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  • Uisce: Characteristic-based Routingin Mobile Ad Hoc Networks

    Guoxian Yang, Stefan WeberSchool of Computer Science and Statistics

    Trinity College DublinDublin, Ireland

    Email: gyang@tcd.ie, sweber@tcd.ie

    AbstractThe goal of communication in computer networks isthe delivery of information to endpoints with certain properties.In wired networks, identities such as IP addresses are usedto guide information through a network and the properties ofnetwork nodes are mapped to these identities by service discoverymechanisms. In mobile ad hoc networks (MANETs), identitieslose their guiding ability because of the dynamism of the topology.

    Instead of identities, we introduce a concept called a charac-teristic, which describes the properties of nodes. Characteristicsare disseminated throughout a network, simulating the flow ofwater streams. Messages are forwarded to their destinations- nodes with given properties - following these characteristicslike following a water stream to its source. Characteristic-basedrouting differs from existing content-based routing in that acharacteristic describes features of MANET nodes rather thancontents of data messages. A trace characteristic is left bydata messages along their forwarding path. Subsequent datamessages can be forwarded to the same destination node andreply messages can be delivered back to the sender of datamessages following the trace characteristic.

    We demonstrate that a characteristic-based approach increasesthe rate of successful delivery in comparison to existing identity-based approaches in MANETs. Then we analyze the abilityof trace characteristics to maintain routes for subsequent datamessages in different mobility settings.

    I. INTRODUCTIONIn current communication architectures, the Internet Proto-

    col (IP) suite provides identity-based addressing and routingat network layer. Service discovery is achieved at applicationlayer by maintaining a mapping between the identity of a nodeand the services that the node provides. In wired networks,IP address describes the location of a network node, i.e. thesubnet that the node is attached to[2]. However, the locationof nodes changes frequently in MANETs. IP addresses losethe meaning that they have in wired networks. Rather, theyare used as identities to differentiate nodes.We argue that identity-based end-to-end communication

    does not suit MANETs. First, identity-based routing is misdi-rected: it is the properties rather than the identities of nodesthat are of primary interest. Second, the topology of a MANETconsisting of individual nodes is more dynamic and prone tocommunication failure than the topology of services in such anetwork. Shown in Fig.I, S1 and S2 provide the same servicethat the requestor C is interested in. From the perspectiveof individual nodes (Fig.1(a)), the failure of an intermediatelink disrupts the provision of a service; In contrast, from the

    C

    S1

    S2

    1

    3

    2

    S1, S2: Service providers C: Service requestor

    (a) Identity-based

    C1

    3

    2

    S

    S: Service provided by S1 and S2 C: Service requestor

    (b) Service-based

    Fig. 1. Topologies of a MANET

    perspective of service (Fig.1(b)), the failure of an intermediatelink has less effect on the provision of the service.Content-based routing (CBR) has been proposed to address

    the aforementioned issues of identity-based approaches. In-stead of identities, in CBR data messages are routed based ontheir content and the subscriptions of nodes. Therefore sendersand receivers are loosely coupled and communication is lessaffected by the frequent network changes. A limitation of CBRis that by itself CBR only provides one-way data delivery fromcontent publishers to subscribers. Because the two ends of acommunication are loosely coupled, it does not address theissue of delivering data from subscribers back to publishers,i.e. two-way communication is not supported by CBR.In our protocol Uisce, we introduce a concept called a char-

    acteristic. A characteristic describes information of generalinterest about a MANET node. It can be service description,as used in service discovery, or features of a node such astype of CPU, battery power, etc. We propose a potential-basedapproach for dissemination of characteristic information. Toestablish a two-way communication, a characteristic is leftby data messages along the path. Further communication willfollow the characteristic towards specific nodes.

    II. SERVICE DISCOVERYService information includes service name, service descrip-

    tion, service type, etc. It is advertised periodically with the IPaddress of its provider. A database of this information, called adirectory, can be used to facilitate communication between ser-vice providers and requesters. A service provider registers itsinformation with a directory and a service requester contactsa directory for the IP addresses of providers. The requesterthen contacts a provider for a service invocation. Basedon the existence of directories, service discovery protocolscan be classified into three categories: centralized directory-based solution [3], distributed directory-based solution [4], anddirectory-less solution [1]. Compared to centralized solution,directory-less solution suits more in a dynamic MANET while

    2011 The 10th IFIP Annual Mediterranean Ad Hoc Networking Workshop

    978-14577-0900-5/11/$26.00 2011 IEEE 119

  • distributed solution achieve a better scalability. [5]. However,regardless of the structure, the service discovery is stronglycoupled with IP as a service is associated with an IP address.

    III. CHARACTERISTIC-BASED COMMUNICATION

    Majority of identity-based routing protocols use the distanceof a path, such as hop count and delay experienced, asrouting metric. Resource discovery such as service discovery,on the other hand, uses the capacity of resources as metricwhen selecting providers. Characteristic-based routing uses apotential-based method to combine the two metrics into oneto integrate route discovery with resource discovery.Each characteristic at its source node is assigned a potential

    value called weight to represent its capacity. The disseminationof characteristic information simulates a water stream: waterpropagation is driven by potential value, from high to low.And during the propagation, water flows lose their potentialvalue. Multiple streams merge into one at intersection point,form a new stream and propagate further. The higher capacity acharacteristic contains, the further it may propagate; the closerto a character source, the higher capacity could be sensed.Different characteristics may have different form of weight

    changes during propagation, depending on the nature of thecharacteristics and the network environment they run through.For example, internet connection is preferable to nodes inthe vicinity, while a printing service may function in a muchlarger area. We introduce three "knob" functions to tune theform of weight changes as a characteristic propagates. Dataforwarding is driven by weight gradient. In this way, routeselection balances between the distance to a source node andthe capacity of a characteristic.A. Characteristic FlowPeriodically, every node broadcasts its characteristic infor-

    mation. Its neighbours inherit these characteristics and spreadthem out further during their broadcasts. A node has only theknowledge about the characteristics of its one-hop neighbours.During the one-hop based propagation of characteristic

    flows, a weight cost function is applied to simulate the weightlost over a link. The capacity of a characteristic perceivedby a remote node decreases as the distance to the sourcenode increases. This reduction arises from error-prone wirelesslinks as well as the potential consumption of the resourcealong the path. Consider the example of internet access, aroute that consists of multiple hops will not achieve thesame throughput as a single hop communication. The weightdecrease at each hop forms a weight gradient to the anonymoussources of a characteristic. Characteristic propagation is drivenby weight gradient: the characteristic information can onlyflow from nodes with higher weight to nodes with lowerweight. A predefined constant wdif is given to control thisaspect of propagation: a node which contains a characteristicwith weight w accepts an incoming characteristic of the sametype only if it has a weight no lower than wwdif , see Fig.2.Multiple sources of a characteristic may coexist in a

    MANET. Information about various source of the same char-acteristic will merge when they meet at an intermediate node.

    Fig. 2. PropagationThe resulting weight of the merged characteristic is computedby weight fusion function.The weight cost function and weight fusion function de-

    scribe the inherent behaviour of characteristic flows. Howeverthere are also external factors that should affect the flowsand eventually future route selection, i.e. link reliability andprocessing power of an intermediate node. In order to providethe flexibility for a node to affect a characteristic flow that runsthrough it, we introduce a cost compensation function. Costcompensation function is applied by an intermediate node ofa characteristic flow to reduce the cost along the hop.1) Function Specification: The weight cost function should

    be a monotonic increasing function of weight since the loss ofa strong flow should not be smaller than that of a weak flow.In this paper, we select

    c(x) =

    {e

    x + a if x > threshold

    x, if x threshold(1)

    to describe the weight cost. , and a are parameters to shapethe cost function. The threshold is one of the solution(s) ofequation (2). It can be intuitively understood as the minimumweight value for a characteristic flow to propagate further.