Introduction to Computer ScienceCSCI109

AndrewGoodneyFall2018

China– Tianhe-2

Lecture9:NetworksOctober29th,2018

Schedule

1

Midterm Scores

u 30questions,1pointeachu Maxscore:30

u Minscore:u Mean:

2

Computer Networks

u Computernetworkseverywhere!v HeadedtowardsIoT (*everything*electroniconanetwork)

u Howdidwegethere?

3

1960’s

4

u 1960’sv ”Mainframe”computers=$$MMv UniversitiesandGovernmentfacilitiesinstallatdesignatedplaces

(UIUC,UCSD,etc.)v Allowremoteaccesstoresearchersacrossthecountrytobetterutilizev Mainframes– 100%busy

1960’s Remote Access

u DumbTerminalsandanalogmodems

5

Phone call

1960’s Problems

u Anyissueswiththismodel?v Analogconnectionsslow(300BPS)v Onlysomanyphonelinesv Longdistancecallswereexpensive

6

Digital Circuits

u Midtolate1960’sdigitalcircuitsimprovethingsu Also,computer-to-computercommunication

7

mainframe

(smaller) mainframe

64kbpsdigital circuit

(smaller) mainframe

(smaller) mainframe

Late 1960’s

u Terminalsinoffice->localmainframeu Localmainframe->digitalcircuit->bigmainframe

u Keyobservations:v Multipleusersv Computer-to-computer

8

7

Late 1960’s

u Stillnotideal…u Connectionsare“circuitswitched”(dedicated)betweensourceanddestination

u Whathappenswhennooneisusingthecircuit?

u Reallyonlyonedumb-terminalcanuseconnectionatatimeu Dohumanstimesharewell?

u Digitalcircuitsstillexpensiveu Also,computer-to-computeridentifiedaskeyapplication

9

Design Goals

u Needawayto:v Connectusersatterminalstoremotecomputersv Connectmanyusersatthesametime(fairly)v Allowcomputer-to-computercommunicationsv Maximizetheutilizationofexpensivedigitallinks

10

Solution: the packet

u Whatisapacket?u Chunksofuserdata

u AndVERYIMPORTANTLYu Meta-data(header)aboutwherethepacketisfrom,andwhereitneedstogo

11

How do packets solve the problem?

u Nowwecanbuild“packet-switched”networks

12

64kbpsdigital circuit

= packet switch

Packet Switching

u Whatdowehavenow?u Terminalsconnecttolocalcomputers

u Dataisbrokenupintopacketssenttoswitchu Cangotootherlocalcomputers

u Or,switchcan”forward”packettoremoteswitch

u Remoteswitchsendspackettobig,expensivemainframeu Mainadvantage:linksaresharedamongstallusers,orcomputer-to-computercommunication

u Nothingisdedicatedtooneconnection13

Packet Switching Advantage

u Sharingandutilizationismainadvantageofpacketswitching

14

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

A bit more history…

u Howdidwemakethejumptopacketswitching?u Ideacameaboutlate1960’s

u ARPA(AdvancedResearchProjectsAgency)v US-DODwantedpacketswitchedcomputer-to-computernetworksv SeenascrucialtoColdWareffort

u Thisisqueueingtheory,packetswitchingisalotlikelinesatamusementpark/grocerystore

15

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

16

Interface Message Processor

17

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)

18

Modern Networks

u Howdowebuildmodernnetworks?u TheInternetisanetworkofnetworks!

u Internetedge:v Accessnetworksconnectshoststothe

networkv Accessnetworksconnecttonetworksin

thecore

u InternetCore:v Networksofnetworks(ISPs)connecting

together19

mobile network

global ISP

regional ISP

home network

institutionalnetwork

More Terminology

u AccessnetworksareLocalAreaNetworks(LANs)v Host(desktops,laptops,servers)v Switchesv Routers

u Routersarespecialpacketswitchesthatbridge(connect)multipleLANs,orfromLANtoWAN

u WAN=WideAreaNetworkv ConnectsfromLANtonetworkcore

20

More Terminology

u NetworkCorev LANsandWANconnectionsfromaccessnetworkstoISPsv AndbetweenISPs

21

How do we build modern networks

u Firstlookataccessnetworksv Howdoweconnecthosts(pc,laptop,server,phones)tothenetwork

22

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

23

Home Network

24

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

Institutional Network

25

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

Wireless Network

u Wirelessaccesstophones,etcu Providedbycellularoperators

u 2G(dead?),3G,4G,now5Gu Speedsvary

v 100’skilobits/s(2G)v 5G300Mbps+

26

to Internet

WAN Links

u Howtoconnectaccessnetworkstothecore?

27

DSL

u DigitalSubscriberLine(DSL)v Usesexistingphonelinesv 10- 100Mbpsv Homesandsmallbusinesses

28

central office

ISP

telephonenetwork

DSLAM

voice, data transmittedat different frequencies over

dedicated line to central office

DSLmodem

splitter

DSL access multiplexer

Cable Modem

29

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

Fiber Optical WAN Links

u Institutionalnetworks,ISPtoISPu Laserlightonopticalfiber

u Speeds100Mto100Gu Somehomes!

v ATTFiber,GoogleHome,otherluckypeople!

30

Physical Links

u Communicationlinkscanbemadeupofdifferentphysicalmedia,withdifferentproperties

31

Twisted Pair

u TwistedPair:twocopperwirestwistedaroundeachotheru Examples:oldphonelines,CAT5(6,7)Ethernetcables

u Bandwidth:100Mto2.5Gperpairu Usuallymultiplepairspercable

32

Coaxial cable

u Coaxialcable:centerconductorsurroundedbyinsulationthentubularshield

u Carriesradiofrequencysignals

u “CableTV”(cablemodem)wiresinyourhouse,DS3othersimilartelcoservices

u 100Mto10Gpercable

33

Fiber Optic Cable

u Extremelythinglasscablescalledstrandsu Laserlightusedtotransmitdatau Cancarrymultiplewavelengthsatonce(i.e.multiple”colors”)

u Essentiallyunlimitedbandwidthperstrandu Typically1Gto100Gperwavelengthu 96wavelengthsperstrandu Shortdistance:Ethernetoncampusu Longdistance,underseau Propagatesat60%speedoflight

34

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

35

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

Internet Structureu Toenableend-to-endcommunication,allaccessnets(millions)mustinterconnect… how?

37

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

Internet Structureu Naïvesolution:directconnecteveryISPtogether!

38

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.

Internet Structureu Solution:OneglobalISPconnectsaccessnetshierarchicallyu Notpossible,neverhappened…

39

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

globalISP

Internet Structure

40

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

Internet Structure

u RegionalISPsarosetoconnectaccessnets(schools,businesses,homes)totheInternet

u RegionalISPsconnectedtogetheratIXPs(InternetExchangePoints)formutualeconomicbenefit

u Specializedregionalnetworksconnectsomeaccessnetworkstogetherv Educational:LosNettos (socal .edu)v Metroareanetworks:Citycharterednetworkstolowerconnection

costsforbusinesses

41

Network of Networks

42

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

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

Communicating on the Internet

u Materialsofarishowwebuildpacketswitchednetworksu Fromanotherperspective:howdoesdatagettotheotherside?

44

Protocols

u Allcommunicationonnetworksismoderatedbyoneormoreprotocols

u Humanprotocols:v ”Whattimeisit?”“Excuseme,whereis…”v Introductions,smalltalk,etc.

u NetworkProtocolsv Machinetomachine

u Networkprotocols:definetheformatandorderofmessagessentandreceivedbetweennetworkdevices,andtheactionstobetakenuponmessagetransmissionorreciept

45

Protocols

46

ahumanprotocolandacomputernetworkprotocol:

Hi

Hi

Got thetime?2:00

TCP connectionresponse

<file>time

TCP connectionrequest

Get http://www.awl.com/kurose-ross

Getting Data Across the Internet

u Fromtheprogrammersperspective,thenetworkisablack-box

u AsetofinfrastructurecomponentsthatprovideanAPItogetdatafromapptoserverandback

47

GET http://www.usc.edu/

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!

48

Flying Analogy

u Firstananalogy

49

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Flying Analogy

u Goal:getfromonecitytotheother

layers: eachlayerimplementsaservicev viaitsowninternal-layeractionsv relyingonservicesprovidedbylayerbelow

50

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

Why Layers

u Explicitstructureeasessystemdesigninhighlycomplexsystemv Addsstructuretothedesignprocess

u Modularityv Eachlayerprovidesaservicetothelayerabove,essentiallyabstracting

theconnectiondetailsawayv Layerscanbeupdatedandimprovedaslongasservicesoffereddon’t

changev EachlayerABSTRACTStheconnectionbetweendevicesatthatlayer

u Easeforapplicationprogrammersv Minimumamounttolearnv UsetheAPIprovidedbyupperlayers,lettherestsortitout 51

Data flow through layers

52

Pack

et fl

ow

abstract connection

Packet Headers

u Eachlayeraddsaheaderasapacketflowsdownu Headercontainstheinformationnecessaryforthatlayertodoitsjob

u Headerisremovedasdataflowsup

53

Closer look at the layers

54

Typically software layers

Typically hardware layers

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

55

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

Original Internet Design Goals

u “theInternet”cameoutof(D)ARPAprojectsinthe1960’s,70’sand80

u Primarydriverwastoenablecomputer-to-computercommunications,evenundernuclearattack

57

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

What about now?

u ManyofthesegoalsmadetheInternetasweknowitpossible.

u Especiallythemiddle4:u Supportmultipletypesofcommunicationservices(manydifferent

applications)

u Accommodateavarietyofnetworks(e.g.earlynetworkswerenotallEthernet,today’snetworksuseEthernetatlayer2,layer1isverydifferent)

u PermitdistributedmanagementofInternetresources(nooneentityisrequiredfornetworktofunction)

u Costeffective

59

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

60

Back to Internet History

61

Growth of the Internet

62

Switch to TCP/IP (layer 4 protocol)

63

u ARPANet switchestoTCP/IP– stillinuse!

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

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