## 4 Routing Protocols Using Ant-Like Mobile Agents
### Ant-Colony Based Routing Algorithm (ARA)  
One of the representative reactive routing algorithms based on ant-like mobile agents is Ant-Colony Based Routing Algorithm (ARA) [37]. ARA is a purely reactive MANET routing algorithm. One of its notable characteristics is that it does not use the _hello packet_ to explicitly find its neighbors [38]. Instead, ARA uses a flooding technique. The source node in a MANET that employs this algorithm broadcast _forward ants_ toward the destination. As each _forward ant_ moves toward the destination, it constructs a route from the visited node to the source node. In ARA, a route is indicated by a positive pheromone value in the intermediate nodes' routing tables. When the _forward ant_ reaches the destination, it creates the _backward ant_ and sends it back to the source node in order to construct there the correct forward route. Since the all _forward ants_ that share the same destination from the same source node also share the same sequence number, redundant ants returning via inferior routes are simply discarded. Superiority and inferiority are determined by the pheromone values that are computed based on the number of hops of the route.  
As in AntHocNet, the quality of a path is managed by a pheromone value, but it is maintained by data packets instead of ant-like agents. Each time an intermediate node relays a data packet along a certain path, the corresponding pheromone value is increased. As the pheromone evaporates, the less utilized route has less pheromone value and does not attract data packets.  
When an intermediate node leaves from a route while the route is still used in a communication session, the node that failed to relay data sends an error message to the previous node. On receiving the route failure message, the previous node checks whether there is an alternate route to the destination. If there is one, the previous node simply uses it; if there is not, the previous node asks neighbors to relay the data packet. If the failure notification reaches to the source node, it re-initiates the broadcast of the forward ants.  
[PERSON] _et al._ have demonstrated the effectiveness of ARA over AODV [39]. They have shown that the higher the mobility of the participating nodes in the network, the greater the effectiveness of ARA. [PERSON] _et al._ have improved ARA in order to cope with much higher mobility situations. They propose an improved algorithm called Enhanced Ant Colony Based Routing Algorithm (EARA) [40]. In ARA, the pheromone values are computed by the number of hops, and delay time is not taken into consideration. In EARA, the initial pheromone value is set by not only the total number of hops made by the _forward ant_ that pursued the route, but also by taking account of the time interval between the sending of the _forward ant_ and receiving the _backward_ ant. The inclusion of time period in calculating pheromone values creates a pheromone gradient from the source node to the destination node where only hop count produces similar gradients for entire route. This difference creates the higher delivery ratio [40]. [PERSON] _et al._ have also improved ARA by handling loss of ants and avoiding re-transmission of lost packets by reserving resources at intermediate nodes [41].  
[PERSON] _et al._ have proposed a routing protocol for MANET similar to ARA [42]. In the protocol, the source node establishes the route by using reactive ant-like agents. After establishment, it creates _proactive ants_ and sends them to explore the network in order to discover better routes. When an intermediate node has multiple routes to the destination, it selects the transmission path probabilistically and send data packet along it. This yields better balance of data traffic. If an intermediate node finds routing failure, the intermediate node creates reactive ant agents and dispatches them toward the destination.