Encyclopedia of Computer Science and Technology

file transfer protocols 193a central server still may offer better security and can serveas a central repository from which documents or sourcecode can be “checked out” and updated in an orderly way(version control).Further Reading“Designing and Deploying File Servers.” Microsoft TechNet.Available online. URL: http://technet2.microsoft.com/windowsserver/en/library/42befce4-7c15-4306-8edc-a80b8c57c67d1033.mspx. Accessed August 1, 2007.Eckstein, Robert, David Collier-Brown, and Peter Kelly. UsingSamba. 2nd ed. Sebastapol, Calif.: O’Reilly Media, 2003.Matthews, Martin S. Windows Server 2003: A Beginner’s Guide. 2nded. Berkeley, Calif.: McGraw-Hill Osborne, 2003.“Samba: Opening Windows to a Wider World.” Available online.URL: http://us3.samba.org. Accessed August 1, 2007.Tulloch, Mitch. Introducing Windows Server 2008. Redmond,Wash.: Microsoft Press, 2007.file-sharing and P2P networksFile-sharing services allow participants to provide access tofiles on their personal computers, such as music or video.In turn, the user can browse the service to find and downloadmaterial of interest. The structure is generally that of apeer-to-peer (P2P) network with no central server.The first major file-sharing service was Napster. Thiswas a P2P network but had a central server that providedthe searchable list of files and locations—but not the filesthemselves, which were downloaded from users’ PCs.Napster was forced to close in 2001 by legal action fromcopyright holders (see intellectual property and computing).A new but unrelated for-pay service opened laterunder the same name.Because of the legal vulnerability of centralized-list P2Pservices, a new model was developed, typified by Gnutella.This is a fully P2P model with both indexing and datadecentralized in nodes throughout the network. As of mid-2006, Gnutella and similar services such as Kazaa had anestimated 10 million users.BitTorrentMany services today use the popular BitTorrent file-sharingprotocol. A BitTorrent client (either the program of thatname or another compatible one) can transmit or receiveany type of data. To share a file, the client creates a “torrent”—asmall file that contains metadata describing thefile and an assignment to a “tracker.” The tracker is anothercomputer (node) that coordinates the distribution of thefile. Although this sounds complicated and a request takeslonger to set up than an ordinary HTTP connection, theadvantage is that once set up, downloading is efficientlymanaged even for files for which there is high demand.The downloading client connects to multiple clients thatprovide pieces of the desired file. Because of its efficiency,BitTorrent allows for distribution of substantial amounts ofdata at low cost, particularly since the system “scales up”automatically without having to provide extra resources.BitTorrent is currently being used for a variety of legallydistributed material, including video, sound, and textualcontent (see blogs and blogging, podcasting, and rss).Legal IssuesBecause of their frequent use to share copyrighted music,video, or other material, a variety of organizations of copyrightowners have sued file-sharing services and/or theirusers. The biggest problem for the courts is to determinewhether there is “substantial non-infringing use”—that is,the service is being used to exchange legal data.Some file-sharing services have been accused of distributingmalware (viruses or spyware) or of being usedto distribute material that is illegal per se (such as childpornography).In response to litigation threats, file-sharing serviceshave tended to become more decentralized, and some havefeatures that increase anonymity of users (see anonymityand the Internet) or use encryption.Further ReadingBitTorrent. Available online. URL: http://www.bittorrent.com/.Accessed September 20, 2007.Gardner, Susannah, and Kris Krug. BitTorrent for Dummies. Hoboken,N.J.: Wiley, 2006.Gnutella. Available online. URL: http://www.gnutella.com/.Accessed September 20, 2007.Roush, Wade. “P2P: From Internet Scourge to Savior.” TechnologyReview. December 15, 2006. Available online. URL: http://www.technologyreview.com/Biztech/17904. Accessed September21, 2007.Schmidt, Aernout, Wilfred Dolfsma, and Wim Keuvelaar. Fightingthe War on File Sharing. Cambridge: Cambridge UniversityPress, 2007.Silverthorne, Sean. “Music Downloads: Pirates—or Customers?”[Q&A with Felix Oberholzer-Gee]. Harvard Business SchoolWorking Knowledge. Available online. URL: http://hbswk.hbs.edu/item/4206.html. Accessed September 20, 2007.Wang, Wallace. Steal This File Sharing Book: What They Won’t TellYou About File Sharing. San Francisco: No Starch Press, 2004.file transfer protocolsWith today’s networked PCs and the use of e-mail attachmentsit is easy to send a copy of a file or files from onecomputer to another, because networks already include allthe facilities for doing so. Earlier, many PCs were not networkedbut could be connected via a dial-up modem. Toestablished the connection, a terminal program running onone PC had to negotiate with its counterpart on the othermachine, agreeing on whether data would be sent in 7- or8-bit chunks, and the number of parity bits that would beincluded for error-checking (see error correction). Thesending program would inform the receiving program asto the name and basic type of the file. For binary files(files intended to be interpreted as literal binary codes, aswith executable programs, images, and so on) the contentswould be sent unchanged. For text files, there might be theissue of which character set (7- bit or 8-bit ASCII) was beingused, and whether the ends of lines were to be marked witha CR (carriage return) character, an LF (linefeed), or both(see characters and strings).