P2P Networking and Applications PDF
Preview P2P Networking and Applications
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YoumayalsocompleteyourrequestonlineviatheElsevierhomepage(http://elsevier.com),by selecting“Support&Contact”then“CopyrightandPermission”andthen“ObtainingPermissions.” LibraryofCongressCataloging-in-PublicationData ApplicationSubmitted ISBN:978-0-12-374214-8 ForinformationonallMorganKaufmannpublications, visitourWebsiteatwww.mkp.comorwww.elsevierdirect.com PrintedintheUnitedStatesofAmerica 09 10 11 12 13 14 15 16 5 4 3 2 1 To my wife Gina and our daughter Jacqueline JFB To my lovely daughters Angie and Kiki HY To our adorable son Anthony EKL Preface Rationale Peer-to-peer networking has emerged as a viable business model and systems architectureforInternet-scaleapplications.Althoughits technological rootstrace back through several decades of designing distributed information systems, con- temporary applications demonstrate that it is an effective way to build applica- tions that connect millions of users across the globe without reliance on specially deployed servers. Instead, by combining the resources of each user’s computer, these systems automatically self-organize and adapt to changing peer populations while providing services for content sharing and personal communications. Public attention to peer-to-peer applications came first from highly popular file-sharing systems, in which decentralization was used to support a business model that needed to legitimize licensed content sharing. The subsequent suc- cess of the Skype Internet telephony application showed the generality of the peer-to-peer approach and its feasibility to provide acceptable service quality to millions of users. Subsequently there has been growing interest in improving on these systems as well as considering new designs to attain better performance, security, and flexibility. Today it is anticipated that peer-to-peer technologies will become gen- eral-purpose, widely used vehicles for building a broad range of applications for social networking, information delivery, and personal communications applica- tions in the future. There are many important questions about the evolution of peer-to-peer tech- nologies. What new applications will drive this evolution? Will P2P be used as a general-purpose technique for building any distributed application? How do trends in wireless networking, consumer electronics, home networking, high-definition content, digital rights management, and so forth intersect with peer-to-peer? Is P2P a panacea for designing large-scale applications, or if not, what are the characteristics of applications for which it is well suited? How should other architectures coexist with and adapt to peer-to-peer design? Will the P2P landscape be “Balkanized” by many incompatible peer-to-peer protocols and systems? The topics covered in this book provide a comprehensive survey of both the practice of P2P and main research directions and are intended to frame the answers for these questions. Organization and Approach The first two chapters introduce the main concepts of peer-to-peer systems. We examinetheoperationofabasicP2Psystem,includingbehaviorforself-organizing, routing, and searching. We also describe a number of representative commercial applications. The next four chapters describe the fundamental peer-to-peer overlay architectures, including both unstructured and structured overlays. xvii xviii Preface The last chapter in this group covers important implementation issues such as protocol design, NAT traversal, and peer capability assessment. Detailed discussion of P2P mechanisms to support key applications follow, includingchaptersonsearch,contentdelivery,peercastingandoverlaymulticast- ing,andoverlay-basedInternettelephony.Importantusesofpeer-to-peeroverlays thatwedescribehereincludedifferenttechniquesforcontentsearch,deliveryof real-time streaming content, and session initiation using the overlay. We then dis- cussinseparatechaptershowrequirementsforoverlayperformance,peermobil- ity, security, and management intersect with the P2P overlay design. Throughout the book, to motivate and illustrate the material, we include examples of systems in use and describe important research prototypes. We also refertoopen-sourceimplementationsforreaderswhoseekahands-onillustration of the ideas. In particular we use examples from OverlayWeaver, an open-source toolkit developed by Kazuyuki Shudo that supports a number of important peer- to-peer algorithms. Access to the open-source tools and updates to the book can beobtainedviathecompanionWebsiteathttp://elsevierdirect.com/companions/ 9780123742148. Audience This book is intended for professionals, researchers, and computer science and engineering students at the advanced undergraduate level and higher who are familiar with networking and network protocol concepts and basic ideas about algorithms. For the more advanced parts of the book, the reader should have generalfamiliarity with Internetprotocolssuchas TCPand IP routing but should not need to know the details of network routing protocols such as BGPor OSPF. For some sections of the book such as discussions of mobility or multicasting, familiaritywithmobilityinIPandIPmulticastingwillbehelpfulbutnotrequired. The reader will also find it helpful to be familiar with notation for comparing algorithm performance, such as O(n) or O(log n). For instructors who want to use the book as a textbook in a class on peer-to- peer networking, a set of exercises for each chapter, with an answer key for selectedexercisesareavailablebyregisteringathttp://textbooks.elsevier.com Peer-to-peer networking is generally seen as a new technology with a disrup- tive business model and many possibilities for further innovation. These trends make the subject matter in this book highly relevant to the technology commu- nity. We hope the book is a valuable starting point for readers who are new to the subject and an important reference to those who are active in the field. Throughout the book we conclude each chapter with suggestions for further reading for readers who would like to dig deeper into specific topics. Acknowledgements During the preparation of this book, many people provided help in reviewing portions of the text and the original book proposal. We greatly appreciate their suggestions and efforts in improving the quality of the book. Preface xix First, we would like to thank those individuals who reviewed the original proposal and made important comments about structure, topics, and emphasis: Germano Caronni, Google; Christos Gkantsidis, Microsoft Research; Wolfgang Kellerer, DOCOMO Euro-Labs; Xuemin (Sherman) Shen, University of Waterloo; and Xiaotao Wu, Avaya Labs Research. In addition, Wolfgang Kellerer, DOCOMO Euro-Labs, reviewed a substantial portion of the book and provided many useful suggestions. Wearealsogratefultothosewhoreviewedandcommentedonportionsofthe book: Yi Cui, Vanderbilt University; Anwitaman Datta, Nanyang Technological University;AaronHarwood,UniversityofMelbourne;VanaKalogeraki,University of California Riverside; Mario Kolberg, University of Stirling; Ben Leong, National University of Singapore; Li Li, Communications Research Centre Canada; Lundy Lewis, Southern New Hampshire University; Muthucumaru Maheswaran, McGill University; Kurt Tutschku, University of Vienna; Mengku Yang, Eastern Kentucky University; Wenjun (Kevin) Zeng, University of Missouri-Columbia. The efforts and constructive comments of all the reviewers are greatly appre- ciated. Any mistakes that remain are the responsibility of the authors. Thanks are due to the staff at Morgan Kaufman--Rick Adams, Senior Acquisi- tions Editor; assistant editors Gregory Chalson, Maria Alonso, and Lindsey Gendall; and our project manager, Melinda Ritchie. Finally we thank our families for their support and understanding while we worked on this book. John F. Buford, Princeton, NJ email: [email protected] Heather Yu, Princeton, NJ email: [email protected] Eng Keong Lua, Japan and USA email: [email protected] About the Authors John F. Buford is Research Scientist, Avaya Labs Research, Basking Ridge, NJ. PreviouslyhewasLeadScientistatPanasonicTechnologies,VPofSoftwareDevel- opmentatKadaSystems,DirectorofInternetTechnologiesatVerizon,andAssoc. Prof. of Computer Science at University of Massachusetts Lowell. PhD in Computer Science, Graz University of Technology. Heather Yu is Senior Manager of Media Technologies at Huawei Technologies USA, Bridgewater, NJ where she leads the research on multimedia content net- working and digital media technologies. PhD in Electrical Engineering, Princeton University. Eng Keong Lua is Faculty Member of College of Engineering, Information Net- workingInstituteatCarnegieMellonUniversity,Pittsburgh,USA,andSystemsSci- entist, Carnegie Mellon CyLab USA and Japan. Previously he held research fellowship and industry consulting positions at the NTT Laboratories, Intel Research,MicrosoftResearch,andHewlett-PackardR&D/Consulting.Hisresearch areas include Peer-to-Peer networks, Internet-scale P2P overlay multimedia com- munications, network security and future Internet. PhD in Computer Science, University of Cambridge, UK. xxi CHAPTER 1 Introduction Our discussion of peer-to-peer (P2P) concepts starts with an overview of the key applications and their emergence as mainstream services for millions of users. ThechapterthenexaminestherelationshipofP2PwiththeInternetanditsdistinc- tive features compared toother service architectures.A reviewofP2Peconomics, business models, social impact, and related technology trends concludes the chapter. P2P EMERGES AS A MAINSTREAM APPLICATION The Rise of P2P File-Sharing Applications Nearly 10 years after the World Wide Web became available for use on the Inter- net, decentralized peer-to-peer file-sharing applications supplanted the server- based Napster application, which had popularized the concept of file sharing. Napster’s centralized directories were its Achilles’ heel because, as it was argued incourt, Napster had the means,through its servers,to detect and preventregis- tration of copyrighted content in its service, but it failed to do so. Napster was subsequently found liable for copyright infringement, dealing a lethal blow to its business model. As Napster was consumed in legal challenges, second-generation protocols such as Gnutella, FastTrack, and BitTorrent adopted a peer-to-peer architecture in which there is no central directory and all file searches and transfers are distributed among the corresponding peers. Other systems such as FreeNet also incorporated mechanisms for client anonymity, including routing requests indi- rectlythroughotherclientsandencryptingmessagesbetweenpeers.Meanwhile, the top labels in the music industry, which have had arguably the most serious revenuelossduetotheemergenceoffilesharing,havecontinuedtopursuelegal challenges to these systems and their users. Regardless of the outcome of these court cases, the social perception of the acceptability and benefits of content distribution through P2P applications has 1 2 CHAPTER 1 Introduction beenirrevocablyaltered.InthemusicindustrypriortoP2Pfilesharing,audioCDs werethedominantdistributionmechanism.Webportalsforonlinemusicwerelim- itedintermsofthesizeoftheircatalogs,anddownloadswereexpensive.Although P2P file sharing became widely equated with content piracy, it also showed that consumerswerereadytoreplacetheCDdistributionmodelwithanonlineexperi- ence if it could provide a large portfolio of titles and artists and if it included fea- tures such as a search, previews, transfer to CD and personal music players, and individualtrackpurchase.AsportalssuchasiTunesemergedwiththeseproperties, a tremendous growth in the online music business resulted. InatypicalP2Pfile-sharingapplication,auserhasdigitalmediafilesheorshe wants to share with others. These files are registered by the user using the local application according to properties such as title, artist, date, and format. Later, other users anywhere on the Internet can search for these media files by providingaqueryintermsofsomecombinationofthesameattributes.Aswedis- cuss in detail in later chapters, the query is sent to other online peers in the net- work.Apeerthathaslocalmediafilesmatchingthequerywillreturninformation on how to retrieve the files. It may also forward the query to other peers. Users may receive multiple successful responses to their query and can then select the files they want to retrieve. The files are then downloaded from the remote peer to the local machine. Examples of file-sharing client user interfaces are shown in Figures 1.1 and 1.2. FIGURE1.1 LimeWireclient. P2P Emerges as a Mainstream Application 3 FIGURE1.2 eMuleclientsearchinterface. Despitetheirpopularity,P2Pfile-sharingsystemshavebeenplaguedbyseveral problems for users. First, some of the providers of leading P2Papplications earn revenue from third parties by embedding spyware and malware into the applica- tions. Users then find their computers infected with such software immediately after installing the P2P application. Second, a large amount of polluted or cor- rupted content has been published in file-sharing systems, and it is difficult for a user to distinguish such content from the original digital content they seek. It is generally felt that pollution attacks on file-sharing systems are intended to dis- courage the distribution of copyrighted material. A user downloading a polluted music file might find, for example, noise, gaps, and abbreviated content. A third type of problem affecting the usability of P2P file-sharing applications is the free-rider problem. A free rider is a peer that uses the file-sharing applica- tion to access content from others but does not contribute content to the same degree to the community of peers. Various techniques for addressing the free- rider problem by offering incentives or monitoring use are discussed later in the book. A related issue is that of peer churn. A peer’s content can only be accessed by other peers if that peer is online. When a peer goes offline, it takes time for other peers to be alerted to the change in status. Meanwhile, content queries may go unanswered and time out. 4 CHAPTER 1 Introduction The leading P2Pfile-sharingsystems havenotadopted mechanisms toprotect licensed content or collect payment for transfers on behalf of copyright owners. Several ventures seek to legitimize P2P file sharing for licensed content by incor- porating techniques fordigital rights management (DRM) and superdistribution into P2P distribution architectures. In such systems, content is encrypted, and thoughitcanbefreelydistributed,auser mustseparatelypurchaseanencrypted license file to render the media. Through the use of digital signatures, such license files are not easily transferred to other users. See this book’s Website for links to current P2P file-sharing proposals for DRM-based approaches. Other ventures such as QTrax, SpiralFrog, and TurnItUp are proposing an ad- based model for free music distribution. The usercan freelydownload the music file,whichinsomemodelsisprotectedwithDRM,butmustlistentoorwatchan ad during download or playback. In these schemes, the advertiser instead of the user is paying the content licensing costs. Questions remain about this model, such as whether it will undercut existing music download business models and whethertheadvertisingrevenueissufficienttomatchthelicensingrevenuefrom existing music download sites. Voice over P2P (VoP2P) Desktop VoIP (voice over IP) clients began to appear in the mid-1990s and offered free desktop-to-desktop voice and video calls. These applications, though economicallyattractiveandtechnicallyinnovative,didn’tattractalargefollowing due to factors such as lack of voice quality and limited availability of broadband accessintheconsumer market.Inaddition,theinitiallysmallsizeofthenetwork community limited the potential of such applications to supplant conventional telephony.ThiscontinuestobeapracticalissuefacingnewtypesofP2Papplica- tions—how to create a community of users that can reach the critical mass needed to provide the value proposition that comes with scale. Startingin1996withthelaunchofICQ,anumberofinstant-messageandpres- ence(IMP)applicationsbecamewidelypopular.TheleadingIMPsystems,suchas AIM, Microsoft Messenger, Yahoo! Messenger, and Jabber, all use client/server architectures.6 Although several of these systems have subsequently included telephony capabilities, their telephony features have not drawn a large user community. Skype is a VoP2P client launched in 2003 that has reached more than 10 mil- lionconcurrentusers.TheVoP2PtechnologyofSkypeisdiscussedinChapter11. Compared to earlier VoIP clients, Skype offers both free desktop-to-desktop calls and low-cost desktop-to-public switched telephone network (PSTN) calls, includ- ing international calls. The call quality is high, generally attributed to the audio codec Skype uses and today’s wide use of broadband access networks to reach theInternet.Inaddition,SkypeincludesfeaturesfromIMPapplications,including buddy lists, instant messaging, and presence. Unlike the file-sharing systems, Skype promises a no spyware policy.
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