User Guide

Internet Security and Privacy
56 Internet Guard Dog
Why packets?
Whygothroughallthistrouble,breakingdatadownintopackets?Theanswer
lies in theorigins of TCP/IP. Like the Internet itself, it is aproductof theCold
War. The United States Department of Defense originally developed the
Internet. It was designed to ensure secure communications, even with
multiple communications network failures anticipated in the event of a
nuclearwar.TCP/IPsolvestheproblemofnetworkfailurebyassumingthat
acertain amount of noisealways exists in the network—noise referring either
to random data errors or more serious sys tem crashes. If you have ever tried
to speak in a noisy room, you know the necessity of repeating yourself—and
that is exactly what TCP/IP is designed to do. Breaking the data down into
packets allows the Internet to seek alternate routes if oneroute is inaccessible.
If a packet cannot get through or arrives damaged, the receiving computer
simply requests it again until it arrives successfully.
When you send an e-mail message, for example, it is broken into several
packets. Depending on hownoisy the networkis, each packetm a y needto be
routed over a separate route in order to find its way to it s destination.
Furthermore, network problems may cause some of the packets tobe delayed
so they arrive out of order. To compensate, TCP examines each packet as it
arrives toverify that it's OK. Once a ll thepackets are received, TC P putsthem
back in their original order. Of course, all of this happens quickly and
automatica lly, s o you will never see the process at work.
The Internet and the Web…what is the difference?
Before the Web, the Internet was mostly command-line driven and
character-based— y ou had to type in the e xact Internet address of the place
youwantedtogoatacommandline.In1989,TimBerners-LeeoftheEuropean
ParticlePhysicsLaboratoryproposedanewwaytoshareinformationoverthe
Internet. T he essential feature in Berner-Lee’s vision of the Web is that it links
documents together. When you click a link on a Web page, you are
automaticallyconnectedto anotherWeb site.This linking function,combined
with the increasing graphics abilities of home computers, transformed the
Internet into a graphically rich place, complete with video, sound, and
pictures. Through the linking of information together in a
graphically-appealing package, the Web made the Internet more attractive to
the typical consumer.
The Internet is a network of linked com puters that uses TCP/IP as its
underlying messaging system. The World Wide Web (WWW, or just “Web”
for short) is hosted by the Internet, and is an ever-expanding collection of
documents employing a special coding scheme named Hypertext Markup
Language (HTML).