Design & imlementation of tarf


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Design & imlementation of tarf

  1. 1. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 1 Design and Implementation of TARF: A Trust-Aware Routing Framework for WSNs Guoxing Zhan, Weisong Shi, Senior Member, IEEE, and Julia Deng Abstract—The multi-hop routing in wireless sensor networks (WSNs) offers little protection against identity deception through replaying routing information. An adversary can exploit this defect to launch various harmful or even devastating attacks against the routing protocols, including sinkhole attacks, wormhole attacks and Sybil attacks. The situation is further aggravated by mobile and harsh network conditions. Traditional cryptographic techniques or efforts at developing trust-aware routing protocols do not effectively address this severe problem. To secure the WSNs against adversaries misdirecting the multi-hop routing, we have designed and implemented TARF, a robust trust-aware routing framework for dynamic WSNs. Without tight time synchronization or known geographic information, TARF provides trustworthy and energy-efficient route. Most importantly, TARF proves effective against those harmful attacks developed out of identity deception; the resilience of TARF is verified through extensive evaluation with both simulation and empirical experiments on large-scale WSNs under various scenarios including mobile and RF-shielding network conditions. Further, we have implemented a low-overhead TARF module in TinyOS; as demonstrated, this implementation can be incorporated into existing routing protocols with the least effort. Based on TARF, we also demonstrated a proof-of-concept mobile target detection application that functions well against an anti-detection mechanism. 31 I NTRODUCTION network traffic. Those routing packets, including their original headers, are replayed without any modification.Wireless sensor networks (WSNs) [2] are ideal can- Even if this malicious node cannot directly overhear thedidates for applications to report detected events of valid node’s wireless transmission, it can collude withinterest, such as military surveillance and forest fire other malicious nodes to receive those routing packetsmonitoring. A WSN comprises battery-powered senor and replay them somewhere far away from the originalnodes with extremely limited processing capabilities. valid node, which is known as a wormhole attack [5].With a narrow radio communication range, a sensor Since a node in a WSN usually relies solely on thenode wirelessly sends messages to a base station via packets received to know about the sender’s identity,a multi-hop path. However, the multi-hop routing of replaying routing packets allows the malicious node toWSNs often becomes the target of malicious attacks. forge the identity of this valid node. After “stealing” thatAn attacker may tamper nodes physically, create traffic valid identity, this malicious node is able to misdirectcollision with seemingly valid transmission, drop or the network traffic. For instance, it may drop packetsmisdirect messages in routes, or jam the communication received, forward packets to another node not supposedchannel by creating radio interference [3]. This paper to be in the routing path, or even form a transmissionfocuses on the kind of attacks in which adversaries loop through which packets are passed among a fewmisdirect network traffic by identity deception through malicious nodes infinitely. It is often difficult to knowreplaying routing information. Based on identity decep- whether a node forwards received packets correctly evention, the adversary is capable of launching harmful and with overhearing techniques [4]. Sinkhole attacks are an-hard-to-detect attacks against routing, such as selective other kind of attacks that can be launched after stealingforwarding, wormhole attacks, sinkhole attacks and Sybil a valid identity. In a sinkhole attack, a malicious nodeattacks [4]. may claim itself to be a base station through replaying As a harmful and easy-to-implement type of attack, a all the packets from a real base station [6]. Such a fakemalicious node simply replays all the outgoing routing base station could lure more than half the traffic, creatingpackets from a valid node to forge the latter node’s iden- a “black hole”. This same technique can be employed totity; the malicious node then uses this forged identity to conduct another strong form of attack - Sybil attack [7]:participate in the network routing, thus disrupting the through replaying the routing information of multiple legitimate nodes, an attacker may present multiple iden-• G. Zhan and W. Shi are with the Department of Computer Science, Wayne State University, Detroit, MI, 48202. tities to the network. A valid node, if compromised, can E-mail:, also launch all these attacks.• J. Deng is with Intelligent Automation Inc., Rockville, MD 20855. The harm of such malicious attacks based on the Email: technique of replaying routing information is furtherThis work is in part supported by AFRL contract FA8650-10-C-1740 and aggravated by the introduction of mobility into WSNsNSF Career Award CCF-0643521. We also would like to thank the programmanager Mr. John Woods for his great support as well as the anonymous and the hostile network condition. Though mobility isreviewers for their constructive comments and suggestions. introduced into WSNs for efficient data collection and
  2. 2. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 2various applications [8], [9], [10], [11], it greatly increases TARF can be developed into a complete and indepen-the chance of interaction between the honest nodes dent routing protocol, the purpose is to allow existingand the attackers. Additionally, a poor network connec- routing protocols to incorporate our implementation oftion causes much difficulty in distinguishing between TARF with the least effort and thus producing a securean attacker and a honest node with transient failure. and efficient fully-functional protocol. Unlike other se-Without proper protection, WSNs with existing routing curity measures, TARF requires neither tight time syn-protocols can be completely devastated under certain cir- chronization nor known geographic information. Mostcumstances. In an emergent sensing application through importantly, TARF proves resilient under various attacksWSNs, saving the network from being devastated be- exploiting the replay of routing information, which iscomes crucial to the success of the application. not achieved by previous security protocols. Even under Unfortunately, most existing routing protocols for strong attacks such as sinkhole attacks, wormhole attacksWSNs either assume the honesty of nodes and focus as well as Sybil attacks, and hostile mobile networkon energy efficiency [12], or attempt to exclude unau- condition, TARF demonstrates steady improvement inthorized participation by encrypting data and authen- network performance. The effectiveness of TARF is ver-ticating packets. Examples of these encryption and au- ified through extensive evaluation with simulation andthentication schemes for WSNs include TinySec [13], empirical experiments on large-scale WSNs. Finally, weSpins [14], TinyPK [15], and TinyECC [16]. Admittedly, it have implemented a ready-to-use TARF module withis important to consider efficient energy use for battery- low overhead, which as demonstrated can be integratedpowered sensor nodes and the robustness of routing into existing routing protocols with ease; the demon-under topological changes as well as common faults stration of a proof-of-concept mobile target detectionin a wild environment. However, it is also critical to program indicates the potential of TARF in WSN appli-incorporate security as one of the most important goals; cations.meanwhile, even with perfect encryption and authen- We start by stating the design considerations of TARFtication, by replaying routing information, a malicious in Section 2. Then we elaborate the design of TARFnode can still participate in the network using another in Section 3, including the routing procedure as wellvalid node’s identity. The gossiping-based routing proto- as the EnergyWatcher and TrustManager components. Incols offer certain protection against attackers by selecting Section 4, we present the simulation results of TARFrandom neighbors to forward packets [17], but at a against various attacks through replaying routing in-price of considerable overhead in propagation time and formation in static, mobile and RF-shielding use. Section 5 further presents the implementation of TARF, In addition to the cryptographic methods, trust and empirical evaluation at a large sensor network and areputation management has been employed in generic resilient proof-of-concept mobile target detection appli-ad hoc networks and WSNs to secure routing protocols. cation based on TARF. Finally, we discuss the relatedBasically, a system of trust and reputation management work in Section 6 and conclude this paper in Section 7.assigns each node a trust value according to its pastperformance in routing. Then such trust values are usedto help decide a secure and efficient route. However, 2 D ESIGN C ONSIDERATIONSthe proposed trust and reputation management systems Before elaborating the detailed design of TARF, wefor generic ad hoc networks target only relatively pow- would like to clarify a few design considerations first,erful hardware platforms such as laptops and smart- including certain assumptions in Section 2.1 and thephones [18], [19], [20], [21]. Those systems can not be goals in Section 2.3.applied to WSNs due to the excessive overhead forresource-constrained sensor nodes powered by batteries.As far as WSNs are concerned, secure routing solutions 2.1 Assumptionsbased on trust and reputation management rarely ad- We target secure routing for data collection tasks, whichdress the identity deception through replaying routing are one of the most fundamental functions of WSNs. Ininformation [22], [23]. The countermeasures proposed so a data collection task, a sensor node sends its sampledfar strongly depends on either tight time synchronization data to a remote base station with the aid of other inter-or known geographic information while their effective- mediate nodes, as shown in Figure 1. Though there couldness against attacks exploiting the replay of routing be more than one base station, our routing approach isinformation has not been examined yet [4]. not affected by the number of base stations; to simplify At this point, to protect WSNs from the harmful our discussion, we assume that there is only one baseattacks exploiting the replay of routing information, we station. An adversary may forge the identity of any legalhave designed and implemented a robust trust-aware node through replaying that node’s outgoing routingrouting framework, TARF, to secure routing solutions in packets and spoofing the acknowledgement packets,wireless sensor networks. Based on the unique character- even remotely through a wormhole.istics of resource-constrained WSNs, the design of TARF Additionally, to merely simplify the introduction ofcenters on trustworthiness and energy efficiency. Though TARF, we assume no data aggregation is involved.
  3. 3. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 3 Node Base station the base station broadcast packets which are asymmet- rically authenticated. The asymmetric authentication of those broadcast packets from the base station is crucial to any successful secure routing protocol. It can be achieved through existing asymmetrically authenticated broadcast schemes that may require loose time synchronization. As an example, µTESLA [14] achieves asymmetric authen- ticated broadcast through a symmetric cryptographic algorithm and a loose delay schedule to disclose the keys from a key chain. Other examples of asymmetricFig. 1. Multi-hop routing for data collection of a WSN. authenticated broadcast schemes requiring either loose or no time synchronization are found in [25], [26].Nonetheless, our approach can still be applied to cluster- Considering the great computation cost incurred by abased WSNs with static clusters, where data are aggre- strong asymmetric authentication scheme and the diffi-gated by clusters before being relayed [24]. Cluster-based culty in key management, a regular packet other than aWSNs allows for the great savings of energy and band- base station broadcast packet may only be moderatelywidth through aggregating data from children nodes and authenticated through existing symmetric schemes withperforming routing and transmission for children nodes. a limited set of keys, such as the message authenticationIn a cluster-based WSN, the cluster headers themselves code provided by TinySec [13]. It is possible that anform a sub-network; after certain data reach a cluster adversary physically captures a non-base legal node andheader, the aggregated data will be routed to a base reveals its key for the symmetric authentication [27].station only through such a sub-network consisting of With that key, the adversary can forge the identity ofthe cluster headers. Our framework can then be applied that non-base legal node and joins the network “legally”.to this sub-network to achieve secure routing for cluster- However, when the adversary uses its fake identity tobased WSNs. TARF may run on cluster headers only falsely attract a great amount of traffic, after receivingand the cluster headers communicate with their children broadcast packets about delivery information, other le-nodes directly since a static cluster has known relation- gal nodes that directly or indirectly forwards packetsship between a cluster header and its children nodes, through it will start to select a more trustworthy paththough any link-level security features may be further through TrustManager (Section 3.4).employed. Finally, we assume a data packet has at least the 2.3 Goalsfollowing fields: the sender id, the sender sequencenumber, the next-hop node id (the receiver in this one- TARF mainly guards a WSN against the attacks mis-hop transmission), the source id (the node that initiates directing the multi-hop routing, especially those basedthe data), and the source’s sequence number. We insist on identity theft through replaying the routing informa-that the source node’s information should be included tion. This paper does not address the denial-of-servicefor the following reasons because that allows the base (DoS) [3] attacks, where an attacker intends to damagestation to track whether a data packet is delivered. It the network by exhausting its resource. For instance, wewould cause too much overhead to transmit all the one- do not address the DoS attack of congesting the networkhop information to the base station. Also, we assume the by replaying numerous packets or physically jammingrouting packet is sequenced. the network. TARF aims to achieve the following desir- able properties: High Throughput Throughput is defined as the ratio of2.2 Authentication Requirements the number of all data packets delivered to the baseThough a specific application may determine whether station to the number of all sampled data packets. Indata encryption is needed, TARF requires that the pack- our evaluation, throughput at a moment is computedets are properly authenticated, especially the broadcast over the period from the beginning time (0) until thatpackets from the base station. The broadcast from the particular moment. Note that single-hop re-transmissionbase station is asymmetrically authenticated so as to may happen, and that duplicate packets are consideredguarantee that an adversary is not able to manipulate or as one packet as far as throughput is concerned. Through-forge a broadcast message from the base station at will. put reflects how efficiently the network is collecting andImportantly, with authenticated broadcast, even with delivering data. Here we regard high throughput as onethe existence of attackers, TARF may use TrustManager of our most important goals.(Section 3.4) and the received broadcast packets about Energy Efficiency Data transmission accounts for a ma-delivery information (Section 3.2.1) to choose trustwor- jor portion of the energy consumption. We evaluate en-thy path by circumventing compromised nodes. Without ergy efficiency by the average energy cost to successfullybeing able to physically capturing the base station, it is deliver a unit-sized data packet from a source node to thegenerally very difficult for the adversary to manipulate base station. Note that link-level re-transmission should
  4. 4. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 4be given enough attention when considering energy cost necessary to delete some neighbors’ entries to keep thesince each re-transmission causes a noticeable increase in table size acceptable. The technique of maintaining aenergy consumption. If every node in a WSN consumes neighborhood table of a moderate size is demonstratedapproximately the same energy to transmit a unit-sized by Woo, Tong and Culler [28]; TARF may employ thedata packet, we can use another metric hop-per-delivery same evaluate energy efficiency. Under that assumption, the In TARF, in addition to data packet transmission, thereenergy consumption depends on the number of hops, are two types of routing information that need to be ex-i.e. the number of one-hop transmissions occurring. To changed: broadcast messages from the base station aboutevaluate how efficiently energy is used, we can measure data delivery and energy cost report messages fromthe average hops that each delivery of a data packet each node. Neither message needs acknowledgement. Atakes, abbreviated as hop-per-delivery. broadcast message from the base station is flooded to theScalability & Adaptability TARF should work well with whole network. The freshness of a broadcast messageWSNs of large magnitude under highly dynamic con- is checked through its field of source sequence number.texts. We will evaluate the scalability and adaptability of The other type of exchanged routing information is theTARF through experiments with large-scale WSNs and energy cost report message from each node, which isunder mobile and hash network conditions. broadcast to only its neighbors once. Any node receiving Here we do not include other aspects such as latency, such an energy cost report message will not forward it.load balance, or fairness. Low latency, balanced network For each node N in a WSN, to maintain such a neigh-load, and good fairness requirements can be enforced in borhood table with trust level values and energy cost val-specific routing protocols incorporating TARF. ues for certain known neighbors, two components, Ener- gyWatcher and TrustManager, run on the node (Figure 2).3 D ESIGN OF TARF EnergyWatcher is responsible for recording the energyTARF secures the multi-hop routing in WSNs against in- cost for each known neighbor, based on N ’s observationtruders misdirecting the multi-hop routing by evaluating of one-hop transmission to reach its neighbors and thethe trustworthiness of neighboring nodes. It identifies energy cost report from those neighbors. A compromisedsuch intruders by their low trustworthiness and routes node may falsely report an extremely low energy cost todata through paths circumventing those intruders to lure its neighbors into selecting this compromised nodeachieve satisfactory throughput. TARF is also energy- as their next-hop node; however, these TARF-enabledefficient, highly scalable, and well adaptable. Before in- neighbors eventually abandon that compromised next-troducing the detailed design, we first introduce several hop node based on its low trustworthiness as trackednecessary notions here. by TrustManager. TrustManager is responsible for trackingNeighbor For a node N , a neighbor (neighboring node) trust level values of neighbors based on network loopof N is a node that is reachable from N with one-hop discovery and broadcast messages from the base stationwireless transmission. about data delivery. Once N is able to decide its next-Trust level For a node N , the trust level of a neighbor is a hop neighbor according to its neighborhood table, itdecimal number in [0, 1], representing N ’s opinion of that sends out its energy report message: it broadcasts toneighbor’s level of trustworthiness. Specifically, the trust all its neighbors its energy cost to deliver a packetlevel of the neighbor is N ’s estimation of the probability from the node to the base station. The energy cost isthat this neighbor correctly delivers data received to the computed as in Section 3.3 by EnergyWatcher. Such anbase station. That trust level is denoted as T in this paper. energy cost report also serves as the input of its receivers’Energy cost For a node N , the energy cost of a neighbor the average energy cost to successfully deliver a unit-sized data packet with this neighbor as its next-hop One-hop Deliverynode, from N to the base station. That energy cost is Neighbordenoted as E in this paper. Energy Cost EnergyWatcher Energy Cost Report Neighborhood Next-hop Table3.1 Overview Network Loop Selection DiscoveryFor a TARF-enabled node N to route a data packet to the TrustManager Neighbor Trust Level Energy Costbase station, N only needs to decide to which neighbor- Base Station Report Broadcasting node it should forward the data packet consideringboth the trustworthiness and the energy efficiency. Once Fig. 2. Each node selects a next-hop node based on itsthe data packet is forwarded to that next-hop node, the neighborhood table, and broadcast its energy cost within itsremaining task to deliver the data to the base station is neighborhood. To maintain this neighborhood table, Energy-fully delegated to it, and N is totally unaware of what Watcher and TrustManager on the node keep track of relatedrouting decision its next-hop node makes. N maintains events (on the left) to record the energy cost and the trust levela neighborhood table with trust level values and energy values of its neighbors.cost values for certain known neighbors. It is sometimes
  5. 5. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 53.2 Routing Procedure explained as follows: the fact that an attacker attracts aTARF, as with many other routing protocols, runs as a great deal of traffic from many nodes often gets revealedperiodic service. The length of that period determines by at least several of those nodes being deceived withhow frequently routing information is exchanged and a high likelihood. The undelivered sequence interval [a,updated. At the beginning of each period, the base b] is explained as follows: the base station searches thestation broadcasts a message about data delivery during source sequence numbers received in last period, identi-last period to the whole network consisting of a few fies which source sequence numbers for the source nodecontiguous packets (one packet may not hold all the with this id are missing, and chooses certain significantinformation). Each such packet has a field to indicate interval [a, b] of missing source sequence numbers ashow many packets are remaining to complete the broad- an undelivered sequence interval. For example, the basecast of the current message. The completion of the base station may have all the source sequence numbers for thestation broadcast triggers the exchange of energy report source node 2 as {109, 110, 111, 150, 151} in last this new period. Whenever a node receives such a Then [112, 149] is an undelivered sequence interval;broadcast message from the base station, it knows that [109, 151] is also recorded as the sequence boundarythe most recent period has ended and a new period has of delivered packets. Since the base station is usuallyjust started. No tight time synchronization is required connected to a powerful platform such as a desktop, afor a node to keep track of the beginning or ending of a program can be developed on that powerful platform toperiod. During each period, the EnergyWatcher on a node assist in recording all the source sequence numbers andmonitors energy consumption of one-hop transmission finding undelivered sequence its neighbors and processes energy cost reports from Accordingly, each node in the network stores a tablethose neighbors to maintain energy cost entries in its of <node id of a source node, a forwarded sequence interval [a, b] with a significant length> about lastneighborhood table; its TrustManager also keeps track ofnetwork loops and processes broadcast messages from period. The data packets with the source node and thethe base station about data delivery to maintain trust sequence numbers falling in this forwarded sequence interval [a, b] have already been forwarded by this node.level entries in its neighborhood table. When the node receives a broadcast message about data To maintain the stability of its routing path, a node delivery, its TrustManager will be able to identify whichmay retain the same next-hop node until the next fresh data packets forwarded by this node are not delivered tobroadcast message from the base station occurs. Mean- the base station. Considering the overhead to store suchwhile, to reduce traffic, its energy cost report could a table, old entries will be deleted once the table is configured to not occur again until the next fresh Once a fresh broadcast message from the base stationbroadcast message from the base station. If a node does is received, a node immediately invalidates all the ex-not change its next-hop node selection until the next isting energy cost entries: it is ready to receive a newbroadcast message from the base station, that guaran- energy report from its neighbors and choose its newtees all paths to be loop-free, as can be deducted from next-hop node afterwards. Also, it is going to select athe procedure of next-hop node selection. However, as node either after a timeout is reached or after it hasnoted in our experiments, that would lead to slow received an energy cost report from some highly trustedimprovement in routing paths. Therefore, we allow a candidates with acceptable energy cost. A node imme-node to change its next-hop selection in a period when diately broadcasts its energy cost to its neighbors onlyits current next-hop node performs the task of receiving after it has selected a new next-hop node. That energyand delivering data poorly. cost is computed by its EnergyWatcher (see Section 3.3). Next, we introduce the structure and exchange of A natural question is which node starts reporting itsrouting information as well as how nodes make routing energy cost first. For that, note that when the base stationdecisions in TARF. is sending a broadcast message, a side effect is that its neighbors receiving that message will also regard this3.2.1 Structure and Exchange of Routing Information as an energy report: the base station needs 0 amountA broadcast message from the base station fits into at of energy to reach itself. As long as the original basemost a fixed small number of packets. Such a message station is faithful, it will be viewed as a trustworthyconsists of some pairs of <node id of a source node, an candidate by TrustManager on the neighbors of the baseundelivered sequence interval [a, b] with a significant station. Therefore, those neighbors will be the first nodeslength>, <node id of a source node, minimal sequence to decide their next-hop node, which is the base station;number received in last period, maximum sequence they will start reporting their energy cost once thatnumber received in last period>, as well as several node decision is intervals of those without any delivery record in lastperiod. To reduce overhead to an acceptable amount, 3.2.2 Route Selectionour implementation selects only a limited number of Now, we introduce how TARF decides routes in a WSN.such pairs to broadcast (Section 5.1) and proved effec- Each node N relies on its neighborhood table to select antive (Section 5.3, 5.4). Roughly, the effectiveness can be optimal route, considering both energy consumption and
  6. 6. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 6reliability. TARF makes good efforts in excluding those relation:nodes that misdirect traffic by exploiting the replay of EN b = EN →b + Ebrouting information. For a node N to select a route for delivering data Since each known neighbor b of N is supposed toto the base station, N will select an optimal next-hop broadcast its own energy cost Eb to N , to compute EN b ,node from its neighbors based on trust level and energy N still needs to know the value EN →b , i.e., the averagecost and forward the data to the chosen next-hop node energy cost of successfully delivering a data packet fromimmediately. The neighbors with trust levels below a N to its neighbor b with one hop. For that, assumingcertain threshold will be excluded from being considered that the endings (being acknowledged or not) of one-as candidates. Among the remaining known neighbors, hop transmissions from N to b are independent withN will select its next-hop node through evaluating each the same probability psucc of being acknowledged, weneighbor b based on a trade-off between TN b and EN b , T Nb first compute the average number of one-hop sendingswith EN b and TN b being b’s energy cost and trust level needed before the acknowledgement is received as fol-value in the neighborhood table respectively (see Sec- lows: ∞tion 3.3, 3.4). Basically, EN b reflects the energy cost of 1delivering a packet to the base station from N assuming i · psucc · (1 − psucc )i−1 = 1 i=1 psuccthat all the nodes in the route are honest; TN b approx-imately reflects the number of the needed attempts to Denote Eunit as the energy cost for node N to send asend a packet from N to the base station via multiple unit-sized data packet once regardless of whether it ishops before such an attempt succeeds, considering the received or not. Then we havetrust level of b. Thus, EN b combines the trustworthiness Nb Eunit T EN b = + Eband energy cost. However, the metric EN b suffers from T Nb psuccthe fact that an adversary may falsely reports extremely The remaining job for computing EN b is to get the proba-low energy cost to attract traffic and thus resulting in a bility psucc that a one-hop transmission is acknowledged.low value of EN b even with a low TN b . Therefore, TARF T Nb Considering the variable wireless connection amongprefers nodes with significantly higher trust values; this wireless sensor nodes, we do not use the simplisticpreference of trustworthiness effectively protects the net- averaging method to compute psucc . Instead, after eachwork from an adversary who forges the identity of an transmission from N to b, N ’s EnergyWatcher will updateattractive node such as a base station. For deciding the psucc based on whether that transmission is acknowl-next-hop node, a specific trade-off between TN b and EN b T Nb edged or not with a weighted averaging technique. Weis demonstrated in Figure 5 (see Section 5.2). use a binary variable Ack to record the result of current Observe that in an ideal misbehavior-free environ- transmission: 1 if an acknowledgement is received; oth-ment, all nodes are absolutely faithful, and each node erwise, 0. Given Ack and the last probability value of anwill choose a neighbor through which the routing path acknowledged transmission pold succ , an intuitive way isis optimized in terms of energy; thus, an energy-driven to use a simply weighted average of Ack and pold succ asroute is achieved. the value of pnew succ . That is what is essentially adopted in the aging mechanism [29]. However, that method used3.3 EnergyWatcher against sleeper attacks still suffers periodic attacks [30]. To solve this problem, we update the psucc value usingHere we describe how a node N ’s EnergyWatcher com- two different weights as in our previous work [30], aputes the energy cost EN b for its neighbor b in N ’s relatively big wdegrade ∈ (0, 1) and a relatively smallneighborhood table and how N decides its own energy wupgrade ∈ (0, 1) as follows:cost EN . Before going further, we will clarify some notations. EN b mentioned is the average energy cost  (1 − wdegrade ) × pold succ + wdegrade × Ack,  if Ack = 0. of successfully delivering a unit-sized data packet from pnew succ =N to the base station, with b as N ’s next-hop node  (1 − wupgrade ) × pold succ + wupgrade × Ack,  if Ack = .1 being responsible for the remaining route. Here, one-hopre-transmission may occur until the acknowledgement The two parameters wdegrade and wupgrade allow flexibleis received or the number of re-transmissions reaches application requirements. wdegrade and wupgrade repre-a certain threshold. The cost caused by one-hop re- sent the extent to which upgraded and degraded per-transmissions should be included when computing EN b . formance are rewarded and penalized, respectively. IfSuppose N decides that A should be its next-hop node any fault and compromise is very likely to be associatedafter comparing energy cost and trust level. Then N ’s with a high risk, wdegrade should be assigned a relativelyenergy cost is EN = EN A . Denote EN →b as the average high value to penalize fault and compromise relativelyenergy cost of successfully delivering a data packet from heavily; if a few positive transactions can’t constitute ev-N to its neighbor b with one hop. Note that the re- idence of good connectivity which requires many moretransmission cost needs to be considered. With the above positive transactions, then wupgrade should be assignednotations, it is straightforward to establish the following a relatively low value.
  7. 7. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 73.4 TrustManager are forwarded by this node to the number of thoseA node N ’s TrustManager decides the trust level of forwarded data packets, denoted as DeliveryRatio. Theneach neighbor based on the following events: discovery N ’s TrustManager updates its next-hop node b’s trustof network loops, and broadcast from the base station level as follows: about data delivery. For each neighbor b of N , TN b  (1 − wdegrade ) × Told N b denotes the trust level of b in N ’s neighborhood table.  +wdegrade × DeliveryRatio,   At the beginning, each neighbor is given a neutral trust  if DeliveryRatio < Told N b .  level 0.5. After any of those events occurs, the relevant Tnew N b =neighbors’ trust levels are updated.  (1 − wupgrade ) × Told N b    Note that many existing routing protocols have their  +wupgrade × DeliveryRatio,   own mechanisms to detect routing loops and to react  if DeliveryRatio >= Told N b .accordingly [31], [32], [28]. In that case, when integratingTARF into those protocols with anti-loop mechanisms, 3.5 Analysis on EnergyWatcher and TrustManagerTrustManager may solely depend on the broadcast fromthe base station to decide the trust level; we adopted Now that a node N relies on its EnergyWatcher andsuch a policy when implementing TARF later (see Sec- TrustManager to select an optimal neighbor as its next-tion 5). If anti-loop mechanisms are both enforced in hop node, we would like to clarify a few importantthe TARF component and the routing protocol that in- points on the design of EnergyWatcher and TrustManager.tegrates TARF, then the resulting hybrid protocol may First, as described in Section 3.1, the energy cost reportoverly react towards the discovery of loops. Though so- is the only information that a node is to passively receivephisticated loop-discovery methods exist in the currently and take as “fact”. It appears that such acceptance of en-developed protocols, they often rely on the comparison ergy cost report could be a pitfall when an attacker or aof specific routing cost to reject routes likely leading to compromised node forges false report of its energy cost.loops [32]. To minimize the effort to integrate TARF and Note that the main interest of an attacker is to preventthe existing protocol and to reduce the overhead, when data delivery rather than to trick a data packet into a lessan existing routing protocol does not provide any anti- efficient route, considering the effort it takes to launchloop mechanism, we adopt the following mechanism to an attack. As far as an attack aiming at preventing datadetect routing loops. To detect loops, the TrustManager delivery is concerned, TARF well mitigates the effect ofon N reuses the table of <node id of a source node, this pitfall through the operation of TrustManager. Notea forwarded sequence interval [a, b] with a significant that the TrustManager on one node does not take any rec-length> (see Section 3.2) in last period. If N finds that a ommendation from the TrustManager on another node.received data packet is already in that record table, not If an attacker forges false energy report to form a falseonly will the packet be discarded, but the TrustManager route, such intention will be defeated by TrustManager:on N also degrades its next-hop node’s trust level. If when the TrustManager on one node finds out the manythat next-hop node is b, then Told N b is the latest trust delivery failures from the broadcast messages of the baselevel value of b. We use a binary variable Loop to record station, it degrades the trust level of its current next-hopthe result of loop discovery: 0 if a loop is received; 1 node; when that trust level goes below certain threshold,otherwise. As in the update of energy cost, the new trust it causes the node to switch to a more promising next-level of b is hop node.  Second, TrustManager identities the low trustworthi-  (1 − wdegrade ) × Told N b + wdegrade × Loop,  ness of various attackers misdirecting the multi-hop if Loop = 0. Tnew N b = routing, especially those exploiting the replay of routing  (1 − wupgrade ) × Told N b + wupgrade × Loop,  information. It is noteworthy that TrustManager does not if Loop = 1.  distinguish whether an error or an attack occurs to the Once a loop has been detected by N for a few times next-hop node or other succeeding nodes in the that the trust level of the next-hop node is too low, It seems unfair that TrustManager downgrades the trustN will change its next-hop selection; thus, that loop is level of an honest next-hop node while the attack occursbroken. Though N can not tell which node should be somewhere after that next-hop node in the route. Con-held responsible for the occurrence of a loop, degrading trary to that belief, TrustManager significantly improvesits next-hop node’s trust level gradually leads to the data delivery ratio in the existence of attack attempts ofbreaking of the loop. On the other hand, to detect the preventing data delivery. First of all, it is often difficult totraffic misdirection by nodes exploiting the replay of identify an attacker who participates in the network us-routing information, TrustManager on N compares N’s ing an id “stolen” from another legal node. For example,stored table of <node id of a source node, forwarded se- it is extremely difficult to detect a few attackers colludingquence interval [a, b] with a significant length> recorded to launch a combined wormhole and sinkhole attack [4].in last period with the broadcast messages from the Additionally, despite the certain inevitable unfairnessbase station about data delivery. It computes the ratio involved, TrustManager encourages a node to chooseof the number of successfully delivered packets which another route when its current route frequently fails to
  8. 8. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 8deliver data to the base station. Though only those legal Matlab to test TARF. We have conducted extensive sim-neighboring nodes of an attacker might have correctly ulation experiments; however, due to the page limit,identified the adversary, our evaluation results indicate interested readers may refer to our technical report [33]that the strategy of switching to a new route without and the conference version of paper [1] for detailedidentifying the attacker actually significantly improves simulation settings and experimental results. In our ex-the network performance, even with the existence of periments, initially, 35 nodes are randomly distributedwormhole and sinkhole attacks. Fig 3 gives an example to within a 300*300 rectangular area, with unreliable wire-illustrate this point. In this example, node A, B, C and D less transmission. All the nodes have the same powerare all honest nodes and not compromised. Node A has level and the same maximal transmission range of 100m.node B as its current next-hop node while node B has an Each node samples 6 times in every period; the timingattacker node as its next-hop node. The attacker drops gap between every two consecutive samplings of theevery packet received and thus any data packet passing same node is equivalent. We simulate the sensor networknode A will not arrive at the base station. After a while, in 1440 consecutive periods.node A discovers that the data packets it forwarded did Regarding the network topology, we set up three typesnot get delivered. The TrustManager on node A starts to of network topologies. The first type is the static-locationdegrade the trust level of its current next-hop node B case under which all nodes stand still. The second typealthough node B is absolutely honest. Once that trust is a customized group-motion-with-noise case based onlevel becomes too low, node A decides to select node C Reference Point Group Mobility (RPGM) model thatas its new next-hop node. In this way node A identifies a mimics the behavior of a set of nodes moving in onebetter and successful route (A - C - D - base). In spite of or more groups [34], [35]. The last type of dynamicthe sacrifice of node B’s trust level, the network performs network incorporated in the experiments is the additionbetter. Further, concerning the stability of routing path, of scattered RF-shielded areas to the aforementioned group-motion-with-noise case. The performance of TARF is compared to that of a B link connectivity-based routing protocol adapted from Attacker what is proposed by Alec Woo, Terence Tong and A David Culler [28]. We denote the link connectivity- Base Station based routing protocol as Link-connectivity. With the Link-connectivity protocol, each node selects its next-hop C D node among its neighborhood table according to anFig. 3. An example to illustrate how TrustManager works. link estimator based on exponentially weighted moving average (EWMA). The simulation results show, in the presence of misbehaviors, the throughput in TARF is oftenonce a valid node identifies a trustworthy honest neigh- much higher than that in Link-connectivity; the hop-per-bor as its next-hop node, it tends to keep that next-hop delivery in the Link-connectivity protocol is generally atselection without considering other seemingly attractive least comparable to that in TARF.nodes such as a fake base station. That tendency is Under a misbehavior-free environment, the simula-caused by both the preference to maintain stable routes tion results show that TARF and Link-connectivity haveand the preference to highly trustable nodes. comparable performance when there is no adversary. Finally, we would like to stress that TARF is designed Both protocols are also evaluated under three commonto guard a WSN against the attacks misdirecting the types of attacks: (1) a certain node forges the identity ofmulti-hop routing, especially those based on identity the based station by replaying broadcast messages, alsotheft through replaying the routing information. Other known as the sinkhole attack; (2) a set of nodes colludes totypes of attacks such as the denial-of-service (DoS) [3] form a forwarding loop; and (3) a set of nodes drops re-attacks are out of the discussion of this paper. For ceived data packets. These experiments were conductedinstance, we do not address the attacks of injecting in the static case, the group-motion-with-noise case, andinto the network a number of data packets containing the addition of RF-shielded areas to the group-motion-false sensing data but authenticated (possibly through with-noise case separately. Generally, under these com-hacking). That type of attacks aim to exhaust the network mon attacks, TARF produces a substantial improvementresource instead of misdirecting the routing. However, if over Link-connectivity in terms of data collection and en-the attacker intends to periodically inject a few routing ergy efficiency. Further, we have evaluated TARF underpackets to cause wrong route, such attacks can still be more severe attacks: multiple moving fake bases anddefended by TARF through TrustManager. multiple Sybil attackers. As before, the experiments are conducted under all the three types of network topology. Under these two types of most severe attacks which4 S IMULATION almost devastates the Link-connectivity protocol, TARFWe have developed a reconfigurable emulator of wire- succeeds in achieving a steady improvement over theless sensor networks on a two-dimensional plane with Link-connectivity protocol. Finally, we have conducted
  9. 9. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 9certain experiments to explore the choice of the period sensing frequency, the energy efficiency and trustworthi-length and the trust updating scheme. Our experiments ness requirement. TrustManagerC provides two inter-reveal that a shorter period or a faster trust updating faces (see Figure 4), TrustControl and Record, whichscheme may not necessarily benefit TARF. are implemented in other modules. The TrustControl interface provides the commands to enable and disable the trust evaluation, while the Record interface provides5 I MPLEMENTATION AND E MPIRICAL E VALUA - the commands for a root, i.e., a base station, to addTION delivered message record, for a non-root node to add forwarded message record, and for a node to retrieve theIn order to evaluate TARF in a real-world setting, we trust level of any neighboring node. The implementationimplemented the TrustManager component on TinyOS on a root node differs from that on a non-root node: a2.x, which can be integrated into the existing routing pro- root node stores the information of messages receivedtocols for WSNs with the least effort. Originally, we had (delivered) during the current period into a record tableimplemented TARF as a self-contained routing proto- and broadcast delivery failure record; a non-root nodecol [1] on TinyOS 1.x before this second implementation. stores the information of forwarded messages during theHowever, we decided to re-design the implementation current period also in a record table and compute theconsidering the following factors. First, the first imple- trust of its neighbors based on that and the broadcast in-mentation only supports TinyOS 1.x, which was replaced formation. Noting that much implementation overheadby TinyOS 2.x; the porting procedure from TinyOS 1.x for a root can always be transferred to a more powerfulto TinyOS 2.x tends to frustrate the developers. Second, device connected to the root, it is reasonable to assumerather than developing a self-contained routing protocol, that the root would have great capability of processingthe second implementation only provides a TrustMan- and storage.ager component that can be easily incorporated into theexisting protocols for routing decisions. The detection configuration TrustManagerC { interface TrustControlof routing loops and the corresponding reaction are provides { {excluded from the implementation of TrustManager since interface TrustControl; //enable trust evaluation interface Record; command error_t start();many existing protocols, such as Collection Tree Proto- } //disable trust evaluationcol [32] and the link connectivity-based protocol [28], implementation { command error_t stop(); ........................................... }already provide that feature. As we worked on the first }implementation, we noted that the existing protocols }provide many nice features, such as the analysis oflink quality, the loop detection and the routing decision interface Record {mainly considering the communication cost. Instead of //for a root to add delivered record <source node id, source sequence number>providing those features, our implementation focuses command void addDelivered(am_addr_t src, uint8_t seq); //for a non-root node to add forwarded record <source id, source sequence, next-hop id>on the trust evaluation based on the base broadcast command void addForwarded(am_addr_t src, uint8_t seq, am_addr_t next);of the data delivery, and such trust information can //return the trust level of a node command uint16_t getTrust(am_addr_t id);be easily reused by other protocols. Finally, instead of }using TinySec [13] exclusively for encryption and au-thentication as in the first implementation on TinyOS 1.x, Fig. 4. TrustManager component.this re-implementation let the developers decide whichencryption or authentication techniques to employ; the A root broadcasts two types of delivery failure record:encryption and authentication techniques of TARF may at most three packets of significant undelivered intervalsbe different than that of the existing protocol. for individual origins and at most two packets of the id’s of the origins without any record in the current period. For each origin, at most three significant undelivered5.1 TrustManager Implementation Details intervals are broadcast. For a non-root node, consideringThe TrustManager component in TARF is wrapped the processing and memory usage overhead, the recordinto an independent TinyOS configuration named table keeps the forwarded message intervals for up to 20TrustManagerC. TrustManagerC uses a dedicated source nodes, with up to 5 non-overlapped intervals forlogic channel for communication and runs as a periodic each individual origin. Our later experiments verify thatservice with a configurable period, thus not interfering such size limit of the table on a non-root node produces awith the application code. Though it is possible to im- resilient TARF with moderate overhead. The record tableplement TARF with a period always synchronized with on a node keeps adding entries for new origins until itthe routing protocol’s period, that would cause much in- is full.trusion into the source code of the routing protocol. The With our current implementation, a valid trust value iscurrent TrustManagerC uses a period of 30 seconds; an integer between 0 and 100, and any node is assignedfor specific applications, by modifying a certain header an initial trust value of 50. The weigh parameters are:file, the period length may be re-configured to reflect the wupgrade = 0.1, wdegrade = 0.3. The trust table of a
  10. 10. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 10non-root node node keeps the trust level for up to 10 candidate has a trust level above certain threshold. Theneighbors. Considering that an attacker may present reason is, the sole trust/cost criteria could be exploitedmultiple fake id’s, the implementation evicts entries with by an adversary replaying the routing information froma trust level close to the initial trust of any node. Such a base station and thus pretending to be an extremelyeviction policy is to ensure that the trust table remembers attractive node. As for Step 2, compared to the CTPthose neighbors with high trust and low trust; any other implementation, we add two more circumstances whenneighbor not in this table is deemed to have the initial a node decides to switch to the optimal candidate foundtrust value of 50. at Step 1: that candidate has a higher trust level, or the current next-hop neighbor has a too low trust level.5.2 Incorporation of TARF into Existing Protocols //Step 1. traverse the neighborhood table for an optimal candidate for the next hopTo demonstrate how this TARF implementation can be optimal_candidate = NULL //the cost of routing via the optimal candidate provided by the existing protocol, initially infinityintegrated into the exiting protocols with the least effort, optimal_cost = MAX_COSTwe incorporated TARF into a collection tree routing //the trust level of the optimal candidate, initially 0 optimal_trust = MIN_TRUSTprotocol (CTP) [32]. The CTP protocol is efficient, robust, for each candidate in the neighborhood table if link is congested, or may cause a loop, or does not pass quality thresholdand reliable in a network with highly dynamic link continuetopology. It quantifies link quality estimation in order better = false if >= optimal_trust && candidate.cost < optimal_costto choose a next-hop node. The software platform is better = true //prefer trustworthy candidatesTinyOS 2.x. To perform the integration, after proper if >= TRUST_THRESHOLD && optimal_trust < TRUST_THRESHOLDinterface wiring, invoke the TrustControl.start better = true if >= ESSENTIAL_DIFFERENCE_THRESHOLD + optimal_trustcommand to enable the trust evaluation; call the better = true //effective when all nodes have low trust due to network change or poor connectivityRecord.addForwarded command for a non-root node if >= 3 * optimal_trust / 2to add forwarded record once a data packet has been better = true //add restriction of trust level requirementforwarded; call the Record.addDelivered command if >= TRUST_THRESHOLD && / candidate.cost > optimal_trust / optimal_costfor a root to add delivered record once a data packet has better = truebeen received by the root. Finally, inside the CTP’s task if better == true optimal_candidate = candidateto update the routing path, call the Record.getTrust optimal_cost = candidate.cost optimal_trust = candidate.trustcommand to retrieve the trust level of each next-hopcandidate; an algorithm taking trust into routing consid- //Step 2. decide whether to switch from the current next-hop node to the optimal candidate found: if optimal_trust >= eration is executed to decide the new next-hop neighbor || <= TRUST_THRESHOLD || current link is congested and switching is not likely to cause loops (see Figure 5). || optimal_cost + NEXTHOP_SWITCH_THRESHOLD < currentNextHop.cost Similar to the original CTP’s implementation, the im- currentNextHop = optimal_candidateplementation of this new protocol decides the next-hopneighbor for a node with two steps (see Figure 5): Step Fig. 5. Routing decision incorporating trust management.1 traverses the neighborhood table for an optimal candi-date for the next hop; Step 2 decides whether to switch This new implementation integrating TARF requiresfrom the current next-hop node to the optimal candidate moderate program storage and memory usage. We im-found. For Step 1, as in the CTP implementation, a plemented a typical TinyOS data collection application,node would not consider those links congested, likely MultihopOscilloscope, based on this new protocol. Theto cause a loop, or having a poor quality lower than MultihopOscilloscope application, with certain modifieda certain threshold. This new implementation prefers sensing parameters for our later evaluation purpose,those candidates with higher trust levels; in certain periodically makes sensing samples and sends out thecircumstances, regardless of the link quality, the rules sensed data to a root via multiple routing hops. Orig-deems a neighbor with a much higher trust level to inally, MultihopOscilloscope uses CTP as its routingbe a better candidate (see Figure 5). The preference of protocol. Now, we list the ROM size and RAM sizehighly trustable candidates is based on the following requirement of both implementation of MultihopOscillo-consideration: on the one hand, it creates the least chance scope on non-root Telosb motes in Table 1. The enablingfor an adversary to misguide other nodes into a wrong of TARF in MultihopOscilloscope increases the size ofrouting path by forging the identity of an attractive ROM by around 1.3KB and the size of memory bynode such as a root; on the other hand, forwarding data around 1.2KB.packets to a candidate with a low trust level would resultin many unsuccessful link-level transmission attempts, TABLE 1thus leading to much re-transmission and a potential Size comparison of MultihopOscilloscope implementationwaste of energy. When the network throughput becomeslow and a node has a list of low-trust neighbors, the Protocol ROM (bytes) RAM (bytes)node will exclusively use the trust as the criterion to CTP 31164 3579 TARF-enabled CTP 34290 4767evaluate those neighbors for routing decisions. As showin Figure 5, it uses trust/cost as a criteria only when the
  11. 11. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 115.3 Empirical Evaluation on Motelab addition to programming those 91 motes with CTP, weWe evaluated the performance of TARF against a com- also programmed the five fake base stations so that theybined sinkhole and wormhole attack on Motelab [36] at stole the id the base station through replaying. In theHarvard University. 184 TMote Sky sensor motes were last experiment, we programmed those 91 motes withdeployed across many rooms at three floors in the de- the TARF-enabled CTP, and programmed the five fakepartment building (see Figure 6), with two to four motes base stations as in the second experiment. Each of ourin most rooms. Around 97 nodes functioned properly programs run for 30 minutes.while the rest were either removed or disabled. Each As illustrated in Figure 7(a), the existence of the fivemote has a 2.4GHz Chipcon CC2420 radio with an wormhole attackers greatly degraded the performanceindoor range of approximately 100 meters. In Figure 6, of CTP: the number of the delivered data packets inthe thin green lines indicate the direct (one-hop) wireless the case of CTP with the five-node wormhole is no moreconnection between motes. Certain wireless connection than 14% that in the case of CTP without adversaries.also exists between nodes from different floors. The TARF-enabled CTP succeeded in bringing an immense improvement over CTP in the presence of the five-node wormhole, almost doubling the throughput. That improvement did not show any sign of slowing down as time elapsed. The number of nodes from each floor that delivered at least one data packet in each six-minute sub-period is plotted in Figure 7(a), 7(b) and 7(c) separately. On each floor, without any adversary, at least 24 CTP nodes were able to find a successful route in each six minute. However, with the five fake base stations in the wormhole, the number of CTP nodes that could find a successful route goes down to 9 for the first floor; it decreases to no more than 4 for the second floor; as the worst impact, none of the nodes on the third floor ever found a successful route. A further look at the data showed that all the nine nodes from the first floor with successful delivery record were allFig. 6. Connectivity map of Motelab (not including the inter- close to the real base station. The CTP nodes relativelyfloor connectivity), adapted from Motelab[Motelab]. far away from the base station, such as those on the second and the third floor, had little luck in making We developed a simple data collection application in good routing decisions. When TARF was enabled onTinyOS 2.x that sends a data packet every five seconds to each node, most nodes made correct routing decisionsa base station node (root) via multi-hop. This application circumventing the attackers. That improvement canwas executed on 91 functioning non-root nodes on Mote- be verified by the fact that the number of the TARF-lab. For comparison, we used CTP and the TARF-enabled enabled nodes with successful delivery record underCTP implementation as the routing protocols for the data the threat of the wormhole is close to that of CTP nodescollection program separately. The TARF-enabled CTP with no attackers, as shown in Figure 7(a), 7(b) and 7(c).has a TARF period of 30 seconds. We conducted an attackwith five fake base stations that formed a wormhole. Asin Figure 6, whenever the base station sent out any 5.4 Application: Mobile Target Detection in the Pres-packet, three fake base stations which overheard that ence of an Anti-Detection Mechanismpacket replayed the complete packet without changing To demonstrate how TARF can be applied in networkedany content including the node id. Other fake base sensing systems, we developed a proof-of-concept re-stations overhearing that replayed packet would also silient application of target detection. This applicationreplay the same packet. Each fake base station essentially relies on a deployed wireless sensor network to detectlaunched a sinkhole attack. Note that there is a distinction a target that could move, and to deliver the detectionbetween such malicious replay and the forwarding when events to a base station via multiple hops with the TARF-a well-behaved node receives a broadcast from the base enabled CTP protocol. For simplification, the target is astation. When a well-behaved node forwards a broadcast LEGO MINDSTORM NXT 2.0 vehicle robot equippedpacket from the base station, it will include its own with a TelosB mote that sends out an AM (Activeid in the packet so that its receivers will not recognize Message) packet every three seconds. A sensor nodethe forwarder as a base station. We conducted the first receiving such a packet from the target issues a detectionexperiment by uploading the program with the CTP report, which will be sent to the base station with theprotocol onto 91 motes (not including those 5 selected aforementioned TARF-enabled CTP protocol.motes as fake bases in later experiments), and no attack The experiment is set up within a clear floor space ofwas involved here. Then, in another experiment, in 90 by 40 inches with 15 TelosB motes (see Figure 8(a)). To
  12. 12. IEEE TRANSACTIONS ON DEPENDABLE AND SECURE COMPUTING 12 4 x 10 3 CTP without adversaries power of all the Telosb motes except two fake base Number of delivered packets CTP with 5-node wormhole stations in the network is reduced through both software 2.5 TARF-enabled CTP with 5-node wormhole reduction and attenuator devices to within 30 inches. The 2 target uses an anti-detection mechanism utilizing a fake base station close to the real base station, and another 1.5 remote base station close to the target and mounted on another LEGO vehicle robot. The two fake base stations, 1 with a transmission range of at least 100 feet, collude 0.5 to form a wormhole: the fake base station close to the base station replays all the packets from the base station 0 0 5 10 15 20 25 30 immediately; the remote fake base station, after receiving Time in minutes those packets, immediately replays it again. This anti- (a) All three floors. detection mechanism tricks some network nodes into sending their event reports into these fake base stations 26 26 instead of the real base station. Though the fake base Number of nodes with 25 25 25 25 24 24 station close to the real base station is capable of cheating delivery record 21 20 the whole network alone by itself with its powerful radio for a certain amount of time, it can be easily recognized 9 9 9 9 9 by remote nodes as a poor next-hop candidate soon by most routing protocols based on link quality: that fake [0min, 6min] [6min, [12min, [18min, [24min, base station does not acknowledge the packets “sent” 12min] 18min] 24min] 30min] to it from remote nodes with a weak radio via a single Time CTP without adversaries hop since it can not really receive them. Thus, the anti- CTP with 5-node wormhole TARF-enabled CTP with 5-node wormhole detection mechanism needs to create such a wormhole to (b) First floor. replay the packets from the base station remotely. 36 36 36 36 37 36 34 Number of nodes with 33 32 30 delivery record 4 3 2 2 2 [0min, 6min] [6min, [12min, [18min, [24min, 12min] 18min] 24min] 30min] Time CTP without adversaries CTP with 5-node wormhole TARF-enabled CTP with 5-node wormhole (c) Second floor. (a) A snapshot of the network. Number of nodes with 25 25 25 25 25 24 24 23 23 22 delivery record 0 0 0 0 0 [0min, 6min] [6min, [12min, [18min, [24min, 12min] 18min] 24min] 30min] Time CTP without adversaries CTP with 5-node wormhole TARF-enabled CTP with 5-node wormhole (d) Third floor.Fig. 7. Empirical comparison of CTP and TARF-enabled CTP (b) A closer look.on Motelab: (a) number of all delivered data packets since thebeginning; number of nodes on (b) the first floor, (c) the second Fig. 8. Deployment of a TARF-enabled wireless sensor networkfloor and (d) the third floor that delivered at least one data packet to detect a moving target under the umbrella of two fake basein sub-periods. stations in a wormhole. The target node 14 and the fake base station 13 close to it move across the network along two parallel tracks ofmake the multi-hop delivery necessary, the transmission 22 inches back and forth (see Figure 8(b)); they travel