# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 3 Routing Protocols in Connected CubeSat network
### Single-Layer Routing for Satellite Networks
For example, the Location-Assisted On-Demand routing (LAOR) protocol that was proposed for the IP-based LEO satellite systems [36] establishes the path discovery process based on route request and route reply among each pair of source and destination, while considering the shortest paths. Even though on-demand routing might create precise information about the network current situation, high routing overheads are expected due to the flooding of path discovery messages. The weakness of LAOR is that it does not consider the node or link failures required for path reestablishment. However, Destruction-resistant on-Demand Routing (DODR) routing has been introduced to solve this issue by using a node-based repair technique [37]. In the case of failure, the local repair technique of DODR protocol allows for a quick response from the current node right before its failure to define a new path to maintain the stream to the destination based on the information that was collected in the path discovery phase. A Datagram Routing Algorithm (DRA) was proposed to maintain minimum propagation links in the LEO environment [35; 36]. Basically, decisions at DRA are made locally at a node for each individual packet. The network is modelled as a static grid of logical locations above the Earth. Each satellite has two neighbours on the same orbital trajectory and two neighbours on nearby trajectories. The packet travels from one fixed logical location to another until it reaches its destination over the minimum propagation delay path. Routing overhead is zero in DRA due to the absence of signalling for collecting topology information. In Reference [39; 40], the Low-Complexity Routing Algorithm (LCRA) and Low-Complexity Probabilistic Routing (LCPR) were both proposed for polar orbit constellations that were based on the concept of logical locations introduced by the DRA. LCRA considers both propagation delay and queuing delay as well as balancing the traffic load to avoid congestion. Likewise, LCPR allows for each node to take its next hop decision based on a minimum propagation delay and the congestion status information collected from its neighbours. The LCPR can reasonably reduce the computational and storage complexity on board when compared to the DV-DVTR and FSA protocols that compute the routing tables off-line and then load them to the nodes. A localized Routing Scheme (LRS) is used for the LEO satellite network with the network modelled as a mesh-like Manhattan Street Network (MSN) [41]. The LRS aims to save processing resources by reducing the size of routing tables that were calculated onboard a satellite to decrease the overall onboard computation complexity, including paths definitions and exchange. This LRS algorithm utilizes two routing techniques that are divided into intra-zone routing and inter-zone routing. The whole network is divided into zones; any adjacent virtual nodes are grouped in one zone. In the intra-routing stage, LRS elects a node to coordinate the routing within the zone and also selects a minimum hop path to the other zones utilizing the MSN binary method. In the inter-routing stage, data will be routed among the zones that are based on the shortest path; however, the traffic load among the zones is not considered in the inter-zone routing, which might lead to performance degradation. A Satellite network Link State Routing (SLSR) was proposed to address the high overhead and long delays on satellite networks while using the same MSN model [42]. Routing includes two phases with propagation delay routing tables, based on the satellite dynamics, calculated off-line in the first phase. In the second phase, the topology is regularly updated to determine the broken links or failed satellites, as well as any congestion information. An enhanced flooding mechanism is used in the second phase to exchange network information to ensure reliability and reduce the high overhead that is caused by the original flooding technique.