bubble rap: social-based forwarding in delay tolerant ......social metrics are intrinsic properties...
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BUBBLE Rap: Social-based Forwarding in DelayTolerant Networks
Pan Hui, Member, IEEE Jon Crowcroft, Fellow, IEEE and Eiko Yoneki, Member, IEEE
Abstract—The increasing penetration of smart devices with networking capability form novel networks. Such networks, also referred asPocket Switched Networks (PSNs), are intermittently connected and represent a paradigm shift of forwarding data in an ad-hoc manner.The social structure and interaction of users of such devices dictate the performance of routing protocols in PSNs. To that end, socialinformation is an essential metric for designing forwarding algorithms for such types of networks. Previous methods relied on buildingand updating routing tables to cope with dynamic network conditions. On the downside it has been shown that such approaches endup being cost ineffective due to the partial capture of the transient network behavior. A more promising approach would be to capturethe intrinsic characteristics of such networks and utilize them in the design of routing algorithms.In this paper, we exploit two social and structural metrics, namely centrality and community, using real human mobility traces. Thecontributions of this paper are two-fold. First we design and evaluate BUBBLE, a novel social-based forwarding algorithm, thatutilizes the aforementioned metrics to enhance delivery performance. Second we empirically show that BUBBLE can substantiallyimprove forwarding performance compared to a number of previously proposed algorithms including the benchmarking history-basedPROPHET algorithm, and social-based forwarding SimBet algorithm.
Index Terms—Social Networks, Forwarding Algorithms, Delay Tolerant Networks, Pocket Switched Networks, Centrality, CommunityDetection.
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1 INTRODUCTION
W E envision a future in which a multitude of devicescarried by people are dynamically networked. We aim
to build Pocket Switched Networks (PSN) [1], a type ofDelay Tolerant Networks (DTN) [2] for such environments.A PSN utilizes contact opportunities to allow humans tocommunicate without network infrastructure.We propose anefficient data forwarding mechanism over time evolving graphsof the PSN [3], that copes with dynamical, repeated disconnec-tion and re-wiring. With such scenarios, end-to-end deliverythrough traditional routing algorithms is rarely applicable.
Many MANET and some DTN routing algorithms [4] [5]provide forwarding by building and updating routing tableswhenever mobility occurs. We believe this approach is not ap-propriate for a PSN, since mobility is often unpredictable,andtopology structure is highly dynamic. Rather than exchangemuch control traffic to create unreliable routing structures,which may only capture the “noise” of the network, we preferto search for some characteristics of the network which areless volatile than mobility. A PSN is formed by people. Hence,social metrics are intrinsic properties to guide data forwardingin such kinds of human networks. Furthermore, if we candetect these social mobility patterns online in a decentralisedway, we can apply these algorithms in practice.
In this paper, we focus on two key social metrics: commu-nity and centrality. Co-operation binds, but also divides human
• P. Hui is with Deutsche Telekom Laboratories, Ernst-Reuter-Platz 7, 10587,Berlin, GermanyE-mail: [email protected]
• J. Crowcroft and E. Yoneki are with Computer Laboratory, University ofCambridge, William Gates Building, 15 JJ Thomson Avenue, Cambridge,CB3 0FD, UK
society into communities. For an ecological community, theidea of correlated interaction means that an organism of agiven type is more likely to interact with another organismof the same type than with a randomly chosen member ofthe population [6]. This correlated interaction concept alsoapplies to human, so we can exploit this kind of communityinformation to select forwarding paths. Within a community,some people are more popular, and interact with more peoplethan others (i.e., have highcentrality); we call them hubs. Inthis paper, we will exploit community and centrality for dataforwarding in PSNs.
Methodologically, community detection [7] [8] can help usto uncover and understand local community structure in bothoff-line mobile trace analysis and online applications, and istherefore helpful in designing good strategies for informationdissemination. Freeman [9] defined several centrality metricsto measure the importance of a node in a network. Between-ness centrality measures the number of times a node falls onthe shortest path between two other nodes. This concept is alsovalid in a DTN. In a PSN, it can represent the importanceof a node as a potential traffic relay for other nodes in thesystem. The main contributions of this paper are to answerthese questions:
1) How does the variation in node popularity help us toforward in a PSN?
2) Are communities of nodes detectable in PSN traces?3) How well does social based forwarding work, and how
does it compare to other forwarding schemes in a real(emulated) environment?
4) Can we devise a fully decentralised way for suchschemes to operate?
To preview our answers to the above questions, we eval-uate the impact of community and centrality on forwarding,
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and proposeBUBBLE, a hybrid algorithm, that selects highcentrality nodes and community members of destination asrelays. We demonstrate a significant improvement in forward-ing efficiency over a number of previously proposed state-of-the-arts algorithms includingPROPHETalgorithm [5], whichuses patterns of movement, rather than the longer term socialrelationships on which ourBUBBLE scheme rests, and thesocial-based forwardingSimBetalgorithm [10]. In a PSN, theremay be noa priori information. By definition, we are also in adecentralised world without access to infrastructure. Thereforedistributed detection and dissemination of node popularityand node communities, and the use of these for forwardingdecisions are crucial. We verify that this is not only possible,but works well in terms of message delivery performance andefficiency compared to prior schemes.
The rest of this paper is structured as follows. We introducethe experimental data sets in Section 2, describe the contactgraphs and inferring human communities in Section 3. InSection 4, we examine the heterogeneity in centrality. We showevaluation methodology in Section 5 and results includingdiscussions from Section 6 to 9 followed by the related worksin Section 10. Finally, we conclude the paper with a briefsummary of contributions in Section 11.
2 EXPERIMENTAL DATASETS
In this paper, we use four experimental datasets gatheredby the Haggle Project1 over two years, referred to asInfo-com05, HongKong, Cambridge, Infocom06; one dataset fromthe MIT Reality Mining Project [11], referred to asReality.Previously, the characteristics of these datasets such as inter-contact and contact distribution have been explored in severalstudies [12] [13] [14], to which we refer the reader forfurther background information. These five datasets cover arich diversity of environments, ranging from busy metropolitancity (HongKong) to quiet university town (Cambridge), with anexperimental period from several days (Infocom06) to almostone year (Reality).
• In Infocom05, the devices were distributed to approxi-mately fifty students attending the Infocom student work-shop. Participants belong to different social communities(depending on their country of origin, research topic,etc.).
• In Hong-Kong, the people carrying the wireless deviceswere chosen independently in a Hong-Kong bar, to avoidany particular social relationship between them. Thesepeople have been invited to come back to the same barafter a week. They are unlikely to see each other duringthe experiment.
• In Cambridge, the iMotes were distributed mainly totwo groups of students from University of CambridgeComputer Laboratory, specifically undergraduate year1and year2 students, and also some PhD and Mastersstudents. This dataset covers 11 days.
• In Infocom06, the scenario was very similar toInfocom05except that the scale is larger, with 80 participants.
1. http://www.haggleproject.org
Participants were selected so that 34 out of 80 form 4subgroups by academic affiliations.
• In Reality, 100 smart phones were deployed to studentsand staff at MIT over a period of 9 months. These phoneswere running software that logged contacts with otherBluetooth enabled devices by doing Bluetooth devicediscovery every five minutes.
The five experiments are summarised in Table 1. A remarkabout the datasets is that the experiments do not have the samegranularity and the finest granularity is limited to 120 seconds.This is because of the trade-off between the duration of theexperiments and the accuracy of the samplings.
3 INFERRING HUMAN COMMUNITIESIn PSN, the social network could map to the computer networksince people carry the computing devices. To answer thequestion whether communities of nodes are detectable inPSN traces we need community detection algorithms. In thissection, we introduce and evaluate two centralised communitydetection algorithms:K-CLIQUE by Palla et al. [15] andweighted network analysis (WNA) by Newman [16]. We usethese two centralised algorithm to uncover the communitystructures in the mobile traces. We believe our evaluation ofthese algorithms can be useful for future traces gathered bythe research community.
Many centralised community detection methods have beenproposed and examined in the literature (see the recent reviewpapers by Newman [7] and Danonet al. [8]). The criteriawe use to select a centralised detection method are the abilityto uncover overlapping communities, and a high degree ofautomation (low manual involvement). In real human societies,one person may belong to multiple communities and hence itis important to be able to detect this feature. TheK-CLIQUEmethod satisfies this requirement, but was designed for binarygraphs, thus we must threshold the edges of the contactgraphs in our mobility traces to use this method, and it isdifficult to choose an optimum threshold manually [15]. On theother hand, (WNA) can work on weighted graphs directly, anddoes not need thresholding, but it cannot detect overlappingcommunities [16]. Thus we chose to use bothK-CLIQUE andWNA; they each have useful features that complement oneanother.
3.1 Contact GraphsIn order to help us to present the mobility traces and makeit easier for further processing, we introduce the notion ofcontact graph. The way we convert human mobility traces intoweighted contact graphs is based on the number of contactsand the contact duration, although we could use other metrics.The nodes of the graphs are the physical nodes from thetraces, the edges are the contacts, and the weights of theedges are the values based on the metrics specified such asthe number of contacts during the experiment. We can measurethe relationship between two people by how many times theymeet each other and how long they stay with each other. Wenaturally think that if two people spend more time together orsee each other more often, they are in a closer relationship.
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Experimental data set Infocom05 Hong-Kong Cambridge Infocom06 RealityDevice iMote iMote iMote iMote Phone
Network type Bluetooth Bluetooth Bluetooth Bluetooth BluetoothDuration (days) 3 5 11 3 246
Granularity (seconds) 120 120 600 120 300Number of Experimental Devices 41 37 54 98 97
Number of internal contacts 22,459 560 10,873 191,336 54,667Average # Contacts/pair/day 4.6 0.084 0.345 6.7 0.024Number of external devices 264 868 11,357 14,036 NANumber of external contacts 1,173 2,507 30,714 63,244 NA
TABLE 1Characteristics of the five experimental data sets
First, we find the distribution of contact durations andnumber of contacts for the two conference scenarios are quitesimilar. To prevent redundancy, in the later sections we onlyselectively show one example, in most casesInfocom06, sinceit contains more participants.
Figure 1 and Figure 2 show the contact duration and numberof contacts distribution for each pair in four experiments.For theHongKong experiment we include the external devicebecause of the network sparseness, but for the other threeexperiments we use only the internal devices. These contactgraphs created are used for the community detection in thefollowing subsections.
Fig. 1. The distribution of pair-wise contact durations
3.2 K-CLIQUE Community DetectionPallaet al. [15] define ak-clique community as a union of allk-cliques (complete subgraphs of sizek) that can be reachedfrom each other through a series of adjacentk-cliques, wheretwo k-cliques are said to be adjacent if they sharek−1 nodes.As k is increased, thek-clique communities shrink, but onthe other hand become more cohesive since their membernodes have to be part of at least onek-clique. We haveapplied this on all the datasets above. Figure 3 shows the3-clique communities in theInfocom06 dataset. More detaileddescriptions about thek-clique communities on these datasetscan be found in our previous work [17] [18].
Fig. 2. The distribution of pair-wise number of contacts
Barcelona Group
Paris Group A
Paris Group B
Lausanne Group
Paris Groups
Barcelona Group
Lausanne Group
Fig. 3. 3-clique communities based on contact durations with weightthreshold that equals 20 000 s (Infocom06; circles, Barcelona group;squares, Paris group A; triangles, Paris group B; diamonds, Lausannegroup)
3.3 Weighted Network Analysis
In this section, we implement and apply Newman’s weightednetwork analysis (WNA) for our data analysis [16]. This isan extension of the unweightedmodularity proposed in [19]to a weighted version, and use this as a measurement of thefitness of the communities it detects.
For each community partitioning of a network, one can com-pute the corresponding modularity value using the following
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definition of modularity (Q):
Q =∑
vw
[
Avw
2m−
kvkw
(2m)2
]
δ(cv,cw) (1)
whereAvw is the value of the weight of the edge betweenverticesv and w, if such an edge exists, and 0 otherwise;the δ-function δ(i, j) is 1 if i = j and 0 otherwise;m =1
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∑
vw Avw ; kv is the degree of vertexv defined as∑
w Avw;andci denotes the community vertexi belongs to.Modularityis defined as the difference between this fraction and, thefraction of the edges that would be expected to fall withinthe communities if the edges were assigned randomly, but wekeep the degrees of the vertices unchanged. The algorithm isessentially a genetic algorithm, using the modularity as themeasurement of fitness. Rather than selecting and mutatingcurrent best solutions, we enumerate all possible merges ofany two communities in the current solution, and evaluate therelative fitness of the resulting merges, and choose the bestsolution as the seed for the next iteration.
Table 2 summarises the communities detected by applyingWNA on the four datasets. According to Newman [16],nonzeroQ values indicate deviations from randomness; valuesaround 0.3 or more usually indicate good divisions. For theInfocom06 case, theQmax value is low; this indicates thatthe community partition is not very good in this case. Thisalso agrees with the fact that in a conference the communityboundary becomes blurred. For theReality case, theQ valueis high; this reflects the more diverse campus environment.For the Cambridge data, the two groups spawned by WNAis exactly matched the two groups (1st year and 2nd year) ofstudents selected for the experiment.
Dataset Info06 Camb Reality HKQmax 0.2280 0.4227 0.5682 0.6439
Max. Community Size 13 18 23 139No. Communities 4 2 8 19
Avg. Community Size 8.000 16.500 9.875 45.684No. Community Nodes 32 33 73 868
Total No. of Nodes 78 36 97 868
TABLE 2Communities detected from the four datasets
These centralised community detection algorithms give usrich information about the human social clustering and areuseful for offline data analysis on mobility traces collected.We can use them to explore structures in the data and hencedesign useful forwarding strategies, security measures, andkiller applications.
4 HETEROGENEITY IN CENTRALITYIn human society, people have different levels of popularity:salesmen and politicians meet customers frequently, whereascomputer scientists may only meet a few of their colleaguesonce a year [17]. Here, we want to employ heterogeneityin popularity to help design more efficient forwarding strate-gies: we prefer to choose popular hubs as relays rather thanunpopular ones.SimBet routing algorithm [10] also uses theconcept of centrality for choosing relays. We will compare theperformance of both algorithms in Section 8
A temporal network or time evolving network is a kindof weighted network. The centrality measure in traditionalweighted networks may not work here since the edges arenot necessarily concurrent (i.e., the network is dynamic andedges are time-dependent). Hence we need a different wayto calculate the centrality of each node in the system. Ourapproach is as follows:
1) Carry out a large number of emulations of unlimitedflooding with different uniformly distributed traffic pat-terns created using theHaggleSim emulator(Section 5.1).
2) Count the number of times a node acts as a relay forother nodes on all the shortest delay deliveries. Here theshortest delay delivery refers to the case when the samemessage is delivered to the destination through differentpaths, where we only count the delivery with the shortestdelay.
We call the number calculated above thebetweenness cen-trality of this node in this temporal graph. Of course, it can benormalised to the highest value found. Here we use unlimitedflooding since it can explore the largest range of deliveryalternatives with the shortest delay. This definition capturesthe spirit of Freeman centrality [9].
Initially, we only consider a homogeneous communicationpattern, with equal likelihood of every destination, and wedonot weight the traffic matrix by locality. We then calculate theglobal centrality value for the whole homogeneous system.Later, we will analyse the heterogeneous system (Section 8).
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Fig. 4. Number of times a node as relays for others on four datasets
Figure 4 shows the number of times a node falls on theshortest paths between all other node pairs. We can treatthis simply as the centrality of a node in the system. Weobserve very wide heterogeneity in each experiment. Thisclearly shows that there is a small number of nodes whichhave extremely high relaying ability, and a large number ofnodes that have moderate or low centrality values, acrossall experiments. One interesting point from the HK data is
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that the node showing highest delivery power in the figure isactually an external node. This node could be some popularhub for the whole city, i.e., postman or a newspaper manin a popular underground station, which relayed a certainamount of cross city traffic. The 30th, 70th percentiles andthe means of normalised individual node centrality are shownin Table 3. These numbers summarise the statistical propertyof the centrality values for each system shown in Figure 4.
Experimental dataset 30th percentile Mean 70th percentileCambridge 0.052 0.220 0.194
Reality 0.005 0.070 0.050Infocom06 0.121 0.188 0.221Hong Kong 0.000 0.017 0.000
TABLE 3Statistics about normalised node centrality in 4 experiments
5 INTERACTION AND FORWARDINGIn the first half of this paper, we have shown the existenceof heterogeneity at the level of individuals and groups, inall the mobility traces. This motivates us to consider a newheterogeneous model of human interaction and mobility.Cliques and Community: we explored the community struc-tures inside different social environments, and found thesecommunity structures match quite well with the real under-lying social structures.Popularity Ranking: We observed the heterogeneity for nodecentralities in both global and local scales. We shall see thatpopular hubs are as useful in thePSN context as they are inthe wireline Internet and in the Web.
From Section 6 to Section 9, we will look at how can weuse this information to make smart forwarding decisions. Thefollowing three pre-existing schemes provide lower and upperbounds in terms of cost and delivery success. All of theseschemes are inefficient because they assume a homogeneousenvironment. If the environment is homogeneous then everynode isstatistically equivalent (i.e., every node has the samelikelihood of delivering the messages to the destination).Aswe showed in the first half of this paper, the environmentsand nodes are diverse, and hence all these naive schemesare doomed to have poor performance. We need to designalgorithms which make use of this rich heterogeneity.
• WAIT: Hold onto a message until the sender encountersthe recipient directly, which represents the lower boundfor delivery cost.WAIT is the only single-copy algorithmin this paper.
• FLOOD: Messages are flooded throughout the entire sys-tem, which represents the upper bound for delivery andcost.
• Multiple-Copy-multiple-hoP (MCP): Multiple copies aresent subject to a time-to-live hop count limit on thepropagation of messages. By exhaustive emulations, the4-copy-4-hopMCP scheme is found to be the most cost-effective scheme in terms of delivery ratio and cost for allnaive schemes among most of the datasets. Hence for faircomparison, we would like to evaluate our algorithms and
the comparison algorithms (e.g.PROPHET and SimBet)against the 4-copy-4-hopMCP scheme in most of thecases.
Explicit Social Structure
Structure in Degree
Structure in Cohesive Group
Label
Rank, Degree
Clique Label
Bubble
Network Plane
Hu
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Fig. 5. Design space for forwarding algorithms
Figure 5 shows the design space for the forwarding algo-rithms. The vertical axis represents the explicit social structure.This is the social or human dimension. The two horizontalaxes represent the network structural plane, which can beinferred purely from observed contact patterns. The Structure-in-Cohesive Group axis indicates the use of localised cohesivestructure, and the Structure-in-Degree axis indicates theuse ofnode ranking and degree. These are observable physical char-acteristics. In our design framework, it is not necessary thatphysical dimensions are orthogonal to the social dimension,but since they represent two different design parameters, wewould like to separate them. The design philosophy here is toconsider both the social and physical aspects of mobility.
We introduce four forwarding algorithms in this paper,namelyLABEL , RANK, DEGREE, andBUBBLE.
• LABEL: Explicit labels are used to identify forwardingnodes that belong to the same organisation. Optimisationsare examined by comparing label of the potential relaynodes and the label of the destination node.This is in thehuman dimension, although an analogous version can bedone by labelling ak-clique community in the physicaldomain.
• RANK: The forwarding metric used in this algorithm isthe node centrality. A message is forwarded to nodes withhigher centrality values than the current node. It is basedon observations in the network plane, although it alsoreflects the hub popularity in the human dimension.
• DEGREE:The forwarding metric used in this algorithm isthe node degree, more specifically the observed averageof the degree of a node over a certain time interval. Eitherthe last interval window (S-Window), or a long-term cu-mulative estimate, (C-Window) is used to provide a fullydecentralised approximation for each node’s centrality,and then that is used to select forwarding nodes.
• BUBBLE: TheBUBBLE family of protocols combines theobserved hierarchy of centrality of nodes and observedcommunity structure with explicit labels, to decide on thebest forwarding nodes.BUBBLE is an example algorithm
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which uses information from both human aspects and alsothe physically observable aspects of mobility.
BUBBLE is a combination ofLABEL and RANK. It usesRANK to spread out the messages and usesLABEL to identifythe destination community. For this algorithm, we make twoassumptions:
• Each node belongs to at least one community. Here weallow single node communities to exist.
• Each node has a global ranking (i.e., global centrality)in the whole system and also a local ranking within itscommunity. It may belong to multiple communities andhence may have multiple local rankings.
5.1 HaggleSim EmulatorIn order to evaluate different forwarding algorithms, we de-veloped an emulator calledHaggleSim [20], which can replaythe mobility traces collected and emulate different forwardingstrategies on every contact event. This emulator is driven bycontact events. The original trace files are divided into discretesequential contact events, and fed into the emulator as inputs.
In all the simulations in this work, we divided the tracesinto discrete contact events with granularity of 100 seconds.Our simulator reads the file line by line, treating each line as adiscrete encounter event, and makes a forwarding decision onthis encounter based on the forwarding algorithm under study.
5.2 Simulation ParametersThere are three parameters we used in our simulation toachieve controlled flooding inMCP strategy.
• Number of copies (m): The maximum number of dupli-cates of each message created at each node.
• Number of hops (Hop-TTL): The maximum number ofhops, counted from the source, that a message copy cantravel before reaching the destination; this is similar toTTL value in the Internet.
• Time TTL: The maximum time a message can stay inthe system after its creation. This is to prevent expiredmessages from further circulation.
For all the emulations conducted to compare forwardingefficiency in this paper, we have the following two metrics.
• Delivery Ratio: The proportion of messages that havebeen delivered out of the total unique messages created.
• Delivery Cost: The total number of messages (includeduplicates) transmitted across the air. To normalise this,we divide it by the total number of unique messagescreated.
For some cases, we also compute the Hop-count distributionfor the deliveries, which is the distribution of the number ofhops needed for all the deliveries, and which reveals the socialdistance between sources and destinations.
In the following sections, we will introduce several for-warding algorithms and evaluate their performances with theabove metrics. For each emulation,1,000 unique messages arecreated, with the source and destination randomly chosen orchosen based on specific grouping, which will be specifiedin each evaluation case. The creating time of the messages is
uniformly distributed within the simulation duration. Since theexperimental durations for the datasets we used for simulationare in the order of days or even weeks, so we ignore the tran-sient period and average the results thoughout the simulationperiods.
6 GREEDY RANKING ALGORITHMThe contribution of this section is to introduceRANK algo-rithm in detail and evaluate its performance using differentdatasets.
RANK is similar to the greedy strategy introduced byAdamic et al. [21]. A PSN is not a static network like theInternet: we do not know when a local maximum is reachedsince the next encounter is unexpected. We cannot employprecisely the same strategy as they proposed, of traversingupthe hierarchy until reaching the maximum, and then down astep. In RANK, we assume each node knows only its ownranking and the rankings of those it encounters, but does notknow the ranking of other nodes it does not encounter, anddoes not know which node has the highest rank in the system.RANK is very simple: we keep pushing traffic on all paths tonodes which have a higher ranking than the current node, untileither the destination is reached, or the messages expire.
If a system is small enough, the global ranking of each nodeis actually the local ranking. If we consider only the SystemsResearch group (around 40 people), a subset of the CambridgeComputer Laboratory (235 people), this is the ranking of eachnode inside the group. If we consider the whole ComputerLaboratory, we are considering a larger system of manygroups, but they all use the same building. A homogeneousranking can still work. But when we consider the whole cityof Cambridge, a homogeneous ranking system would excludemany small scale structures. In this section, we show that inrelatively small and homogeneous systems, a simple greedyranking algorithm can achieve good performance.
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Fig. 6. Comparison of delivery ratio (left) and cost (right) of MCP andRANK on 4 copies and 4 hops case (Reality )
Figure 6(left) shows thatRANK performs almost as well asMCP for delivery. Figure 6(right) also shows that the cost isat maximum of around 40% that ofMCP, which representsa marked improvement. The audiences may notice that thedifference in cost between these two algorithms is not constant.This is because they have different spreading mechanismswhich may affect their abilities to find necessary number ofrelays within a certain time TTL.
RANK appears to work in small and homogeneous systems,but when we look at a more diversified system, for example
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the Hong Kong dataset, it may work differently. In theHongKong experiment, the 37 participants are intentionally selectedwithout any social correlation. They live and work throughoutthe whole city. With FLOOD, we can deliver more than 40%of the total traffic across the whole city by using only the37 iMotes and the external devices detected by these iMotes.But in this case, greedy ranking can only deliver 10% of themessages, although the cost is much lower as well. In termsof delivery and cost, greedy ranking is still more cost-effectivethan flooding, but clearly the delivery success rate is stilltoolow. One explanation for this low performance is that since theparticipants have no social correlation, and belong to differentsocial communities, high global ranking of a node may notrepresent a good choice of relay for some local communities.Messages keep being pushed up to some globally higherranking nodes, and getting stuck at some maxima, rather thanthen trickling down to some local community.
Figure 7(a) shows that the maximum number of hopsfor greedy Rank is 4 hops and after that the messages getstuck. Figure 7(b) shows the rank distribution of the sources,destinations and dead-ends of all the undelivered messages,indicating that message delivery has typically failed at highly-ranked nodes. This supports our hypothesis concerning thedilemma of the messages getting stuck at maxima.
Fig. 7. The hop distribution of the delivered (left) and the rankdistribution of undelivered (right) on HK data
Hierarchical organisation is a common feature of manycomplex systems. The defining feature of a hierarchical organ-isation is the existence of a hierarchical path connecting anytwo of its nodes. Trusinaet al. [22] address how to detect andmeasure the extent of the hierarchy manifested in the topologyof a given complex network. They defined the hierarchical pathbased on node degrees. A path between two nodes in a networkis called hierarchical if it consists of anup path where one isallowed to step from nodei to nodej only if their degreeski, kj satisfy ki ≤ kj , followed by adown path where onlysteps to nodes of lower or equal degree are allowed. Eitherthe up or down path is allowed to have zero length. Becauseof the good results from the greedy ranking algorithm, weanalysed the percentage of hierarchical paths inside all theshortest paths. Table 4 summarises the results.
The percentage of hierarchical paths is calculated as thenumber of hierarchical paths divided by the number of non-direct deliveries. We can see that forCambridge data andReality, the percentage of hierarchical paths is very high, soour strategy of pushing the messages up the ranking tree
Experimental dataset % hierarchical pathsCambridge 87.2 (-2.4,+4.3)
Reality 81.9 (-3.1,+3.3)Infocom05 62.3 (-2.5,+2.5)Infocom06 69.5 (-4.1,+2.4)Hong Kong 33.5 (-4.0,+4.0)
TABLE 4Hierarchical paths analysis of all shortest paths
can find a lot of these paths, and the performance of theranking strategy here is not much different from that ofMCP.For Infocom06 and Infocom05, the percentage of hierarchicalpaths is also high, hence the greedyRANK strategy can aswell discover many of the shortest paths. However, for theHong Kong experiment, the network is too sparse and a lotof shortest paths are hidden. (This occurs because we couldnot know the devices detected by the external devices, andmost of the resulting paths used for delivery are actually notthe shortest) We can see that percentage of hierarchical pathscontrols the delivery success achieved by the greedyRANKalgorithm. We conclude from this that a high percentage ofthe shortest paths are actually hierarchical paths.
7 DIRECT LABELLING STRATEGYIn the LABEL strategy, each node is assumed to have a labelthat tells others its affiliation, just like a name badge in aconference. The directLABEL strategy refers to the exclusivelyusing of labels to forward messages to destinations: next-hopnodes are selected if they belong to the same group (samelabel) as the destination. It was demonstrated thatLABELsignificantly improves forwarding efficiency over “oblivious”forwarding usingInfocom06 dataset [20]. This is a beginningof social based forwarding inPSN. The limitation ofLABELis the lack of mechanisms to move messages away from thesource when the destinations are socially far away (such asReality). The contribution of this section is to demonstratethe limitations of LABEL strategy, which motivates a newforwarding algorithm using both community and centralityinformation.
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Fig. 8. Comparison of delivery ratio (left) and cost (right) of MCP andLABEL on 4 copies and 4 hops case (Reality )
We evaluate theLABEL strategy on theReality dataset. Herewe use the community information detected usingK-CLIQUEalgorithm to label the nodes. We can see from Figure 8 thatLABEL only achieves around 55% of the delivery ratio of theMCP strategy and only 45% of the flooding delivery although
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the cost is also much lower. However, it is not an ideal scenariofor LABEL . In this environment, people do not mix as well asin a conference. A person in one group may not meet membersin another group so often, so waiting until the members of theother group appear to do the transmission is not effective here.
Figure 9 shows the correlation of thenth-hop relay nodesto the source and destination groups for the messages on allthe shortest paths, that is the percentage of the nth-hop relaynodes that are still in the same group as the source or alreadyin the same group as the destination. We can see that more than50% of the nodes on the first hops (from the S-Group plot)are still in the same group as the source group of the messageand only around 5% of the first hop nodes (from the D-Groupplot) are in the same group as the destination. We can alsosee that on going to the 2nd hop, S-Group correlation dropsto slightly less than 30%, and when going to 4th-hops, almostall (90%) messages have escaped from this source group. Tocalculate the percentage for each hop we divide the number ofmessages which belong to that group (S-Group or D-Group)by the total number of messages destined beyond nodes at thatparticular hop, but not the total messages created. In the 4-hopcase, there are perhaps only 100 messages to forward further,and only 10 out of these 100 relay nodes belong to the sourcegroup. This explains whyLABEL is not effective, since it isfar from discovering the shortest path. In the next section,wewill talk about how to use centrality to improve the deliveryratio of LABEL .
Fig. 9. Correlation of nth-hop nodes with the source group anddestination group (Reality )
8 CENTRALITY MEETS COMMUNITY
The contribution of this section is to combine the knowledgeofboth centralities of nodes and community structure, to achievefurther performance improvements in forwarding. We showthat this avoids the occurrence of the dead-ends encounteredwith pure global ranking schemes. We call the protocols hereBUBBLE, to capture our intuition about the social structure.Messages bubble up and down the social hierarchy, based onthe observed community structure and node centrality, togetherwith explicit label data. Bubbles represent a hybrid of socialand physically observable heterogeneity of mobility over timeand over community.
8.1 Two-community CaseIn order to make the study more systematic, we start withthe two-community case. We use theCambridge dataset forthis study. By experimental design, and confirmed using ourcommunity detection algorithm, we can clearly divide theCambridge data into two communities: the undergraduateyear-one and year-two group. In order to make the experimentmore fair, we limit ourselves to just the two 10-clique groupsfound with a number-of-contact threshold of 9; that is whereeach node at least meet another 9 nodes frequently. Somestudents may skip lectures and cause variations in the results,so this limitation makes our analysis yet more plausible.
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First we look at the simplest case, for the centrality of nodeswithin each group. In this case, the traffic is created only formembers within the same community and only members inthe same community are chosen as relays for messages. Wecan clearly see from Figures 10(a) and 10(b) that inside acommunity, the centrality of each node is different. In GroupB, there are two nodes which are very popular, and haverelayed most of the traffic. All the other nodes have lowcentrality value. Forwarding messages to the popular nodeswould make delivery more cost effective for messages withinthe same community.
Then we consider traffic which is created within each groupand only destined for members in another group. To eliminateother outside factors, we use only members from these twogroups as relays. Figure 11 shows the individual node cen-trality when traffic is created from one group to another andthe correlation of node centrality within an individual groupand inter-group (for data deliveries only to other groups butnot to its only group) centrality. We can see that points liemore or less around the diagonal line. This means that theinter- and intra- group centralities are quite well correlated.Active nodes in a group are also active nodes for inter-groupcommunication. There are some points on the left hand side ofthe graph which have low intra-group centrality but moderateinter-group centrality. These are nodes which move acrossgroups. They are not important for intra-group communicationbut can perform certainly well when we need to move trafficfrom one group to another.
We can show now why homogeneous global ranking inSection 6 does not work perfectly. Figure 12 shows thecorrelation of the local centrality of Group A and the global
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centrality of the whole population. We can see that quite anumber of nodes from Group A lie along the diagonal line. Inthis case the global ranking can help to push the traffic towardGroup A. However the problem is that some nodes which havevery high global rankings are actually not members of GroupA, for example node D. Just as in real society, a politiciancould be very popular in the city of Cambridge, but not amember of the Computer Laboratory, so may not be a verygood relay to deliver message to the member in the ComputerLaboratory. Now we assume there is a message at node A todeliver to another member of Group A. According to globalranking, we would tend to push the traffic toward B, C, D, andE in the graph. If we pushed the traffic to node C, it wouldbe fine, and to node B it would be perfect. But if it push thetraffic to node D and E, the traffic could get stuck there andnot be routed back to Group A. If it reaches node B, that is thebest relay for traffic within the group, but node D has a higherglobal ranking than B, and would tend to forward the trafficto node D, where it would probably get stuck again. Here wepropose theBUBBLE algorithm to avoid these dead-ends.
Forwarding is carried out as follows. If a node has a messagedestined for another node, this node would first bubble thismessage up the hierarchical ranking tree using the globalranking until it reaches a node which has the same label(community) as the destination of this message. Then the localranking system will be used instead of the global rankingand continue to bubble up the message through the localranking tree until the destination is reached or the messageexpired. This method does not require every node to know
the ranking of all other nodes in the system, but just tobe able to compare ranking with the node encountered, andto push the message using a greedy approach. We call thisalgorithm BUBBLE, since each world/community is like abubble. Figure 13 illustrates theBUBBLE algorithm and thepseudo code can be found in our previous work [18].
Ranking
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Fig. 13. Illustration of the BUBBLE forwarding algorithm.
This fits our intuition in terms of real life. First you try toforward the data via people more popular than you aroundyou, and then bubble it up to well-known popular people inthe society, such as a postman. When the postman meets amember of the destination community, the message will bepassed to that community. This community member will tryto identify the more popular members within the communityand bubble the message up again within the local hierarchyuntil the message reaching a very popular member, or thedestination itself, or the message expires.
A modified version of this strategy is that whenever amessage is delivered to the community, the original carriercandelete this message from its buffer to prevent it from furtherdissemination. This assumes that the community memberwould be able to deliver this message. We call this protocolwith deletion, strategyBUBBLE-B, and the original algorithmintroduced aboveBUBBLE-A.
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Fig. 14. Comparisons of several algorithms on Cambridge dataset
We can see from Figure 14 that bothBUBBLE-A andBUBBLE-B achieve almost the same delivery success rate asthe 4-copy-4-hopMCP. AlthoughBUBBLE-B has the messagedeletion mechanism, it achieves exactly the same delivery asBUBBLE-A. BUBBLE-A only has 60% the cost ofMCP andBUBBLE-B is even better, with only 45% the cost ofMCP.Both have almost the same delivery success asMCP.
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8.2 Multiple-community CasesTo study the multiple-community cases, we use theRealitydataset. To evaluate the forwarding algorithm, we extract a3-week session during term time from the whole 9-monthdataset. Emulations are run over this dataset with uniformlygenerated traffic.
There is a total of 8 groups within the whole dataset.Figure 15 shows the node centrality in 4 groups, from small-size to medium-size and large-size group. We can see thatwithin each group, almost every node has different centrality.
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Fig. 16. Comparisons of several algorithms on Reality dataset, singlegroup
In order to make our study easier, we first isolate the largestgroup in Figure 15, consisting of 16 nodes. In this case, all thenodes in the system create traffic for members of this group.We can see from Figure 16 thatBUBBLE-A and BUBBLE-Bperform very similarly toMCP most of the time in the singlegroup case, and even outperformMCP when the timeTTL isset to be larger than 1 week.BUBBLE-A only has 70% andBUBBLE-B only 55% of the cost ofMCP. We can say that theBUBBLE algorithms are much more cost effective thanMCP,with high delivery ratio and low delivery cost.
After the single group case, we start looking at the caseof every group creating traffic for other groups, but not forits own members. We want to find the upper cost bound forthe BUBBLE algorithm, so we do not consider local ranking(i.e., only global ranking); messages can now be sent to allmembers in the group. This is exactly a combination of directLABEL and greedyRANK, using greedyRANK to move the
messages away from the source group. We do not implementthe mechanism to remove the original message after it hasbeen delivered to the group member, so the cost here willrepresent an upper bound for theBUBBLE algorithms.
From Figure 17, we can see that of course flooding achievesthe best performance for delivery ratio, but the cost is 2.5times that ofMCP, and 5 times that ofBUBBLE. BUBBLEis very close in performance toMCP in multiple groups caseas well, and even outperforms it when the timeTTL of themessages is allowed to be larger than 2 weeks.2 However,the cost is only 50% that ofMCP.Figure 18 shows the sameperformance evaluations with theInfocom06 dataset. In thiscase, the delivery ratio ofRANK is approaching that ofMCPbut with less than half of the cost. The performance ofBUBBLE overRANK is not as significant as in theReality casebecause in a conference scenario the people are very mixingand hence the community factors are less dominating. We canalso see that even in this case, the delivery cost forBUBBLEonly increases slightly, which indicates that even in a mixingenvironment,BUBBLE is still very robust towards the possiblemisleading of the community factors.
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Fig. 17. Comparisons of several algorithms on Reality dataset, allgroups
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Fig. 18. Comparisons of several algorithms on Infocom06 dataset, allgroups
In order to further justify the significance of social basedforwarding, we also compareBUBBLE with a benchmark‘non-oblivious’ forwarding algorithm,PROPHET[5], and an-other state-of-the-arts social-based forwarding algorithm, Sim-Bet [10]. PROPHETuses the history of encounters and tran-
2. Two weeks seems to be very long, but as we have mentioned before,the Reality network is very sparse. We choose it mainly because it has longexperimental period and hence more reliable community structures can beinferred. The evaluations here can serve as a proof of concept of the BUBBLEalgorithm, although the delays are large in this dataset.
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sitivity to calculate the probability that a node can delivera message to a particular destination.SimBet is similar inconcept asBUBBLE for leveraging social contexts. It exploitsthe exchange of pre-estimate ‘betweenness’ centrality metricsand locally determined social ‘similarity’ to the destinationnode to guide the message delivery. SincePROPHET hasbeen evaluated against other algorithm before andSimBet isanother well-credited social-based algorithm, and both havethe same contact-based nature asBUBBLE (i.e., does not needlocation information), they are good candidates to comparewith BUBBLE.
PROPHEThas four parameters. We use the defaultPROPHETparameters as recommended in [5]. However, one parameterthat should be noted is the time elapsed unit used to age thecontact probabilities. The appropriate time unit used differsdepending on the application and the expected delays in thenetwork. Here, we age the contact probabilities at every newcontact. In a real application, this would be a more practicalapproach since we do not want to continuously run a thread tomonitor each node entry in the table and age them separatelyat different time. ForSimBetrouting, we use the default valuesfor the SimBetutility parameters as specified in the paper [10](i.e., α = β = 0.5) which assigns an equal importantce tothe similarity and betweenness utility. And sinceBUBBLE is amulti-copy algorithm, we also implement a multi-copySimBetfor better comparison. For all the comparisons, we use thesame settings of number of hops and number of copies for allthe algorithms, more particularly we show the results of the4-hop-4-copy case. The results are also valid for the othercombinations of number of hops and number of copies inour simulations. As most of the traces have long experimentaldurations (i.e. in the order of days or even weeks), we averagethe results throughout the simulation period and ignore thetransient period.
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Fig. 19. Comparisons of BUBBLE, PROPHET, and SimBet onReality dataset
Figures 19 shows the comparison of the delivery ratio anddelivery cost ofBUBBLE, PROPHETandSimBetfor the 4-hop-4-copy case.3 Here, for the delivery cost, we only count thenumber of copies created in the system for each message aswe have done before for the comparison with the “oblivious”algorithms. We do not count the control traffic created byPROPHETfor exchanging routing table during each encounter,
3. For one more perspective on the data, here we show the 4-hop-4-copycase instead of the unlimited case in Figure 17. The trend is very similar forthe other case.
which can be huge if the system is large (PROPHET usesflat addressing for each node and its routing table containsentry for each known node). We also do not count themessage exchange inSimBet for updating the similarity andbetweenness values. We can see that most of the time,BUBBLEachieves a similar delivery ratio toPROPHETand around10%better thanSimBet, but with only half of the cost ofPROPHETand70% of the cost ofSimBet. Considering thatBUBBLE doesnot need to keep and update an routing table for each nodepairs, the performance achievement is significant.
PROPHETrelies on encountering history and transient de-livery predictability to choose relays. This can efficientlyidentify the routing paths to the destinations, but the dynamicenvironment may result in many nodes having a lot of slightlyfluctuation of probabilities. This results in more redundantnodes being chosen as relays, which can be reflected fromthe delivery cost. Instead,BUBBLE use social information andhence filter out these noises due to the temporal fluctuationsofthe network.SimBet can successfully leverage social context,but it fails for identifying the sequence of using betweennessand similarity.BUBBLE explicitly identify centrality and com-munity, and first use centrality metric to spread out the mes-sages and then use community metric to focus the messagesto the destinations. This approach effectively guarantee ahighdelivery ratio and a low delivery cost.
A remark here is that the centrality values used for theBUB-BLE simulations in this section are calculated in a centralisedway, whilePROPHETandSimBetuse mainly online estimation,but we will show that this can be effectively approximatedin a low-cost distributed manner in the next section. Overall,we evaluatedBUBBLE againstWAIT , FLOOD, the optimisedMCP, LABEL , RANK, the benchmarkPROPHET, and SimBet.This provides us a reasonable variety of samples to illustratethe performance ofBUBBLE.
9 MAKING CENTRALITY PRACTICALFor practical applications, we want to look further into howBUBBLE can be implemented in a distributed way. To achievethis, each device should be able to detect its own communityand calculate its centrality values. In [23], we have proposedthree algorithms, namedSIMPLE, K-CLIQUE andMODULAR-ITY , for distributed community detection, and we have provedthat detection accuracy can be up to 85% of the centralisedK-CLIQUE algorithm. The next step is to ask how can eachnode know its own centrality in a decentralised way, and howwell past centrality can predict the future.
The final contribution of this paper is to provide answers tothese two questions.
9.1 Approximating CentralityWe found that the total degree (unique nodes) seen by a nodethroughout the experiment period is not a good approximationfor node centrality. Instead the degree per unit time (forexample the number of unique nodes seen per 6 hours, startedfrom midnight) and the node centrality have a high correlationvalue. We can see from Figure 20 that some nodes with a veryhigh total degree are still not good carriers. It also shows thatthe per 6 hour degree is quite well correlated to the centrality
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Fig. 20. Correlation of rank with total degree and rank with unit timedegree (Reality )
value, with correlation coefficient as high as 0.9511. Thatmeans how many people you know does not matter too much,but how frequently you interact with these people does matter.
In order to verify that the average unit-time degree is asgood as or close toRANK, we run another sets of emulationsusing greedy average unit-time degree (or we simply call itDEGREE) instead of the pre-calculated centrality. We find thatRANK andDEGREEperform almost the same with the deliveryand cost lines overlapping each other. They not only havesimilar delivery but also similar cost.
However, the average unit-time degree calculated through-out the whole experimental period is still difficult for eachnode to calculate individually. We then consider the degreefor the previous unit-time slot (we call this the slot window)such that when two nodes meet each other, they comparehow many unique nodes they have met in the previous unit-time slot (e.g. 6 hours). We call this approach single window(S-Window). Another approach is to calculate the averagevalue on all previous windows, such as from yesterday tonow, then calculate the average degree for every 6 hours.We call this approach cumulative window (C-Window). Thistechnique is similar to a statistics technique called exponentialsmoothing [24] and we would like to do further theoreticalinvestigation.
We can see from Figure 21 that the S-Window approachreflects more recent context, and achieves a maximum of 4%improvement in delivery ratio overRANK, but at double thecost. The C-Window approach measures more of the cumula-tive effect, and gives more stable statistics about the averageactiveness of a node. However, its cumulative measurement isnot as good an estimate asRANK, which averages throughoutthe whole experimental period. It does not achieve as gooddelivery asRANK (not more than 10% less in term of delivery),but it also has lower cost.
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Fig. 21. Comparisons of delivery (left) and cost (right) of RANK, S-Window and C-Window (Reality )
9.2 Predictability of CentralityIn order to further verify whether the centrality measured inthe past is useful as a predictor for the future, we extractedthree temporally consecutive 3-week sessions from theRealitydataset and then run a set of greedyRANK emulations on thelast two data sessions, but using the centrality values fromfirstsession.
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Fig. 22. Delivery ratio (left) and cost (right) of RANK algorithm on2nd data session, all groups (Reality )
Figure 22 shows the delivery ratio and cost ofRANK on the2nd data session using the centrality values from the 1st datasession. It seems that the performance ofRANK is not far fromMCP but with much lower cost, i.e., it is as good as running theemulation on the original dataset which the centrality valuesderived from. Similar performance is also observed in the 3rddata session. These results imply some level of predictabilityof human mobility, and show empirically that past contactinformation can be used in the future.
All these approaches, (DEGREE, S-Window, C-Window andpredictability of human mobility) provide us with a decen-tralised way to approximate the centrality of nodes in thesystem, and hence help us to design appropriate forwardingalgorithms. Combining these approximate methods and thedistributed community detection, we can putBUBBLE intoreality. We will briefly discuss how distributedBUBBLE worksfor a city wide environment, but leave the evaluation detailsas future work when we can get a larger scale of dataset.
Suppose there is a network of mobile users, perhaps span-ning an entire city, each device can detect its own localcommunity or knowing its social graph from online socialnetworks [25]. At the same time, it also counts its own 6-hour-averaged degree (i.e., C-Window). Its global ranking can beapproximated as its 6-hour-averaged degree for all nodes andits local ranking can be approximated as its 6-hour-averageddegree only for nodes inside its community. With all thesemetrics, each node can forward messages usingBUBBLE. Orwe simply call it DiBuBB algorithm, which uses labels fromits social graph, affiliation, or distributed community detectionfor community information and C-Window to approximate itsown global and local centrality values. Besides that, it operateexactly likeBUBBLE. Figure 23 shows the plotting of deliveryratio against the delivery cost forBUBBLE, DiBuBB, SimBet,and PROPHET. Here, DiBuBB uses the 6-hour C-Windowapproach to approximate the centrality values and the socialgraph information for the communities. In general, the largerthe slopes of the lines, the more efficient the algorithm is, interm of delivery and cost. We can see that the performance
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of DiBuBB is very close toBUBBLE and outperforms bothSimBet, andPROPHET.
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Fig. 23. Delivery ratio against cost for several algorithms, (Reality )
10 RELATED WORKFor distributed search for nodes and content in power-lawnetworks, Sarsharet al. [26] proposed using a probabilisticbroadcast approach: sending out a query message to an edgewith probability just above the bond4 percolation threshold ofthe network. They show that if each node caches its directoryvia a short random walk, then the total number of accessiblecontents exhibits a first-order phase transition, ensuringveryhigh hit rates just above the percolation threshold.
For routing and forwarding inDTNs and mobile ad hocnetworks, there is much existing literature. Vahdatet al.proposed epidemic routing, which is similar to the “oblivious”flooding scheme we evaluated in this paper [27]. Spray andWait is another “oblivious” flooding scheme but with a self-limited number of copies [28]. Grossglauseret al. proposed thetwo-hop relay schemes to improve the capacity of dense ad hocnetworks [29]. Many approaches calculate the probability ofdelivery to the destination node, where the metrics are derivedfrom the history of node contacts, spatial information and soforth. The pattern-based Mobyspace Routing by Leguayetal. [30], location-based routing by Lebrunet al. [31], context-based forwarding by Musolesiet al. [32] and PROPHETRouting [5] fall into this category.PROPHET uses pastencounters to predict the probability of future encounters. Thetransitive nature of encounters is exploited, where indirectlyencountering the destination node is evaluated. Message Ferryby Zhaoet al. [33] takes a different approach by controllingthe movement of each node.
Recent attempts to uncover a hidden stable network struc-ture in DTNs such as social networks have been emerged.For example, SimBet Routing [10] uses ego-centric centralityand its social similarity. Messages are forwarded towardsthe node with higher centrality to increase the possibilityoffinding the potential carrier to the final destination.LABELforwarding [20] uses affiliation information to help forwarding
4. A percolation which considers the lattice edges as the relevant entities.
in PSNs based on the simple intuition that people belongingto the same community are likely to meet frequently, and thusact as suitable forwarders for messages destined for membersof the same community. We have comparedBUBBLE withLABEL and SimBet in this paper, and demostrate that by theexploitation of both community and centrality information,BUBBLE provide further improvement in forwarding effi-ciency. The mobility-assisted Island Hopping forwarding [34]uses network partitions that arise due to the distribution ofnodes in space. Their clustering approach is based on thesignificant locations for the nodes and not for clustering nodesthemselves. Clustering nodes is a complex task to understandthe network structure for aid of forwarding.
Finally, we emphasise that we take an experimental ratherthan theoretical approach, which contrasts with other workdescribed above.
11 CONCLUSIONWe have shown that it is possible to uncover importantcharacteristic properties of social network from a diversesetof real world human contact traces. We have demonstratedthat community and centrality social metrics can be effec-tively used in forwarding decisions. OurBUBBLE algorithmis designed for a delay tolerant network environment, builtout of human-carried devices, and we have shown that it hassimilar delivery ratio to, but much lower resource utilisationthan flooding, control flooding,PROPHET, andSimBet.
BUBBLE is designed to work better with a hierarchicalcommunity structure. The limitation imposed by the size of thedatasets (each experiment is not large enough for us to extracthierarchical structure) does not allow us to optimally evaluateit. The current evaluation on a flat community structure didstill provide us satisfactory performance improvement. Wewill further verify our results when more mobility traces areavailable. Synthetic mobility models can also be useful forfurther evaluating the algorithm, but currently there is nobenchmark models. Another aspect we want to look using ourmobility traces is to compare them with the available mobilitymodels and find out the one which can represent most of realmobility scenarios. We believe that this approach represents anearly step in combining rich multi-level information of socialstructures and interactions to drive novel and effective meansfor disseminating data in DTNs. A great deal of future researchcan follow.
ACKNOWLEDGMENTSThis research is funded in part by the the Deutsche Telekom Labora-tories MADNet project, the ITA project, the EU grants for theHaggleproject, IST-4-027918, and the SOCIALNETS project, 217141. Wewould like also to acknowledge comments from Steven Hand, BradKarp, Frank Kelly, Richard Mortier, Pietro Lio, Andrew Moore,Nishanth Sastry, Derek Murray, Sid Chau, Andrea Passarella, HamedHaddadi, and Georgios Smaragdakis, and SimBet source codesfromThrasyvoulos Spyropoulos.
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Pan Hui is a senior research scientist inDeutsche Telekom Laboratories, Berlin. He fin-ished his PhD in Computer Laboratory, Univer-sity of Cambridge in 2007. During his PhD, healso affiliated with Intel Research Cambridgeand Thomson Research Laboratory in Paris. Be-fore that he was with University of Hong Kong forhis Mphil and bachelor degree. His research in-terests include delay tolerant networking, mobilenetworking and systems, planet-scale mobilitymeasurement, social networks, and the applica-
tion of complex network science in communication system design. Moreinformation about his profile and his research work can be found athttp://www.deutsche-telekom-laboratories.de/∼panhui/.
Jon Crowcroft is the Marconi Professor of Com-munications Systems in the Computer Lab, atthe University of Cambridge. Jon graduated inPhysics from Trinity College, Cambridge Univer-sity in 1979, and got an MSc in Computing in1981, and PhD in 1993 both from UCL.He is afellow of the ACM, the British Computer Society,the IE[ET] the royal academy of engineering andthe IEEE. Jon is a recipient of ACM SigcommAward in 2009.
Eiko Yoneki is an EPSRC Research Fellowin the University of Cambridge Computer Lab-oratory, Systems Research Group. Eiko hasreceived her Ph.D. degree from the Univer-sity of Cambridge in December, 2006 anda Postgraduate Diploma in Computer Sci-ence from the University of Cambridge in2002. Previously, she has spent several yearswith IBM (US, Japan, Italy and UK) work-ing on various networking products. Her re-search interests span distributed systems, net-
working, and databases over mobile/wireless networks including com-plex and time-dependent networks. Further details are available athttp://www.cl.cam.ac.uk/∼ey204.