PoS(ISCC2015)003 it would

widespread

compared tocompared http://pos.sissa.it/ our program road network. Thus, it's . with which considers the between (LBS) s LBS in

the Peer to Peer (P2P) model without Exposing without the anonymous time, the size of cloaking algorithm P2P P2P network efficiently. In addition,

spatial- issue in

of location. a anonymizing Beijing University of PostsBeijing and Telecommunications University of Beijing Posts and Telecommunications University , , P2P P2P network. The key difference communication cost, the for for LBS the aforementioned the National Great Science Specific Project (Grants No. is that the an is is hisnot collaborator map location into which index one- In this paper, we propose a s show that our proposed algorithm is more efficient when when that efficient our algorithm show is more proposed s

it can avoid 1 scheme so 2 ing lows users to collect other collaborators' locations through network P2P l exist Spatial-cloaking Algorithm Spatial-cloaking the the [email protected] [email protected] . A well-known preserving architecture for LBS is generated region success rate the cloaking and the being of the malicious collectors in and one mapping, our algorithm can also provide a personalized privacy-preserving for The experiment result some existing work in terms of User’s User’s privacy issue is an important concern in Location-based Service use which a incur the serious privacy disclosing collaborators'if locations. the users in P2P network maliciously collect those P2P P2P Copyright owned by the author(s) under the terms of the Creative Commons Creative of by author(s)Copyright the terms the the under owned

Speaker by is supported study This 2014ZX03002002-004) and National 2014ZX03002002-004) and Natural Science Foundation (Grants No. of China No. and NSFC-61170176) NSFC-61471067  4.0). 4.0 International (CC BY-NC-ND License Attribution-NonCommercial-NoDerivatives ISCC 2015 ISCC 2015 December, 18-19, China Guangzhou, 1 2 E-mail: E-mail: Beijing, China E-mail: Fan Chunxiao Center Research Communication and Network Beijing, China the Collaborators Collaborators the Zhengyuan Zhai Center Research Communication Network and A PoS(ISCC2015)003 the the P2P not , have been rapid

using is ve ally the roviding privacy. ’ blurred p of the P2P mobile P2P summarized their collaborators' network knowledge of ddition the -. To ZhengyuanZhai are desired services. A 1 (LBS) ha P2P P2P s − k cloaking have been the k location query [1]. the with the help of of privacy as well as s too much s ’s location index, he locationindex, can he ’s

based on their position gain the collaborators privacy in spatial- es

feasible privacy-preserving s to ving of this paper a s ha , we can generate different CRs location s ' Wi-Fi received signal received strength of Wi-Fi or and extracting accurate results for f

users users submitted

a collaborator users mobile devices, as well as the

s most existing approaches s to other peers in must pay for the loss of ocation-based service s s l locations are submitted to the LBS server 1 and users’ location users’ is first suggested by Chow et al. [8], and moving moving on the road. Motivated by this, we location index elying on

location − precise location k k location their 2 same user’s loaction index, we can retrieve retrieve some can loaction index, we same user’s their LBS. s’ location s’ and personalized privacy requirements. To our they are the of

s , mobile user tracing ed by an adversary for example, Che et al. proposes the dual-active model he model P2P , T when

different dummy location s network are trustworthy. In this case, an adversary would exact location are submitted, user’s individual information by anonymizing attack quality LBS, many approaches based on constraints an of

P2P these if pretending to be a normal user. To this end, some cloaking

s high the collaborator privacy concern s compute and share . Therefore, it is imperative to design mobile users utilize computing ], which mostly employ a privacy metric called queries [5-7]. They all limiationshave for r 2 users’ location users’ the ]. ]. the is inside the cell, but doesn’t know the exact location; (3) Besides, these methods are performed in Euclidean space, which -1 under road-network. The main contribution it's likely to be communication network, . infer the identity by (QoS) centralized or Peer to Peer (P2P) architecture. The centralized architecture

8 and e the

esistance to location leaking in P2P model of privay-preserving LBS. In our he . s s i r -anonymity, queries related -anonymity, with the a service of the 13, 14 or time difference of arrival of beacon signal and upporting road-network environment. We divide the whole road-network into s all peers in the large-scale k of LBS . In LBS, ing the collaborator personalized privacy requirement. personalized privacy Moreover, home address, work place, and interests) can be linked with the cell. In this way, when a LBS query peer peer receive queryLBSwhen a this cell.Inway,

the . a hold ecently, due to the ubiquity of smart phones Spatial-cloaking Algorithm Spatial-cloaking To solveTo the privacy issue R techniques, users’ location users’ Introduction dummy locations. By covering meetto environment; (2) program, inside a determine where requirements.personality on privacy Based network, which considers road knowledge, this is the first paper architectureproposed to solve the location privacy asissue below: in(1) many cells based on the density of the road, so our work is appropriate in road-network been [ proposed adjacent peers region to form a CR. toefficient protect present a novel spatial-cloaking approach to preserve enabling users to collect neighbor peers' gathered location [11], which avoids the deficiencies caused by the centralized anonymizer. Nervertheless,model easily collect others’ location algorithms without exposing locations to form a CR via multi-hop communication. vulnerable to However, the location anonymizer is all users the greatly extended in some papers [9-12] well studied [2-4, achieve mainly through builds a cloaking region (CR) enclosing other trusted location anonymizer, while the P2P architecture collects other some LBS users are always moving on the road and requirequality differentof level service mechanism to address development of widely used However, to get Because when a LBS query and (such as AP2P AP2P 1. PoS(ISCC2015)003 . . is the . ) an . i and v G ( j LBS 14 server (a), v d Database junction is the set 3 ω to } or a n which has rchitecture i v in Fig. 1, the , ZhengyuanZhai A v in Fig. ... 2,. between population density is as shown in Fig.shownin1 as v candidate results is 5. This way can’t anonymous query ystem connects two adjacent 1, S . The network distance v as its degree the as shown { ij , fi e i base station = - as shown v road-network graph wi V V junction intermediate node to isa an P 5 The edge v , where result of their location , differing from the Euclid distance extracting anonymity processing larger CR when eview of P2P LBS P2P of eview E partitioning the space into cells of various is the sum of weights a R V

node 5 end node, A querier

v : and n 3 P 2 the the 2

P H-value a network denoting the Euclid distance theEuclid fromdistance denoting other users V As depicted in Fig. 1, a curve to achieve privacy-preserving, ure it's . ij junction the ω Fig set junction is the set of edges. the -filling 22 more, 21 the v v } the to ij 20

2 e v 2 18 Hilbert-filling curve is an end isan node, end 17 16 v { v v

1 cell 5, so to 19 6 1 P v odel = 4 1 the 1 M v E 14 4 6 v v the 5 15 8 10 the Hilbert v 11 v v 2 13

3 v 5 to 1, 2, 3 or point

12 with awith weight of 2 2 comprised of v s node Road-network graph a

j 9 8 10 v v v utilized the the 3 7

equal v 8 5 the node , is defined as the sum of weights from oad-network oad-network v 3 8 from v ) has R 6 a variation of i 4 4 45 ) and 4 v 6 v is  v ) ( A A 3 V i  , 4 V d 3 v . 14 , 1:  work P (b) Spatial-cloaking Algorithm Spatial-cloaking The Hilbert-filling curves, as showed in Fig. 3 (a), first partition a 2-d space into some Definition 2: Network distance from points to road junctions We define the number of edges connecting with the node Definition 1: In this section, we review some knowledge background related with our proposed ( b and c locate in P s 3 2 1 45 s ( E Hilbert-filling Curves Road-network . v v ure

Preliminaries ω d d obvious drawback for mapping users in the same cell to a H-value, user differentiate people inside the cell well, resulting in low cells of the same size, and then traverse through all cells in a Fig.certain order without crossing sizes based on the road density 4, which means each cell including at least 4 road Somesegements. Fig 2.2 distance from respectively, i.e. respectively, of network nodes, while node When program, then present the P2P system architecture and users’ privacy requirements. users’ architecture and system theP2P program,then present 2.1 an undirected graph AP2P AP2P 2. PoS(ISCC2015)003 to . ) LBS } urve ) and 1 j n j n C r v { The , 21 22 , v send false n v . with other } s j n 20 e CS v 18 17 v the query and v 4 -a anonymity, v different road 19 road network. { 16 v 5 15 , ZhengyuanZhai v loc he submits the v 7 segements n ( l k 14 the he untrusted LBS v E N 11 , d v 10 13 v v n 6 ) 12 3 v b R deliberately ( filling , 9 8 n v v 7 v CS and finally, 5 and v 3 1 terms of v 2 4 loc v 6 in , v n ariation of Hilbert-filling ) V H candidate results with his precise

( min A 1 2

v v inference attack or collud A = , the which can be a base station in the center . An anonymous processing CR is l location in CR is indistinguishable from (b): , the base stations. Afterbase receivingstations. in an ascend order byanin ascend order

k ( CS real param } the j 3 5 5 9 n 10 16 value 4 Our paper mainly focuses on t - semi-trusted peers can’t v H equirements { perform R of CR.of locations of CR locate in at least f a b c d e uthority (TA) recursively partitions the whole road of CR. of Points

a all min A rder 4(4×4 cells) rder rivacy rivacy min indistinguishability O rust P central server t - A 16 12 11 13 a f

s a detailed process of partitioning ) P2P network. In our framework, each mobile user has its own

peers who can min , there are two important entities in the P2P architecture of LBS: c 2 A community by anonymity processing, show 9 , a urve withurve l We provide that the 14 15 10 , with other peers to aorganize network P2P via ad hoc network rooting, d C s k ) a ( a lgorithms minimum area minimum ( 3 7 extracts demanding results from 2 A a cells, cells, and sets 8 rchitecture and and rchitecture A semi-trusted user Hilbert Hilbert -indistinguishability, if the user’s N submitted locations and to the LBS servers forwarding by A c

once they join a ) the 1 4 6 5 1 b loaking loaking min ystem ystem - (a): S A . The C k 3 , and

and CR s Spatial-cloaking Algorithm Spatial-cloaking l In the initialization phase, Definition 3 In the P2P system architecture of LBS, there are 3 types of adversaries: untrusted LBS As illustrated in Fig. hide his location in , -diversity and -diversity user Initialization P2P P2P mobile users. For mobileusers. simplicity,we arrange For ure , the LBS servers find a set of candidate results in their databases and return these results to

k Our Our ( l In the end, TA broadcasts the system parameter the 3.1 network into of each cell. In the process of our partition, each cell includes at least 4 road road junction. Algorithm1 Fig 3. other size within segmentsthe desired peers in P2P network. profile which includes three parameters the location. servers, semi-trusted peers and eavesdroppers. server mobile users and LBS database servers. Mobile users are equipped with cellphone user first communicate then query CR AP2P AP2P 2.3 PoS(ISCC2015)003 in in n the and and k t n . After cell and n n cell G CS includes other 3-tuple record in the the ZhengyuanZhai a 1 in num the CR to shares } s . ) registered collaborators n n 1,2,3,4 Regist cell = , defined , k and his location to generate a CS , k s k n , and then 0 k G num cell a list of G ( s cell { < -indistinguishability. is a minimum area including 4 users cell ) } ) for j 2 n r min { ) , return to progress 1 progress toreturn , , CR } A update square lgorithm 2, } k ij , , , j . When a mobile user is willing to join the ly n l ω in connectionin order v , organizes mobile users locating { ) { edges and edges k , } in A k , 5 t num n ndicates the unique identity of the user, } ( ij n i 0 k ij > ) ω } e first 0 1,2,3,4 N G { n { ij , cell , , G these dummy location = n launches a query, he first refer n } periodical } ω k ij se { k i R of registered collaborators , , UID e , and the Table theTable T and n v into four squares,four into k CS Fig. 4 elaborates we can generate three different CR { } n { meets of of , n ij ( G } 0 CS e i = 3 G n list { cell k v G , { loc } H

G ( the n CR , i and then

n = v , where { horizontal angles or vertical angles of each edge in ( hase H G ( P ) = in the k n , the number of it junctions insidethenumber , of the requirements. t including , G k 0 user G with other mobile users by submitting tosubmitting other withmobileusers the by and relevant into relevant cellTable T and h haring G , S ) 0 k t G , UID ( is close to his current time and retrieve these collaborators’ dummy locations as user h = nitialize is the location index of user at time , i Spatial-cloaking Algorithm Spatial-cloaking 2: determine2: the H-value 9: return9: Table T Hilbert-filling phase Input: Table T: Output: parameters Hilbert-filling connect usecurve1:in Table cells T the 5: get sub-road network 5: get sub-road 6:is if there 7:add sub-network eachfor 8: else 1: ifjunctions 2:thenumber insideof partitioning 3:recursively of endat 4: the interface add points Algorithm1: Partitioning and Hilbert-filling the Road Partitioning andHilbert-fillingNetwork Algorithm1: Road the Partitioningphase Input: network road Table relevant cells T Output: sub-road and graphs a Based on user’s location index, our algorithm can retrieve dummy locations in one cell to When a mobile user After initialization phase, each positions of nodes, Cooperative user UID Regist Regist ( h lgorithm 3. Then, the query user can u and the real LBS user, while anduser, LBSthe real meet different privacy meeting different privacy levels based on a location index table, where four users and the LBS user at one road segments, the receiving these parameters, he can choose some collaborators whoseA time stamp requirement. satisfying CRhis privacy 3.3 Anonymous QueryPhase Anonymous3.3 TA respectively for obtaining to form a cooperative group, cooperative group, he computes his location as AP2P AP2P 3.2 PoS(ISCC2015)003 - 5) 3. (3.1) edges ) least net ) n ' j n CS v and the , ' n j n loc j ZhengyuanZhai n v , N Alice r (3.2) , . loc ) 20 20 n Alice .. j v v loc n n ' ( v N s j , loc N d n ( 1,2,. . 15 15 E 14 + user v 14 ' v param v 7 n v 7 = d j d and compute cell j H (3.4) , loc 13 13 position v + v ( } ) c n j = c j n n ) d d ' v v n i 1 1 H ( 1 1 { , v v Alice θ (3.3) = ( h ) ' 7 7 n i n Alice b v for each v 1 j with n Alice junction 5 θ 5 cos f N r CR v ( v } h n e j , e . loc n ) } . b , 1 1 ) ( R ) f n v .. 3 j 6 6 sin ' v ndex ndex d v n user n i N I CR . ocations n user a y a ) . , system parameter system , ω L y , j 6 G 1,2,. CL ) ' , Alice j n n i , compute, − ) , junctions set = h user ) ) j j Alice 2 ω 2 n H c a user ( x v ( v ) v , ' n x , compute, ( to user on each on usertopath each − size and j 20 , ocation ocation n − { n ) v ( ummy ummy loc j j N v n { L ' ' n , D j N n i n v user user , compute , , v r CS h for eachfor collaborators a LBS user LBS a , computedistance, the Euclid + ( 15 n Alice n 14 ≤ loc v v j ser’s ser’s 7 user ) n loc d ( from the user theuser to from sers' ' = n R H j v ) s U n

d j R U loc < n 13 v loc v ( ≤ )

( , v c n ( , 2 5 4 8 6 8 d + ...... ' value d n i 1 1 1 1 7 7 CS - user 1 1 v H ( v h Alice and user n x , collaborators’ list + loc ' < n i n e f a c 7 b d , = v v loc Points loc 1 n etrieving CS ( possible locationspossible cell 2 y 5 ( f User location index Table - v user to junctionto R N e d Alice possible path possible CR b j d

= x 1 loc < the j n 6 a 1 of v + loc user r - the the param n ' ( y j Examples of Generating Different CRs Based on a Same CRsLocationBaseda Index on Generating Examples Different of n n ij ute E H r θ d ) 2 b ( v : comp : and: return: all : compute : find:all determine : node the nearest : : if : if: : if Spatial-cloaking Algorithm Spatial-cloaking 5 6 7 2 3 4 Input: locationspossible Output: user’s 1 4: else findtheleast 4: else Algorithm 3: 1: for eachfor1: 2 3 Algorithm 2: Algorithm Computing 2: location current Alice’s Input: user locationindex current Alice’s Output: 3: compute center compute3: position center 4: find the number of junctions Table Figure 4: Figure directions work work distance AP2P AP2P PoS(ISCC2015)003 of are and 8 phase A 400 in a cell with the 6 area ofarea CR = [400,20000] λ over this road users ommunication in thein ZhengyuanZhai 4 compared = The c communication cost, l . 4 l [4,8] the . Our simulated data the ratio of road density road [14] . in respect of s 5 [5,20] k anonymity degree 7 algorithm users in the cell, the broadcasting numbers follow X-region algorithm xperiment k E for the EDA and the Dual-active ( existing n 20 with a certain privacy level privacy certain with a k [5,40] and the λ . ], ! )

2 success rate of anonymous indicates k λ users’number cell)(a

− e ( the efaults of our our efaults of D EDA [1 while the size of the CR and the success rate of anonymous N 10000 the 7035 edges. We build 1758 cells of various sizes with [5000,25000] ], time, 1 users’ number users’ and sharing, successfuly anonymized anonymized successfuly The Communication Cost, (a) the Average hops (b) the Average Size of Messages Size of the hops(b) Average theAverageCommunication(a) The Cost, Parameters and Parameters the average communitation hops and average size of transmitted ofmessages hops average size theaverage communitation and periments Spatial-cloaking Algorithm Spatial-cloaking Finally, we experimentally evaluate the success rate of anonymity for our scheme and Fig. 5 (a) shows the comparison between our algorithm with the EDA and the Dual-active We compare our algorithm with This section evaluates the performance of our cloaking algorithms operative parameter ranges defaults compare it with the other two methods' result, as showed in Table From2. Table 2, drawwe can Figure 5: Figure transmitting an obscured region. Fig. 6 and Fig. 7 respectively show the response time and the size of region with different privacy levelsFigures, we can draw thatwhen the response time and the anonymizingsize of cloaking region increase witha more location. From thesestricted privacytwo levels. We can also obersve that our scheme increases less than the other two retrievingshown the Algorithm locationsin3. methodsas many dummy for respectively), while the user only communicates with the TA and some central servers model.in Fig. 5 (b)our reveals our algorithm is superior to the EDA and the X-region on the average size of transmitted messages. This is because the EDA transmit user’s message,2D coordinateswhereas in the we transmit 1D value. The X-region algorithm has the worst result for algorithm in the average communication hops, which indicates our algorithms has little change, but the EDA and the Dual-active increase before the number of users reaching 15000. In cooperative the sharing phase, if there are the poisson distribution the anonymous costis co be canwho Table 1: Table 1: Dual-active [1 generated by the Thomas Brinkhoff generator with a road network of Oldenburg which includes 6105 nodes network. Table 1 presents the parameter settings and defaults in our experiment. Our algorithm RAM. CPUIntel 4GB7, Windows environment and of i3 2.50GHz an with Javain isperformed AP2P AP2P 4.Ex PoS(ISCC2015)003 P2P al locations to k ZhengyuanZhai EDA 98.1% 92.6% 90.8% 87.1% our framework can reduce the 8 97.6% 91.8% 90.1% 85.9% Dual-active

he experiment result turns out that

T Ours 99.1% 95.4% 92.3% 90.4% users in the cooperative sharing group, the user receives 4 4 4 k 4 = = = = l l l mapping collaborators' location into an unrecovered index, and retrieving dummy l The Anonymizing Time against Different Privacy Anonymizing againstLevels TimeThe Different The Size of Generated Cloaking Region with Different k CloakingwithRegion The Generated Different Size of 5, 20, 10, 15, The Success Rate of Anonymous with Different Privacy Levels Anonymous ofRatewith Different Privacy Success The = = = = dummy locations as illustrated in the Formulas (3.3.4) and (3.3.5), which greatly k k k k Spatial-cloaking Algorithm Spatial-cloaking In this paper, we propose a novel method of privacy-preserving LBS in mobile P2P k Figure 6:Figure Conclusion Conclusion 4 Figure Figure 7: network. Our program can solve the issue of exposing cooperators’ location in tradition architecture, also provides a personalized cloaking region for mobile users on the road by two mechanisms: locations based on an index. 5. Table 2: Table 2: when there are form a CR for the Dual-active and the EDA, while the user in our scheme can retrieve at most group. anonymousanlocating userin of a probabilitythe increases AP2P AP2P a conclusion that our method reaches a higher success rate of anonymity with different privacy levels when 40 users scatter randomly in the cell. The cause accounting for such superior is that PoS(ISCC2015)003 ZhengyuanZhai , Shanghai, China,, Shanghai, [J]. IEEE Transactions on Transactions IEEE [J]. ICDE 2009 ICDE [C]. Proceedings of the VLDB VLDB of[C]. the Proceedings our research focus in thefuture. infocus research our 2012: 2098-2102. , Alberta, Canada, May, 2007: 371-380 Alberta, Canada, May, , Wireless Communications and and Communications Wireless Hiding in the Mobile Crowd: Location Location Mobile the Hiding Crowd: in we don’t consider the moving speed of will be April, April, , [C]. IEEE Proc but , , Switzerland, July, 2003: 702-712 2003: July, , Switzerland, Proc IEEEProc 9 [J]. Communication Systems Journal, 2015, 28(1): Communication Journal, [J]. Systems [J]. Journal of[J]. Science, University 2013, Journal Zhejiang Zurich X-Region: location A framework privacy for EDA: an enhanced dual-active algorithm for location EDA: for location an enhanced dual-active algorithm , [C]. Paris, France Paris, efficiently MOBIHIDE: a mobile peer-to-peer system for anonymous system MOBIHIDE: peer-to-peer a mobile A context-aware scheme for privacy-preserving location- for scheme privacy-preserving context-aware A

PRIVE: anonymous distributed queries location-based in ), World Wide Web’07 Wide World Spatial cloaking for cloaking anonymous in services Spatial location-based A peer-to-peer spatial cloaking algorithm cloaking for spatial anonymous peer-to-peer A

time [J]. Advances Computer Science Information for of Geographic [J]. 2012 [J]. IEEE Transactions on Dependable and on Dependable 2014, Secure Computing, Transactions IEEE [J]. Hong, al. et

, Arlington Virginia, USA, November, 2006: 171-178 November, USA, Virginia, Arlington , A dual-active spatial cloaking algorithm for spatial location algorithm privacy cloaking dual-active A Y. .

Anonymous query processing in road networks in road processing Anonymous query INTERACT 2003 INTERACT Location-Based Services for Mobile Telephony: a Study of a Study Users' Privacy for Telephony: Services Mobile Location-Based [C]. Proc ACM International Symposium on Advances in Advances geographic on ACM International Symposium Proc [C]. [C]. Pro Advances in Spatial and Temporal Databases, Boston, USA, July, Databases, July, USA, Boston, Temporal inAdvances Spatial and [C]. Pro

anonymize Yiu. (GIS’06)

Non-exposure location anonymity Non-exposure

Protecting location privacy with personalized k-anonymity: Architecture and Architecture k-anonymity: privacy personalized with location Protecting Privacy-aware mobile services over road networks over services mobile road Privacy-aware K. Dey.

. [C]. Proceedings ACM [C]. Proceedings Xu. [J]. Computer Networks Journal, 2012, 56(11): 2551-2568 Computer Networks Journal,[J]. 2012, 56(11): L. [J]. IEEE Transactions on , 2008, 7(1): 2008, Computing, 1-18 on Mobile Transactions IEEE [J]. [C]. Proc ACM Proc [C]. s peer-to-peer environments peer-to-peer Hu, J. Hu, J. Che, K. Chiew, and X. X. and Chiew, Che, K. B. Z. . Wang, L. Liu. L. Wang, . Spatial-cloaking Algorithm Spatial-cloaking preservation in peer-to-peer mobile networks 167-186 privacy preservation in mobile P2P networks in P2P mobile privacy preservation 14(5): 356-373 H. March, 2009: 1120-1131 Y. Hong, Y. M. He, X. Che, Q. al. Z. and et 2007: 221-238 Hong Y. X. Yang, Che, Q. Y. networks in peer-to-peer mobile preserving Networking Conference (WCNC Y. C. Y. Chow, M. F. Mokbel, Liu. X. M. F. Chow, Y. C. mobile 351-380 15(2): Systems, 2011, S. Kalnis, Skiadopoulos. P. G. Ghinita, location-based queries A. Pingley, W. Yu, and Zhang, N. al. et Yu, W. A. Pingley, based services Mokbel, Liu. X. M. F. Chow, Y. C. location-based service systemsinformation G. Ghinita, P. Kalnis, S. Kalnis, Skiadopoulos. P. G. Ghinita, systemsmobile al. Papadimitratos, et Theodorakopoulos, P. R. Shokri, G. Collaboration Privacy through 266-279 11(3): algorithms T 1042-1053 August, 2009: France, Endowment Lyon, 2009, M. L. K. Mouratidis, Knowledge and 2010, 22(1): Data2-15 Engineering, L. Barkhuus, A Barkhuus,L. Concern Liu.B. L. Gedik, [9] [7] [8] [5] [6] [3] [4] [1] [2] [11] [13] [14] [12] [10] References AP2P AP2P communication cost and server thewhich side, efficiency query theat users and