  
Certain types of application domains rely fully or partially on constrained dynamic networks due to technical, physical or other limitations of financial and regulatory nature. Military and emergency response networks are examples of such application domains, while their study is further motivated due to the nature of the scenarios and areas of their utilization. Four levels of command have been defined across military operations, namely (i) political, (ii) strategic, (iii) operational, and (iv) tactical. The infrastructure serving the three higher levels of command typically relies on comparatively over-provisioned wire-line and wireless networks. Contrary to that, the networks serving the tactical level mainly consist of constrained mobile devices, with self configuring characteristics, and connectivity limitation both locally and towards the infrastructure serving the other levels. In addition to bandwidth limitations and unstable connections, military networks require tailored security solutions, which consequently create additional overhead for the network and can influence the stability of the connections. The capabilities of the nodes are also often limited by low battery life, low storage capacity and limited CPU power. These networks are characterized as Disconnected, Intermittent and Limited networks (DIL) or disruption-tolerant networks (DTN). Trends of data-centric solutions in the civil domain such as Software Defined Networks (SDN) and Network Function Virtualization (NFV) execute the network control in a centralized manner, while CPU intensive services are distributed. These principles have the potential of satisfying the network constraints of military networks. Hence, the main idea behind this literature review was to discover how these trends of SDN have been adopted to constrained military networks, and how they can contribute to solve their operational challenges. Although not all types of tactical nodes are constrained, and sections of the network may operate under a non-constrained mode, communication across the tactical network and towardshigher levels of command must be optimized for the support of the required military capabilities. Accordingly, this generates strict requirements to the deployed service delivery and policy based management systems, both in terms of security and quality of service. This is aggravated by environmental limitations, which arise due to the aforementioned constrained and dynamic nature of military operations. Accordingly, such deployed supporting mechanisms are affected, since they have to operate within severe constraints, in terms of the locally achievable bit-rate and throughput, high packet loss and packet loss variability, asymmetric link characteristics, and high rate of change and unpredictability within the network topology graph. Furthermore, currently used network devices commonly integrate functions related both to the control and data planes. The network control planes are often distributed to enable the networks to operate autonomously. However, SDN opens up for moving the enforcement of policy management and QoS (Quality of Service) control from a service application oriented control plane to a network oriented control plane. Software Defined Networking is defined as the concept of separating the control plane and the data plane. Practically, this separation results in a clear distinction between a network controller and the distributed network devices. However, the separation of the planes is implemented in various versions of SDN where the level of centralized control slightly differs. These different SDN protocols also have different application domains and can contribute to multiple layers in the networking stack. SDN can for example both be a tool to enable distributed DDoS protection and it can be a control plane for routing network traffic. The main part of this literature review classifies SDN into different categories with respect to how SDN can be an enabler for network traffic routing.