Scalable P2P Search Daniel A. Menascé George Mason University

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1 Saling the Web Salable P2P Searh aniel. Menasé eorge Mason University lthough the traditional lient-server model irst established the Web s bakbone, it tends to underuse the Internet s bandwidth and intensiy the burden that dediated servers ae as their load inreases. 1 Peer-to-peer omputing relies on individual omputers omputing power and storage apaity to better utilize bandwidth and distribute this load in a sel-organizing manner. In P2P, nodes (or peers) at as both lients and servers, orm an appliation-level network, and route messages (suh as requests to loate a resoure). The design o these routing protools is o paramount importane to a P2P appliation s eiieny: naive approahes suh as nutella s lood routing, or example an add trai. 1,2 P2P systems that exhibit the small world property in whih most peers have ew links to other peers, but a ew o them have many are robust to random attaks, but an be highly vulnerable to targeted ones. 3,4 P2P omputing also has the potential to enhane reliability and ault tolerane beause it doesn t rely on dediated servers. 1 ah peer maintains a loal diretory with entries to the resoures it manages. It an also ahe other peers diretory entries. Important appliations o P2P tehnologies inlude distributed diretory systems, 1 new e-ommere models, 5 and Web servie disovery, all o whih require eiient resoure-loation mehanisms. We an view the resoure-loation problem simply as, given a resoure name, ind the node or nodes that manage the resoure. I all this the deterministi loation problem. In a very large network, ontating all the nodes to loate a resoure is learly not easible. Instead, onsider this modiied problem statement: given a resoure name, ind with a given probability the node or nodes that manage the resoure. I all this a probabilisti loation approah. I present here a probabilisti resoure-loation protool that an trade perormane and salability or the probability P that a resoure will be loated. The protool behavior depends on probabilisti parameters that we an tune to obtain a desired value o P. The goal is to ahieve a probability P lose to 1 with a ost muh lower than that o the deterministi ase. We an measure ost in terms o the number o messages exhanged, bandwidth used, or number o peers ontated. Probabilisti Searh Protool onsider a large, ully onneted network with N peer nodes and an unknown topology. very peer has a loal diretory (L) that points to loally managed resoures (suh as iles, Web pages, proesses, and devies). The L o peer node in igure 1, or example, indiates that the node stores resoures g and h. ah peer has total ontrol over its loal resoures and its L, whih means that resoures an be added or deleted rom the L without the other iretory ahe () Loal diretory (L) o: igure 1. peer-to-peer omputing system. ah peer node has a loal diretory and a diretory ahe. The diretory ahe o peer, or example, indiates that resoure d is loated in peer. The diretory ahes or nodes,, and are empty. I INTRNT OMPUTIN /03/$ I Published by the I omputer Soiety MR PRIL

2 Saling the Web r: SearhRequest (soure, resoure, (s 1,, s m ), TTL) I resoure in L Then Send Resoureound (soure, resoure, (s 1,, s m 1 ), r) to sm lse I (resoure, lo) in Then /* send request to presumed peer */ Send SearhRequest (soure, resoure, (s 1,, s m, r), TTL 1) to lo. lse I TTL > 0 Then /* probabilisti broadast */ or every v N(r) do Send SearhRequest (soure, resoure, (s 1,, s m, r), TTL 1)] p ; r: Resoureound (soure, resoure, (s 1,, s m ), v) I r soure Then egin Send Resoureound (soure, resoure, (s 1,, s m 1 ), v) to sm; dd (resoure, v) to nd lse request soure rom peer v /* end */ igure 2. The probabilisti resoure-loation protool. It onsiders two ases: in one, peer r reeives a SearhRequest (SR) message; in the other, peer r reeives a Resoureound message. In this algorithm, r: M(, ) means that message M with ields, and is reeived at peer r; [x]p means that x exeutes with probability p. iretory ahe () Loal diretory (L) o: igure 3. xample o the SearhRequest (SR) message s propagation. The SR message arrives at peer in searh o resoure n. In step 2, this request is probabilistially sent to peer, whih inds in its the inormation that resoure n is in peer. Peer then sends the SR diretly to peer. peers authorization. This assumption distinguishes the protool desribed here rom other work, 6,7 whih assumes that resoures an be repliated and an migrate to any node at the P2P system s will. ah resoure has a unique, loation-independent, global identiier (UI) that developers an generate by several means. reenet, or example, uses seure S-1 hashes o ile ontent to generate UIs. 3 In a distributed online bookstore appliation, developers ould use ISNs as UIs. ah peer has a diretory ahe () that points to the presumed loation o resoures managed by other peers. The o node in igure 1, or instane, indiates that resoure will be in node. ntries in the might just hint at the loation o remote resoures. n entry in the is a pair (id, lo) in whih id is the UI o a resoure and lo is the network address o a peer node that might store the resoure loally. iretory entries migrate rom Ls to s at other nodes to adjust to aess patterns. ah peer s has a loal neighborhood N(s) deined as the set o peers lose one hop away or in the same loal area network to peer s. We an extend the notion o neighborhood to indiate a set o peers that have business dealings with a peer, as in an e-ommere setting. Protool Operation igure 2 provides a high-level desription o the probabilisti resoure-loation protool. The soure is where a searh starts in a peer node. The algo- 84 MR PRIL I INTRNT OMPUTIN

3 Salable P2P Searh rithm in the igure uses the ollowing messages: SearhRequest (s, res, RevPath, TTL) is a request sent by soure s to loate resoure res. RevPath is the sequene o peers that have reeived this message so ar. This sequene is used as a reverse path to the soure. The parameter TTL (time to live) limits this message s propagation. Resoureound (s, res, RevPath, v) indiates that the resoure res being searhed by soure s was ound at peer v. RevPath is the reverse path to the soure. When the peer reeives the SearhRequest message, it searhes its L irst and then its. I it inds the resoure in the L, it sends a Resoureound message along the path the SearhRequest message traverses until it reahes the soure. This message updates the at every peer it visits. I the peer inds the resoure in the, it sends the SearhRequest message to the peer pointed to by that. This peer might no longer have the resoure, so the searh ontinues rom that point orward. I a peer does not ind the resoure in its L or, it will send the request to eah peer in its neighborhood with a ertain probability p, alled the broadast probability. This probability ould vary with the length o the path that the request traverses. igure 3 shows how the SearhRequest message is propagated when a request or resoure n arrives at peer. Resoure n is not in peer s L, so a probabilisti broadast sends the SearhRequest message on to peer. This node heks its L, but does not ind resoure n. owever, the in node has an indiation that resoure n might be in peer. Peer then sends the SearhRequest message to. igure 4 shows how the Resoureound message travels bak to the soure (node ) via the reverse path used by the SearhRequest message rom to to. Note how the in node gets updated: the next time resoure n is requested, node an ontat node diretly. Protool Perormane igure 5 shows two urves generated rom models that a student and I developed. 8 The igure s graphs assume a network with 120 peers, with an average node degree equal to 5 and a dereasing hit ratio at the. The irst urve in igure 5 illustrates the variation o the probability P that a resoure will be ound with the broadast probability p. s the R(n) iretory ahe () Loal diretory (L) broadast probability p inreases, the probability that the resoure will be ound inreases and reahes 0.9 or a value o p equal to 0.6. t that value, only 10.5 perent o the peers are involved in the searh, as shown in the seond urve, whih displays the variation o the ration o peers involved in the searh. s the broadast probability approahes one, the probability that a resoure will be ound also approahes one. t that point, R(n) o: igure 4. xample o the Resoureound (R) message s propagation. The R message returns to the soure (node ) ollowing the reverse path used by the SR message. The R message updates the in eah o the nodes. In step 3, the in node is updated to relet the at that resoure n is in peer % igure 5. Probability that the resoure will be ound and the ration o peers ontated versus the broadast probability. The urves show that a relatively high probability o inding a resoure an be ahieved by ontating a small ration o all peer nodes roadast probability (p) Probability resoure is ound ration o peers ontated I INTRNT OMPUTIN MR PRIL

4 Saling the Web Ishould mention several relevant P2P systems here, inluding nutella, reenet, ridella, and Sun s JXT Searh. nutella is an open, deentralized group membership and searh protool used mainly or ile sharing. Its goals are dynami operability, perormane and salability, reliability, and anonymity, 1 but it uses lood routing to broadast queries, whih an generate unneessary trai. 2 Ripeanu and olleagues 1 identiied a mismath between nutella s overlay network topology and the Internet inrastruture, whih they show has ritial perormane impliations. berer and olleagues 2 disuss ridella a nutella-ompatible P2P system that distributes repliation over a ommunity o Other P2P orts peers and redues bandwidth requirements beause o its superimposition o a binary tree on top o the P2P network. reenet is a distributed inormation storage system designed to address inormation privay and survivability onerns. 3 reenet operates a sel-organizing P2P network that pools unused disk spae aross potentially hundreds o thousands o omputers to reate a ollaborative virtual ile system. JXT 4 is a suite o protools that ailitates P2P ommuniation. JXT Searh is a deentralized P2P searh engine in whih inormation providers publish a desription o queries they are willing to answer. JXT Searh uses speialized peers (alled hubs) that an register with other hubs as inormation providers.this searh engine is a middle ground between the deentralized nutella model and the entralized Napster approah. Reerenes 1. M. Ripeanu,. Iamnithi, and I. oster, Mapping the nutella Network, I Internet omputing, vol. 5, no. 1, 2002, pp K.berer et al., Improving ata ess in P2P Systems, I Internet omputing, vol. 5, no. 1, 2002, pp I. larke et al., Proteting ree xpression Online with reenet, I Internet omputing,vol.5,no.1, 2002, pp S.Waterhouse et al., istributed Searh in P2P Networks, I Internet omputing, vol. 5, no. 1, 2002, pp verage number o hops (number o peers 1) Peers urther away are ontated Resoure ound loser due to inreased ahe hit ratio roadast probability (p) igure 6. Normalized average number o hops versus broadast probability. s the broadast probability inreases, more nodes are ontated, and the probability o inding a resoure loser to the soure inreases thanks to an inreased ahe hit ratio. a little over 50 perent o the peer nodes are ontated beause o the s eet. What is most interesting here is that a relatively small broadast probability an generate a reasonable probability o inding a resoure at a very low ost (meaning a small ration o the peer nodes are involved). To urther evaluate the probabilisti searh protool, I implemented it in a LN in whih eah mahine simulates multiple peer nodes. I also experimented with randomly generated topologies. To generate the topology or a network with N peers I start with a regular graph with k neighbors in every node and then rewire the graph by taking eah edge in turn. With a ertain probability, I move it to onnet to a randomly hosen vertex. I the resulting graph is not onneted, I restart the proess. You an use this approah to generate graphs that exhibit the small-world behavior mentioned earlier. Suh networks are haraterized by a power-law distribution o graph degree (x) ~ x t in whih (x) is the distribution o the number o routing table entries. 3 or eah ombination o the set o parameters, I generated 10 random topologies with the same initial node degree k and rewiring probability and then generated as many requests per peer as neessary to ahieve at least a ive-perent preision with a 95-perent onidene interval. The results in igure 6 assume 30 peers, k = 4, a rewiring probability equal to 0.1, and a ahe size per peer equal to 1 perent o the total number o resoures stored by all peers. Least reently used (LRU) is the ahe replaement poliy. igure 6 shows the variation o the average number o hops needed to ind a resoure normalized by the total number o peers that must be ontated i all peers (exept the soure) reeived a searh request message. The urve also shows the measurements 95-perent onidene intervals. Initially, the average number o hops inreases as peers urther away rom the soure are ontated; the resoure tends to be ound at a greater distane rom the soure. s p ontinues to inrease, the 86 MR PRIL I INTRNT OMPUTIN

5 Salable P2P Searh inreased value o the ahe hit ratio allows resoures to be ound loser to the soure, dereasing the value o. Note that the maximum value o the urve in igure 6 ours or p = 0.5. t this value, a resoure will be ound with probability 0.84 within 3.4 hops rom the soure on average. onlusions The protool desribed here does not address ahe replaement poliies in the or ahe invalidation options. I ould extend the protool to allow or invalidation o entries when a SearhRequest is reeived beause the resoure was ound in the. The igure doesn t show other details o the protool suh as a provision to avoid broadasting a message more than one. n optimization to the protool would be to notiy the soure as soon as the resoure is ound in addition to sending the Resoureound message to the soure through the reverse path. This would redue searh response time. The design o salable P2P resoure loation protools has drawn the attention o many researhers given that resoure loation is at the heart o any middleware servie. iient middleware servies or P2P are essential glue or appliations running on a multitude o devies inluding wireless devies and all sorts o applianes. Reerenes 1. L. ong, Peer-to-Peer Networks in tion, I Internet omputing, vol. 5, no. 1, 2002, pp K. berer et al., Improving ata ess in P2P Systems, I Internet omputing, vol. 5, no. 1, 2002, pp I. larke et al., Proteting ree xpression Online with reenet, I Internet omputing, vol. 5, no. 1, 2002, pp T. ong, Perormane, Peer-to-Peer: arnessing the Power o isruptive Tehnologies,. Oram, ed., O Reilly, 2001, pp W. Meira Jr. et al., -Representative: Salability Sheme or -ommere, Pro. 2nd Int l Workshop on dvaned Issues o -ommere and Web-ased Inormation Systems (WWIS 2000), I S Press, S. Ratnasamy et al., Salable ontent ddressable Network, Pro. M SIOMM, M Press, 2001, pp Rowstron and P. rushel, Pastry: Salable, istributed, Objet Loation and Routing or Large-Sale Peer-to-Peer Systems, IIP/M Int l on. istributed System Platorms (Middleware), M Press, 2001, pp Menasé and L. Kanhanapalli, Probabilisti Salable P2P Resoure Loation Servies, M Sigmetris Perormane valuation Rev., vol. 30, no. 2, 2002, pp aniel Menasé is a proessor o omputer siene, the odiretor o the -enter or -usiness and the diretor o the MS in -ommere program at eorge Mason University. e reeived a Ph in omputer siene rom UL and published the books apaity Planning or Web Servies and Saling or -usiness (Prentie all, 2002 and 2000). e is a ellow o the M and the reipient o the Mihelson ward rom the omputer Measurement roup. et SP ertiied nnouning I omputer Soiety's new ertiied Sotware evelopment Proessional Program oing Sotware Right emonstrate your level o ability in relation to your peers Measure your proessional knowledge and ompetene The SP Program dierentiates between you and others in a ield that has every kind o redential, but only one that was developed by, or, and with sotware engineering proessionals. "The exam is valuable to me or two reasons: One, it validates my knowledge in various areas o expertise within the sotware ield, without regard to speii knowledge o tools or ommerial produts... Two, my partiipation, along with others, in the exam and in ontinuing eduation sends a message that sotware development is a proessional pursuit requiring advaned eduation and/or experiene, and all the other requirements the I omputer Soiety has established. I also believe in living by the Sotware ngineering ode o ethis endorsed by the omputer Soiety. ll o this will help to improve the overall quality o the produts and servies we provide to our ustomers..." Karen Thurston, ase Two Solutions Register Today Visit the SP web site at or ontat ertiiation@omputer.org