Editing Peer-to-peer (section)

====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.