# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 4 DTN Routing Protocols in Disjointed Satellite Networks
### DTN Quota-Based Techniques  
DTN quota-based techniques were introduced to moderate the negative effect of flooding data and acknowledgment throughout the network by controlling the number of message replicas produced. A Spray and Wait is one of the DTN routing protocols that tries to take the advantage of both the high delivery ratio that is achieved by replication-based routing and the intelligent resources utilisation of forwarding-based routing [61]. It attains the intelligent employment of network resources by using a limited number of replicas that are distributed throughout the network. This can be accomplished by injecting a small number of copies into the network and waiting until the message reaches its destination while accounting for delay circumstances. This protocol has two phases: Spray and Wait. All of the created messages are assigned a number L that shows the maximum acceptable number of copies. In the Spray phase, the source is accountable for delivering a copy of the message to the L selected nodes (relays). These L nodes will hold the message in the Wait phase until the destination comes into direct contact with one of them. The Spray and Wait protocol is divided into two main types: Vanilla and Binary. The two types are almost the same, except in the way that L replicas spread to L selected nodes in the Spray phase. The vanilla type is simple with the source sending copies of the message to the first (L -1) selected nodes it contacts after creating the message. In the Binary type, while the source follows the same process as with the Vanilla type, the selected nodes will then send 50% of the number of copies to the future encountered nodes, if they do not have already copies of the message. The advantage of the Binary Spray and Wait is that messages are more quickly distributed than the Vanilla type and expand their reach to destinations that are two hops away from the sources. The Binary Spray and Wait protocol has been approved as the best among all Spray and Wait schemes in regards to the minimum expected delay. Spray and Focus enhanced the performance of Spray and Wait by developing a Focus phase instead of the Wait phase [62]. In the Focus phase, each node that has received a message from the Sprayphase will search for another intermediate node with a higher probability of meeting the destination to forward the message to it, instead of just waiting to directly meet the destination. This is obtained by using a utility function that counts the elapsed time since the last encounter of each two nodes. A small elapsed time indicates the proximity mobility pattern among the two nodes and incorporating this information increases the probability of delivery in the future. These timers are based on the technique that was originally proposed in the FRESH protocol [54]. Similar to Spray and Wait, an Opportunistic Routing with Window-Aware Replication (ORWAR) utilises the concept of a limited number of the message copies, except that copies of the message will not be randomly disseminated to the first encountered nodes [63]. Instead, the ORWAR evaluates the contact window at the node to select the most suitable message to be forwarded first. The contact window is calculated based on the node speed, movement pattern, and radio capabilities. The selection criteria are based on the utility per bit mechanism and, thereafter, the algorithm selects the message with the size that is suitable for the contact window to ensure complete message delivery and avoid retransmissions that will reduce the resources consumption, such as the power and bandwidth from the overall network view. The ORWAR evaluation against other DTN algorithms, such as Epidemic, Spray and Wait, and MaxProp suggests ORWAR's performance is better in providing greater delivery rates with less overhead. Consideration of the message size and the contact window for networks with asymmetric data rates and variable message lengths, such as the CubeSat networks, is crucial.