Routing in Delay-tolerant Networking - Routing Considerations

Routing Considerations

There are many characteristics DTN protocols, including routing, must take into consideration. A first consideration is if information about future contacts is readily available. For example, in interplanetary communications, many times a planet or moon is the cause of contact disruption, and large distance is the cause of communication delay. However, due to the laws of physics, it is possible to predict the future in terms of the times contacts will be available, and how long they will last. These types of contacts are known as scheduled or predictable contacts. On the contrary, in disaster recovery networks the future location of communicating entities, such as emergency responders, may not be known. These types of contacts are known as intermittent or opportunistic contacts.

A second consideration is if mobility can be exploited and, if so, which nodes are mobile. There are three major cases, classifying the level of mobility in the network. First, it is possible that there are no mobile entities. In this case, contacts appear and disappear based solely on the quality of the communication channel between them. For instance, in interplanetary networks, large objects in space, such as planets, can block communicating nodes for a set period of time. Second, it is possible that some, but not all, nodes in the network are mobile. These nodes, sometimes referred to as Data Mules, are exploited for their mobility. Since they are the primary source of transitive communication between two non-neighboring nodes in the network, an important routing question is how to properly distribute data among these nodes. Third, it is possible that the vast majority, if not all, nodes in the network are mobile. In this case, a routing protocol will most likely have more options available during contact opportunities, and may not have to utilize each one. An example of this type of network is a disaster recovery network where all nodes (generally people and vehicles) are mobile. A second example is a vehicular network where mobile cars, trucks, and buses act as communicating entities.

A third consideration is the availability of network resources. Many nodes, such as mobile phones, are limited in terms of storage space, transmission rate, and battery life. Others, such as buses on the road, may not be as limited. Routing protocols can utilize this information to best determine how messages should be transmitted and stored to not over-burden limited resources. As of April 2008, only recently has the scientific community started taking resource management into consideration, and this is still an active area of research.