# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 5 Conclusions and Research Questions  
Small satellite networks, particularly CubeSat networks, create a challenging environment for efficient data routing while considering that both the CubeSats mobility model, which creates a highly dynamic network that causes the network to be disjointed and the limitations of the CubeSat network, such as the size of the CubeSats, energy, radio range, and bandwidth, resulting in intermittent connectivity among the satellites over time. This paper highlights the main challenges in satellite communications, specifically when the network suffers from intermittent connectivity. Routing is the most challenging issue in satellite communications, where traditional routing protocols do not work. By conducting this comprehensive survey on the state-of-the-art DTN and Non-DTN routing protocols, some issues that are related to applying existing routing protocols on CubeSat networks were identified. Accordingly, single-layer and multi-layer routing protocols continue to face additional issues. In single-layer routing, each CubeSat must maintain its routing table, which will consume some of its limited energy and will be also associated with some computational delay and storage complexity. The multi-layer protocols can solve this issue by transferring the routing table costs from the core satellites to a limited number of satellites in the higher layer. These higher-layer satellites should be featured with higher performance capabilities, so that each one of them can coordinate with a certain number of satellites in the lower layer. However, this poses another issue regarding increasing the traffic on cross-layer links where congesting such important links could raise the possibility of the failure of one or more of these backbone links, leading to the unfortunate result of severe degradation and actually jeopardise the mission. The assumption of network connectivity and the application of IP based routing protocols in the existing single-layer and multi-layer routing protocols make them inefficient for the disjointed CubeSat networks, which results in DTN routing protocols being adopted as a reasonable alternative. Based on the mission requirements, different DTN routing protocols can be chosen, where DTN routing protocols are suitable for different space missions, such as near the earth CubeSat networks, deep space networks, and interplanetary CubeSat network. DTN routing protocols can work on top of both single-layer and multi-layer CubeSat networks. This will open new avenues for routing optimisation under the new communication conditions of CubeSat networks that are based on the intended mission objectives. Accordingly, current DTN routing protocols have been compared and analysed to find that, while the flooding-based routing can provide the highest delivery rates and minimum delivery delay, it is still inefficient for use in the resources-scar CubeSat networks. This has been concluded because of the very expensive overheads that may harvest these limited resources and that conversely lead to rapid network degradation. The DTN quota-based routing protocol can be used to overcome the limitations of flooding by limiting the number of the message copies; however, quota-based routing protocols still produce high overheads that may lead to high congestion in such a restricted environment. Alternatively, the DTN single-copy forwarding protocols can be used and they will not acquire any overheads, as there is only one copy of the message to be forwarded to the destination. Unfortunately, these single-copy forwarding protocols suffer from long delays and a low delivery ratio. The DTN history-based routing protocols can build global knowledge regarding the whole network by exchanging information with all encountered satellites and using negative feedback.