Transcript Computer Networks and Internets with Internet Applications, 4e By Douglas E. Comer © 2007 Pearson Education Inc., Upper Saddle River, NJ.

Computer Networks and Internets with
Internet Applications, 4e
By Douglas E. Comer
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 1
Introduction
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1.1 Growth Of Computer Networking (1)
• NW is used in everywhere such as business, billing,
advertising, federal, state and local government offices.
• Computer networks (NW) have been growing explosively
and its impact include:
– More and more individual and business use
networks to exchange data
– Network industry has emerged to develop NW
technologies, products, and services
– Network has produced a demand in all industries for
network professionals to plan, acquire, install,
operate, and manage the hardware (HW) and
software (SW) systems related to networks
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1.2 Complexity In Network Systems (1)
• Computer NW is a complex subject:
– Many technologies exist, and each has features that distinguish
it from the others
– Multiple organizations have created network standards
independently, which are not all compatible
– Many companies have created commercial network products
and services that use the technologies in unconventional ways
– Finally, network is complex because multiple technologies exist
that can be used to interconnect two or more network
• As a result, many combinations of NW are possible
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1.2 Complexity In Network Systems (2)
• NW can be especially confusing to a beginner
– because there is no single underlying theory that explains the
relationship among all parts.
– Because multiple organizations define NW technologies and
standards, multiple terms exist for a given concept
– Professionals often use a technical term from one technology
when referring to an analogous feature of another
– In addition to a large set of terms and acronyms that contains
many synonyms
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1.3 Mastering The Complexity
• To master the complexity
– one must look beyond the details and concentrate on concepts
• For example (Ex):
– although it is not important to understand the details of wires
used to connect computers to a specific NW
– it is important to understand the few basic categories of wiring
schemes that exist and the advantages of each
• Similarly,
– although it is not important to learn the details of how a particular
communication protocol handles a congested NW
– it is important to know what congestion is and why it must be
handled
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1.4 Concepts And Terminology
• The text focuses on concepts and avoids unnecessary
detail
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explains the purpose of each network technology
gives the advantages and disadvantages
describes some of the consequences of using the technology
uses analogies and illustrations to simplify explanations
introduces networking terminology
notes popular abbreviations and synonyms that professionals
use
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1.5 The Value Of Hands-On Experience
• This text provides a conceptual overview of the material that is
essential for a beginner:
– deep understanding can only result from personal experience
• Readers are strongly encouraged to gain as much hands-on
experience, Possibilities include:
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building application programs that communicate over a NW
configuring network systems
observing protocols in action
and measuring system performance
• The companion lab text, “Hands-On Networking”, contains many
suggestions for experiments and projects
– Some of the exercises throughout the text refer to “Hands-On
Networking”
– and recommend specific experiments that will help the reader gain
deeper appreciation of the material
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 2
Motivation
and
Tools
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2.1 Introduction
This chapter
• discusses the size and rapid growth of the Internet, and
• introduces a few basic tools that can be used to explore
NW
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2.2 Resource Sharing (1)
• Some of the earliest computer network were built to
extend existing computing facilities.
– NW were devised that allowed multiple computers to access a
shared peripheral device such as a printer or a disk
• Main motivation for the first network were share largescale computational power
• The U.S. Department of Defense (DoD)
– Advanced Research Projects Agency (ARPA) was concerned
about the lack of high-powered computers
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2.2 Resource Sharing (2)
• Many of the ARPA research projects needed access to
the latest equipment
– Each research group wanted one of each new computer type
• By the latter 1960s, it became obvious that the ARPA
budget could not keep up with demand
– As an alternative, ARPA started investigating data NW
– The agency decided to give each group one computer
– Interconnect the computer with a data NW
• The ARPA NW research turned out to be revolutionary
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2.3 Growth Of The Internet
• The Internet has grown from the early research
prototype
– to a global communication system that reaches all countries
• Figure 2.1 illustrates how the Internet has grown
– The figure contains a graph of the number of computers attached
to the Internet as a function of the years from 1981-2003
• When plotted on log-scale as in Figure 2.2
– the growth appears approximately linear
• meaning that the Internet has experienced exponential growth over
two decades
– The Internet has been doubling in size every 9-12 months
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2.4 Probing The Internet (1)
• The program begins by walking through the Domain Name System
(DNS),
– system that stores names for computers and then uses a program that
tests to see whether the computer is currently online
• Tools used to probe the Internet are also available to users
• One of the simplest probing tools is a program known as ping:
– Ex: ping www.netbook.cs.purdue.edu
• The ping program sends a message to the specified computer and
then waits a short time for a response.
– If a response arrives, ping reports to the user that the computer is alive
– otherwise, ping reports that the computer is not responding
– Ex: www.netbook.cs.purdue.edu is alive
• Figure 2.3 shows an example of ping output with the timing and
repetition options turned on
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2.4 Probing The Internet (2)
• In Figure 2.3, ping sends one request each second
– and produces one line of output for each response received
• The output tells the size of the packet received
– the sequence number, and the round-trip time in milliseconds
• When the user interrupts the program,
– ping produces a summary that specifies the number of packets
sent and received
– packet loss
– and the minimum
– mean
– and maximum round-trip times
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2.5 Interpreting A Ping Response (1)
• When no response is received, ping cannot help
determine the reason:
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The remote computer could be turned off
disconnected from the NW
its NW interface could have failed
SW running may not respond to ping
the local computer could be disconnected from the NW
the NW to which the remote computer attaches could have failed
failure of an intermediate computer or NW
Finally, ping sometimes fails because the NW has become so
congested with traffic that delays are unreasonably long
• Ping has no way to determine the cause of the problem
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2.5 Interpreting A Ping Response (2)
• Another reason why ping may fail to generate a
response is less subtle:
– some companies configure their site to reject ping packets
• The motivation for disabling ping is security:
– if a corporation allows ping traffic to enter its site
• the site becomes susceptible to a denial-of-service or flooding
attack
– so many ping packets arrive that the company's NW and
computers cannot respond to legitimate requests
– To avoid such attacks,
• the company merely rejects ping packets before they enter
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2.6 “Real” Internet delays and routes
• What do “real” Internet delay & loss look like?
• Traceroute program: provides delay measurement from source to
intermediate router along end-end Internet path towards destination.
For all i:
– sends three packets that will reach router i on path towards destination
– router i will return packets to sender
– sender times interval between transmission and reply.
3 probes
3 probes
3 probes
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2.6 “Real” Internet delays and routes
traceroute: gaia.cs.umass.edu to www.eurecom.fr
Three delay measurements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms
4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms
5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms trans-oceanic
8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms
link
9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms
16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms
17 * * *
* means no response (probe lost, router not replying)
18 * * *
19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
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2.6 Tracing A Route (1)
• Figure shows the output from traceroute with the
destination www.eurecom.fr
• traceroute provides more information than ping
– Each line corresponds to each of intermediate computers
– and one corresponds to the final destination itself
– traceroute prints the name of the intermediate computer, and
gives the minimum, average, and maximum round-trip times
• traceroute cannot be used for all destinations
– NW administrators may choose to disable it
• to prevent outsiders from obtaining detailed information about their
architecture
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2.6 Tracing A Route (2)
• Figure illustrates another features of traceroute:
– a report of packet loss
• The line with asterisks indicates two of three probes
received no response (i.e., packets were lost)
• In a later test, all probes were received successfully
• We can conclude that the loss was a temporary
condition
– probably caused by congestion on one of the paths between the
source and destination
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