# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 4 DTN Routing Protocols in Disjointed Satellite Networks
### DTN Flooding-Based Techniques
The product of these counters is used to calculate the ageing index and bundles with a higher ageing index are dropped first. Bundles with the least hop count to their destination will have higher delivery priority, according to the message scheduling policy of NECTAR. The NECTAR showed a better performance when compared with the Epidemic and PROPHET protocols in terms of having higher data rates and lower resource consumption in networks with restricted resources. Even though some features of NECTAR are reasonably suitable for CubeSat networks, maintaining all of the routing computations onboard and not considering the periodicity of satellite movement make it unfavourable. FResher Encounter Search(FRESH) was originally proposed for the concept of routing based on historical encounters and an ageing index for Mobile Ad-hoc Network (MANET) by [54]. A Shortest Expected Path Routing (SEPR) was introduced to solve the routing problem in semi-connected ad-hoc networks that include randomly dynamic nodes with limited resources [55]. Instead of flooding data, like in Epidemic, routing in SEPR uses Expected Path Length Metric (EPLM) to reduce the message copies, where each node develops its topology map by exchanging link-state information with its neighbouring nodes. Once a complete topology map is collected, the node runs the Dijkstra algorithm to define all of the expected path lengths among all pairs in the network. No recovery techniques were developed to deal with link and node failures, even though these protocols considered the limitation of the resources of the nodes. In a similar manner, MobySpace uses Euclidean space as a method to improve the forwarding decisions in the generic DTN routing protocols for networks that have predictable mobility [56]. Each node is defined in the Euclidean space by coordinates that reflect the probability of its appearance in all different locations. The forwarding decision is made according to historical topology information. Based on this information, MobySpace selects a suitable next hop that is based on the mobility pattern similarity of the node as compared with the mobility of the destination node. Where two nodes have a similar encounter history, this means that they regularly meet with the same connection nodes due to being close to each other in their mobility pattern. Delivery probability is directly proportional to the pattern similarity. A Minimum Estimated Expected Delay (MEED) algorithm was proposed to calculate the estimated average delay to the next-hop nodes for networks with unpredictable behaviour, yet assumes that an encounter with a node in the past indicates the probability of a future encounter [57]. In MEED, global network topology information is gained by flooding link state messages while using a link-state routing algorithm with routing tables dynamically updated after each encounter to consider any unpredictable encounters. MEED outperforms other protocols that have predetermined global knowledge, in terms of providing a minimum delay path, even without prior knowledge about the topology. From here, MEED could be a suitable candidate for dense CubeSat networks, while assuming that there is no restriction on the onboard storage. However, this algorithm still generates high overheads due to the flooding of link state messages and no recovery technique has been proposed. MaxProp is a restricted flooding-based algorithm, which, although originally proposed for a vehicular DTN environment, can be generalised to any other DTN [58]. MaxProp is an extension of the MV protocol [59]. The main idea of MaxProp is in the decision mechanisms proposed for forwarding and dropping messages, where the messages are prioritised based on the probability of meeting the destination in the future. Based on historical information, higher probability is given to a message according to its delivery probability, for example, if it has a minimum hop count to the destination. MaxProp also developed some techniques to boost the delivery rates and decrease delays.