# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 4 DTN Routing Protocols in Disjointed Satellite Networks
### DTN Forwarding-Based Techniques
However, this new definition of the TC problem is quite dissimilar to the typical space-time-routing [73; 77], which focus on finding the most cost-efficient space-time route between one pair of source and destination nodes. The authors also discussed the possibility of addressing the topology control problem in undirected delay-tolerant networks by using the proposed greedy algorithms. The cost-efficient topology control problem (CETC) is also defined in [71], with a new factor being added to the TC problem that the cost of the least-cost path that links any two nodes in the extracted space-time graph should be more cost-efficient when compared with the least cost path in the original space-time graph. Towards this aim, two new greedy algorithms: Greedy Algorithm to Delete Links (GrdDL) and Greedy Algorithm to Add Links (GrdAL), were introduced to minimise the total network topology cost while ensuring the connectivity and the cost-efficient paths among any two nodes during time evolution. All of the aforementioned algorithms were investigated by conducting simulations for each algorithm on both the random DTN networks generated by a classical random graph generator and the real-world DTN networks based on CRAWDAD statistics [80]. The simulation results show that the proposed algorithms can save, in cases of the low-density network, approximately 50 percent of the network cost and 50 percent of the number of edges; however, about 95 percent of costs and edges can be saved in the high-density networks. Even though the results demonstrate a proportional efficiency of the proposed methods, some limitations and weaknesses still exist. Firstly, the CETC considers the network from time slot (0) to time slot (T) on the space-time graph and packets were generated at the time (0). In some cases, packets that may arrive in the middle slots may not be delivered to the destination at the end of the period (T) on that extracted space-time graph. Secondly, the complexity of some algorithms is high, so the algorithm might experience long delays if the number of the nodes and the number of the time slots is increased. Thirdly, the TC problem is defined for predictable DTNs with the consideration of time evolution, where there was an assumption regarding the reliability of future links to deliver data without any errors or distortion and, additionally, no link recovery techniques were suggested. In reality, such a strong assumption might be acceptable for particular forms of DTNs, but it cannot be applied to general DTNs, as, practically speaking, the satellite wireless links are unreliable because of the instability of wireless channels. Although the mobility in some DTN can be prior predicted according to historical statistics or the nature of user movement, in some cases these predictions can be incorrect and, as such, it would be quite remarkable to investigate the TC problem with the consideration of DTN routing reliability as a new and important factor. Finally, all of the proposed algorithms have aimed to reduce the number of links on the extracted space-time graph to minimise the costs. However, this process can reduce the performance of the network in aspects, such as routing. Therefore, it is important to study the effect of cost minimisation resultant from TC on the routing performance to consider more optimum routing. In References [81; 82], heuristic-based DTN algorithms were proposed for drones flight path optimization. These algorithms optimise the route for both assignment and data delivery with higher delivery efficiency compared with other DTN algorithms such as EBR and Epidemic.