# A Credit-Based Congestion-Aware Incentive Scheme for DTNs
###### Abstract  
In Delay-Tolerant Networks (DTNs), nodes may be selfish and reluctant to expend their precious resources on forwarding messages for others. Therefore, an incentive scheme is necessary to motivate selfish nodes to cooperatively forward messages. However, the current incentive schemes mainly focus on encouraging nodes to participate in message forwarding, without considering the node congestion problem. When many messages are forwarded to the nodes with high connection degree, these nodes will become congested and deliberately discard messages, which will seriously degrade the routing performance and reduce the benefits of other nodes. To address this problem, we propose a credit-based congestion-aware incentive scheme (CBCAIS) for DTNs. In CBCAIS, a check and punishment mechanism is proposed to prevent forwarding nodes from deliberately discarding message. In addition, a message acceptance selection mechanism is proposed to allow the nodes to decide whether to accept other messages, according to self congestion degree. The experimental results show that CBCAIS can effectively stimulate selfish nodes to cooperatively forward messages, and achieve a higher message delivery ratio with lower overhead ratio, compared with other schemes. delay-tolerant networks (DTNs); congestion; incentive scheme; security +
Footnote †: journal: Information  
At present, some credit-based incentive schemes [12; 13; 14; 15; 16], which motivate selfish nodes to participate in message forwarding by virtual credits, have been proposed, but these schemes just focus on how to encourage nodes to deliver messages to others with higher delivery probability or smaller delivery delay, without considering the node congestion problem. [PERSON] et al. [17] has shown that in forwarding routing, most messages will be forwarded to the nodes with high connection degree, resulting in an unbalanced load among nodes. When many messages are forwarded to nodes with high connection degree, then these nodes will be congested and discard most of the messages due to limited buffer. Discarding the messages will seriously degrade the routing performance and reduce the benefits to other nodes, which is unfair to other nodes. As shown in Figure 1, suppose node \(N_{3}\) is a congested node with high connection degree. When receiving message \(m_{8}\) from \(N_{4}\), it will discard \(m_{7}\) if which can bring more benefits. The discarding will reduce the benefits of nodes \(N_{1}\), \(N_{2}\), or \(N_{D}\). For example, in the scheme proposed by [PERSON] et al. [14], the message-forwarding nodes are paid some credits only when the message is successfully delivered, so nodes \(N_{1}\) and \(N_{2}\) will not get their credits due to the discarding. In the scheme proposed by [PERSON] et al. [15], the destination node has to pay some credits to the forwarding nodes even if a message is not received, so destination node \(N_{D}\) will lose some credits due to the discarding. To address the above problem, we propose a credit-based congestion-aware incentive scheme (CBCAIS) for DTNs. The CBCAIS scheme will check if a forwarding node has discarded messages deliberately by a check and punishment mechanism, and punish the node seriously if it has. When congested nodes do not accept messages anymore to avoid being punished, these messages will be transferred to other nodes with low congestion, and finally to the destination node. A node is prevented from discarding messages deliberately, but it can decide whether to accept other nodes' messages autonomously in CBCAIS. To obtain more credits, a congested node can decide whether to accept other nodes' messages by a message acceptance selection mechanism, where messages are accepted according to node self congestion degree. In case of congestion, the message acceptance selection mechanism will increase the acceptance threshold to accept the messages that can bring more credits. Otherwise the threshold is decreased to accept as many messages as possible.