A wireless ad hoc network is a self-organized wireless network without centralized infrastructure. In such a network, nodes rely on each other to forward packets to remote destinations. A threat to such multihop transmission is posed by selfish nodes, which may drop others' packets to save their own bandwidth and battery life. Therefore, stimulating packet forwarding is a fundamental problem for wireless ad hoc networks. Most solutions proposed so far are either reputation-based or credit-exchange-based, which are fundamentally context-based solutions. They need to accurately identify selfish behaviors, securely maintain the context, and appropriately punish selfish nodes. These requirements are difficult to satisfy. Recently a solution from a new angle is proposed: a context-free protocol that does not rely on observation and selfish behavior detection. Given a path, a context-free protocol can transmit packets through it without knowing whether the intermediate nodes are selfish or not. It can get rid of all troubles caused by establishing and maintaining the context, and has some significant advantages over context-based solutions. In this article we first provide an overview of traditional context-based solutions, then introduce the recently proposed context-free solution, and finally analyze them extensively.
