TTEM: An Effective Trust-Based Topology Evolution Mechanism for P2P Networks

Transcription

1 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER EM: An Effectve rust-based opology Evoluton Mechansm for P2P Networks Janl Hu Insttute of Networks & Informaton Securty, School of Computer, Natonal Unversty of Defense echnology, Changsha, Chna Emal: lxman82@gmal.com Quanyuan Wu, Bn Zhou Insttute of Networks & Informaton Securty, School of Computer, Natonal Unversty of Defense echnology, Changsha, Chna Emal: {quanyuan, bnzhou@nudt.edu.cn Abstract Current unstructured peer-to-peer (P2P systems lack far topology structures, and take no consderaton for malcous behavors of peers. he man reason s that the topology s not senstve to peer s trust, and cannot accommodate heterogenety of peers over the network. hus, a feedback credblty based global trust model s presented n ths paper. hen, based on the trust model, an adaptve topology evoluton mechansm for unstructured P2P networks s proposed. hrough ths mechansm, trusted peers can mgrate to the centrc poston, whle untrusted peers to the edge of the topology, guaranteeng farness durng topology evoluton. On the other hand, the mechansm can effectvely counter the malcous behavors of peers, and also has the ncentve functonalty, whch ncents peers to provde more hgh-qualty servces n order to get more return on servces. Analyss and smulatons show that, compared wth the current topologes, the resultng topology mechansm demonstrates more effectveness and robustness n combatng the selfsh or malcous behavors of peers. Index erms P2P, topology evoluton, trust, ncentve mechansm I. INRODUCION In recent years, pee-to-peer (P2P computng has acheved ts popularty n many dstrbuted applcatons, ncludng fle-sharng, dgtal content delvery, and P2P Grd computng []. However, current deployed systems lack any vable ncentve structures for encouragng users to behave n the best nterest of the communty. As a result, varous forms of abuse and attack have been observed n practce [2]. he most common ones are free rders [3] and the attacks from some types of malcous peers. For example, some peers n P2P networks only provde nauthentc servces (for example, bogus fles, or use collusve strategy to attack the trust system tself. A free rder s a user who only reduces servces from a P2P. Correspondng author, Janl Hu (Emal: lxman82@gmal.com network whle never sharng any fles. hese users become leeches and dran resources from the communty. hereby, how to mprove the avalablty of the system, compel ratonal and selfsh partcpants to share ther resources actvely, and punsh malcous peers, becomes a challengng task for the healthy development of P2P networks. As for the tradtonal resoluton, some researchers have proposed some trust system for combatng these problems [4-7]. In such a system, each user s assgned a trust value by the communty that reflects ts contrbuton to and ts partcpaton n the communty. In contrast to buldng a trust system wth reputatons or economcs that requres users to cooperate or some central authortes, we offer a trust based topology evoluton mechansm to warrant the avalablty, effectveness and farness of P2P networks. he avalablty of P2P networks has a close relaton wth P2P topology evoluton, whch s tghtly related to the heterogenety of peers. he heterogenety of the peer n P2P network nvolves such features as the mum connecton number, computaton capacty and honesty, etc. Hence, the peer s heterogenety s largely attrbuted to the trust heterogenety of ths peer. Untl now, n desgnng the topology evoluton mechansm, the popular dstrbuted P2P networks [8-0] don t take nto account the factor of heterogenety, and use the same metrc for the mportance of all peers, whch cannot ensure the farness of topology, and cannot prompt peers to share more authentc resources actvely. hus, referrng to the concept of trust n the socal network, we propose a trust-based P2P topology evoluton mechansm (EM, whch can make the normal peers take up good poston, and obtan authentc servces more effectvely, whle the selfsh or malcous peers are ostraczed to the frnge of the network, more dffcult to gan hgh-qualty servces. Analyss and smulaton experments show that EM has advantages n the effectveness of encouragng the normal peers and punshng the selfsh or malcous peers

2 4 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER 2008 over the exstng topology mechansm. he remanng parts of the paper are organzed as follows: Secton II revews the related work. Secton III descrbes the global trust computng model. Secton IV demonstrates EM concretely. Secton V smulates and dscusses EM. Fnally, we conclude the paper make suggestons for further research work. II. LIERAURE REVIEW he related work about the adaptve P2P topology manly ncludes several aspects as follow: ( Peer capacty-based topology evoluton. Cooper BF [] presents a topology evoluton mechansm, n whch peers can be self-organzed nto relatvely effcent networks to cope wth the over-loaded problems. In terms of ths mechansm, the connecton between peers can be dvded nto two categores, ncludng the search connecton (sendng search messages and the ndex connecton (sendng ndex messages. When subectng to the overloaded problem, the peer drops some connectons, accordng to the quantty of messages submtted by the neghborng peers. Smlar mechansms are also put forward by Lv Q, et al. [2] and Chawathe Y, et al. [3]. hese mechansms only conceve the factor of search connectons among peers. Each peer evaluates the request-processng capacty of neghborng peers, and computes the satsfactory degree for the neghborng peers n terms of the peer s own request-processng capacty and neghborng peers capacty. Fnally, ths peer establshes connecton wth peers wth hgh processng capacty to mprove the correspondng satsfactory degree, untl the current set of neghborng peers have met the demands for request-processng capacty. (2 Peer physcal locaton-based topology evoluton. Lu Y, et al. [4,5] gve an adaptve unstructured topology evoluton mechansm, whch solves the marchng problem between the P2P topology and the underlyng physcal network by choosng the nearer peer n real dstance as some peer s neghbor, to mprove the performance of P2P networks. (3 Peer trust-based adaptve topology evoluton. Conde [6] provdes an adaptve P2P topology (AP. he basc dea n AP s that after each transacton, the peer (supposed calculates the trust value of the correspondng peer (supposed, and compares the result wth ts neghborng peers, to determne whether there exsts some peer whose trust value s lower than that of peer. If yes, peer wll send the connecton request to peer. When peer receves the request, t wll use the same prncple to decde whether to receve ths request. Notably, the topology evoluton mechansms provded n ( and (2 take no consderaton for free-rdng or malcous behavors of peers, whle AP n (3 can suppress the free-rdng or malcous behavors to a certan degree. he dfferences between EM and AP are descrbed as follow: ( As for the trust value computaton, the local trust model provde by AP s a lttle rougher, but the feedback credblty based global trust model proposed n ths paper can dentfy and restran more effectvely the attacks from broad malcous peers. (2 When dealng wth the process of topology evoluton, AP only pays attenton to the process of the trust value comparson between the current transacton peer and neghbors to decde whch lnk to be dropped or dsplaced. However, n EM, the peer (supposed not only takes nto account the current peer, but also consders the peers, whch have ever transacted wth peer. In other words, peer can pck up the peer whch has the largest trust value from the whole transacton hstory, to make a contrast wth ts neghbors. III. PEER RUS MODEL A. Defnton of the Model Frstly, the defntons of the satsfactory degree valuaton functon and the local trust value are gven. Secondly we defne feedback credblty (FC, and then the global trust value (GV. Defnton Satsfactory degree valuaton functon. After transactng wth each other, one peer (the servce consumer wll submt ts ratngs of satsfactory degree to the other peer (the servce provder, whch can be defned as the followng map functon f (, :, fully satsfactory f (, = 0, fully unsatsfactory e( (0,, else n whch, we use the method of probablty to dstngush the dfferent QoS provded by dfferent peers. he number denotes peer feels fully satsfactory to the servce provded by peer, whle zero means for the opposte ratng, and the larger the value s, the more satsfactory the peer feels. Defnton 2 Local trust value (namely, the normalzed local satsfactory degree feedback. In the tme fracton t (t s decded by the concrete applcaton. For example, sx months, supposng m denotes the number for wth peer has nteracted wth peer. hus, the local trust value peer puts to peer can be defned: = D m f(, k =, m m 0 0, m = 0 where, m = 0, means there are no transacton records between peer and peer. hereby, we set 0 to the correspondng local trust value. Defnton 3 FC s used to measure the degree of accuracy of the feedback nformaton the feedback peers (servce consumers provde to the trustee (servce provder. Durng the trust evaluaton, the feedbacks provded by the peer wth hgher FC are trustworther, and are weghted more than those from the peer wth the lower FC, and vce versa. Normally, FC s relevant to the followng factors: ( he transacton frequency between peers. Normally, the hgher the frequency, the hgher FC s. (2 he consstency of ratng behavors between peers. he more the smlarty of ratngs between peer and the reference peer s, the more consstently they rate other peers n the network. We ntroduce the transacton densty factor ( (2 Num to

3 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER descrbe the transacton frequency between peer and peer, whch can be defned n (3. m m Num = β (0, m 0 n β (3 where, m denotes the transacton number between peer and peer, and n denotes the total transacton number of peer wth other peers. When the value of m equals 0, we set 0 to Num ; β, beng transacton densty regulatory constant, s used to portray precsely the actual state of the transacton frequency, and reflect exactly the dfference of the transacton densty of peers. We take the ndex for depctng the smlarty of ratngs between peers as Sm, used for representng the consstency of peers actons. Assumng the set of common nteracton peers between peer and peer as CSet(,. herefore, the dfference of ratngs between them can be defned as follows: Df Df D k CSet(, k D k = (4 CSet(, Supposng θ denotes the mum devaton peer can allow for peer, so we can defne Sm as: ( Sm Df Sm +, Df <θ 2 θ Sm = Sm θ Sm, else 2 Df Syntheszng the above two factors, we can gve the computaton formula of FC (denoted Cr as follows: (5 Cr = Num Sm (6 hus, based on the above analyss, we can conclude that the bgger the transacton number of feedback peers s, and the more consstent the ratng actons are, and so s FC. Defnton 4 We can defne the feedback qualty matrx as R = (R, n whch, R = D *Cr. Dfferent from the normal matrx of the network trust relatonshp D, the feedback qualty matrx not only allow for the peers local trust ratngs, but also consder the FC of these peers themselves. he approach of the convergence of these two knds of nformaton better pants the real trust level of the feedback nformaton. Defnton 5 In the network N, the GV of an arbtrary peer (denoted s defned n (7. (7 = D Cr K where, K denotes the peer set whch conssts of peers, who have ever nteracted wth peer, and offered feedbacks to t. We use the FC peer puts to peer and the GV of peer to weght the local trust nformaton provded by peer. Assumng the GV vector s = [, 2,, n ], then the matrx form of (7 s as follows: = R (8 n whch, R s the feedback qualty matrx gven n Defnton 4. B. he Convergence Analyss of GV Iteratve Computaton he teratve convergence feature of (8 determnes whether we can get the computaton results of the GV vector. In the followng, we use the proposton that the norm of the teratve matrx R s less than, to proof the convergence of (8. heorem For an arbtrary ntal vector, we can (0 ( k+ ( k conclude that the smple teratve formula = R of (8 converges. Proof he suffcent condton of the convergence for the above formula s the norm of the matrx R meets the lmtatons: R < [7]. For the computaton relatonshp: we can obtan from (6: R = D Cr Cr D Cr,, Cr = Num Sm < Num Sm < Sm <,,,,, herefore, the proposton of R < has been proofed. IV. RUS BASED OPOLOGY EVOLUION MECHANISM he peer s trust value represents the possblty of collaboratng wth each other n the future. herefore, ntutvely, evolvng the P2P topology, n terms of the peer s trust value, can keep the connectons for the collaboratve peers, exclude the non-collaboratve peers to the frnge of the network, and reach the obectve of ncentve functonalty to encourage the collaboratve peers and to punsh the non-collaboratve peers. As llustrated n Fg., the green, red and yellow small crcle denote the hghly trusted peer (C and D, the lowly trusted peer (A and B, and the normal peer. from the ntal stage (see (a, where peer A and peer B are lyng n the center of the topology, whle peer C and peer D are lyng n the frnge of the topology. hrough the topology evoluton, we can get a possble equlbrum topology (see (b that the hghly trusted peers move to the center, and the lowly trusted peers to the frnge. rust based topology evoluton s mplemented by the request-response mechansm between peers. A peer can locate the needed servce n P2P networks by sendng query requests. However, the real locatng effect for ths peer s largely determned by ts message forwardng mechansm. herefore, n ths secton, we offer a trust orented message forwardng mechansm (MFM to D A B C hghly trusted peer lowly trusted peer normal peer topology evoluton (a Random topology (b Equlbrum topology Fgure. A possble topology evoluton dagram ncent peers to enhance ts trust level, and reach a better D A B C

4 6 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER 2008 search effect. Before ntroducng the topology evoluton mechansm, we frst defne some notatons used later on. N(: he set composed of peer s neghborng peers; M(: he set composed of peers peer has ever transacted wth; CN mn (: he least trust threshold peer can allow to ts neghbors. In other words, f the trust value of a neghborng peer s less than CN mn (, peer wll dsconnect wth t; CR mn (: he least trust threshold other peers have to possess, when ther connecton requests are accepted by peer ; f the trust value of the requestng peer s less than CR mn (, peer wll refuse t; Fv( = { CRmn(, M( N (: he set of peers peer wshes to, but has not bult connectons wth; SNS( = { CNmn (, N( : he set of peers whose trust value s no less than CN mn (; Fv( = { Fv (, k Fv ( and k > k : he peer whose trust value s the mum n Fv(; N ( mn = { N (, k N ( and k k > : he peer whose trust value s the mnmum n N(; τ : he mnmum connecton number peer can mantan; mn τ : he mum connecton number peer can mantan. A. Message Forwardng Mechansm Query messages are propagated va floodng-based broadcast. A search query can be ntated by any node n the network by frst broadcastng to all peers n ts neghbor set. Each node, upon recevng a propagated search query, wll examne ts local servce system for a match. Any matches are returned drectly to the query ntator. he peer may then propagate the query to all ts neghbors except for the node from whch t receved the query. Each query mantans a tme-to-lve (L feld to lmt the scope of the query floodng. At query tme, the ssung peer wll set the L feld to some default value, whch s then decremented by one at each propagaton. A node recevng a query wth L=0 wll not forward the query. Fg. 2 llustrates how a query propagates through the P2P network. In the example, peer k ntates the search query, wth L=, by broadcastng to all of ts neghbors, n ths case peer. Peer decdes not to respond to the query and forwards the query to ts neghbors, except to peer k. Upon recevng the query, peer drectly responds to peer k wth a match. he query floodng termnates at peer snce the L s now 0. peer k peer query query response connecton peer Fgure 2. Query propagaton model o ncent the peer to provde more authentc servces actvely, and ncrease ts trust level, we relate the propagaton scope of the peer wth ts trust value. he peers wth dfferent trust values have dfferent propagaton effect. In other words, the peer wth hgher trust value wll be gven a larger search scope, and can go across many hops to reach more peers. hus, these peers can have more optons of servces offered by dfferent provders, and vce versa. As for the lowly trusted peers, n order to gan more hgh-qualty servces, they have to mprove the qualty of servces and feedbacks they provde to others, to ncrease ther own trust values. Concretely, the desgn dea of MFM s as follows: ( he probablty that a peer forwards the query request of the hghly trusted peer s always hgher than that does for the lowly trusted peer. (2 he query request messages sent by a peer are largely forwarded by the peer whose trust value s less than tself. In whch, the desgn prncple of ( can assure that the hghly trusted peer can get a larger search scope, and (2 can not only provde enough chance for the lowly trusted peer to provde good servces, but also restrct the opportunty for the lowly trusted peers to gan the servces from the hghly trusted peer. hs mechansm can prompt the lowly trusted peer to offer more contrbutons to others to mprove ts trust rank. In the lght of the above desgn prncple, takng advantage of the desgn method of the trust-crtcal broadcast search mechansm (BS [2], we gve a novel forwardng probablty algorthm as followed: r, Pf = r, > n whch, r denotes the current broadcast radus, and, denote the trust value of peer and peer, respectvely. We can get the fact from (9, that for an arbtrary r, the r r formula < s always correct. In other words, as to a peer, the probablty for t to obtan servces from the hghly trusted peer s always more than that does from the lowly trusted peer. he concluson has proved our above desgn dea n MFM. B. Sendng Connecton Request Peer can update the topology lnks every some tme, and when the tme s up, t wll process as the followng steps: Frstly, when peer meets the condton of Fv( N( τ <, t wll send connecton request to ( (9 Fv, whch shows the fact that peer wshes to establsh a desrable connecton wth Fv(, but the fnal result s determned by the negotaton process wth peer Fv(. As to peer Fv(, t wll decde whether to accept ths request or not. When the negotaton s successful, the state of.negotaton( Fv( returns the boolean value true, showng both partcpants hope to establsh the connecton. After buldng the connecton, peer wll remove peer Fv( from Fv(. Secondly, when Fv( N( = τ, the process s same

5 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER wth the above way, and the unque dfference s that peer wll drop the connecton wth peer N( mn, when ts current connecton number has reached the specfed threshold. Fnally, when Fv( = SNS( < τ mn, peer wll submt request to a random peer over the network. If the request s accepted, and the condton N ( > τ s satsfed, peer wll drop the connecton wth peer N( mn. Peer wll try to keep the connectons wth the τ mn neghbors, whose trust values are no less than N( mn. However, when peer has tred for many tmes wthout any response, t wll gve up. As the connecton demander, the topology evoluton algorthm for peer s descrbed as follow: Procedure connect_demander( { f ( Fv( N( < τ f (.negotaton(fv( =true { addconnceton(fv( ; remove Fv( from Fv(; f ( Fv( N( = τ f (.negotaton(fv( =true { addconnceton(fv( ; remove(n( mn ; remove Fv( from Fv(; f ( Fv( = SNS( < τ mn { negotate wth a random peer, and establsh connecton wth t; f ( N ( > τ remove(n( mn ; C. Recevng Connecton Request As the connecton recever peer, when t meets the condton of > CRmn ( N( < τ or > CRmn ( N( = τ, t wll negotate wth peer to determne whether to accept peer s request or not. he followng process s smlar to that n Secton B, and the correspondng algorthm s gven as follow: Procedure connect_recever( { f ( > CRmn ( N( < τ f (.negotaton(=true addconnceton(; f ( > CRmn ( N( = τ f (.negotaton(=true { addconnceton(; remove(n( mn ; In order to fulfll the above operatons, peer need mantan two tables as follow: one s the neghborng peer lst (see 3(a and the other s the transacton record lst (see 3(b, as shown n Fg. 3. In Fg. 3, ID,, IDk and ID ',, ' ID m are the dentfer sequences of peer s neghborng peers and peers who have ever transacted wth peer, respectvely;,, k and ' ',, m the correspondng trust value sequences of these peers, respectvely. Assumng the average mum connecton number of peer s k. We can know from the above topology evoluton algorthm, that, under the extreme crcumstances, the communcaton overhead mantaned by ths algorthm s Ok (. Due to the lmted connecton quantty wth peer, the real communcaton overhead s not very hgh. In terms of the data structure of peer, we can easly know ts storage overhead s Ok ( ( p+ q + Om ( ( p+ q Ok ( + Om (, n whch, p denotes the byte number used for storng the peer s dentfer, q does for the peer s trust value, and m represents the number of transacton record entry, stored n peer. In addton, we can get some storage space by deletng the outdated nformaton and the transacton records of peers wth low trust value perodcally. V. EXPERIMENS AND ANALYSIS A. Smulaton Setup he peers we smulate are categorzed nto three types, ncludng ( the normal peer, marked wth captal N, whch always provde authentc servces and honest recommendatons to others, (2 the free-rdng peer, marked wth captal F, whch doesn t offer any servce and recommendaton to others, and (3 the malcous peer, marked wth captal V, whch always gves low-qualty or fraudulent servces and dshonest recommendatons to others. In the smulaton experments, the proporton of these three types of peers s 3:5:2, respectvely. he related smulaton parameter settngs are gven n able I. he hardware platform of smulaton conssts of CPU for AMD Athlon 64 X2 Dual.9GHZ, and the memory of GMB, and the smulaton software s developed n Java. ID ID ID 2 2 ID 2 2 ID k k ID m m (a Fgure 3. ABLE I. (b he data structure of peer SIMULAION PARAMEERS SEINGS # of the total number of peers n N communty # of the mnmum neghborng connecton τ mn number τ CN mn CR mn β θ P res # of the mum neghborng connecton number #of the least trust threshold to dsconnect from ts neghborng peers #of the least trust threshold to be accepted for other peers connect requests % of the transacton densty regulatory constant # of the mum devaton between peers % of the probablty n response to query requests

6 8 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER 2008 B. Effectveness aganst Free-Rdng Peers hs experment s amed to verfy whether EM can effectvely bansh the F peers to the brm of the network or not. After beng banshed to the brm, F peers have a further topology dstance to N peers, and t s dffcult for them to obtan the servces provded by N peers. We have a comparson smulaton for EM and AP. Here, we apply the shortest path length (SPL as the metrc to measure the locaton change durng the process of P2P topology evoluton. As to an arbtrary peer, we can make use of the followng formula to compute the SPL from peer to the rest peers: spl = N \ ShortestPath(, (0 N\ Fgure 4. Comparson of average SPLs of both F peers and N peers n whch, N denotes the set of N peers, and N \ denotes the set of the remanng peers except peer. As demonstrated n Fg. 4, for EM, after 40 smulaton cycles, the average SPL from F peers to N peers nears a large value (as for AP, more F peers have dsconnected from the network n EM; the average SPL between N peers s close to a constant, roughly 2.8. However, to AP, after the same cycle, the correspondng values are 4.83 and 4.2, respectvely. Obvously, AP cannot effectvely drve F peers to the brm of the network. he man reason s that AP only takes advantage of the current local feedbacks to evolve the P2P topology, and neglect the real trust level n the whole transacton hstory, whch results n the stuaton that F peers cannot be clearly recognzed and dstngushed, and dffcult to be excluded to the brm of the network. Furthermore, he average SPL between N peers n AP s longer than that n EM, whch proves EM can make N peers get closer together to form a cluster, to gan a hgher search effcency than AP does. C. Effectveness aganst Malcous Peers hs smulaton s used to test whether the topology evoluton mechansm can effectvely bansh V peers to the brm of the network. Excludng V peers to the brm can decrease the negatve effect of malcous peers. Due to the long dstance between the peers lyng n the edge and those n the center, the servce requests from N peers are dffcult to reach V peers by settng a less L value. We smulate EM and AP at the same tme, and use the same SPL formula as (0. As shown n Fg. 5, so far as EM s concerned, after 55 smulaton cycles, the average SPL from V peers to N peers approaches a large value. hs result shows EM can bansh V peers to the brm of the network. In addton, the average SPL between N peers n EM s around 2.6. hus, we set a less value (for example, less than 3 to L, preventng N peers from recevng the servce lookups. In AP, after the same cycle, the correspondng values are 5.2 and 4, respectvely. hereby, AP cannot effectvely exclude V peers to the brm of the network. Smlar to the results n Secton B, the average SPL between N peers n AP s much more than that n EM, leadng to lower search effcency and hgher communcaton overhead n AP. Fgure 5. Comparson of average SPLs of both V peers and N peers D. Effectveness for rust Based Search Mechansm Assumng the P2P network s composed of a scale of peers, based on whch, the topology evoluton s executed. he trust values of peers are modeled by random dstrbuton, whose scope are confned to (0,. o smplfy the smulaton process, we choose such three trust value regons, as (0.4,0.5], (0.6,0.7] and (0.9,.0], named S, S 2 and S 3, respectvely. Here, we gve the concept of search effect. Defnton 6 Supposng the peer set peer has travelled across when t sends a search request through MFM s B. For an arbtrary peer ( B, assumng the dstance between peer and peer s d, we defne V = as the search mert peer put to peer. Defnton 7 As for B defned n Defnton 6, we defne E = V B as the search effect of peer at ths tme. Defnton 8 Wth respect to peer set S, we defne V B R = as the average search effect (ASE of peer S. S he varyng tendency of ASE of the three types of peers wth dfferent trust value regons wth topology evoluton s shown n Fg. 6. Wth topology evoluton, peers wth smlar trust values get together to form a cluster. MFM has a stronger effect on peers wth hgher trust values n the search effect. Addtonally, an mplct fact s that, due to the mmoderately search mechansm n classc unstructured P2P applcatons, the topology mposes no lmtatons and restrctons on the hghly trusted peers and the lowly trusted peers, resultng n severe tragedy of commons. On the contrary, n terms of the topology evoluton mechansm n EM, wth the lowly trusted peers dvertng away from the hghly trusted peers, and the effect of MFM, the probablty that the lowly d

7 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER Fgure 7. Comparson of the ERRs Fgure 6. he varyng tendency of ASE for three types of peers wth topology evoluton trusted peer reach the hghly trusted peer s becomng smaller gradually. hus, EM can restrct tragedy of commons at the cost of decreasng the search effect of lowly trusted peers. E. Effectveness for Incentng Well-Behaved Peers he ndex of the network effectveness for N peers s used to descrbe how the collaboratve peers obtan trusted resources. Here, we utlze the effectve response rate (ERR to measure the network effectveness. ERR: Supposng V r good represents the set of the collaboratve peers who ntate search requests durng some cycle, and receve the correspondng responses. As for the request submtted by peer, r responses are receved, r a of whch are provded by the collaboratve peers. hus, we can defne ERR as follow: a r ERR r =. hus, we can get: r ERR ERR = r. V Vgood good In Fg. 7, we compare the ERR for EM wth that for AP, when the proporton between F peers and N peers s :4. Snce AP utlzes the concept of the connecton trust, the ERR for AP ncreases fast at the begnnng. However, wth the smulaton cycles growng, the V peers n EM are dentfed rapdly to be banshed to the brm of the network. herefore, the ERR for EM exceeds that for AP after the 0 th cycle, and reach after the 290 th cycle, whle the ERR for AP s only about 0.84, after 800 smulaton cycles. We can see from Fg. 7, that EM show more effectveness than AP. Snce EM can converge the well-behaved peers wth smlar nterests nto a cluster more effectvely, these peers n the cluster can gan hgh ERR more easly. However, AP regulates P2P topology only based on the current transacton records of neghborng peers, and cannot reflect the real trust level of peers. hus, whether the well-behaved peers or the malcous peers cannot be accurately evaluated and dentfed, whch causes a negatve effect on the topology evoluton mechansm. VI. CONCLUSIONS AND FUURE WORK In ths paper, we provde a FC-based global trust model. Based on ths, a trust-based adaptve topology evoluton mechansm for P2P networks s gven. EM can regulate P2P topology n lght of the peer s global trust level, ensurng the farness of the topology evoluton. he mechansm can suppress the malcous behavors of peers effectvely, and also has the ncentve effect on all peers. Our prelmnary experments show that EM show more effectveness and robustness n ncentve effect than the exstng topology evoluton mechansm. However, we have not dscussed the communcaton overhead. hs ndex s one of the key factors for the successful deployment and effectve mplementaton n the real engneerng envronment for EM. In our future works, we wll present concrete expermental smulaton and theoretcal analyss for the communcaton overhead of EM wth topology evaluaton, and make tests n real local applcatons. o ths end, we should make further mprovements for EM n the mechansm performance. ACKNOWLEDGMEN he authors acknowledge the useful comments from the anonymous revewers. hs research was supported by the Natonal Grand Fundamental Research Program (973 Program of Chna under Grand No. 2005CB32800, the Natonal Hgh echnology Research and Development Program (863 Program of Chna under Grant No. 2007AA0030, and the Natonal Scence Foundaton for Dstngushed Young Scholars of Chna under Grant No REFERENCES [] Bawa M, Cooper BF, Crespo A, Daswan N, Ganesan P, Garca-Molna H, Kamvar S, Mart S, Schlosser M, Sun Q, Vnograd P, Yang B. Peer-to-Peer research at Stanford. ACM SIGMOD Record, 2003,32(3: [2] Dou W. he research on trust-aware P2P topologes and constructng technologes [Ph.D. hess]. Changsha: Natonal Unversty of Defense echnology, 2003 (n Chnese wth Englsh abstract. [3] E. Adar and B. Huberman. Free rdng on gnutella. Frst Monday, 5(0, October [4] F. Cornell, E. Daman, S. D. C. d Vmercat, S. Parabosch, and P. Samarat. Choosng reputable servents n a p2p network. In th Internatonal WWW Conference, [5] R. Dngledne, M. J. Freedman, and D. Molnar. he free haven proect: Dstrbuted anonymous storage servce. In Workshop on Desgn Issues n Anonymty and Unobservablty, [6] Xong L, Lu L. Peerrust: Supportng reputaton-based trust n peer-to-peer communtes. IEEE ransactons on Data and Knowledge Engneerng, Specal Issue on Peer-to-Peer Based Data Management, 2004, 6(7: [7] Kamwar S. D, Schlosser M., Hector Garca-Molna. he egenrust algorthm for reputaton management n P2P networks. In : Proceedngs of the 2th Internatonal

8 0 JOURNAL OF COMMUNICAIONS, VOL. 3, NO. 7, DECEMBER 2008 Conference on World Wde Web, Budapest, Hungary, 2003, [8] Stoca I, Morrs R, Karger D, Kaashoek MF, Balakrshnan H. Chord: A scalable peer-to-peer lookup servce for Internet applcatons. echncal Rept :R-89, MI, [9] S. Ratnasamy et al. A Scalable Content-Addressable Network. Proceedng of ACM SIGCOMM,ACM Press, NewYork, 200.8, pp [0] A.Rowstron,P.Druschel, Pastry: Scalable, dstrbuted obect locaton and routng for large-scale peer-to-peer systems. IFIP/ACM Internatonal Conference on Dstrbuted Systems Platforms, Kluwer Academc Press, 200., pp [] Cooper BF, Garca-Molna H. Ad hoc, self-supervsng peer-to-peer search networks. echncal Report, Stanford Unversty, [2] Lv Q, Ratsnasamy S, Shenker S. Can heterogenety make Gnutella scalable? In: Druschel P, Kaashoek M F, Rowstron AI, eds. Proc. of the st Int l Workshop on P2P Systems. Berln: Sprnger-Verlag, [3] Chawathe Y, Ratnasamy S, Breslau L, Shenker S. Makng Gnutella-lke P2P systems scalable. In: Crowcroft J, Wetherall D, eds. Proc. of the Conf. on Applcatons, echnologes, Archtectures, and Protocols for Computer Communcatons. New York: ACM Press, [4] Lu Y, Zhuang Zh, Xao L, N LM. AOO: Adaptve overlay topology optmzaton n unstructured P2P systems. In: Proc. of the IEEE GLOBECOM San Francsco, [5] Xao L, Lu Y, N LM. Improvng unstructured peer-to-peer systems by adaptve connecton establshment. IEEE rans. on Computers, 2005, 54(9: [6] Conde, Kamvar SD, Garca-Molna H. Adaptve peer-to-peer topologes. In: Lambrx P, Duma C, eds. Proc. of the 4th Int l Conf. on Peer-to-Peer Computng. New York: IEEE Press, [7] J.Altman. PKI Securty for JXA Overlay Networks. Sun Mcrosystem, Palo Alto, ech Rept: R-I ,2003. Hu et al. Dstrbuted and effectve reputaton mechansm n p2p systems. In: Proceedngs of the 2008 Internatonal Conference on Computer Scence and Software Engneerng. Wuhan, Chna, December 2-4, Janl Hu et al. RBrust: a recommendaton belef based dstrbuted trust management model for p2p networks. In: Proceedngs of the 2008 Internatonal Workshop on Massve Network Storage Systems and echnologes. Dalan, Chna, September, 25-7, Hs current research nterests nclude moble agent computng, P2P computng, grd computng, electronc commerce, and network securty. Dr. Hu s a member of CCF. He was awarded the Mechancal Engneerng College Specal Research Prze n Appled Scence n 2006 as the hghest standng graduate n the faculty of Appled Scence. Bn Zhou s an assocate professor n the School of Computer of the Natonal Unversty of Defense echnology (NUD, Changsha, Hunan, Chna. He receved hs BS, MS and Ph.D. degrees n computer scence from NUD, n 994, n 997, and n 2000, respectvely. Hs nterests are n dstrbuted computng technology and data mnng technology. Quanyuan Wu, as a doctoral supervsor, s a professor n computer scence n the School of Computer of the Natonal Unversty of Defense echnology. Hs research nterests are n artfcal ntellgence, dstrbuted computng, mddleware applcaton. Janl Hu was born n Wuhan, Hube, Chna, n January, 9th, 976. He receved hs B. E. degree n computer scence n 999, from the Mechancal Engneerng College, Shazhuang, Chna. In 2003, he receved hs M. E. degree n mltary commend from Mltary Command Insttute, Zhangakou, Chna, and n 2006, he receved hs Ph.D. degree n computer scence from the Mechancal Engneerng College, Shazhuang, Chna. He has attended many proect research, many of whch were n part supported by the Natonal Grand Fundamental Research Program (973 Program of Chna and Natonal Hgh echnology Research and Development Program (863 Program of Chna. He currently concentrates on P2P and trust research n the Computer School of the Natonal Unversty of Defense echnology, Changsha, Hunan, Chna, as a postdoctoral researcher. He has publshed many artcles n domestc and overseas core ournals or academc conferences, three of whch are ncludng: Janl Hu et al. FCrust: a robust and effcent feedback credblty-based dstrbuted trust model for p2p networks. In: Proceedngs of the 2008 Internatonal Symposum on rusted Computng. Zhangae, Chna, November 8-2, Janl