Information Technology Reference
In-Depth Information
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(a)
(b)
FIGURE 4.10
Comparison of the Napster and FastTrack implementations of peer-to-peer file
sharing. (a) In Napster, a central server maintains the index of all files available for sharing.
Retrieving a file is a three-step process: (1) making the request to the central server, (2)
establishing a peer-to-peer connection between the sending and receiving computers, and
(3) transferring the file. (b) In FastTrack, the index of available files is distributed among
many “supernodes.” Each supernode has information about files available for sharing on
“nearby” computers. Different users connect with different supernodes.
For a computer with a broadband connection to the Internet, downloading a file from
the network is about ten times faster than uploading a file to the network. A problem
with FastTrack and other peer-to-peer networking protocols is that when one peer com-
puter shares a file with another peer computer, the file is transferred at the slower, upload
speed rather than the faster, download speed. To solve this problem, Bram Cohen devel-
oped BitTorrent [81].
BitTorrent divides a file into pieces about a quarter megabyte in length. Different
pieces of a file can be downloaded simultaneously from different computers, avoiding
the uploading bottleneck (Figure 4.11). As soon as a user has a piece of a file, the user
can share this piece with other users. Since BitTorrent gives a priority for downloads to
those users who allow uploading from their machines, users tend to be generous. As a
result, downloading speeds increase as more peers get a copy of the file. Put another way,
downloading speeds increase with the popularity of a title.
With its markedly higher downloading rates, BitTorrent has made practical the
exchange of files hundreds of megabytes long. People are using BitTorrent to down-
load copies of computer programs, television shows, and movies. Linspire, a Linux
operating system developer, reduces demand on its servers (and saves money) by us-
ing BitTorrent to distribute its software [82]. BitTorrent was also the vehicle by which
Revenge of the Sith
became available on the Internet before it appeared in movie the-
aters [83].
