Introduction to Computer Science · 1 Mbps link example: §1 Mb/s link §each user: • 100 kb/s...
Transcript of Introduction to Computer Science · 1 Mbps link example: §1 Mb/s link §each user: • 100 kb/s...
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Introduction to Computer ScienceCSCI109
AndrewGoodneyFall2018
China– Tianhe-2
Lecture9:NetworksOctober29th,2018
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Schedule
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Midterm Scores
u 30questions,1pointeachu Maxscore:30
u Minscore:u Mean:
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Computer Networks
u Computernetworkseverywhere!v HeadedtowardsIoT (*everything*electroniconanetwork)
u Howdidwegethere?
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1960’s
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u 1960’sv ”Mainframe”computers=$$MMv UniversitiesandGovernmentfacilitiesinstallatdesignatedplaces
(UIUC,UCSD,etc.)v Allowremoteaccesstoresearchersacrossthecountrytobetterutilizev Mainframes– 100%busy
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1960’s Remote Access
u DumbTerminalsandanalogmodems
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Phone call
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1960’s Problems
u Anyissueswiththismodel?v Analogconnectionsslow(300BPS)v Onlysomanyphonelinesv Longdistancecallswereexpensive
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Digital Circuits
u Midtolate1960’sdigitalcircuitsimprovethingsu Also,computer-to-computercommunication
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mainframe
(smaller) mainframe
64kbpsdigital circuit
(smaller) mainframe
(smaller) mainframe
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Late 1960’s
u Terminalsinoffice->localmainframeu Localmainframe->digitalcircuit->bigmainframe
u Keyobservations:v Multipleusersv Computer-to-computer
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Late 1960’s
u Stillnotideal…u Connectionsare“circuitswitched”(dedicated)betweensourceanddestination
u Whathappenswhennooneisusingthecircuit?
u Reallyonlyonedumb-terminalcanuseconnectionatatimeu Dohumanstimesharewell?
u Digitalcircuitsstillexpensiveu Also,computer-to-computeridentifiedaskeyapplication
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Design Goals
u Needawayto:v Connectusersatterminalstoremotecomputersv Connectmanyusersatthesametime(fairly)v Allowcomputer-to-computercommunicationsv Maximizetheutilizationofexpensivedigitallinks
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Solution: the packet
u Whatisapacket?u Chunksofuserdata
u AndVERYIMPORTANTLYu Meta-data(header)aboutwherethepacketisfrom,andwhereitneedstogo
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How do packets solve the problem?
u Nowwecanbuild“packet-switched”networks
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64kbpsdigital circuit
= packet switch
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Packet Switching
u Whatdowehavenow?u Terminalsconnecttolocalcomputers
u Dataisbrokenupintopacketssenttoswitchu Cangotootherlocalcomputers
u Or,switchcan”forward”packettoremoteswitch
u Remoteswitchsendspackettobig,expensivemainframeu Mainadvantage:linksaresharedamongstallusers,orcomputer-to-computercommunication
u Nothingisdedicatedtooneconnection13
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Packet Switching Advantage
u Sharingandutilizationismainadvantageofpacketswitching
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Nusers
1 Mbps link
example:§ 1Mb/slink§ eachuser:
• 100kb/swhen“active”• active10%oftime
u circuit-switching:v 10users
u packetswitching:v with35users,probability>10
activeatsametimeislessthan.0004
v with50users,P=0.01
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A bit more history…
u Howdidwemakethejumptopacketswitching?u Ideacameaboutlate1960’s
u ARPA(AdvancedResearchProjectsAgency)v US-DODwantedpacketswitchedcomputer-to-computernetworksv SeenascrucialtoColdWareffort
u Thisisqueueingtheory,packetswitchingisalotlikelinesatamusementpark/grocerystore
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Len Kleinrock/UCLA
u LenKleinrock writesPhDthesisonqueuingtheory/packetswitchingv Alltheoretical,didn’texistyet
u ARAPcontactsandsays“Heyyouseemlikejusttherightguy,here’satonofmoney,inventtheARPANet…”
u GetsfacultypositionatUCLA(~1963),embarksonproject
u WithBBN,buildsInterfaceMessageProcessor(IMP)(packetswitch)
u 49yearsago(today!),sendsfirstmessagefromUCLAtoStanford
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Interface Message Processor
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Network Terminology
u Everythingonthenetworkiseither:v Host
u sendsorreceivesdatav Packetswitch
u receivespacketsoninterfacesandforwardstootherswitchesorhostsv Communicationlink
u Physicalconnectionbetweentwodevicesthatcarriesdata
u Storeandforwardv Packetswitches“storeandforward”
u Receiveapacketu Storeit(store)u Figureoutwheretosendit(forward)
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Modern Networks
u Howdowebuildmodernnetworks?u TheInternetisanetworkofnetworks!
u Internetedge:v Accessnetworksconnectshoststothe
networkv Accessnetworksconnecttonetworksin
thecore
u InternetCore:v Networksofnetworks(ISPs)connecting
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mobile network
global ISP
regional ISP
home network
institutionalnetwork
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More Terminology
u AccessnetworksareLocalAreaNetworks(LANs)v Host(desktops,laptops,servers)v Switchesv Routers
u Routersarespecialpacketswitchesthatbridge(connect)multipleLANs,orfromLANtoWAN
u WAN=WideAreaNetworkv ConnectsfromLANtonetworkcore
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More Terminology
u NetworkCorev LANsandWANconnectionsfromaccessnetworkstoISPsv AndbetweenISPs
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How do we build modern networks
u Firstlookataccessnetworksv Howdoweconnecthosts(pc,laptop,server,phones)tothenetwork
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Ethernet LANs
u In2018(almost?)allLANsarebuiltwithEthernettechnologies
u Homes,businesses,universitycampus
u Builtwithv twistedpair(“Ethernetcable”)v Wifiv Opticalfiberv Powerlines
u Widerangeofspeedsv 10M,100M,1G,10G,40G,100G
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Home Network
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to/from headend or central office
Cable, DSL modem, or fiber
router, firewall, NAT
wired Ethernet (100 Mbps or 1G)
wireless access point (100->1000
Mbps)
wirelessdevices
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Institutional Network
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Ethernet switch
institutional mail,web servers
institutional router
institutional link to ISP (Internet)
u typicallyusedincompanies,universities,etcv 10Mbps,100Mbps,1Gbps,10Gbps,40G,100Gtransmissionratesv today,endsystemstypicallyconnectintoEthernetswitch
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Wireless Network
u Wirelessaccesstophones,etcu Providedbycellularoperators
u 2G(dead?),3G,4G,now5Gu Speedsvary
v 100’skilobits/s(2G)v 5G300Mbps+
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to Internet
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WAN Links
u Howtoconnectaccessnetworkstothecore?
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DSL
u DigitalSubscriberLine(DSL)v Usesexistingphonelinesv 10- 100Mbpsv Homesandsmallbusinesses
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central office
ISP
telephonenetwork
DSLAM
voice, data transmittedat different frequencies over
dedicated line to central office
DSLmodem
splitter
DSL access multiplexer
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Cable Modem
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data, TV transmitted at different frequencies over shared cable
distribution network
cablemodem
splitter
…cable headend
CMTS
ISP
cable modemtermination system
v HFC: hybrid fiber coax§ asymmetric: up to 1Gbps downstream transmission rate, <20 Mbps
upstream transmission ratev network of cable, fiber attaches homes to ISP router
§ homes share access network to cable headend § unlike DSL, which has dedicated access to central office
v Homes and small business
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Fiber Optical WAN Links
u Institutionalnetworks,ISPtoISPu Laserlightonopticalfiber
u Speeds100Mto100Gu Somehomes!
v ATTFiber,GoogleHome,otherluckypeople!
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Physical Links
u Communicationlinkscanbemadeupofdifferentphysicalmedia,withdifferentproperties
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Twisted Pair
u TwistedPair:twocopperwirestwistedaroundeachotheru Examples:oldphonelines,CAT5(6,7)Ethernetcables
u Bandwidth:100Mto2.5Gperpairu Usuallymultiplepairspercable
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Coaxial cable
u Coaxialcable:centerconductorsurroundedbyinsulationthentubularshield
u Carriesradiofrequencysignals
u “CableTV”(cablemodem)wiresinyourhouse,DS3othersimilartelcoservices
u 100Mto10Gpercable
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Fiber Optic Cable
u Extremelythinglasscablescalledstrandsu Laserlightusedtotransmitdatau Cancarrymultiplewavelengthsatonce(i.e.multiple”colors”)
u Essentiallyunlimitedbandwidthperstrandu Typically1Gto100Gperwavelengthu 96wavelengthsperstrandu Shortdistance:Ethernetoncampusu Longdistance,underseau Propagatesat60%speedoflight
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Wireless
u Alsoknownasradiowavesu Connectsthroughfreespace(nowires)
u Wi-fi,cellular,satellite,specializedmicrowaveservicesu Bandwidthdependsonfrequency,channelwidth,modulation,etc.(kilobitstogigabits)v 900MHz,2.4GHz,5GHz,60GHz
u Propagationaffectedbywalls,buildings,trees,ground,water,othertransmissionsources
u Somefree-spacelaserlinksexist,notverycommon
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Internet structure
u Keepsaying“networkofnetworks”u Whatdowemean?u BasicgoalofInternet“endtoend”communication
v Anyhostcantalktoanyhost
u EndsystemsconnecttoInternetviaaccessISPs(InternetServiceProviders)v Residential,businessanduniversityISPs
u AccessISPsinturnmustbeinterconnected.v Sothatanytwohostscansendpacketstoeachother
u Resultingnetworkofnetworksisverycomplexv Evolutionwasdrivenbyeconomics andnationalpolicies
u Let’stakeastepwiseapproachtodescribecurrentInternetstructure36
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Internet Structureu Toenableend-to-endcommunication,allaccessnets(millions)mustinterconnect… how?
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accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnetaccess
net
accessnet
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Internet Structureu Naïvesolution:directconnecteveryISPtogether!
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accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnetaccess
net
accessnet
connecting each access ISP to each other directly doesn’t
scale: O(N2) connections.
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Internet Structureu Solution:OneglobalISPconnectsaccessnetshierarchicallyu Notpossible,neverhappened…
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accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnetaccess
net
accessnet
globalISP
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Internet Structure
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accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnetaccess
net
accessnet
ISP B
ISP A
ISP C
IXP
IXP
peering link
Internet exchange point
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Internet Structure
u RegionalISPsarosetoconnectaccessnets(schools,businesses,homes)totheInternet
u RegionalISPsconnectedtogetheratIXPs(InternetExchangePoints)formutualeconomicbenefit
u Specializedregionalnetworksconnectsomeaccessnetworkstogetherv Educational:LosNettos (socal .edu)v Metroareanetworks:Citycharterednetworkstolowerconnection
costsforbusinesses
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Network of Networks
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accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnet
accessnetaccess
net
accessnet
ISP B
ISP A
ISP C
IXP
IXP
regional net
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Hierarchical View
u atcenter:small#ofwell-connectedlargenetworks
v “tier-1” commercialISPs (e.g.,Level3,Sprint,AT&T,NTT),national&internationalcoverage
v contentprovidernetwork(e.g,Google):privatenetworkthatconnectsitdatacenterstoInternet,oftenbypassingtier-1,regionalISPs 43
accessISP
accessISP
accessISP
accessISP
accessISP
accessISP
accessISP
accessISP
Regional ISP Regional ISP
IXP IXP
Tier 1 ISP Tier 1 ISP Google
IXP
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Communicating on the Internet
u Materialsofarishowwebuildpacketswitchednetworksu Fromanotherperspective:howdoesdatagettotheotherside?
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Protocols
u Allcommunicationonnetworksismoderatedbyoneormoreprotocols
u Humanprotocols:v ”Whattimeisit?”“Excuseme,whereis…”v Introductions,smalltalk,etc.
u NetworkProtocolsv Machinetomachine
u Networkprotocols:definetheformatandorderofmessagessentandreceivedbetweennetworkdevices,andtheactionstobetakenuponmessagetransmissionorreciept
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Protocols
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ahumanprotocolandacomputernetworkprotocol:
Hi
Hi
Got thetime?2:00
TCP connectionresponse
<file>time
TCP connectionrequest
Get http://www.awl.com/kurose-ross
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Getting Data Across the Internet
u Fromtheprogrammersperspective,thenetworkisablack-box
u AsetofinfrastructurecomponentsthatprovideanAPItogetdatafromapptoserverandback
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GET http://www.usc.edu/
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How to provide that service?
u Togetfromdevicetoserverandbackv Datamustbepacketizedv Traverseaccessnetworkà ISPà ISPà ISPà accessnetworkà
server(andback)v Travelacrossheterogeneousnetworktypes,physicallayers
u Does(should?)theprogrammerworryaboutanyofthis?
u No!Itjustworks.u Whatisthesolution?
u AseriesofAbstractions!
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Flying Analogy
u Firstananalogy
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ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
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Flying Analogy
u Goal:getfromonecitytotheother
layers: eachlayerimplementsaservicev viaitsowninternal-layeractionsv relyingonservicesprovidedbylayerbelow
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ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departureairport
arrivalairport
intermediate air-trafficcontrol centers
airplane routing airplane routing
ticket (complain)
baggage (claim
gates (unload)
runway (land)
airplane routing
ticket
baggage
gate
takeoff/landing
airplane routing
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Why Layers
u Explicitstructureeasessystemdesigninhighlycomplexsystemv Addsstructuretothedesignprocess
u Modularityv Eachlayerprovidesaservicetothelayerabove,essentiallyabstracting
theconnectiondetailsawayv Layerscanbeupdatedandimprovedaslongasservicesoffereddon’t
changev EachlayerABSTRACTStheconnectionbetweendevicesatthatlayer
u Easeforapplicationprogrammersv Minimumamounttolearnv UsetheAPIprovidedbyupperlayers,lettherestsortitout 51
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Data flow through layers
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Pack
et fl
ow
abstract connection
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Packet Headers
u Eachlayeraddsaheaderasapacketflowsdownu Headercontainstheinformationnecessaryforthatlayertodoitsjob
u Headerisremovedasdataflowsup
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Closer look at the layers
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Typically software layers
Typically hardware layers
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TCP/IP
u TCP/IPMostfamousprotocolsuiteu TCP
v Layer4protocolv Guaranteesin-orderdeliveryofpackets
u Eithertheygettotheothersideinorderu Orbothhostsknowapacketwaslost
v Offers”datastream”abstractiontolayersaboveu IP(InternetProtocol)
v Layer3protocolv All“Internet”trafficisIPtrafficv Handlesaddressing(IPaddresses)androutingofpacketsacrossthe
Internetv Noguarantees,packetsroutedbesteffortv Offershost-to-hostpacketservicetolayersabove
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Network Performance
u PerformanceMetricsforNetworksu Bandwidth- ”howfastismynetwork”
v Howmuchdata(bits)canIgettotheothersidepersecondv Appliestocommunicationlinks,aswellasacrossthenetwork
u Latency– “Howslowismynetwork”v Howlongdoesittakefordatatogetfromonesidetotheotherv Measuredintime:s,ms,us
u Loss– “Howbadismynetwork”v Fractionofpacketsthatgetdropped(don’tmakeittotheotherside)v Needstobe<<1%forgoodnetworkoperation
u Throughput– “Howgoodismynetwork”v Consideringhowbandwidth,latency,losscombinewithaprotocolto
measurehowmuchisactuallydeliveredperunittimev Thisiswhatapplicationssee 56
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Original Internet Design Goals
u “theInternet”cameoutof(D)ARPAprojectsinthe1960’s,70’sand80
u Primarydriverwastoenablecomputer-to-computercommunications,evenundernuclearattack
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Original Internet Design Goals
u Paperfor1988:“TheDesignPhilosophyoftheDARPAInternetProtocols”(DavidClark,1988).
u InPRIORITYorderu 1.Continuedespitelossofnetworkinfrastructure(degrade,butstillfunction
underattack)
u 2.Supportmultipletypesofcommunicationservices(manydifferentapplications)
u 3.Accommodateavarietyofnetworks(e.g.earlynetworkswerenotallEthernet,today’snetworksuseEthernetatlayer2,layer1isverydifferent)
u 4.PermitdistributedmanagementofInternetresources(nooneentityisrequiredfornetworktofunction)
u 5.Costeffective
u 6.Hostattachmentshouldbeeasy(onlylocalchangesrequiredtoaddhost)
u 7.Resourceaccountability(security) 58
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What about now?
u ManyofthesegoalsmadetheInternetasweknowitpossible.
u Especiallythemiddle4:u Supportmultipletypesofcommunicationservices(manydifferent
applications)
u Accommodateavarietyofnetworks(e.g.earlynetworkswerenotallEthernet,today’snetworksuseEthernetatlayer2,layer1isverydifferent)
u PermitdistributedmanagementofInternetresources(nooneentityisrequiredfornetworktofunction)
u Costeffective
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What about now?
u However,somemakethingsinterestingu 4.PermitdistributedmanagementofInternetresources
u Originallyindentedtomakethenetworkresilientinamilitaryscenario
u Now:basicallynooneisinchargev 7.Resourceaccountability
u Ifyoureadthepaper,theybasicallynevergottothisu ThisiswhyInternetsecurityissuchadisaster,itwasn’tdesignedinoriginally
u Networkagentsimplicitlytrusted,thisstillcausesproblems,seere:BGPattacks
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Back to Internet History
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Growth of the Internet
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Switch to TCP/IP (layer 4 protocol)
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u ARPANet switchestoTCP/IP– stillinuse!
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USC/ISIu InformationSciencesInstitute
v PartofUSCunderViterbiv Researchers,gradstudents,undergradsv InbeautifulMDR
u BecameprominentindevelopingearlyInternetprotocolsv IP
u “Theinternetprotocolprovidesfortransmittingblocksofdatacalleddatagramsfromsourcestodestinations,wheresourcesanddestinationsarehostsidentifiedbyfixedlengthaddresses.”
v ICMPv DomainNameSystemv SimpleMailTransportProtocol
u ICANNneeIANAv InternetAssignedNumbersAuthority->ICANNv (USCbasicallyrantheInternet) 64
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Takeaways
u Networksallowcomputerstocommunicate(exchangedata)u Dataisbrokenintopackets(smallchunks)andsentoversharedcircuits
(packetswitching)byswitchesandroutersu Internetisanetwork-of-networks– Nooneowner/regulatoru TheInternetprovidesanAPIforaservice(networking)thatletsapp
developerswritesoftware(abstraction)u Servicesareimplementedwithprotocolsthatarestackedinlayers(more
abstraction)u TCP/IPisthecoreprotocolonwhichmostinternetworkingisbasedu SecuritywasnotanimportantdesignfeaturewhentheInternetwasinvented
– we’restillpayingforthisnowu Bandwidth,latency,loss,throughputaretheimportantnetworkperformance
metricsu USC/ISIhasbeeninvolvedininventingthemodernInternetsince(notquite)
thebeginning(e.g DNS,IANA,ICANN)65