Vehicular ad hoc network which can provide safety and comfort applications was being one of the research topics in recent years. There is no infrastructure for routing in vehicular ad hoc networks. Routing and forwarding packets have to be done with the contribution of nodes. Cooperation in this operation can consume node's resources like computation power, bandwidth and storage space. Each node may belong to an individual user and thus can be selfish. A selfish node is unwilling to forward packets of others for nothing. Existence of an incentive system seems necessary in distributed and self-managed networks. Reputation-based and credit-based are two types of incentive system. In reputation approach cooperation is assumed as a duty. This approach stimulates cooperation by detecting and punishing selfish nodes. Punishment can be implemented by dropping their packets. On the other side in credit approach, cooperation is a service that the relaying node gives to the source of the packet. Cooperative nodes are being rewarded with credit or virtual money. Each of these approaches has some advantages and disadvantages. We have proposed a hybrid incentive system for stimulating packet forwarding in vehicular ad hoc networks. The main purpose of proposed system is to solve problems like fairness, credit inflation, credit depletion that are common in existing incentive systems. The result of simulation shows that the proposed system can improve performance in the case of packet delivery ratio in presence of selfish nodes. It also shows that proposed system can increase nodes level of cooperation. Keywords: Vehicular ad hoc network, selfish node, cooperation, incentive system
