Consistency and Update in Mobile Overlay Networks. Zhou SU, Jiro KATTO, Yasuhiko YASUDA, and Yu CHEN, Members, IEEE

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1 1 Consstency and Update n Moble Overlay Networs Zhou SU, Jro KATTO, Yasuho YASUDA, and Yu CHEN, Members, IEEE Contact Emal: zhousu@asag.waseda.p Abstract Wth the development whereby moble content dstrbuton systems can manage dynamcally changng fles, an mportant ssue to be resolved s consstency management, whch means the cached replcas on dfferent moble stes must be updated f the orgnals change. Ths paper s to desgn an ntegrated consstency-control algorthm for moble contents delvery networ (M-CDN to enhance the effcent utlzaton of networ resource and support the clent moblty. Frstly, by carryng out an analyss of moble surrogates characterstcs, for a gven content whch has been changed on ts orgnal node, only a lmted number of ts replcas nstead of all replcas are updated. Secondly, f a replca has been selected for update, the latest verson wll be sent from an algorthm-decded ste nstead of from ts orgnal server. Smulaton results show that the proposal outperforms other conventonal methods. Index Terms Moble contents delvery, Consstency Algorthm, Web Performance, Networ Traffc I. INTRODUCTION W the rapd developments of wreless communcaton technologes, a moble contents delvery networ has appeared to facltate a set of movng nodes to communcate wth each other va short-range wreless transmsson protocols such as IEEE , Blue-tooth or Ultra Wde Band (UWB. In the moble CDN, a movng node receves nformaton from ts neghbors, or from remote nodes by mult-hop transmsson relayed by ntermedated movng nodes [1][2]. Ths technology has attracted many attentons and applcatons: for example, the Noa moble server has been proposed to mae a web-server runnng on a moble phone accessble from the Internet usng any web browsers. Furthermore, the Helx moble server led by Real Networs s optmzed for delvery of all maor fle formats to any wreless networ, wth the broadest support for moble delvery standards. However, although the above companes advocate Zhou SU, Jro KATTO, Yasuho YASUDA are wth Department of Computer Scence, Waseda Umversty, Oubo 3-4-1, Shnuu-u, Toyo , JAPAN Yu CHEN., s wth Department of Electrcal and Computer Engneerng State Unversty of New Yor Bnghamton Bnghamton, NY 13902, USA ther wreless support for contents delvery, ther techncal detals on how to eep the content consstency have not yet clarfed nor verfed. In a moble CDN, an mportant ssue to be resolved s consstency management, whch means the cached replcas on dfferent stes must be updated f the orgnals change. If a reuested obect can be provded to the user wth the latest verson, the user wll not need to contact the remote orgn server to get the updated obect. Therefore, the approprate consstency control of server replcas benefts content provders by reducng latency for ther clents, and benefts ISPs by reducng bandwdth consumpton and transmsson cost. The conventonal consstency methods for wred networs could not be effcently carred out n moble envronment. It s because: n contrast to conventonal CDN systems n wred networs that consst of surrogates, moble CDN are subected to the lmtatons of long-lved nature of typcal multmeda sessons, wreless bandwdth, and the dynamcally changed networ topology. Challenges arse n how to mprove the data consstency n a moble CDN. In ths paper, we therefore propose an optmal algorthm for controllng Web consstency n moble contents delvery overlay. Frstly, we mae an analyss on the Web access dstrbuton n moble contents delvery networ. When a gven content changes at ts orgnal server, a consstency prorty, whch s proposed based on the analyss of moblty, wll be calculated. For a gven content s each replca over the whole overlay networ, only when ts prorty s beyond the threshold, the replca of content at ths surrogate wll be updated. Secondly, the process of the delvery of contents n M-CDN s dscussed. After consderng the avalable sessons, receved sgnal strength and user satsfacton degree, an update prorty s proposed, where the surrogate (nstead of the orgnal server wth the hghest update prorty wll provde the reuested contents to the clent. Ths paper s organzed as follows: n Sect.II, related wor wth regard to moble contents delvery and consstency management algorthms s revewed. Sect.III presents mathematcal analyses of contents selecton and surrogate update. And our proposed algorthm s also presented n Sect.IV. In Sect.V, smulaton results are gven and conclusons

2 2 are presented n Sect.VI. Fg1: Moble Dynamc Contents Delvery Overlays II. PREVIOUS WORK Content dstrbuton networs (CDNs have been playng an mportant role n managng content dstrbuton to large numbers of users n wred networs. However, unlmted n the wred networ, wth the development of wreless technology, the demands for moble content delvery to dynamcally changng clent, has appeared n recent years. [11] taled about the mportance of the Web consstency and also proposed a method of Consstency-preservng Cachng for Dynamc Database Content. However, the above wor doesn t support the wreless communcaton. In [12], an optmzed uery plannng of contnuous aggregaton ueres n dynamc data dssemnaton networs was presented. Ths method of uery executon can be mplemented usng schemes smlar to that used n CDNs. However, how to optmally control the uery to mprove consstency s stll not resolved. [4] proposed a moble surrogate-selecton scheme that can sgnfcantly reduce the number of server-handoffs necessary for moble multmeda contents delvery va a hgh densty M-CDN. In ths method, all surrogates are dvded nto 2 ters: Lower-ter servers are topologcally closer to the clents, and hence can delver better QoS n terms of end-to-end delay and tter. On the other hand, hgher ter servers have a larger coverage area and hence ther clents ncur fewer server handoffs. However, the crteron for dvdng surrogates nto dfferent ters s stll not clearly presented. Furthermore, the cooperaton among dfferent ters s not taled about. [1] proposed a M-CDN model for the replcaton n enterprse networs, whch taes clent demand varatons nto account to dynamcally add/remove content replcas to/from the networ n order to mnmze the total traffc over the bacbone. It provded two solutons: an offlne optmal and a heurstc onlne soluton. Although the proposed model s helpful to contents replcaton, ts analyss s only based on the networ traffc. The correspondng analyss about networ bandwdth and topology should be carred out. In [8], t concluded that usng the fxed cachng proxes to serve moble nodes may defeat the entre purpose of usng caches for traffc localzaton, therefore t s mportant that the moble node can swtch to approprate cachng proxy as t moves. Ther results showed that delay experenced by moble nodes could be sgnfcantly reduced f moble surrogate eeps swtchng to nearest cachng proxy. Thought ths wor proved that eepng the replca on dfferent proxes s helpful, how to eep the consstency of the replcas on dfferent proxy has not been mentoned. A cooperatve cachng and data consstency scheme [2], called Proxmty Regons for Cachng n Cooperatve (PReCnCt, has been proposed to support scalable data retreval n large-scale Moble Peer to Peer (MP2P networs. Ths cooperatve cachng scheme enables peers n a regon to share ther data, thus provdng a unfed vew of the cache. Ths helps n allevatng the message latency and lmted accessblty problems n MP2P networs. The cachng scheme consders data popularty, data sze and regon-dstance durng replacement to optmze cache content of peers. The above method used a parameter called Tme-to-Refresh (TTR to eep the consstency among the replcas, however, how to decde an optmal value of Tme-to-Refresh (TTR was not resolved. In Propagaton method, the updated verson of a document s delvered to all copes whenever a change s made to the document on the orgn server. Although the copes always eep the latest verson of the orgnals by the Propagaton, ths method may generate sgnfcant levels of unnecessary traffc f documents are updated more freuently than accessed. In Invaldaton [7], an nvaldaton message s sent to all copes when a document s changed on the orgn server. Ths method does not mae full use of the delvery networ for content delvery and each replca needs to fetch an updated verson ndvdually at a later tme. Therefore, the user-delay may get worse f a freuently accessed document can not be updated on tme. We ourselves proposed an ntegrated pre-fetchng and searchng algorthm for moble P2P model, where the uery can be effcently used [9]. We also presented a scheme to manage web consstency n a contents delvery networ, n whch all nodes are wred wthout moble contents [10]. The former s lmted to P2P networ and the later s desgned for wred networ, whch are both dfferent from the proposal n ths paper. III. THEORY ANALYSIS A. Contents Selecton We consder an overlay networ that has K moble nodes. Each node (=1,, K, has a λ (bytes/second whch denotes an aggregate reuest rate from clents to the server. As for the contents, we assume that there are J dfferent contents n our moble CDN. A parameter P defnes the reuest probablty for content, wth a data sze B. Accordng to the Zpf dstrbuton whch the dstrbuton of many Web accesses follows, the probablty that the content s reuested can be obtaned as follows: V P = (1 w α

3 3 where V, α are parameters of the Zpf dstrbuton, and w s the ranng of reuest tmes. We assume that there s a Reuest Routng (RR functon n each node. The RR mantans the resdence tme value r, for each clent n node s zone. Then, the relatve resdence of a clent wth a node can be r, r = (2 r, Assume that a clent frstly enters n the area of node and then reuests contents, we can get the probablty that the content s reuested from node to be: r, V θ, = (3 α w r, We call the above one consstency prorty,. Note that ths prorty can support node-moblty snce both the resdence tme and the enterng area are taen nto consderaton. The usage of t wll be ntroduced n later sectons. B. Surrogate Update In a conventonal method, f a surrogate decdes to fetch the latest verson of a modfed document, t wll contact the modfed document s orgnal server to fetch ts latest verson. However, ths method causes a bad load balance f all reuests gong to the orgnal server. Furthermore, fetchng the contents from the orgnal server ( whch s far from the clent also ncreased the extra networ traffc. To select a proper surrogate to provde the latest verson of the reuested content to the clent becomes an mportant ssue. Here, we thn the proper surrogate should at least satsfy the followng three reurements: 1 be avalable: The surrogate should have enough free sessons to receve the reuest when a latest verson s wanted from the clent. 2 be reachable: The receved sgnal strength (RSS of the surrogate should be strong enough to let the reuested content arrve at the clent. 3 be satsfed: The clent should be satsfed wth the ualty (such as user delay after recevng the updated content from the surrogate. We mae analyses of the above reurements as follows: 1 Analyss of the Avalablty Assume that the node has a coverage area denoted by the radus of ca and ts dstance between node s D,. For each node, f t s n the coverage area of node, then we can get: D, ca (4 ={1,2, K} & Let Z denote the capacty (the number of smultaneous sessons of node, and Z s the number of total sessons happened n the whole networ wthn a testng past perod. θ Then the average sessons happened n node durng the past perod can be: ( λ / λ Z (5 A free-degree s defned as follows to show the number of free sessons n node. f = ( Z ( λ / λ Z Z (6 2 Analyss of the Reachable Ablty Assume that the receved sgnal strength (RSS of the node s RSS.The probablty that ts sgnal s better than the threshold s: { } { } l = P RSS Thresh = P RSS RSS Thresh T where Thresh s the set threshold and T s the total detectng tmes wthn the testng perod. 3 Analyss of the Satsfacton Recent study [6] shows that user s satsfacton (or utlty wth recevng the reuested content depends on the user-delay (t (. to obtan content from surrogate. r t (, S (, = e (8 Here, r s a satsfacton parameter that s vared accordng the user s senstvty. As the user-delay s very mportant to reflect the user s satsfacton, we calculate the user-delay, whch s caused durng the delvery from a surrogate to the clent (reuested surrogate as follows. Assume that content s orgnally stored n server o( and C (,, s the average bandwdth (per hop durng the path from surrogate (, to surrogate. Here, surrogate s the node where the clent sent the reuest, whle surrogate (,s a node whch has the latest verson of content. Then, when a reuest for content happens at surrogate and surrogate (, sends the latest verson to satsfy ths reuest, the user delay durng the delvery from surrogate (, to surrogate s gven by 1 t = λ B P D / C Λ (,,, (,, (,, Here, C (,, means the shortest dstance (hop count surrogate (, to surrogate. If we contnue to defne: 1 G = B P Λ (10 (7 (9

4 4 U = D / C (11 (,, (,, (,, E.(9 becomes as follows: t = λ G U (12 (,,, (,, Then, we can obtan the user s satsfacton degree ( S (, when surrogate (, provdes the latest verson of content to surrogate (where the user s r t (, S (, = e (13 r ( λ G U = e (,, For smplcty, we use S to represent S (, n the followng secton. 4 Node Update Prorty We defne an ndex for update as follows: = f l S (14 Let X,,, be a parameter whch taes a bnary value of X, = 1 (f content s stored n node X, = 0 (otherwse (15 Then, we can get a matrx X of whch element s X,, whch represents a placement pattern of contents. For the moble surrogates, whch are n the coverage of node, and also have the replca of content, we can select the moble surrogate wth the maxmum update ndex ( by Assume that there are K (X, =1, ={1,,K} replcas of content slected to be updated, for a replca at moble surrogate, an update prorty wll be calculated accordng to Eua.16. The latest verson of content wll be sent to the surrogate (whch replca need to be replcated from moble surrogate wth the maxmum. Therefore, the latest verson wll be sent from an algorthm-decded ste nstead of ts orgnal server resultng n the hghest satsfacton. V. SIMULATION RESULTS In the smulaton scenaro the moble CDN comprses 21 nodes arranged by the Power-Law dstrbuton [3]. For each node, the radus of coverage area s 2/3 of ts longest dstance to other nodes. RSS s set to be one of values of the set {1,2,3}, where 3 means the strongest value of RSS. The capacty of each node s vared between 50 to 300 smultaneous sessons [4]. As for the reuest, t arrves accordng to a Posson process. All clents reuests are always redrected to the closest server. Resdence tme s decded at random from 1 mnute to 2.5 mnutes at random [4]. There are 1000 dfferent contents n the networ. The Web reuest for each content s decded by ths Zpf dstrbuton wth a Zpf parameter 0.6[5]. The update rate of a gven obect s decded at random. The total reuest tmes n the smulatons are There are 3 replcaton algorthms we wll study: Invaldaton Polcy TTR Polcy Proposal & X, = 1 D, ca max( ( Old Ht Old Ht IV. PROPOSAL We present our algorthm as follows: Step1: Update Selecton When a gven content changes at ts orgnal server o(, a consstency prorty θ, wll be calculated accordng to Eua.3. For content s each replca (X, =1 over the whole moble networ, only when ts prorty θ, s beyond the threshold, the replca of content at moble surrogate wll be updated. Otherwse, ths replca wll not be updated untl a new reuest for content happens at moble surrogate next tme. Therefore, when a gven content s changed at ts orgnal server, not all ts replcas (X, = 1 over all overlay networ wll be updated accordng to the analyss of Web access dstrbuton. Step2: Hghest Satsfaton Update Old Ht Invaldaton TTR Proposal Fg.2: Comparson of Old Ht between Invaldaton and Proposal. It can be found that our proposal can get better Old Ht than the Invaldaton n Fg.2. The reason s that: the proposal selects parts of (not all of contents to be updated as whenever ts orgnal s changed on the orgnal ste. Then the Old Ht can be mproved. On the other hand, why the proposal outperforms TTR s because the proposal uses Resdence tme to decde the consstency prorty. However, TTR only uses update-nterval

5 5 whch does not support the moblty of nodes n M-CDN. In Fg.3, the proposal can also reduce networ traffc most than other two algorthms. In the proposal, the latest verson the reuested data s sent from the surrogate decded by the algorthm. Ths selected surrogate, whch s n the coverage area of the moble node (clent, can provde hgh user s satsfacton wth low user-delay. Snce t s not necessary to fetch the date from the orgnal server (whch s necessary both n Invaldaton and TTR, whch s far way from the node where the reuest happens. Compared wth the Invaldaton, the TTR causes worst traffc snce t fetch the data tself from the orgnal server whle the Invaldaton only send an Invaldaton nformaton. Fnally, we conclude that our proposal can get better Old Ht and less Networ traffc than the Invaldaton and the TTR. Networ Traffc (Byte Networ Traffc Invaldaton TTR Proposal Networ Traffc Fg.3: Comparson of Networ Traffc between Invaldaton and Proposal. VI. CONCLUSIONS AND FURTHER WORK In ths paper, based on mathematcal analyss we proposed an ntegrated algorthm to manage the consstency of dfferent replcas n M-CDN. On one hand, the proposal can decde whether a replca should be updated when ts orgnal content changes. On the other hand, f a replca has been decded for update, from whch surrogate the latest verson should be sent to the clent can also be determned. We compared our proposal wth other conventonal methods by smulatons. More smulaton results wll be carred out from the practcal vewpont. Furthermore, theoretcal modelng for consstency control should be expanded and sophstcated. REFERENCES [1] Wagner M. Aoff, Geraldo R. Mateus, Jussara M. de Almeda, and Antono A. F. Lourero. Dynamc Content Dstrbuton for Moble Enterprse Networs. IEEE Trans on Selected Areas n Communcatons, NO. 10, Oct 2005 [2] H.Shen, M.S.Joseph, M.Kumar, and S.K. Das, PReCnCt: A Scheme for Cooperatve Cachng n Moble Peer-to-Peer Systems, Proceedngs of the 19th IEEE Internatonal Parallel and Dstrbuted Processng Symposum, 2005 [3] G. Antonol, G. Casazza, G. D Lucca, M. D Penta, and E. Merlo, Predctng Web Ste Access: An Applcaton of Tme Seres, Proceedngs of IEEE the thrd Internatonal Worshop on Web Ste Evoluton, Florence, Nov [4] M. Tar, R. Jan and T. Kawahara: Moblty Aware Server Selecton for Moble Streamng Multmeda Content Dstrbuton Networs, Proceedngs of Internatonal Web Cachng Worshop (IWCW, Sep [5] L. Breslao, P. Cao, L. Fan, G. Phllps, and S. Shener Web Cachng and Zp-le Dstrbutons: Evdence and Implcatons Proceedngs of IEEE INFOCOM 99, New Yor, Apr [6] K.Nomura, K.Yamor, E.Taahash, T.Myosh, and Y.Tanaa, "Watng Tme versus Utlty to Download Images," Proceedngs of 4th Asa-Pacfc Symposum on Informaton and Telecommuncaton Technologes (APSITT 2001, Kathmandu, Nepal / Atam, Japan [7] P. Cao and C. Lu, Mantanng Strong Cache Consstency n theworld-wde Web, Proceedngs of the 17th Int l Conf. Dstrbuted ComputngSystems, May [8] M. Tar and A. Taeshta, Management of cacheable streamng multmeda content n networs wth moble hosts, Proceedngs of IEEE GLOBECOM, Tape, Tawan, 2002, pp [9] Zhou SU, Jro KATTO and Yasuho YASUDA, An ntegrated Retreval and Pre-fetchng algorthms for Segmented Streamng n Moble Peer-to-Peer Networs Proceedngs of IEEE Cluster2005, September, Boston, Massachusetts, USA, 2005 [10] Zhou SU, Jro KATTO and Yasuho YASUDA, "Scalable Mantenance for Strong Web Consstency n Dynamc Content Delvery Overlays", Proceedngs of 2007 IEEE Internatonal Conference on Communcatons (ICC 2007, Scotland, Jun, 2007 [11] Nra Tola and M. Satyanarayanan, Consstency-preservng Cachng of Dynamc Database Content, Proceedngs of WWW2007, Banff, Canada, May, 2007 [12] Raeev Gupta, Krth Ramamrtham Optmzed Query Plannng of Contnuous Aggregaton Queres n Dynamc Data Dssemnaton Networs, Proceedngs of WWW2007, Banff, Canada, May, 2007