# Enhanced Message Replication Technique for DTN Routing Protocols
## 2 Related Works
### Quota-Based Protocols  
One example of a quota-based protocol is the single-copy scheme proposed by [PERSON] et al. [26], in which the source delivers messages directly to the destination node. However, this protocol can be redundant if the destination is located far from the source node, and the messages may never be delivered. To address this issue, [PERSON] et al. [15] proposed the Spray and Wait routing protocol, which is a multi-copy two-hop scheme. In the Spray and Wait protocol, the number of replicas is fixed at the time of message creation. The protocol consists of two phases: the spray phase, in which message replicas are disseminated, and the waiting phase, in which the node waits with a single replica message to meet the destination. While the Spray and Wait protocol is resource-efficient, it may still fail to deliver messages if the destination is in a different area than the source node. To address this issue, [PERSON] proposed the binary Spray and Wait routing protocol, in which the node forwards half of the message's replicas during each contact. [PERSON] et al. [27] proposed the Adaptive Spray and Wait (QoN-ASW) routing algorithm, which adaptively allocates the number of message copies between encountered nodes based on the quality of node (QoN) metric during the spray phase. In the waiting phase, a forwarding scheme is implemented to increase flexibility. Unlike the direct transmission approach, the QoN-ASW strategy makes use of encounter opportunities and, when a node is left with only one copy, it forwards the copy to a suitable candidate node with higher delivery predictability instead of waiting for the destination to be encountered. While the QoN-ASW algorithm improves network performance in terms of delivery rate and average delay, it has a slightly higher overhead than the Spray and Wait protocol. Another example of a quota-based protocol is Spray and Focus, which was proposed by [PERSON] et al. [16]. This protocol uses a timer-based utility value to track the intervals between nodes' encounters and assumes that nodes with similar mobility patterns have short intervals between encounters. Spray and Focus also has two phases: the spray phase, and the focus phase, in which a single copy is forwarded to maximize a utility function. Both Spray and Wait and Spray and Focus aim to reduce the high overhead caused by flooding based protocols but have low delivery ratios due to low message dissemination [28]. [PERSON] et al. [29] proposed an algorithm for transmitting replicas over various periods, in which the source node forwards \(n\) copies to the first \(n\) encountered nodes and waits for an acknowledgment to confirm successful delivery. If delivery fails, more copies are injected into the network in subsequent periods to increase the delivery probability. However, if a message is successfully delivered, the source node's acknowledgment may not be received on time due to the significant delays in the DTN, leading to the forwarding of many replicas to nodes and causing buffer congestion and dropped messages. [PERSON] et al. [30] proposed the Encounter- and Distance-based Routing (EDR) protocol for DTNs, which uses a forward parameter to select the next hop based on the number of encounters and distance to the destination. However, EDR assumes that all nodes have sufficient energy, which may not be the case, and it was not evaluated in terms of delivery ratio--a crucial evaluation metric in DTNs. In [31], the Adaptive Message Replication Technique (AMRT) was proposed for use with quota-based protocols. The AMRT controls network traffic by assigning different numbers of replicas for each generated message based on network conditions (such as congestion among the sender's neighbors), using historical traffic information as an estimate of future capacity. The AMRT was implemented and evaluated with various quota-based routing protocols, improving the delivery ratio and reducing the delivery delay.