Algorithms and Protocols for Wireless and Mobile Ad Hoc Networks (Boukerche/Mobile) || Routing Protocols in Intermittently Connected Mobile Ad Hoc Networks and Delay-Tolerant Networks
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<ul><li><p>CHAPTER 8</p><p>Routing Protocols in IntermittentlyConnected Mobile Ad Hoc Networksand Delay-Tolerant Networks</p><p>ZHENSHENG ZHANGSan Diego Research Center, San Diego, CA 92121</p><p>8.1 INTRODUCTION</p><p>In the last few years, there has been much research activity in mobile (wireless) ad hocnetworks (MANETs). MANETs are infrastructureless, and nodes in the networks areconstantly moving. In MANETs, nodes can directly communicate with each otherif they enter each others communication range. A node can terminate packets orforward packets (serve as a relay). Thus, a message traverses an ad hoc network bybeing relayed from one node to another, until it reaches its destination. As nodesare moving, this becomes a challenging task, since the topology of the network isin constant change. How to find a destination, how to route to that destination, andhow to ensure robust communication in the face of constant topology change aremajor challenges in mobile ad hoc networks. Routing in mobile ad hoc networksis a well-studied topic. To accommodate the dynamic topology of mobile ad hocnetworks, an abundance of routing protocols have recently been proposed, such asOLSR , AODV , DSR , LAR , EASE [5, 6], ODMRP , and manyothers [8, 9]. For all these routing protocols, it is implicitly assumed that the networkis connected and there is a contemporaneous end-to-end path between any source anddestination pair. However, in a physical ad hoc network, the assumption that thereis a contemporaneous end-to-end path between any source and destination pair maynot be true as illustrated below. In MANETs, when nodes are in motion, links canbe obstructed by intervening objects. When nodes must conserve power, links areshut down periodically. These events result in intermittent connectivity. At any giventime, when no path exists between source and destination, network partition is said</p><p>Algorithms and Protocols for Wireless and Mobile Ad Hoc Networks, Edited by Azzedine BoukercheCopyright 2009 by John Wiley & Sons Inc.</p><p>219</p></li><li><p>220 ROUTING PROTOCOLS IN INTERMITTENTLY CONNECTED MOBILE AD HOC NETWORKS</p><p>8:00 am ma00:11 0 am1:01</p><p>2</p><p>3</p><p>2</p><p>3</p><p>1</p><p>2</p><p>3</p><p>4</p><p>1 3</p><p>S</p><p>D</p><p>S</p><p>D</p><p>S</p><p>D</p><p>2</p><p>Figure 8.1. Illustration of time-evolving behavior of the ad hoc networks.</p><p>to occur. Thus, it is perfectly possible that two nodes may never be part of the sameconnected portion of the network. Figure 8.1 illustrates the time evolving behavior inintermittently connected networks (ICNs). In Figure 8.1, there is no direct path fromnode S to node D at any given time. Packets from node S can be delivered to node D ifintermediate nodes can hold/carry the packets (at 8:00 am, node S sends the packetsto node 2, at 10:10 am, node 2 forwards the packets to node 3, and at 11:00 am, node3 forwards the packets to node D). Examples of an intermittently connected networkare as follows:</p><p>(a) An interplanet satellite communication network where satellites and groundnodes may only communicate with each other several times a day.</p><p>(b) A sensor network where sensors are not powerful enough to send data to acollecting server all the time or scheduled to be wake/sleep periodically.</p><p>(c) A military ad hoc network where nodes (e.g., tanks, airplanes, soldiers) maymove randomly and are subject to being destroyed.</p><p>Applications in ICNs must tolerate delays beyond conventional IP forwardingdelays, and these networks are referred to as delay/disruption tolerant networks(DTNs). Routing protocols such AODV and OLSR do not work properly in DTNs,since under these protocols, when packets arrive and no contemporaneous end-to-end paths for their destinations can be found, these packets are simply dropped. Newrouting protocols and system architectures should be developed for DTNs.</p><p>There are many potential applications in DTNs, such as interplanetary network(IPN), Zebranet, DataMule and village networks. IPN  consists of both terrestrialand interplanetary links, which suffers from long delays and episodic connectivity. InZebranet , wild-life researchers drive through a forest collecting information aboutthe dispersed zebra population. In the DataMule project , DataMules randomlymove and collect data from low power sensors. For village networks, for example,a recent project in developing nations uses rural buses to provide Internet connec-tivity to otherwise isolated and remote villages that do not have any communication</p></li><li><p>INTRODUCTION 221</p><p>infrastructure . Another example of village networks is presented in reference 14,in which the Wizzy digital courier service provides disconnected Internet access topeople/students in remote villages of South Africa. A courier on a motorbike, equippedwith a USB storage device, travels from a village to a large city which has high-speedInternet connectivity. Typically, it takes a few hours for the courier to travel from thevillage to the city.</p><p>There are many different terminologies used for DTNs in the literature, such aseventual connectivity, space-time routing, partially connected, transient connection,opportunistic networking, extreme networks, and end-to-end communication. Thedata unit in DTNs can be a message, a packet, or bundle, which is defined as anumber of messages to be delivered together. For simplicity, throughout this chapter,we use bundles, messages, and packets interchangeably.</p><p>The characteristics of DTNs are very different from the traditional Internet inthat the latter implicitly has some well-known assumptions: (1) continuous con-nectivity, (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 indesigning efficient 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 thedata transmission efficiently in DTNs. In DTNs, end-to-end communication usingTCP/IP protocol may not work, because packets whose destinations cannot be foundare usually dropped. If packet dropping is too severe, TCP eventually ends the ses-sion. UDP provides no reliable service and cannot hold and forward. New protocolsand algorithms need to be developed. There are several different types of DTN dueto their different characteristics. For instance, the satellite trajectories in example(a) are predictable, while the movement of a solider or tank in example (c) maybe random. Therefore, for different types of DTN, different solutions may need tobe proposed. Recently, the DTN research group under the Internet Research TaskForce (IRTF) has proposed several research documents including a DTN architecture[10, 1517]. This architecture addresses communication issues in extreme networksor networks encompassing a wide range of architectures. The architecture is a net-work of regional networks, with an overlay on top of a transport layer or on top ofthese regional networks. It provides key services, such as in-network data storageand retransmission, interoperable naming, and authenticated forwarding. The DTNsolutions in reference 15 are concerned with message transport between infrastruc-tures of disparate architectures by using gateways that handle bundles of messagesbetween these infrastructures. The DTN architecture addresses the issues of eventualconnectivity 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 anaddressing scheme that exploits the late binding of addresses. Local addresses arenot bound to nodes until the message is in the local area of the destination. Thiscreates a hierarchical routing structure that makes routing across networks easier toimplement. There are many activities in the DTN working group; and due to spacelimitation, readers are referred to reference 15 (and the references therein) for moreinformation.</p></li><li><p>222 ROUTING PROTOCOLS IN INTERMITTENTLY CONNECTED MOBILE AD HOC NETWORKS</p><p>Routing in DTNs is one of the key components in the architecture document. Basedon different types of DTN, deterministic or stochastic, different routing protocols arerequired. Due to intermittent connectivity, it is likely that paths to some of the des-tinations may not exist from time to time. When a packet arrives and its destinationcannot be found in the routing table, the packet is simply dropped under the rout-ing protocols mentioned above (developed with the assumption that the network isconnected). Therefore, these routing protocols will not work efficiently in DTNs. Inthis chapter, we provide an overview of the state of the art in DTN routing protocols.</p><p>To cope with intermittent connectivity, one natural approach is to extend the store-and-forward routing to a store-carry-forward (SCF) routing. In storecarryforwardrouting, a next hop may not be immediately available for the current node to forwardthe data. The node will need to buffer the data until the node gets an opportunityto forward the data, and it must be capable of buffering the data for a considerableduration. The difficulty in designing a protocol for efficiently and successfully deliv-ering messages to their destinations is to determine, for each message, the best nodesand time to forward. If a message cannot be delivered immediately due to networkpartition, the best carriers for a message are those that have the highest chance ofsuccessful deliverythat is, the highest delivery probabilities. Because ad hoc net-works could be very sparse, SCF routing could mean that the node may have to bufferdata for a long period of time. This condition can get worse if the next hop is notselected properly. A bad forwarding decision may cause the packets to be delayedindefinitely. If messages must be stored somewhere, a buffer management schemeshould be proposed.</p><p>If all the future topology of the network (as a time-evolving graph) is deterministicand known, or at least predictable, the transmission (when and where to forwardpackets) can be scheduled ahead of time so that some optimal objective can beachieved. If the time-evolving topology is stochastic, SCF routing performs routingby moving the message closer to the destination one hop at a time. If the nodes knownothing about the network states, then all that the nodes can do is to randomly forwardpackets to their neighbors, protocols in this category are referred to as epidemic. Ifone can estimate the forwarding probability of its neighbors, a better decision couldbe made. Protocols in this category are referred to as history- or estimation-basedforwarding. Furthermore, if the mobility patterns can be used in the forwarding prob-ability estimation, an even better decision may be made. Protocols in this categoryare referred to as model-based forwarding. In some cases, network efficiency can beachieved if the movements of certain nodes are controlled, and these protocols are inthe category of controlling node movements. Recently, coding-based routing proto-cols are also proposed for DTNs. These protocols can be categorized as follows. Inthis chapter, we will review some of the routing protocols.</p><p>8.2 DETERMINISTIC ROUTING</p><p>In this section, we review a few routing protocols assuming that future movementand connections are completely known (that is the entire network topology is knownahead of time).</p></li><li><p>DETERMINISTIC ROUTING 223</p><p>281</p><p>DTN routing protocols </p><p>Deterministic case (where network topology is deterministic and known ahead of time) </p><p>Stochastic case Coding-based approaches </p><p>Space-time routing  </p><p>Tree approach  </p><p>Modified shortest- path approach  </p><p>Epidemic/ Random spray  </p><p>Predication-based </p><p>Model-based [30, 31] </p><p>Network coding  </p><p>Erasure coding [38, 39, 41]</p><p>One-hop information- based ,  </p><p>End-to-end information- based[25, 28, 29] </p><p>Control movement [12, 3237] </p><p>In a tutorial article , Ferreira describes a simple combinatorial reference modelthat captures most characteristics of the time-varying networks. A notion of evolvinggraphs, which consists of formalizing time domain graphs, is introduced. Modelingtime in mobile ad hoc networks gives rise to several different matrices that may serveas objective functions in routing strategies, such as earliest time to reach one or allthe destinations or minimum hop paths (with or without the condition that packetsarrive before a predefined time period). The readers are referred to reference 42 formore details and references therein. In Figure 8.2, originally shown in reference 42,the min-hop path from S to D takes 4 hops at time interval 1 while it takes one hop attime interval 4, where the number next to each link denotes the time interval duringwhich the link is active.</p><p>In reference 19, algorithms selecting the path of message delivering are presented,depending on the available knowledge about the motion of hosts. Three cases areconsidered. In the first case, it assumes that global knowledge of the characteristicprofiles with respect to space and time (that is, the characteristic profiles of the motionand availability of the hosts as functions of time) are completely known by all the</p></li><li><p>224 ROUTING PROTOCOLS IN INTERMITTENTLY CONNECTED MOBILE AD HOC NETWORKS</p><p>SA  [1-2]</p><p> </p><p>[1-3] [1-4] [1-3] [1-2] </p><p>HGFEDCB</p><p>Figure 8.2. Illustration of different metrics.</p><p>hosts. Paths are selected by building a tree first, such an approach is referred to as thetree approach. Under the tree approach, a tree is built from the source host by addingchildren nodes and the time associated with nodes. Each node records all the previousnodes the message has to travel and records the earliest time to reach it. A final pathcan be selected from the tree by choosing the earliest time (or minimum hop) to reachthe desired destination. The results for the time-evolving graph in Figure 8.3 usingthe tree approach are given in Figure 8.3a. From the figure, one can easily notice thata path from A to D can be set up at time period 1 with 3 hops or at time period 4 withone hop.</p><p>In the second case, it assumes that characteristic profiles are initially unknown tohosts, hosts gain this information through learning the future by letting neighbor hostsexchange the characteristic profiles available between them. Paths are selected basedon this partial knowledge. In the third case, to enhance the algorithm in the secondcase, it also requires hosts to record the past, that is, it stores the sequence of hosts amessage has transited within the message itself.</p><p>For DTNs, several routing algorithms are proposed in reference 16, dependingon the amount of knowledge about the network topology characteristics and trafficdemand. They define four knowledge oracles; each oracle represents certain knowl-edge of the network. Contacts Summary Oracle contains information about aggregatestatistics of the contacts (resulting in time-invariant information). A contact is definedas an opportunity to send data. Contacts Oracle contains informa...</p></li></ul>
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