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BitTorrent
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=== Downloading and sharing === Users find a torrent of interest on a torrent index site or by using a search engine built into the client, download it, and open it with a BitTorrent client. The client connects to the tracker(s) or seeds specified in the torrent file, from which it receives a list of seeds and peers currently transferring pieces of the file(s). The client connects to those peers to obtain the various pieces. If the swarm contains only the initial seeder, the client connects directly to it, and begins to request pieces. Clients incorporate mechanisms to optimize their download and upload rates. The effectiveness of this data exchange depends largely on the policies that clients use to determine to whom to send data. Clients may prefer to send data to peers that send data back to them (a "[[tit for tat]]" exchange scheme), which encourages fair trading. But strict policies often result in suboptimal situations, such as when newly joined peers are unable to receive any data because they do not have any pieces yet to trade themselves or when two peers with a good connection between them do not exchange data simply because neither of them takes the initiative. To counter these effects, the official BitTorrent client program uses a mechanism called "optimistic unchoking", whereby the client reserves a portion of its available bandwidth for sending pieces to random peers (not necessarily known good partners, or "preferred peers") in hopes of discovering even better partners and to ensure that newcomers get a chance to join the swarm.<ref name = "Tamilmanistudy" >{{cite web| url=http://mnl.cs.stonybrook.edu/home/karthik/BitTorrent/Robustness_of_BT.doc |title=Studying and enhancing the BitTorrent protocol |first=Karthik |last=Tamilmani |publisher=Stony Brook University |date=25 October 2003 |access-date=6 May 2006 |format=DOC |archive-url=https://web.archive.org/web/20041119150847/http://mnl.cs.stonybrook.edu/home/karthik/BitTorrent/Robustness_of_BT.doc |archive-date=19 November 2004}}</ref><!-- mnl.cs.stonybrook.edu down, subst archive.org link --> Although "swarming" scales well to tolerate "flash crowds" for popular content, it is less useful for unpopular or [[niche market]] content. Peers arriving after the initial rush might find the content unavailable and need to wait for the arrival of a "seed" in order to complete their downloads. The seed arrival, in turn, may take long to happen (this is termed the "seeder promotion problem"). Since maintaining seeds for unpopular content entails high bandwidth and administrative costs, this runs counter to the goals of publishers that value BitTorrent as a cheap alternative to a client-server approach. This occurs on a huge scale; measurements have shown that 38% of all new torrents become unavailable within the first month.<ref>{{cite arXiv |eprint=0912.0625 |title=Unraveling BitTorrent's File Unavailability: Measurements and Analysis |last=Kaune |first=Sebastian |display-authors=etal |class=cs.NI |year=2009 }}</ref> A strategy adopted by many publishers which significantly increases availability of unpopular content consists of bundling multiple files in a single swarm.<ref>{{cite conference |url=http://conferences.sigcomm.org/co-next/2009/papers/Menasche.pdf |title=Content Availability and Bundling in Swarming Systems |author=D. Menasche |display-authors=etal |publisher=ACM via sigcomm.org |work=CoNEXT'09 |date=1β4 December 2009 |location=Rome, Italy |isbn=978-1-60558-636-6 |access-date=18 December 2009 |archive-url=https://web.archive.org/web/20110501082904/http://conferences.sigcomm.org/co-next/2009/papers/Menasche.pdf |archive-date=1 May 2011 |url-status=live |df=dmy-all }}</ref> More sophisticated solutions have also been proposed; generally, these use cross-torrent mechanisms through which multiple torrents can cooperate to better share content.<ref>{{cite web |url=http://www.eecs.qmul.ac.uk/~tysong/files/ICCCN09.pdf |title=The Seeder Promotion Problem: Measurements, Analysis and Solution Space |last=Kaune |first=Sebastian |display-authors=etal |publisher=Queen Mary's University London |access-date=20 July 2017 |archive-url=https://web.archive.org/web/20140809010501/http://www.eecs.qmul.ac.uk/~tysong/files/ICCCN09.pdf |archive-date=9 August 2014 |url-status=live |df=dmy-all }}</ref>
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