# A Systematic Literature Review on Military Software Defined Networks
## 5 Additional Contributions
### SDN Control Plane Resilience and Security  
[PERSON] et al. [81] presented a survey of disaster resilient SDN networks that is transferable to the military context. The survey points out unresolved issues concerning security considerations and disaster scenarios. To overcome the single point of failure (SPoF) of a network controller, multi-controller and multi-domain topologies have been suggested, but, according to the author, this approach introduces a new security problem on the orchestration and management layers. Trends of cloud computing services also apply to military services, where SDN networks with one network controller often are pointed out as a critical part of the infrastructure [74]. [PERSON] et al. [84] suggests an efficient way to overcome the SPoF problem by putting a redundant load balancer in front of a pair of network controllers. However, multi-controller orchestration is not mentioned in the article. A similar approach was also demonstrated by [PERSON] et al. [35] (see Section 4.5) that presented an SDN load balancing design for naval networks. In military networks, it is critical to maintain access to network services, especially under disasters and critical operations. The SPoF problem does not only concern the availability and the load on the controller, but it also applies when the network devices do not have network connectivity to the controller. Distributed content caching in vehicles, can assure that information can be distributed to devices around the vehicles if the vehicles lose up-link network connections. [PERSON] et al. [67] suggests to use many distributed SDN controllers and to alter the OpenFlow rules during online and offline caching operations. The paper demonstrates the concept, but it introduces a security concern due to operating multiple SDN domains that is not mentioned in the article. However, [PERSON] et al. [80] presents a survey of the relationship between SDR, SDN and NFV and points out security and inter-domain communication of SDN controllers as the main research challenges, something that is also transferable to the military context. [PERSON] et al. [75] showed how SDN in a federated network can enforce network policy control across domain borders in military networks. However, in a multi-domain SDN topology with distributed SDN controllers, the SDN flow rules can come in conflict with each other. In networks such as military networks, security precedence can have higher prioritization than the current OpenFlow specification allows. The authors of an architecture named Brew [64] suggested to extend the OpenFlow specification in order to calculate reconciliations of conflicting SDN rules between controllers. In addition to conflicting rules, the integrity of the rules is also pointed out as a security weakness in multi-domain topologies. This is presented by [PERSON] et al. [72] who is suggesting to use a BGP route reflector with route filtering capabilities based on allocation, reputation and path analysis. SDN with OpenFlow can then contribute to making route filtering policies. However, [PERSON] et al. [76] also show that within a single OpenFlow domain, multiple SDN applications must also meet stringent security requirements. The authors here presented a security framework to handle conflicting SDN rules between different applications. Furthermore, [PERSON] et al. [85] focused on mitigating DNS Query-Based DDoS Attacks with machine learning on SDN, and implemented a prototype based on traffic features using Dirichlet process mixture model (DPMM) for clustering traffic applications flow, including those used for DDoS attacks in an unsupervised manner. Designing a critical infrastructure topology also includes placing network devices strategically, in order to achieve network redundancy and to have backup sites ready when disaster happens. [PERSON] et al. [65] suggests utilizing SDN flow-rule parameters combined with monitoring, in order to calculate where it is most efficient to place backup switches to make the network more resilient. [PERSON] et al.