# A Systematic Literature Review on Military Software Defined Networks
## 4 Identified Significant Research Contributions
### Control Systems
The authors describe in detail three logical topologies for the SDN controller, namely: (i) centralized, (ii) partially-meshed, and (iii) extension to static hosts. Within the same application domain, [PERSON] et al. [31] summarized a set of research challenges related to Content Based Networking (CBN) in military coalition networks. As presented by the authors, in a joint military operation, the key focus is to bring together different partners into a single mission, maintaining the ability to securely share data across the distinct teams. Furthermore, it is required that the network operators are capable of performing analytics and maintain service related situational awareness in a resource constrained network. SDN is promoted by the authors as an enabler for CBN in respect of easing the complexity, and allowing the dynamic discovery of distributed information. However, centralized network control, such as OpenFlow, does not fit with disruptive networks. The paper suggests placing future research focus on: (i) hybrid SDN in order to allow network independence from the controllers, (ii) east-west control plane communication between network controllers, (iii) security implications related to dynamic distributed services, (iv) content aware networking by mapping the service request to the network control, and (v) distributed analysis in order to derive human situational understanding. Also focusing on coalition networks, [PERSON] et al. [40] examined how the principles of SDN can be utilized in order to improve cyber situational awareness in coalition environments within military networks. The authors discuss the adaptation of the Observe, Orient, Decide, Act (OODA) loop within SDN in order to improve security awareness. The OODA loop describes a decision-making process and is reflected in an SDN controller application. The authors also suggest that an east-west communication protocol is required for controllers to share what they have learned. The article explains comprehensively the concepts related to the OODA loop but does not provide an extensive discussion over the corresponding requirements and constraint for its adaptation on SDN. Consequently, [PERSON] et al. [28] examined the application of SDN across military coalition operations, proposing a mechanism for enabling dynamic Communities of Interest (CoI) within these environments, and evaluating such interoperability architectures in accordance with key performance metrics. Furthermore, this article provides a comprehensive overview of tactical communities of interest and the underlying constraints towards their deployment and management. The authors proceed by merging the two, and discussing the topic of Software Defined Coalitions (SDC), as the mechanism that is capable of facilitating the operation of dynamic tactical CoIs. Accordingly, they identify three interoperability levels, namely: (i) network, (ii) network and storage, and (iii) network, storage, and compute. They clarify that (ii) combines mechanisms from SDN and Software Defined Storage, while (iii) builds upon the concept of Software Defined Environments. Consequently, the article proposed three types of architectures, namely: (i) simplification, (ii) brokered, and (iii) federated, comparing the three in terms of complexity, trust, and standardization. The presentation and discussion of the proposed mechanisms is sound and comprehensive, although further scenario based validation/verification can be desirable. With focus on aerial networks, [PERSON] et al. [58] proposed an SDN and SDR architecture that can predict network outage in aerial networks. Under the assumption that aerial flights have fixed orbits, the authors suggest that the radio can inform the centralized controller about its future position,allowing for the flow routes and radio links to be switched in advance and prior to network outlege. Hence, the availability is expected to be increased by the use of predictive SDN. The presented architecture is based on the assumption that the radio link outage can be predicted.