# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
###### Abstract
Indeed, the largest batch that was launched into space on a single rocket was Flock-3p, which consisted of 88 CubeSats [6]. Figure 2 shows the current and future NanoSats launch statistics. The advancement of miniaturised space technology and the capability of multi-satellite missions have allowed for the establishment of enormous Nano-satellite networks. These satellites can then communicate with each other without human intervention by means of Inter-Satellite Links (ISL), where the ISL provides the essential capabilities of real-time or quasi-real-time communication. Deploying low-cost swarms or constellations of Nano-satellites with ISL opportunities can simultaneously improve the spatial and temporal data collection from different parts of space [7]. Figure 1: Standard CubeSat Sizes (1U-12U). Figure 2: Nano-satellite launch by size statistics [4]. CubeSat networks can be deployed in different formations/topologies according to mission objectives. In reference [8], [PERSON] et al. categorised 39 CubeSat missions, according to the mission objectives, satellite number, and type of formation flying. The topology of small satellites can be mostly divided into three types, namely: leader-follower; cluster; and, constellation. In leader-follower missions, all satellites are deployed on the same orbit, are separated by a specific distance and all follow the leader. A cluster mission is when a group of satellites is deployed close to each other on different orbits to cover an appointed area on the Earth, while a constellation mission includes a group of satellites that are distributed in different orbits to provide full coverage of the Earth [1]. In said topologies, the densities of CubeSats and mobility pattern play an important role in forming the connectivity of the network. when the density of CubeSats is high in the network, the probability of discovering a route from a source to a destination increases. In such cases, Non Delay Tolerant (Non-DTN) routing protocols effectively increase the network performance in terms of high delivery ratio, low delay, and low overhead. However, when the density of CubeSats in the network decreases and/or the topology of the network is highly dynamic, the network experiences frequent disconnectivity, which will cause the Non-DTN protocols to not be efficient anymore. This is because the data packets will be dropped when a route is not discovered. Thus, Delay Tolerant (DTN) protocols as a new approach were introduced to store, carry, and forward the data packets when a route is not discovered. In this survey, we consider both DTN and Non-DTN protocols, as CubeSats' missions may create a connected or disjointed network. The relevant space missions DTN and Non-DTN protocols are classified, as depicted in Figure 3. The CubeSat networks create a different network topology based on the intended mission. Both types of protocols are considered where the network is connected or disjointed, respectively. Hence, all of the protocols discussed in this survey can be used in satellite networks, depending on the network resources and conditions. The Non-DTN based routing protocols are divided into two categories that are based on the satellite network architecture, namely: (i) single-layer routing protocols where a certain number of satellites are distributed on one standard orbit, for example, LEO; and, (ii) multi-layer routing protocols, where the satellites are distributed over two or three standard orbits, such as LEO/MEO/GEO. The said protocols can be efficient if the missions create a connected network topology. However, in many missions, the network might become disjointed, which requires the store-carry-forward manner as a route towards a destination may not be contemporaneously discovered. Thus, the DTN protocols can be used.