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infrastructure [13]. Another example of village networks is presented in reference 14, in which the Wizzy digital courier service provides disconnected Internet access to people/students in remote villages of South Africa. A courier on a motorbike, equipped with a USB storage device, travels from a village to a large city which has high-speed Internet connectivity. Typically, it takes a few hours for the courier to travel from the village to the city. There are many different terminologies used for DTNs in the literature, such as eventual connectivity, space-time routing, partially connected, transient connection, opportunistic networking, extreme networks, and end-to-end communication. The data unit in DTNs can be a message, a packet, or bundle, which is de ned as a number of messages to be delivered together. For simplicity, throughout this chapter, we use bundles, messages, and packets interchangeably. The characteristics of DTNs are very different from the traditional Internet in that the latter implicitly has some well-known assumptions: (1) continuous connectivity, (2) very low packet loss rate, and (3) reasonably low propagation delay. DTNs do not satisfy all of these assumptions, and sometimes none. The challenges in designing ef cient protocols in the DTNs are extremely long delay (up to days), frequent disconnection, and opportunistic or predicable connections. Consequently, the existing protocols developed for the wired Internet are not able to handle the data transmission ef ciently in DTNs. In DTNs, end-to-end communication using TCP/IP protocol may not work, because packets whose destinations cannot be found are usually dropped. If packet dropping is too severe, TCP eventually ends the session. UDP provides no reliable service and cannot hold and forward. New protocols and algorithms need to be developed. There are several different types of DTN due to their different characteristics. For instance, the satellite trajectories in example (a) are predictable, while the movement of a solider or tank in example (c) may be random. Therefore, for different types of DTN, different solutions may need to be proposed. Recently, the DTN research group under the Internet Research Task Force (IRTF) has proposed several research documents including a DTN architecture [10, 15 17]. This architecture addresses communication issues in extreme networks or networks encompassing a wide range of architectures. The architecture is a network of regional networks, with an overlay on top of a transport layer or on top of these regional networks. It provides key services, such as in-network data storage and retransmission, interoperable naming, and authenticated forwarding. The DTN solutions in reference 15 are concerned with message transport between infrastructures of disparate architectures by using gateways that handle bundles of messages between these infrastructures. The DTN architecture addresses the issues of eventual connectivity and partitioned networks by the use of a store and forward mechanism, and it handles the diverse addressing needs of the overlay architecture by using an addressing scheme that exploits the late binding of addresses. Local addresses are not bound to nodes until the message is in the local area of the destination. This creates a hierarchical routing structure that makes routing across networks easier to implement. There are many activities in the DTN working group; and due to space limitation, readers are referred to reference 15 (and the references therein) for more information.
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Routing in DTNs is one of the key components in the architecture document. Based on different types of DTN, deterministic or stochastic, different routing protocols are required. Due to intermittent connectivity, it is likely that paths to some of the destinations may not exist from time to time. When a packet arrives and its destination cannot be found in the routing table, the packet is simply dropped under the routing protocols mentioned above (developed with the assumption that the network is connected). Therefore, these routing protocols will not work ef ciently in DTNs. In this chapter, we provide an overview of the state of the art in DTN routing protocols. To cope with intermittent connectivity, one natural approach is to extend the storeand-forward routing to a store-carry-forward (SCF) routing. In store carry forward routing, a next hop may not be immediately available for the current node to forward the data. The node will need to buffer the data until the node gets an opportunity to forward the data, and it must be capable of buffering the data for a considerable duration. The dif culty in designing a protocol for ef ciently and successfully delivering messages to their destinations is to determine, for each message, the best nodes and time to forward. If a message cannot be delivered immediately due to network partition, the best carriers for a message are those that have the highest chance of successful delivery that is, the highest delivery probabilities. Because ad hoc networks could be very sparse, SCF routing could mean that the node may have to buffer data for a long period of time. This condition can get worse if the next hop is not selected properly. A bad forwarding decision may cause the packets to be delayed inde nitely. If messages must be stored somewhere, a buffer management scheme should be proposed. If all the future topology of the network (as a time-evolving graph) is deterministic and known, or at least predictable, the transmission (when and where to forward packets) can be scheduled ahead of time so that some optimal objective can be achieved. If the time-evolving topology is stochastic, SCF routing performs routing by moving the message closer to the destination one hop at a time. If the nodes know nothing about the network states, then all that the nodes can do is to randomly forward packets to their neighbors, protocols in this category are referred to as epidemic. If one can estimate the forwarding probability of its neighbors, a better decision could be made. Protocols in this category are referred to as history- or estimation-based forwarding. Furthermore, if the mobility patterns can be used in the forwarding probability estimation, an even better decision may be made. Protocols in this category are referred to as model-based forwarding. In some cases, network ef ciency can be achieved if the movements of certain nodes are controlled, and these protocols are in the category of controlling node movements. Recently, coding-based routing protocols are also proposed for DTNs. These protocols can be categorized as follows. In this chapter, we will review some of the routing protocols.
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8.2 DETERMINISTIC ROUTING In this section, we review a few routing protocols assuming that future movement and connections are completely known (that is the entire network topology is known ahead of time).
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