8/4/2019 Computer Science 37 Lecture 29

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Lecture29

ComputerNetworks

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ThePurposeofNetworking

• Goal: remotedataexchange.

• Afewdesirableproperties:

– Interoperability,

– Flexibility,

– Geographicalrange,

– Scalability,

– Privacyandsecurity.

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Mattersof Protocol

Everythinginnetworkinghappensthrough protocols:

Aprotocoldetermineshowhostsshareandaccessthemedium,

Aprotocoldetermineshowhostsdealwiththemediabandwidth,

errors,flowcontrol,etc,

Aprotocoldetermineshowconnectionsbetweenhostsareestablished

andmaintained,

Aprotocoldetermineshowinformationisroutedacrossshortandlong

distances.

Etc,etc,etc…

Question: Ok,butwhatis protocol?

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ISO:InternationalStandardsOrganizationOSI:OpenSystemsInterconnection

Physical

Presentation

Session

Transport

Network 

Datalink 

Application

TheISO/OSIReferenceModelSource:ComputerNetworks,AndrewTanenbaum

Theprotocolstack :

Theideabehindthemodel: Breakupthe

designtomakeimplementationsimpler.

Eachlayerhasawell-definedfunction.

Layerspasstooneanotheronlythe

informationthatisrelevantateachlevel.

Communicationhappensonlybetween

adjacentlayers.

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TheLayersintheISO/OSIRFModel

Physical: Transmitrawbitsoverthemedium.

DataLink: Implementstheabstractionofanerrorfreemedium(handlelosses,duplication,errors,flowcontrol).

Network: Routing.

Transport: Breakupdataintochunks,sendthemdowntheprotocolstack,

receivechunks,putthemintherightorder,passthemup.

Session: Establishconnectionsbetweendifferentusersanddifferent

hosts.

Presentation: Handlesyntaxandsemanticsoftheinfo;examples:

encoding,encrypting.

Application: Protocolscommonlyneededbyapplications(cddb,http,

ftp,telnet,etc).

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Layern-1

Layern+1

Layern

CommunicationBetweenLayerswithinaHost

SAP SAP…

SAP SAP…

SAPs (serviceaccesspoints)

Note: ThisisISOterminology.

It’simportanttospecify

theservicesoffered

tohigherlayersin

thehierarchy.What

theyare+howto

usethem=interface.

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CommunicationBetweenLayersinDifferentHosts

sender receiver

data data

AH data

PH data

SH data

TH data

NH data

DHDTdata

BITSPhysical

Presentation

Session

Transport

Network 

Datalink 

Application

Physical

Presentation

Session

Transport

Network 

Datalink 

Application

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TheLayersintheTCP/IPProtocolSuiteSource:TheTCP/IPProtocolSuite,Behrouz A.Forouzan

Physical

Datalink 

IPARP RARP

ICMP IGMP

Transport TCP UDP

Network 

Session

Presentation

Application

FTP HTTP DNS NFS …

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DesignAlternatives

Point-to-pointchannels: physicallinks(asinwiring)connect

everytwocommunicatingpartieswitha“private”channel.

Broadcastchannels: communicatingpartiesareconnected

byasharedmedium;hostscanheartransmissionsnot

necessarilyaddressedtothem.

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NetworkTopology

Wecanclassifycomputernetworksaccordingtotheirtopology:

bus star

ring

mesh

hypercube

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RangeofCoverage

LAN: localareanetwork 

WLAN: wirelesslocalareanetwork 

MAN: metropolitanareanetwork 

WAN: wideareanetwork(longhaulnetwork)

Wecanalsoclassifycomputernetworksaccordingto

theirgeographicalcoverage:

Mostcommonly,we’reinterestedintheseamlessintegration

ofalltheselevels(asintheInternet).

Note: Differentlevelsuseverydifferenttechnologies.

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Technology

NetworkInterfaceCard(NIC): I/Odeviceinthecomputer

systemthatallowsittojoinanetwork.

TheNICworkswithaspecificmedium

(twisted-pair,coaxialcable,opticfiber,etc).

Host

NIC

NIC

…

Tothehost,theNICisjustanotherI/Odevice,whichhasits

ownaddress.A protocol determineshowtheNICaccesses

themedium.

Aslongasitsbus

allows,ahostcan

havemultiple

NICs.

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Ethernet

BusArbitrationbyCollisionDetection:

CarrierSenseMultipleAccesswith

CollisionDetection(CSMA/CD)

time

HostAlistensand

findsthebusidle.

HostBlistensand

findsthebusidle.

HostAstartsTX.

HostBstartsTX.

COLLISION!

HostAdetects

collision.

HostBdetects

collision.

HostBlistensand

findsthebusidle.

HostBstartsTX.

HostBcompletesTX.

HostBbacksoff.

HostAbacksoff.

Ethernethasabustopology.

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COTSEthernet

Ethernethasabustopology

Themediumcouldbeanythingthatallowsforabusimplementation.

(someoptionsareeasiertoworkwiththanothers)

Host NIC

Host NIC

Host NIC

… Hub:

Anout-of-theboxbus

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SwitchedLANs

Host NIC

Host NIC

Host NIC

… Switch:allowsformorethan

onepairtotalkatthesame

time.

Thebusbandwidthislimited,sogoforaswitchedarchitecture.

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TokenRing

Hostsconnecttoaswitchwhichclosesthering.

Mediumaccessisdonebypassingaroundatoken.

A B C

DEF

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NetworkArchitectureforPerformance

Thekeywordishierarchy:

Switch

Subnet1

Subnet2

SubnetN

…

hub

hub

hub

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NetworkArchitectureforPerformanceandCoverage

Again,thekeywordishierarchy:

Switch

Subnet1

Subnet2

SubnetN

…

Switch

Subnet1

Subnet2

SubnetN

…

…

RouterConnectiontoothernetworks

hub

hub

hub

hub

hub

hub