# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 3 Routing Protocols in Connected CubeSat network
### Multi-Layer Routing Protocols in Connected CubeSat Network
In Reference [30], a Multi-Layer Satellite Network (MLSN) that is based on the Walker Delta satellite constellation was developed and tested with the introduced Adaptive Routing Strategy (ARS) with the proposed structure providing continuous connectivity among satellites on the same layer and intermittent connectivity between different layers. The ARS design is based on the Bellman-Ford backwards-forward routing protocol for selecting optimal paths. The path with the least total path weight is considered to be the most optimal path from among all other available paths. A Satellite Grouping and Routing Protocol (SGRP) was proposed for two-layered LEO/MEO structures [31]. The \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Routing Algorithm** & **Protocol Type** & **Topology Information** & **Decision Criteria** & **Resources Consumption** & **Links Recovery** \\ \hline HSRP [27] & Connection oriented & logical locations/ & Node info/Topology state/resource status & High & Yes \\ \hline MLSR [28] & Connection oriented & logical locations/ & Node info/Topology state/resource status & High & Yes \\ \hline RSSBN [29] & Connection oriented & logical locations/ & Node info/Topology & High & No \\ \hline ARS-MLSN [30] & Connectionless & logical locations/ & Link load & High & No \\  & & Mesh-grid & & & \\ \hline SGRP [31] & Connectionless & Predictable/logical locations/ Meshgrid & Link metrics & Medium & Yes \\ \hline SARA [32] & Connectionless & Predictable/logical locations/ Meshgrid & Link metrics & Medium & Yes \\ \hline PSNR [33] & Connectionless & Predictable/dynamic & Link metrics & Medium & Yes \\ \hline NDRA [34] & Connectionless & logical location/ Meshgrid &
\begin{tabular}{l} Data-driven Link \\ metrics \\ \end{tabular} & High & No \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Multi-layer routing algorithms. LEO satellites are modelled as a logical location grid that changes over time in order to show the different snapshots of the topology. The LEO satellites are then grouped according to the footprints of MEO satellites. The MEO satellites are in charge of routing table calculations and updates and these tables are then sent and stored at LEO satellites. SGRP route the data over paths with minimum delay while considering link propagation delay and congestion as well as transmission delays at the satellites. SGRP uses the same recovery technique of MLSR in the case of detected satellite failure. However, only routes that are affected by a satellite failure will be rerouted by the MEO satellites to reduce the computational complexity. A Snap-based Autonomous Routing Algorithm (SARA) was introduced to enhance satellite failure link recovery. When compared to SGRP, SARA achieved a similar propagation delay, but it outperforms SGRP in terms of overhead minimization and boosting delivery rates [32]. A Predictable Satellite Network Routing Protocol (PSNRP) was proposed for two-layer LEO/MEO MLSNs [33]. PSNR addresses some drawbacks of the previous multi-layered routing protocols for enhancing routing robustness and network survivability. For example, the lack of effective application of satellite movement predictability in the previously mentioned multilayered protocols. Additionally, instead of assigning the routing table management to the MEO satellites, control centres on the ground are employed in conjunction with the MEO satellites to fulfil this task. In this case, the computational burden on the MEO satellites is reduced to avoid any congestion at these major nodes, which may otherwise lead to performance degradation. PSNR utilizes a simple link recovery technique that separates the user data from control data and floods link states to all LEO satellites as a response to any detected disruption. Additionally, to enhance QoS requirements, such as delay and bandwidth, a routing algorithm based on heuristic techniques was adapted by PSNRP and it is considered to be an NP-complete problem.