## 2 Existing Routing Methods for the DTN  
The application area of the DTN routing has been expanding in recent years, and many application-dependent protocols have been studied. This paper proposes a new routing method and compares it to existing routing methods in terms of performance. The existing methods compared are \"epidemic routing\" [6] and \"spray and wait\" [7], both of which are well known as original routing methods, and \"location-based routing\" [8] and \"motion vector routing (MoVe)\" [8], both of which are representative routing methods that use location information as the source of routing knowledge. In \"epidemic routing\", each node tries to deliver a bundle to the destination node by unconditionally propagating bundle copies to nodes in its neighborhood. This routing can achieve the theoretically highest bundle arrival rate, but it propagates so many copies that the network can become congested. It requires an enormous amount of network resources, such as radio bandwidths and memory buffers. To remedy this problem, \"spray and wait routing\" restricts consumption of resources by setting an upper limit to the number of copies. The downside is that it cannot deliver bundles to as far places as the epidemic routing can. The \"location-based routing\" assumes that all relay nodes know the location of the destination node. Each relay node selects a node that is closer to the destination node than itself and sends a bundle to it. While the routing algorithm is the simplest, there is no assurance that the bundle can reach the destination node quickly with a small number of hops. The \"motion vector routing\" tries to increase the probability at which each selected relay node approaches the destination node quickly by using moving speed vectors instead of simply using information on the location of the destination node. One of the existing routing methods that uses dedicated relay nodes is \"message ferry routing\" [9; 10; 11]. It defines the places where mobile terminal owners or chased wild animals tend to come together as \"clusters\" and defines a relay node that travels from one cluster to another as a \"ferrry node\". It always achieves the highest bundle arrival rate among the different routing methods because the ferry node ensures that the bundle reaches the destination. This routing is also the most efficient, because the number of bundle copies and the number of hops are theoretically the smallest. However, since it assumes that the network is dedicated to a specific purpose, the ferry node needs to visit every node or cluster. This requirement restricts its feasibility and extensibility. The characteristics of the different routing methods are summarized in Table 1. Table 2 compares these routing methods in terms of the bundle arrival rate, number of copies, number of hops and the buffer size required at each node. In addition, the existing routing methods are intended only for point-to-point transfer or dissemination, _i.e._, transfer of data to unspecified nodes. They do not consider point-to-multipoint transfer, which is needed for content delivery. The classification of the proposed and the existing routing methods is illustrated in Figure 1.