Computer Networking: Principles, Protocols and Practice

282 Pages·10.747 MB·English

by  Bonaventure Olivier.

Computer Networking : Principles,
Protocols and Practice
Release 0.25
Olivier Bonaventure
October30,2011
Saylor URL: http://www.saylor.org/courses/cs402/ The Saylor Foundation
Saylor URL: http://www.saylor.org/courses/cs402/ The Saylor Foundation
Contents
1 Preface 3
2 Introduction 5
2.1 Servicesandprotocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Thereferencemodels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Organisationofthebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3 TheapplicationLayer 27
3.1 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Application-levelprotocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 Writingsimplenetworkedapplications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4 Thetransportlayer 67
4.1 Principlesofareliabletransportprotocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.2 TheUserDatagramProtocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.3 TheTransmissionControlProtocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5 Thenetworklayer 127
5.1 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
5.2 InternetProtocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
5.3 RoutinginIPnetworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
5.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
6 ThedatalinklayerandtheLocalAreaNetworks 211
6.1 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
6.2 MediumAccessControl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
6.3 Datalinklayertechnologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
6.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
7 Glossary 249
8 Bibliography 255
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9 Indicesandtables 257
Bibliography 259
Index 273
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ComputerNetworking: Principles,ProtocolsandPractice,Release0.25
Contents 1
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ComputerNetworking: Principles,ProtocolsandPractice,Release0.25
2 Contents
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1
CHAPTER
Preface
Thistextbookcamefromafrustrationofitsmainauthor. Manyauthorschosetowriteatextbookbecausethere
are no textbooks in their field or because they are not satisfied with the existing textbooks. This frustration
has produced several excellent textbooks in the networking community. At a time when networking textbooks
weremainlytheoretical,DouglasComerchosetowriteatextbookentirelyfocusedontheTCP/IPprotocolsuite
[Comer1988], a difficult choice at that time. He later extended his textbook by describing a complete TCP/IP
implementation,addingpracticalconsiderationstothetheoreticaldescriptionsin[Comer1988]. RichardStevens
approached the Internet like an explorer and explained the operation of protocols by looking at all the packets
thatwereexchangedonthewire[Stevens1994]. JimKuroseandKeithRossreinventedthenetworkingtextbooks
bystartingfromtheapplicationsthatthestudentsuseandlaterexplainedtheInternetprotocolsbyremovingone
layeraftertheother[KuroseRoss09].
The frustrations that motivated this book are different. When I started to teach networking in the late 1990s,
studentswerealreadyInternetusers,buttheirusagewaslimited.Studentswerestillusingreferencetextbooksand
spent time in the library. Today’s students are completely different. They are avid and experimented web users
who find lots of information on the web. This is a positive attitude since they are probably more curious than
theirpredecessors. ThankstotheinformationthatisavailableontheInternet,theycancheckorobtainadditional
informationaboutthetopicsexplainedbytheirteachers. Thisabundantinformationcreatesseveralchallengesfor
ateacher.Untiltheendofthenineteenthcentury,ateacherwasbydefinitionmoreknowledgeablethanhisstudents
and it was very difficult for the students to verify the lessons given by their teachers. Today, given the amount
ofinformationavailableatthefingertipsofeachstudentthroughtheInternet,verifyingalessonorgettingmore
informationaboutagiventopicissometimesonlyafewclicksaway. Websitessuchaswikipediaprovidelotsof
informationonvarioustopicsandstudentsoftenconsultthem. Unfortunately,theorganisationoftheinformation
onthesewebsitesisnotwellsuitedtoallowstudentstolearnfromthem. Furthermore,therearehugedifferences
inthequalityanddepthoftheinformationthatisavailablefordifferenttopics.
Thesecondreasonisthatthecomputernetworkingcommunityisastrongparticipantintheopen-sourcemove-
ment. Today,therearehigh-qualityandwidelyusedopen-sourceimplementationsformostnetworkingprotocols.
This includes the TCP/IP implementations that are part of linux, freebsd or the uIP stack running on 8bits con-
trollers,butalsoserverssuchasbind,unbound,apacheorsendmailandimplementationsofroutingprotocolssuch
asxorporquagga. Furthermore,thedocumentsthatdefinealmostalloftheInternetprotocolshavebeendevel-
opedwithintheInternetEngineeringTaskForce(IETF)usinganopenprocess. TheIETFpublishesitsprotocol
specificationsinthepubliclyavailableRFCandnewproposalsaredescribedinInternetdrafts.
This open textbook aims to fill the gap between the open-source implementations and the open-source network
specificationsbyprovidingadetailedbutpedagogicaldescriptionofthekeyprinciplesthatguidetheoperationof
the Internet. The book is released under a creative commons licence. Such an open-source license is motivated
by two reasons. The first is that we hope that this will allow many students to use the book to learn computer
networks. The second is that I hope that other teachers will reuse, adapt and improve it. Time will tell if it is
possible to build a community of contributors to improve and develop the book further. As a starting point, the
firstreleasecontainsallthematerialforaone-semesterfirstupperundergraduateoragraduatenetworkingcourse.
Asofthiswriting,mostofthetexthasbeenwrittenbyOlivierBonaventure. LaurentVanbever,VirginieVanden
3
Saylor URL: http://www.saylor.org/courses/cs402/ The Saylor Foundation
ComputerNetworking: Principles,ProtocolsandPractice,Release0.25
Schriek, Damien Saucez and Mickael Hoerdt have contributed to exercises. Pierre Reinbold designed the icons
usedtorepresentswitchesandNipaulLonghasredrawnmanyfiguresintheSVGformat. StephaneBortzmeyer
sent many suggestions and corrections to the text. Additional information about the textbook is available at
http://inl.info.ucl.ac.be/CNP3
4 Chapter1. Preface
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2
CHAPTER
Introduction
When the first computers were built during the second world war, they were expensive and isolated. However,
after about twenty years, as their prices gradually decreased, the first experiments began to connect computers
together. Intheearly1960s,researchersincludingPaulBaran,DonaldDaviesorJosephLickliderindependently
published the first papers describing the idea of building computer networks [Baran] [Licklider1963] . Given
the cost of computers, sharing them over a long distance was an interesting idea. In the US, the ARPANET
started in 1969 and continued until the mid 1980s [LCCD09]. In France, Louis Pouzin developed the Cyclades
network [Pouzin1975]. Many other research networks were built during the 1970s [Moore]. At the same time,
thetelecommunicationandcomputerindustriesbecameinterestedincomputernetworks. Thetelecommunication
industry bet on the X25. The computer industry took a completely different approach by designing Local Area
Networks (LAN). Many LAN technologies such as Ethernet or Token Ring were designed at that time. During
the 1980s, the need to interconnect more and more computers led most computer vendors to develop their own
suiteofnetworkingprotocols. Xeroxdeveloped[XNS],DECchoseDECNet[Malamud1991],IBMdeveloped
SNA[McFadyen1976],MicrosoftintroducedNetBIOS[Winston2003],ApplebetonAppletalk[SAO1990]. In
theresearchcommunity,ARPANETwasdecommissionedandreplacedbyTCP/IP[LCCD09]andthereference
implementationwasdevelopedinsideBSDUnix[McKusick1999]. UniversitieswhowerealreadyrunningUnix
couldthusadoptTCP/IPeasilyandvendorsofUnixworkstationssuchasSunorSiliconGraphicsincludedTCP/IP
intheirvariantofUnix. Inparallel,theISO,withsupportfromthegovernments,workedondevelopinganopen
1 Suiteofnetworkingprotocols. Intheend,TCP/IPbecamethedefactostandardthatisnotonlyusedwithinthe
research community. During the 1990s and the early 2000s, the growth of the usage of TCP/IP continued, and
today proprietary protocols are seldom used. As shown by the figure below, that provides the estimation of the
numberofhostsattachedtotheInternet,theInternethassustainedlargegrowththroughoutthelast20+years.
Figure2.1: EstimationofthenumberofhostsontheInternet
1OpeninISOtermswasincontrastwiththeproprietaryprotocolsuiteswhosespecificationwasnotalwayspubliclyavailable. TheUS
governmentevenmandatedtheusageoftheOSIprotocols(seeRFC1169),butthiswasnotsufficienttoencouragealluserstoswitchtothe
OSIprotocolsuitethatwasconsideredbymanyastoocomplexcomparedtootherprotocolsuites.
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Recent estimations of the number of hosts attached to the Internet show a continuing growth since 20+ years.
However, although the number of hosts attached to the Internet is high, it should be compared to the number
of mobile phones that are in use today. More and more of these mobile phones will be connected to the Inter-
net. Furthermore, thanks to the availability of TCP/IP implementations requiring limited resources such as uIP
[Dunkels2003],wecanexpecttoseeagrowthofTCP/IPenabledembeddeddevices.
Figure2.2: Estimationofthenumberofmobilephones
Before looking at the services provided by computer networks, it is useful to agree on some terminology that
is widely used in networking literature. First of all, computer networks are often classified in function of the
geographicalareathattheycover
• LAN:alocalareanetworktypicallyinterconnectshoststhatareuptoafewormaybeafewtensofkilome-
tersapart.
• MAN :ametropolitanareanetworktypicallyinterconnectsdevicesthatareuptoafewhundredkilometers
apart
• WAN :awideareanetworkinterconnecthoststhatcanbelocatedanywhereonEarth2
Anotherclassificationofcomputernetworksisbasedontheirphysicaltopology. Inthefollowingfigures,physical
linksarerepresentedaslineswhileboxesshowcomputersorothertypesofnetworkingequipment.
Computer networks are used to allow several hosts to exchange information between themselves. To allow any
hosttosendmessagestoanyotherhostinthenetwork,theeasiestsolutionistoorganisethemasafull-mesh,with
a direct and dedicated link between each pair of hosts. Such a physical topology is sometimes used, especially
whenhighperformanceandhighredundancyisrequiredforasmallnumberofhosts. However,ithastwomajor
drawbacks:
• for a network containing n hosts, each host must have n-1 physical interfaces. In practice, the number of
physicalinterfacesonanodewilllimitthesizeofafull-meshnetworkthatcanbebuilt
• foranetworkcontainingnhosts, n×(n−1) linksarerequired. Thisispossiblewhenthereareafewnodes
2
inthesameroom,butrarelywhentheyarelocatedseveralkilometersapart
The second possible physical organisation, which is also used inside computers to connect different extension
cards, is the bus. In a bus network, all hosts are attached to a shared medium, usually a cable through a single
interface.Whenonehostsendsanelectricalsignalonthebus,thesignalisreceivedbyallhostsattachedtothebus.
Adrawbackofbus-basednetworksisthatifthebusisphysicallycut, thenthenetworkissplitintotwoisolated
networks. Forthisreason,bus-basednetworksaresometimesconsideredtobedifficulttooperateandmaintain,
especiallywhenthecableislongandtherearemanyplaceswhereitcanbreak. Suchabus-basedtopologywas
usedinearlyEthernetnetworks.
A third organisation of a computer network is a star topology. In such topologies, hosts have a single physical
interface and there is one physical link between each host and the center of the star. The node at the center of
thestarcanbeeitherapieceofequipmentthatamplifiesanelectricalsignal,oranactivedevice,suchasapiece
2 Inthisbook, wefocusonnetworksthatareusedonEarth. Thesenetworkssometimesincludesatellitelinks. Besidesthenetwork
technologiesthatareusedonEarth,researchersdevelopnetworkingtechniquesthatcouldbeusedbetweennodeslocatedondifferentplanets.
SuchanInterPlanetaryInternetrequiresdifferenttechniquesthantheonesdiscussedinthisbook. SeeRFC4838andthereferencestherein
forinformationaboutthesetechniques.
6 Chapter2. Introduction
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