Editing Peer-to-peer (section)

==Architecture==
A peer-to-peer network is designed around the notion of equal ''[[Peer group (computer networking)|peer]]'' nodes simultaneously functioning as both "clients" and "servers" to the other nodes on the network.<ref>{{Cite web |title=Practical Applications of Peer-to-Peer Networks in File Sharing and Content Distribution {{!}} SimpleSwap |url=https://simpleswap.io/blog/practical-applications-of-peer-to-peer-networks-in-file-sharing-and-content-distribution |access-date=2024-08-21 |website=SimpleSwap {{!}} Cryptocurrency exchange blog |language=en}}</ref> This model of network arrangement differs from the [[client–server]] model where communication is usually to and from a central server. A typical example of a file transfer that uses the client-server model is the [[File Transfer Protocol]] (FTP) service in which the client and server programs are distinct: the clients initiate the transfer, and the servers satisfy these requests.

===Routing and resource discovery===
Peer-to-peer networks generally implement some form of virtual [[overlay network]] on top of the physical network topology, where the nodes in the overlay form a [[subset]] of the nodes in the physical network.<ref>{{Cite web |title=Overlay Network - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/computer-science/overlay-network |access-date=2024-08-21 |website=www.sciencedirect.com}}</ref> Data is still exchanged directly over the underlying [[TCP/IP]] network, but at the [[application layer]] peers can communicate with each other directly, via the logical overlay links (each of which corresponds to a path through the underlying physical network). Overlays are used for indexing and peer discovery, and make the P2P system independent from the physical network topology. Based on how the nodes are linked to each other within the overlay network, and how resources are indexed and located, we can classify networks as ''unstructured'' or ''structured'' (or as a hybrid between the two).<ref>{{cite book |editor-last=Ahson |editor-first=Syed A. |editor2-last=Ilyas |editor2-first=Mohammad |title=SIP Handbook: Services, Technologies, and Security of Session Initiation Protocol |publisher=Taylor & Francis |year=2008 |isbn=9781420066043 |page=204 |url=https://books.google.com/books?id=CKzPq3-wVdcC&pg=PA204}}</ref><ref>{{cite book |editor-last=Zhu |editor-first=Ce |title=Streaming Media Architectures: Techniques and Applications: Recent Advances |publisher=IGI Global |year=2010 |isbn=9781616928339 |page=265 |url=https://books.google.com/books?id=Cb4dWYVJ_8AC&pg=PA265 |display-editors=etal}}</ref><ref>{{cite book |last=Kamel |first=Mina |chapter=Optimal Topology Design for Overlay Networks |editor-last=Akyildiz |editor-first=Ian F. |title=Networking 2007: Ad Hoc and Sensor Networks, Wireless Networks, Next Generation Internet: 6th International IFIP-TC6 Networking Conference, Atlanta, GA, USA, May 14-18, 2007 Proceedings|publisher=Springer |year=2007 |isbn=9783540726050 |page=714 |chapter-url=https://books.google.com/books?id=r4V2G7yPLIAC&pg=PA714 |display-authors=etal}}</ref>

====Unstructured networks====
[[File:Unstructured peer-to-peer network diagram.png|thumb|right|300px|Overlay network diagram for an '''unstructured P2P network''', illustrating the ad hoc nature of the connections between nodes]]
''Unstructured peer-to-peer networks'' do not impose a particular structure on the overlay network by design, but rather are formed by nodes that randomly form connections to each other.<ref>{{cite book |last=Filali |first=Imen |chapter=A Survey of Structured P2P Systems for RDF Data Storage and Retrieval |editor-last=Hameurlain |editor-first=Abdelkader |title=Transactions on Large-Scale Data- and Knowledge-Centered Systems III: Special Issue on Data and Knowledge Management in Grid and PSP Systems |publisher=Springer |year=2011 |isbn=9783642230738|page=21 |chapter-url=https://books.google.com/books?id=pjQr7BHtbCoC&pg=PA21 |display-authors=etal|display-editors=etal}}</ref> ([[Gnutella]], [[Gossip protocol|Gossip]], and [[Kazaa]] are examples of unstructured P2P protocols).<ref name=":0">{{cite book |last=Zulhasnine |first=Mohammed |chapter=P2P Streaming Over Cellular Networks: Issues, Challenges, and Opportunities |editor=Pathan |title=Building Next-Generation Converged Networks: Theory and Practice |publisher=CRC Press |year=2013 |isbn=9781466507616 |page=99 |chapter-url=https://books.google.com/books?id=tr5PGJk-swIC&pg=PA99 |display-authors=etal|display-editors=etal}}</ref>

Because there is no structure globally imposed upon them, unstructured networks are easy to build and allow for localized optimizations to different regions of the overlay.<ref>{{cite book |last1=Chervenak |first1=Ann |last2=Bharathi |first2=Shishir |chapter=Peer-to-peer Approaches to Grid Resource Discovery |editor-last=Danelutto |editor-first=Marco |title=Making Grids Work: Proceedings of the CoreGRID Workshop on Programming Models Grid and P2P System Architecture Grid Systems, Tools and Environments 12-13 June 2007, Heraklion, Crete, Greece|publisher=Springer |year=2008 |isbn=9780387784489 |page=67 |chapter-url=https://books.google.com/books?id=adN0pm_BBuYC&pg=PA67 |display-editors=etal}}</ref> Also, because the role of all peers in the network is the same, unstructured networks are highly robust in the face of high rates of "churn"—that is, when large numbers of peers are frequently joining and leaving the network.<ref name="Jin-Unstructured-2010">{{cite book |last1=Jin |first1=Xing |last2=Chan |first2=S.-H. Gary |chapter=Unstructured Peer-to-Peer Network Architectures |editor=Shen |title=Handbook of Peer-to-Peer Networking |publisher=Springer |year=2010 |isbn=978-0-387-09750-3 |page=119 |display-editors=etal}}</ref><ref name="lv-2002">{{cite book |last=Lv |first=Qin |chapter=Can Heterogeneity Make Gnutella Stable? |editor-last=Druschel |editor-first=Peter |title=Peer-to-Peer Systems: First International Workshop, IPTPS 2002, Cambridge, MA, USA, March 7-8, 2002, Revised Papers |publisher=Springer |year=2002 |isbn=9783540441793 |page=[https://archive.org/details/peertopeersystem0000iptp/page/94 94] |chapter-url=https://books.google.com/books?id=f57AwpUIctcC&pg=PA94 |display-authors=etal |display-editors=etal |url=https://archive.org/details/peertopeersystem0000iptp/page/94 }}</ref>

However, the primary limitations of unstructured networks also arise from this lack of structure. In particular, when a peer wants to find a desired piece of data in the network, the search query must be flooded through the network to find as many peers as possible that share the data. Flooding causes a very high amount of signaling traffic in the network, uses more [[CPU]]/memory (by requiring every peer to process all search queries), and does not ensure that search queries will always be resolved. Furthermore, since there is no correlation between a peer and the content managed by it, there is no guarantee that flooding will find a peer that has the desired data. Popular content is likely to be available at several peers and any peer searching for it is likely to find the same thing. But if a peer is looking for rare data shared by only a few other peers, then it is highly unlikely that the search will be successful.<ref>{{cite book |last1=Shen |first1=Xuemin |last2=Yu |first2=Heather |last3=Buford |first3=John |last4=Akon |first4=Mursalin |title=Handbook of Peer-to-Peer Networking |publisher=Springer|edition=1st |year=2009 |location=New York |page=118 |isbn=978-0-387-09750-3}}</ref>

====Structured networks====
[[File:Structured (DHT) peer-to-peer network diagram.png|thumb|right|300px|Overlay network diagram for a '''structured P2P network''', using a [[distributed hash table]] (DHT) to identify and locate nodes/resources]]

In ''structured peer-to-peer networks'' the overlay is organized into a specific topology, and the protocol ensures that any node can efficiently<ref>{{Cite web |last1=Dhara |first1=Krishna |last2=Kolberg |first2=Mario |date=January 2010 |title=Overview of Structured Peer-to-Peer Overlay Algorithms |url=https://www.researchgate.net/publication/226809025 }}</ref> search the network for a file/resource, even if the resource is extremely rare.<ref name=":0" />

The most common type of structured P2P networks implement a [[distributed hash table]] (DHT),<ref name="CP2P" /><ref>R. Ranjan, A. Harwood, and R. Buyya, "Peer-to-peer based resource discovery in global grids: a tutorial," ''IEEE Commun. Surv.'', vol. 10, no. 2. and P. Trunfio, "Peer-to-Peer resource discovery in Grids: Models and systems," ''Future Generation Computer Systems'' archive, vol. 23, no. 7, Aug. 2007.</ref> in which a variant of [[consistent hashing]] is used to assign ownership of each file to a particular peer.<ref>{{cite book |last1=Kelaskar |first1=M. |last2=Matossian |first2=V. |last3=Mehra |first3=P. |last4=Paul |first4=D. |last5=Parashar |first5=M. |year=2002 |url=http://portal.acm.org/citation.cfm?id=873218 |title=A Study of Discovery Mechanisms for Peer-to-Peer Application |pages=444– |publisher=IEEE Computer Society |isbn=9780769515823 }}</ref><ref name="P2P_API">{{cite book |last1=Dabek |first1=Frank |first2=Ben |last2=Zhao |first3=Peter |last3=Druschel |first4=John |last4=Kubiatowicz |first5=Ion |last5=Stoica |title=Peer-to-Peer Systems II |chapter=Towards a Common API for Structured Peer-to-Peer Overlays |year=2003 |volume=2735 |series=Lecture Notes in Computer Science |pages=33–44 |doi=10.1007/978-3-540-45172-3_3 |isbn=978-3-540-40724-9 |citeseerx=10.1.1.12.5548 }}</ref> This enables peers to search for resources on the network using a [[hash table]]: that is, (''key'', ''value'') pairs are stored in the DHT, and any participating node can efficiently retrieve the value associated with a given key.<ref>Moni Naor and Udi Wieder. [http://www.wisdom.weizmann.ac.il/~naor/PAPERS/dh.pdf Novel Architectures for P2P Applications: the Continuous-Discrete Approach] {{Webarchive|url=https://web.archive.org/web/20191209032152/http://www.wisdom.weizmann.ac.il/~naor/PAPERS/dh.pdf |date=2019-12-09 }}. Proc. SPAA, 2003.</ref><ref>Gurmeet Singh Manku. [http://www-db.stanford.edu/~manku/phd/index.html Dipsea: A Modular Distributed Hash Table] {{webarchive|url=https://web.archive.org/web/20040910154927/http://www-db.stanford.edu/~manku/phd/index.html |date=2004-09-10 }}. Ph. D. Thesis (Stanford University), August 2004.</ref>
[[File:DHT en.svg|thumb|left|250px|Distributed hash tables]]

However, in order to route traffic efficiently through the network, nodes in a structured overlay must maintain lists of neighbors<ref>{{Cite web|url=https://sites.cs.ucsb.edu/~ravenben/publications/pdf/impact-iptps.pdf|title=Impact of Neighbor Selection on Performance and Resilience of Structured P2P Networks|author=Byung-Gon Chun, Ben Y. Zhao, John D. Kubiatowicz|date=2005-02-24|access-date=2019-08-24}}</ref> that satisfy specific criteria. This makes them less robust in networks with a high rate of ''churn'' (i.e. with large numbers of nodes frequently joining and leaving the network).<ref name="lv-2002" /><ref>{{cite book|last=Li |first=Deng |title=An Efficient, Scalable, and Robust P2P Overlay for Autonomic Communication |editor-last=Vasilakos |editor-first=A.V. |publisher=Springer |year=2009 |isbn=978-0-387-09752-7 |page=329 |url=https://books.google.com/books?id=c02mTcXW_U4C&pg=PA329 |display-authors=etal|display-editors=etal}}</ref> More recent evaluation of P2P resource discovery solutions under real workloads have pointed out several issues in DHT-based solutions such as high cost of advertising/discovering resources and static and dynamic load imbalance.<ref>{{cite journal |last1=Bandara |first1=H. M. N. Dilum |first2=Anura P. |last2=Jayasumana |title=Evaluation of P2P Resource Discovery Architectures Using Real-Life Multi-Attribute Resource and Query Characteristics |journal=IEEE Consumer Communications and Networking Conf. (CCNC '12) |date=January 2012}}</ref>

Notable distributed networks that use DHTs include [[Tixati]], an alternative to [[BitTorrent (protocol)|BitTorrent's]] distributed tracker, the [[Kad network]], the [[Storm botnet]], and the [[YaCy]]. Some prominent research projects include the [[Chord project]], [[Kademlia]], [[PAST storage utility]], [[P-Grid]], a self-organized and emerging overlay network, and [[CoopNet content distribution system]].<ref>{{cite book | last1=Korzun | first1=Dmitry| last2=Gurtov| first2 = Andrei| title= Structured P2P Systems: Fundamentals of Hierarchical Organization, Routing, Scaling, and Security|publisher=Springer | isbn = 978-1-4614-5482-3 |date= November 2012 | url=https://www.springer.com/gp/book/9781461454823}}</ref> DHT-based networks have also been widely utilized for accomplishing efficient resource discovery<ref>{{cite web |last1=Ranjan |first1=Rajiv |last2=Harwood |first2=Aaron |last3=Buyya |first3=Rajkumar |date=1 December 2006 |url=http://www.cs.mu.oz.au/%7Erranjan/pgrid.pdf |title=A Study on Peer-to-Peer Based Discovery of Grid Resource Information |access-date=25 August 2008 |archive-date=14 May 2011 |archive-url=https://web.archive.org/web/20110514055004/http://www.cs.mu.oz.au/%7Erranjan/pgrid.pdf |url-status=dead }}</ref><ref>{{cite web |url=http://gridbus.org/papers/DecentralisedDiscoveryGridFed-eScience2007.pdf |first1=Rajiv |last1=Ranjan |first2=Lipo |last2=Chan |first3=Aaron |last3=Harwood |first4=Shanika |last4=Karunasekera |first5=Rajkumar |last5=Buyya |title=Decentralised Resource Discovery Service for Large Scale Federated Grids |url-status=dead |archive-url=https://web.archive.org/web/20080910170417/http://gridbus.org/papers/DecentralisedDiscoveryGridFed-eScience2007.pdf |archive-date=2008-09-10 }}</ref> for [[grid computing]] systems, as it aids in resource management and scheduling of applications.

====Hybrid models====
Hybrid models are a combination of peer-to-peer and [[Client–server model|client–server]] models.<ref>{{cite book |last=Darlagiannis |first=Vasilios |chapter=Hybrid Peer-to-Peer Systems|editor-last1=Steinmetz |editor-first1=Ralf |editor-last2=Wehrle |editor-first2=Klaus |title=Peer-to-Peer Systems and Applications |publisher=Springer |year=2005 |isbn=9783540291923 |chapter-url=https://books.google.com/books?id=A8CLZ1FB4qoC&pg=PA353 }}</ref> A common hybrid model is to have a central server that helps peers find each other. [[Spotify]] was an example of a hybrid model [until 2014].<ref>{{Cite web |last=Pejchinovski |first=Gordan |title=Spotify – Combining Cache, Peer-To-Peer and Server-Client Architectures for Users’ Satisfaction |url=https://www.researchgate.net/publication/260281013_Spotify_-_Combining_Cache_Peer-To-Peer_and_Server-Client_Architectures_for_Users'_Satisfaction |website=Researchgate}}</ref> There are a variety of hybrid models, all of which make trade-offs between the centralized functionality provided by a structured server/client network and the node equality afforded by the pure peer-to-peer unstructured networks. Currently, hybrid models have better performance than either pure unstructured networks or pure structured networks because certain functions, such as searching, do require a centralized functionality but benefit from the decentralized aggregation of nodes provided by unstructured networks.<ref>{{cite journal |last1=Yang |first1=Beverly |last2=Garcia-Molina |first2=Hector |year=2001 |title=Comparing Hybrid Peer-to-Peer Systems |journal=Very Large Data Bases |url=http://infolab.stanford.edu/~byang/pubs/hybridp2p_long.pdf |access-date=8 October 2013}}</ref>

====CoopNet content distribution system====
'''CoopNet (Cooperative Networking)''' was a proposed system for off-loading serving to peers who have recently [[download]]ed content, proposed by computer scientists Venkata N. Padmanabhan and Kunwadee Sripanidkulchai, working at [[Microsoft Research]] and [[Carnegie Mellon University]].<ref>{{Cite book| last1 = Padmanabhan| first1 = Venkata N.| last2 = Sripanidkulchai| first2 = Kunwadee| publication-date = March 2002| year = 2002| title = The Case for Cooperative Networking (PostScript with addendum)| volume = Proceedings of the First International Workshop on Peer-to-Peer Systems| series = Lecture Notes in Computer Science| location = Cambridge, MA| publisher = Springer| pages = [https://archive.org/details/peertopeersystem0000iptp/page/178 178]| isbn = 978-3-540-44179-3| doi = 10.1007/3-540-45748-8_17| url = https://archive.org/details/peertopeersystem0000iptp/page/178}}  [http://research.microsoft.com/projects/CoopNet/papers/iptps02-with-addendum.pdf PDF (Microsoft, with addendum)] {{Webarchive|url=https://web.archive.org/web/20070417140616/http://research.microsoft.com/projects/CoopNet/papers/iptps02-with-addendum.pdf |date=2007-04-17 }} [https://doi.org/10.1007%2F3-540-45748-8_17 PDF (Springer, original, fee may be required)] {{Webarchive|url=https://web.archive.org/web/20230101095443/https://link.springer.com/chapter/10.1007/3-540-45748-8_17 |date=2023-01-01 }}</ref><ref>{{Cite web|url=http://research.microsoft.com/projects/CoopNet/|title=CoopNet: Cooperative Networking|publisher=Microsoft Research}}  Project home page.</ref> When a [[Server (computing)|server]] experiences an increase in load it redirects incoming peers to other peers who have agreed to [[mirror site|mirror]] the content, thus off-loading balance from the server. All of the information is retained at the server. This system makes use of the fact that the bottleneck is most likely in the outgoing bandwidth than the [[CPU]], hence its server-centric design. It assigns peers to other peers who are 'close in [[IP Address|IP]]' to its neighbors [same prefix range] in an attempt to use locality. If multiple peers are found with the same [[Computer file|file]] it designates that the node choose the fastest of its neighbors. [[Streaming media]] is transmitted by having clients [[web cache|cache]] the previous stream, and then transmit it piece-wise to new nodes.

===Security and trust===
Peer-to-peer systems pose unique challenges from a [[computer security]] perspective. Like any other form of [[software]], P2P applications can contain [[vulnerability (computing)|vulnerabilities]]. What makes this particularly dangerous for P2P software, however, is that peer-to-peer applications act as servers as well as clients, meaning that they can be more vulnerable to [[remote exploits]].<ref name="vu-p2p-principles-p8">{{cite book |last=Vu |first=Quang H. |title=Peer-to-Peer Computing: Principles and Applications |publisher=Springer |year=2010 |isbn=978-3-642-03513-5 |page=8 |display-authors=etal}}</ref>

====Routing attacks====
Since each node plays a role in routing traffic through the network, malicious users can perform a variety of "routing attacks", or [[denial of service]] attacks. Examples of common routing attacks include "incorrect lookup routing" whereby malicious nodes deliberately forward requests incorrectly or return false results, "incorrect routing updates" where malicious nodes corrupt the routing tables of neighboring nodes by sending them false information, and "incorrect routing network partition" where when new nodes are joining they bootstrap via a malicious node, which places the new node in a partition of the network that is populated by other malicious nodes.<ref name="vu-p2p-principles-p8" />

====Corrupted data and malware====
{{See also|Data validation|Malware}}
The prevalence of [[malware]] varies between different peer-to-peer protocols.<ref>{{Cite web |title=Malware Propagation Modelling in Peer-to-Peer Networks: A Review |url=https://bradscholars.brad.ac.uk/bitstream/handle/10454/16651/Malware%20Propagation%20Modelling%20in%20Peer%20to%20Peer%20Networks.pdf?sequence=4&isAllowed=y}}</ref> Studies analyzing the spread of malware on P2P networks found, for example, that 63% of the answered download requests on the [[gnutella]] network contained some form of malware, whereas only 3% of the content on [[OpenFT]] contained malware. In both cases, the top three most common types of malware accounted for the large majority of cases (99% in gnutella, and 65% in OpenFT). Another study analyzing traffic on the [[Kazaa]] network found that 15% of the 500,000 file sample taken were infected by one or more of the 365 different [[computer viruses]] that were tested for.<ref>{{cite book |last=Goebel |first=Jan |chapter=Measurement and Analysis of Autonomous Spreading Malware in a University Environment |editor-last=Hämmerli |editor-first=Bernhard Markus |editor2-last=Sommer |editor2-first=Robin |title=Detection of Intrusions and Malware, and Vulnerability Assessment: 4th International Conference, DIMVA 2007 Lucerne, Switzerland, July 12-13, 2007 Proceedings |publisher=Springer |year=2007 |isbn=9783540736134 |page=112 |chapter-url=https://books.google.com/books?id=M0PfEaVa9QIC&pg=PA112 |display-authors=etal}}</ref>

Corrupted data can also be distributed on P2P networks by modifying files that are already being shared on the network. For example, on the [[FastTrack]] network, the [[RIAA]] managed to introduce faked chunks into downloads and downloaded files (mostly [[MP3]] files). Files infected with the RIAA virus were unusable afterwards and contained malicious code. The RIAA is also known to have uploaded fake music and movies to P2P networks in order to deter illegal file sharing.<ref>{{cite news |url=https://www.nytimes.com/2003/05/04/business/04MUSI.html |title=Software Bullet Is Sought to Kill Musical Piracy |last=Sorkin |first=Andrew Ross |date=4 May 2003 |newspaper=New York Times |access-date=5 November 2011}}</ref> Consequently, the P2P networks of today have seen an enormous increase of their security and file verification mechanisms. Modern [[hash chain|hashing]], [[File verification|chunk verification]] and different encryption methods have made most networks resistant to almost any type of attack, even when major parts of the respective network have been replaced by faked or nonfunctional hosts.<ref>{{cite tech report |first=Vivek |last=Singh |first2=Himani |last2=Gupta |title= Anonymous File Sharing in Peer to Peer System by Random Walks |number=123456789/9306 |institution=SRM University |year=2012 }}</ref>

===Resilient and scalable computer networks===
{{See also|Wireless mesh network|Distributed computing}}
The decentralized nature of P2P networks increases robustness because it removes the [[Reliability engineering|single point of failure]] that can be inherent in a client–server based system.<ref name="ms-overlay-survey">{{cite web |url=http://academic.research.microsoft.com/Publication/2633870/a-survey-and-comparison-of-peer-to-peer-overlay-network-schemes |title=A survey and comparison of peer-to-peer overlay network schemes |last1=Lua |first1=Eng Keong |last2=Crowcroft |first2=Jon |last3=Pias |first3=Marcelo |last4=Sharma |first4=Ravi |last5=Lim |first5=Steven |year=2005 |url-status=dead |archive-url=https://web.archive.org/web/20120724222234/http://academic.research.microsoft.com/Publication/2633870/a-survey-and-comparison-of-peer-to-peer-overlay-network-schemes |archive-date=2012-07-24 }}</ref> As nodes arrive and demand on the system increases, the total capacity of the system also increases, and the likelihood of failure decreases. If one peer on the network fails to function properly, the whole network is not compromised or damaged. In contrast, in a typical client–server architecture, clients share only their demands with the system, but not their resources. In this case, as more clients join the system, fewer resources are available to serve each client, and if the central server fails, the entire network is taken down.

===Distributed storage and search===
[[File:Yacy-resultados.png|thumb|right|300px|Search results for the query "[[software libre]]" using [[YaCy]], a free [[distributed search engine]] that runs on a peer-to-peer network instead of making requests to centralized index servers]]
There are both advantages and disadvantages in P2P networks related to the topic of data [[backup]], recovery, and availability. In a centralized network, the system administrators are the only forces controlling the availability of files being shared. If the administrators decide to no longer distribute a file, they simply have to remove it from their servers, and it will no longer be available to users. Along with leaving the users powerless in deciding what is distributed throughout the community, this makes the entire system vulnerable to threats and requests from the government and other large forces.

For example, [[YouTube]] has been pressured by the [[RIAA]], [[MPAA]], and entertainment industry to filter out copyrighted content. Although server-client networks are able to monitor and manage content availability, they can have more stability in the availability of the content they choose to host. A client should not have trouble accessing obscure content that is being shared on a stable centralized network. P2P networks, however, are more unreliable in sharing unpopular files because sharing files in a P2P network requires that at least one node in the network has the requested data, and that node must be able to connect to the node requesting the data. This requirement is occasionally hard to meet because users may delete or stop sharing data at any point.<ref>{{cite journal |year=2003 |title=Looking up data in P2P systems |journal=Communications of the ACM |volume=46 |issue=2 |pages=43–48 |doi=10.1145/606272.606299 |url=http://www.nms.lcs.mit.edu/papers/p43-balakrishnan.pdf |access-date=8 October 2013|last1=Balakrishnan |first1=Hari |last2=Kaashoek |first2=M. Frans |last3=Karger |first3=David |last4=Morris |first4=Robert |last5=Stoica |first5=Ion |citeseerx=10.1.1.5.3597 |s2cid=2731647 }}</ref>

In a P2P network, the community of users is entirely responsible for deciding which content is available. Unpopular files eventually disappear and become unavailable as fewer people share them. Popular files, however, are highly and easily distributed. Popular files on a P2P network are more stable and available than files on central networks. In a centralized network, a simple loss of connection between the server and clients can cause a failure, but in P2P networks, the connections between every node must be lost to cause a data-sharing failure. In a centralized system, the administrators are responsible for all data recovery and backups, while in P2P systems, each node requires its backup system. Because of the lack of central authority in P2P networks, forces such as the recording industry, [[RIAA]], [[MPAA]], and the government are unable to delete or stop the sharing of content on P2P systems.<ref>{{cite web |url=http://www.p2pnews.net/2012/06/14/art-thou-a-peer/ |title=Art thou a Peer? |author=<!--Staff writer(s); no by-line.--> |date=14 June 2012 |website=www.p2pnews.net |access-date=10 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131006022409/http://www.p2pnews.net/2012/06/14/art-thou-a-peer/ |archive-date=6 October 2013 }}</ref>