Computer Networks 54 (2010) 1923–1925

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Computer Networks

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Editorial

Guest editorial for the special issue collaborative P2P systems

The last few years have witnessed a rapid expansion ofcollaborative applications, going from popular applicationssuch as Twitter to international research projects like theLarge Hadron Collider, which have made collaboration apart of the daily life of many people.

Unfortunately, collaborative applications commonlyuse centralized infrastructures which, in practice, do notscale. One notorious example is Twitter. Twitter has a his-tory of substantial downtime, particularly during times ofhigh load. For instance, as many as 3% of page requestsin June 2008 yielded ‘‘over capacity” errors. To accommo-date its rapid growth, Twitter has been spending thou-sands of dollars to improve its performance, though itsgrowing pool of users greedily consumes the extra capac-ity. Such situations have generated a great interest in thedesign and development of scalable infrastructures for col-laborative applications.

It has been argued that peer-to-peer (P2P) technologycould provide the substrate to facilitate large-scale collab-oration on the Internet. However, providing the same func-tionality of a centralized service on a fully distributedsystem is not trivial: the difficulty in managing collabora-tive knowledge, the inability to both keep track of concur-rent changes and to encourage peers to contribute theirresources, the need to develop new infrastructures forsearching and delivering ephemeral content such as newsand microblog discussions are questions that requiresophisticated algorithms.

Recently, P2P systems have also found their way intosocial networks, basically because social networks can bedriven, for example, to improve collaborative P2P applica-tions by leveraging the inherent trust associated with so-cial links. We see social connectivity as yet anothervariable to tackle, which can be particularly useful tosearch information, to aggregate community data or tofacilitate communication between users sharing the sameinterests. Again, we may witness a promising future if col-laborative applications can harness social connectivity.

This special issue has its origins in the 4th InternationalWorkshop on Collaborative Peer-to-Peer Systems (COPS),which was held within IEEE WETICE in Rome (Italy) on June23–25, 2008. The focus of the workshop was to discuss newresearch ideas and prepare position papers on various hot

1389-1286/$ - see front matter � 2010 Published by Elsevier B.V.doi:10.1016/j.comnet.2010.06.011

topics in collaborative P2P systems. In particular, theworkshop ‘‘gave birth” to a much deeper insight into futuretrends than what was initially expected, making apparentthe need for a broader view of the state of the art. This isthe principal motivation of this special issue.

The Call for Papers attracted 47 submissions from Asia,Europe, and the Americas covering a broad range of topicsin the field of collaborative P2P systems. With the help ofexpert reviewers, the submissions were carefully evalu-ated with at least three reviewers for each paper. The re-sult of this rigorous evaluation process allowed us toselect eleven high-quality articles. The acceptance ratefor this special issue was around 23%. We believe that thiscompendium of papers will become a valuable referencefor researchers for years to come.

Collaborative P2P information systems P2P technology isparticularly appealing for developing collaborative infor-mation systems. In this line, P2P can be used to create so-cial networks and communities, to discover users andinformation, to aggregate community contents or even asa communication medium for large groups. We outline fiverelevant works exemplifying this growing trend:

In ‘‘SocialVPN: Enabling Wide-Area Collaboration withIntegrated Social and Overlay Networks”, Pierre St Juste etal. present a novel P2P architecture that leverages existingsocial infrastructures like Facebook to build ad hoc VirtualPrivate Networks (VPNs). The authors create self-configur-ing virtual network overlays providing seamless bi-direc-tional IP-layer connectivity to socially connected parties.

In ‘‘Building a Collaborative Peer-to-Peer Wiki Systemon a Structured Overlay”, Gerald Oster, Rubén Mondéjar,Pascal Molli and Sergiu Dumitriu construct a novel decen-tralized wiki infrastructure on top of a structured overlay.The authors use optimistic replication and a robust consis-tency algorithm called Woot to ensure that concurrentchanges are correctly propagated and merged for everyreplica. Furthermore, they implement their solution withan adaptive P2P middleware based on AOP (Aspect Ori-ented Programming) and validate the infrastructure inlarge-scale settings.

In ‘‘Badumna: A decentralized network engine for vir-tual environments”, Santosh Kulkarni, Scott Douglas andDavid Churchill present a P2P infrastructure for massively

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multiplayer online games. Badumna is constructed on topof a Distributed Hash Table (DHT) and introduces novelinterest management techniques to reduce network load.Interest management in Badumna consists of a tiered ap-proach that switches between three different protocols(cell, dynamic bounded region, gossip) depending on thenetwork conditions in the virtual environment.

In ‘‘p2pWeb: An open, decentralized infrastructure ofWeb servers for sharing ephemeral Web content”, Marc Sán-chez-Artigas, Jordi Pujol-Ahulló, Lluis Pamies-Juarez, andPedro García-López explore how the collaboration of Webservers as a P2P platform can disseminate ephemeral Webcontent, such as micronews and microblogs, in a timelyand scalable manner. To confirm their thesis, the authors de-vise novel hierarchical abstractions such as a hierarchicalpub/sub system to support the active role of Web servers.Simulation results show the viability of the approach.

Finally, in ‘‘Multi-objective Optimization of MulticastOverlays for Collaborative Applications”, Krzysztof Rzadca,Jackson Tan Teck Yong, Anwitaman Datta present a decen-tralized communication infrastructure for collaborativeapplications. To achieve optimal performance, their multi-cast solution performs multi-objective optimization overinformation gathered about nodes (concretely, their insta-bilities and frequency of sending updates) and communi-cation links (concretely, latencies and average costs).

Collaborative techniques for engineering large-scale dis-tributed applications. The popularity of P2P networkscomes from their collaborative nature. From their very firstbeginning, researchers around the world saw the opportu-nity to utilize the power of collaboration to design large-scale distributed applications. This vision gave rise to theflourishing of new distributed applications without centralcontrol and single points of failures, bringing unprece-dented scalability to the Internet community. The list ofexamples is too extensive to go into. However, the selec-tion of papers for this special issue reflects the many facesof collaboration in distributed systems: streaming, datamanagement, NAT traversal and sampling.

In ‘‘Redesigning multi-channel P2P live video systemswith view-upload decoupling”, Di Wu, Chao Liang, YongLiu and Keith W. Ross present a novel approach for P2P livestreaming called view-upload decoupling (VUD). VUDstrictly decouples what a peer uploads from what it views,bringing stability to multi-channel systems and facilitatingcross-channel resource sharing. Cooperation among peersis hence key to VUD and very relevant for the community:a related paper won the Best Paper Award at Infocom’09 byproviding a theoretical analysis of VUD.

Another area where collaboration techniques havemade interoperability easier to achieve is data manage-ment. For this topic, we found the following three signifi-cant articles:

In ‘‘PCIR: Combining DHTs and peer clusters for efficientfull-text P2P indexing”, Odysseas Papapetrou, Wolf Siberskiand Wolfgang Nejdl propose a solution to reduce the repub-lishing costs of full-text indexing on top of DHTs. Based ondocument similarity, their solution organizes the partici-pating peers into clusters, making use of superpeers formaintaining the cluster index on behalf of the regular peers.

An extensive evaluation shows a significant traffic reduc-tion in republishing and the nice scalability of PCIR.

In his manuscript entitled ‘‘Zone Indexing: Optimizingthe Balance between Searching and Indexing in a LooselyStructured Overlay”, Nicklas Beijar presents a novel P2Psystem, based on a ring structure divided into overlappingzones, to support complex queries with an optimal balancebetween search and indexing costs.

In ‘‘Affinity P2P: A Self-Organizing Content-Based Local-ity-Aware Collaborative Peer-to-Peer Network”, Juan M.Tirado, Daniel Higuero, Florin Isaila, Jesús Carretero andAdriana Iamnitchi introduce a scalable system for themanagement of collaboratively-generated content. AffinityP2P pursues two major aims: to diminish search latencyand to increase content locality in YouTube-like on-lineapplications. The building blocks of Affinity are locality-based clustering and an affinity-based metric for contentlocality. Experimental results confirm the scalability ofthe approach.

Another interesting application of P2P collaboration isNAT traversal. In the current Internet, more than 70% ofthe hosts are located behind NATs and cannot establish di-rect communication. The simplest means for solving thisproblem is to use a third entity, called relay, that forwardsthe traffic between the NATed hosts. In ‘‘A CollaborativeP2P Scheme for NAT Traversal Server Discovery based onTopological Information”, Rubén Cuevas et. al. develop anew protocol, called Gradual Proximity Algorithm (GPA),to identify the topologically closest relay when two NATedhosts cannot establish direct communication. Performanceanalysis demonstrate the strength of this simple and effec-tive algorithm.

Finally, ‘‘Secure Peer Sampling” by Gian Paolo Jesi, Al-berto Montresor and Maarten van Steen presents a novelsecurity protection algorithm for gossip-based protocols.The authors extend peer sampling protocols with a detec-tion mechanism that identifies and blacklists nodes thatare suspected to behave maliciously.

The preparation of this special issue has benefited fromthe joint contribution of many people. We want to thankthem all for their cooperation and timeliness.

Pedro Garcia is professor at the ComputerEngineering and Mathematics Department atthe University Rovira i Virgili (Spain) and IEEEMember. He obtained his PhD in 2003 in theUniversity of Murcia about CollaborativeDistributed Systems under the guidance ofAntonio F. Gomez Skarmeta. His researchtopics are distributed systems, peer-to-peer,software architectures and middleware andcollaborative environments. He has publishedmore than 30 papers in conferences like IEEEP2P, ICDCS, LCN, Europar and in journals like

Elsevier Computer Networks, Elsevier Computer Communications, IEEEComputer and IEEE Internet Computing among others. He currently leadsin Tarragona the ‘‘Architectures and Telematic Services” research group

and coordinates the URV team in the project P2PGRID (Self-Adjusting P2Pand Grid Systems). Furthermore, he chaired the IEEE WETICE Collabora-tive Peer-to-Peer Systems Workshop (2008, 2009, 2010) and is member ofthe Program Committee of the IEEE P2P (2009, 2010) Conference. Contacthim at pedro.garcia@urv.cat.

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Michael Sobolewski received his Ph.D. fromthe Institute of Computer Science, PolishAcademy of Sciences. He is the PrincipalInvestigator and Director of the SORCER lab-oratory (SORCERsoft.org) focused on researchin distributed service-centric metacomputing.Currently he is a World Class Collaborator atthe Air Force Research Lab, Air VehiclesDirectorate where he is advancing his SOR-CER-based research. From 2002 till 2009 hewas a Professor of Computer Science at TexasTech University. In May 2007 he was awarded

the title of Honorary Professor of Computer Science at Ulyanovsk StateUniversity, Russia, in appreciation of his pioneering contribution to thefield of globally distributed service-oriented computing environments. In

Editorial / Computer Net

the period 1994–2002 he worked with General Electric Global ResearchCenter (GRC) as a Senior Computer Scientist. From 1999 till 2003 heworked on service-oriented, intelligent computing systems and devel-oped a P2P service-based programming methodology and architecture forthe FIPER/NIST (Federated Intelligent Product Environment) project.While at GE GRC, Mike was a FIPER chief architect and lead developer.Later he continued the FIPER development under subcontract from GEGRC to TTU. In the period of 1997–2000 he lead and developed a genericweb-based computing framework and demonstrated 17 successfulapplications for various GE businesses including a document manage-ment system for the family of F110 engines–GE Aircraft Engines, a Web-based GE engineering specification system–GE Power Systems, an Engi-neering Calculator–GE Plastics. He led GE’s successful CAMnet/DARPAproject (1995–1996), developing tools and methodology to deliver engi-neering web services. Also, in 1996 he led a successful Lockheed MartinEDN Web Toolkit project that provides enablers to built Web-basedworkbooks and record books.

In November 1989 until February 1994 he was invited to work on theDICE program at Concurrent Engineering Center (CERC), West VirginiaUniversity, where he developed a knowledge-based environment forconcurrent engineering (DICEtalk) based on his novel percept knowledgerepresentation scheme; a generic GUI framework; a GUI client for DICEinformation sharing system; and a GUI user agent for medical informaticssystems.

Prior to coming to U.S., during his 18-year tenure with the PolishAcademy of Sciences, Warsaw, Poland, he was the head of the Pic-ture Recognition and Processing Department, the head of the ExpertSystems Laboratory, the head of Pattern Recognition Methodology Labo-ratory, and was active in the area of knowledge representation, knowl-edge-based systems, pattern recognition, image processing, neuralnetworks, and graphical interfaces. Just before joining CERC/WVU, he wasa research professor at the Institute of Computer Science of the Polish

Academy of Sciences. He has served as a visiting professor, lecturer andconsultant in Sweden, Finland, Italy, Switzerland, Germany, Hungary,Czechoslovakia, Poland, Russia, China, and the USA. Dr. Sobolewski hasover hundred publications in journals, conferences and books in the areasof distributed computing, AI, knowledge representation, expert systems,object oriented programming, and concurrent engineering.

Marc Sánchez-Artigas is a PhD. candidate inthe Department of Computer Engineering andMaths at Universitat Rovira i Virgili, Spain. Hisresearch interests include peer-to-peer sys-tems, Distributed Hash Tables and Grid com-puting. He received the best paper awardfrom the 2007 Intl. Conference of Local Com-puter Networks (LCN), held in Dublin. Cur-rently, he is a visiting researcher at ÉcolePolytechnique Fédérale de Lausanne (EPFL),LSIR lab, under the supervision of Prof. KarlAberer.

Pedro García-LópezDepartment of Computer Engineering and Maths,

Universitat Rovira i Virgili, Av. Països Catalans 26,43007 Tarragona,

SpainE-mail address: pedro.garcia@urv.cat

Michael W. SobolewskiDepartment of Computer Science, Texas Tech University,

Boston and 8th St. Lubbock, TX 79409-3104,USA

E-mail address: sobol@cs.ttu.edu

Marc Sánchez-ArtigasDepartment of Computer Engineering and Maths,

Universitat Rovira i Virgili, Av. Països Catalans 26,43007 Tarragona,

SpainE-mail address: marc.sanchez@urv.cat