# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
###### Abstract
The DTN routing protocols are categorised into three groups, namely: (i) flooding-based protocols, where the number of packet's replicas is dependent on the number of encountered node, (ii) quota-based protocols, where the number of packet's replicas is fixed, and (iii) forwarding-based protocols, where there is only a single copy of the packet throughout the network. Figure 3: Delay Tolerant (DTN) and Non Delay Tolerant (Non-DTN) routing protocols categories. A wide-range of routing protocols in CubeSats communications have been studied in recent years due to the economic benefits and greater spatial and temporal performance. Most of the previous work on inter-CubeSat communications has focused on developing physical layer links i.e., antenna design. In Reference [9], CubeSat missions that are based on cross-link communication are surveyed. The main focus is on the physical connectivity among CubeSats, such as frequency allocation. Similarly, in Reference [10], the suitability and the challenges of intra-satellite and inter-satellite links in wireless sensor space networks are discussed. Additionally, different types of planer antennas for CubeSats communication applications are studied in [11; 12; 13]. In Reference [1], the communication capabilities of small satellites over inter-satellite links are presented from the view of the last three lower layers of the OSI. In Reference [14], some of the routing approaches for conventional satellite networking are reviewed with a focus on networking challenges in the space environment. Most recently, some features of small satellites and CubeSats are reviewed, including mission objectives, CubeSat subsystems, topology examples, and communication protocols [15]. In Reference [16], energy-aware routing protocols are proposed for inter-CubeSat swarms communication. However, there was no sufficient focus on the different routing protocols that can be used for CubeSat networking. From the above, prior work has not comprehensively investigated the suitability and applicability of both Non-DTN and DTN routing protocols for the different topologies of CubeSat networks. Hence, this paper discusses and analyses the suitability and scalability of using the Non-DTN based single-layer and multi-layer routing, as well as DTN routing protocols in CubeSat networks. This survey provides a comprehensive review of the state of the art space routing protocols under DTN and Non-DTN architectures. The review qualitatively compared different TCP/IP single-layer and multi-layer routing protocols, as well as DTN routing protocols. The applicability of the compared protocols is discussed for both connected and disjointed CubeSat networks in order to highlight the cases where DTN and Non-DTN protocols can be used. Most of the other CubeSat communication surveys focus on the CubeSats physical layer capabilities to improve the performance of the network; however, this survey investigates routing issues in the network layer level. The performance of the abovementioned routing protocols in different CubeSat network sittings is analysed and discussed accordingly. Section 2 of this paper describes the existing challenges in CubeSats communications and, in Section 3, particular focus is given to Non-DTN based single-layer and multi-layer routing protocols by providing a comprehensive review. In Section 4, we compare the existing DTN routing protocols and highlight the drawbacks and advantages. Lastly, we conclude our survey by raising open research questions from the existing gaps within the current state of the art in Section 5.