A mobile ad hoc network (MANET) consists of a group of nodes, which are able to communicate without a centralized infrastructure. When a mobile node wants to communicate to a distant node not within its reach, the source node has to rely on other nodes to reach the destination in a multi-hop fashion. Cooperation among the nodes is a necessary condition: every node must be willing to forward packets and act as a router. However, a “selfish” node may not be willing to forward packets, in order to preserve its own resources. Therefore, a billing mechanism is needed to stimulate cooperation: a node forwarding packets is rewarded, and a node originating packets is charged.

One solution is to install in each node a packet counter that is increased whenever a packet is forwarded, and decreased whenever a packet is originated. If the counter becomes negative, a node is isolated, and is no longer allowed to originate packets. Such a hardware-based approach has a number of disadvantages, because new hardware must be installed in each node, thus increasing its complexity.

Instead, the authors propose a metric “load,” measured as the number of packets buffered per interface. This load is considered to be a good indication of the number of packets forwarded by that node. If the routing protocol is based on this metric, and prefers a least loaded path in order to reduce end-to-end delay, then a node with minimum load will have to forward more packets, and is forced to cooperate, or it will be isolated.

Because a node does not necessarily know all nodes at the other end of the network, and because of the use of caching, it is not possible for each node to have up-to-date knowledge of which nodes are isolated. Therefore, the zone routing protocol (ZRP) is selected from a set of 15 routing protocols that have been proposed for MANETs. In a particular zone, each node knows all other nodes, such that least loaded path routing can be used within that zone. If the destination node is in another zone, then a request query is forwarded to the peripheral node, which in turn forwards it to the next peripheral node or to the destination. Because peripherals belong to more than one zone, they can return which nodes are isolated.

Using simulation, it is demonstrated that this hybrid routing approach outperforms other protocols, both in terms of end-to-end delay and throughput, because it does not rely on caching or flooding, and does not require a lot of control messages.