lecture05-principles-goals- design goals spring 2018 rachit agarwal. ... recap: four fundamental...
TRANSCRIPT
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ComputerNetworks:ArchitectureandProtocols
CS4450
Lecture5-ThreeArchitecturalPrinciples
-DesignGoals
Spring2018RachitAgarwal
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Announcements
• Youhavebeenagreatclasssofar• Mostofyouarequietandpayingattention
• Youaregivinggreatanswers!• Evenmoreimportantly,youareaskinggreatquestions!
• Thankyou!
• Admin:
• Theofficehoursareposted,andareon!• Weareonschedule:
• ProblemSet1isposted!
• Solutionswillbereleasedin1week.
• Remember:in-classquizzescanhappenatanytime
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MoreAnnouncements
• Threequestions:• Doanyofyouconsideryourselfh4x0r?• Doyoufeellikeyouhavetoomuchtimeinyourlife?
• Wouldyouliketogetexposedtonetworkingresearch?
• Ifyes,talktome—Iamwillingtotakeontwoundergradresearchers
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ContextforToday’sLecture
• Sofar,wehavediscussedseveralhigh-levelconcepts• Networksharing• End-to-endworkingoftheInternet• Addressing,Routing,Switch/Routerfunctionality,etc.
• And,havediveddeepintoseveraltopics:• Circuitswitchingandpacketswitching(especiallythe“why”)• Delays(transmission,propagation,queueing)
• Youknowmoreaboutcomputernetworksthanyoumayrealize!
• Today:Laythefoundationforrestofthecourse
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GoalsforToday’sLecture
• Threearchitecturalprinciples:• Layering• End-to-endprinciple• FateSharingprinciple
• Designgoalsforcomputernetworks:
• Eightofthem
• Wewillcomebacktotheseoverandoveragain
• Almosteverylectureinthesemester
• Beforewestart,letmeoutrightlyadmit….
• FirsttimeIlearntthese,Isaid—whatthe@#$%….
• …thereareeasierwaystotorturestudents!• Now,thesehavebecometheguidingprinciplesofmycareer!
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Quickrecapfromlastlecture
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• Locatingthedestination:Naming,addressing
• MappingofnamestoaddressesusingDomainNameSystem
• Findingapathtothedestination:Routing• Distributedalgorithmthatcomputesandstoresroutingtables
• Sendingdatatothedestination:Forwarding• Inputqueues,virtualoutputqueues,outputqueues• Enablers:Packetheader(address),androutingtable(outgoinglink)• Queueingdelay:dependenton“networkload”
• Reliability:Failurehandling• Notmuchdiscussion,butthequestion:hostsornetworks?
Recap:fourfundamentalproblems!
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Recap:thefinalpieceinthestory—Hostnetworkstack
OfSocketsandPorts
• Whenaprocesswantsaccesstothenetwork,itopensasocket,whichisassociatedwithaport
• Socket:anOSmechanismthatconnectsprocessestothenetworkingstack
• Port:numberthatidentifiesthatparticularsocket
• TheportnumberisusedbytheOStodirectincomingpackets
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• Applicationopensasocketthatallowsittoconnecttothenetworkstack
• MapsnameofthewebsitetoitsaddressusingDNS
• Thenetworkstackatthesourceembedstheaddressandportforboththesourceandthedestinationinpacketheader
• Eachrouterconstructsaroutingtableusingadistributedalgorithm
• Eachrouterusesdestinationaddressinthepacketheadertolookuptheoutgoinglinkintheroutingtable
• Andwhenthelinkisfree,forwardsthepacket
• Whenapacketarrivesthedestination:
• Thenetworkstackatthedestinationusestheporttoforwardthepackettotherightapplication
Recap:theend-to-endstory
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Questions?
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ThreeArchitecturalPrinciples
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• Howtobreaksystemintomodules?
• Classicdecompositionintotasks
• Wherearemodulesimplemented?
• Hosts?• Routers?• Both?
• Whereisstatestored?
• Hosts?• Routers?• Both?
NetworkModularityDecisions
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• Howtobreaksystemintomodules
• Layering
• Wherearemodulesimplemented
• End-to-EndPrinciple
• Whereisstatestored?• Fate-Sharing
Leadstothreedesignprinciples
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Layering
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• Bitsonwire
• Packetsonwire
• Deliverpacketstohostsacrosslocalnetwork
• Deliverpacketstohostacrossnetworks
• Deliverpacketsreliably,tocorrectprocess
• Dosomethingwiththedata
Breakdownend-to-endfunctionalityintotasks
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• Bitsonwire
• Packetsonwire
• Deliverpacketstohostsacrosslocalnetwork
• Deliverpacketstohostacrossnetworks
• Deliverpacketsreliably,tocorrectprocess
• Dosomethingwiththedata
Breakdownend-to-endfunctionalityintotasks
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• Bitsonwire(Physical)
• Packetsonwire
• Deliverpacketstohostsacrosslocalnetwork(Datalink)
• Deliverpacketstohostacrossnetworks(Network)
• Deliverpacketsreliably,tocorrectprocess(Transport)
• Dosomethingwiththedata(Application)
ResultingModules(Layers)
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• Application:Providingnetworksupportforapps
• Transport(L4):(Reliable)end-to-enddelivery
• Network(L3):Routing
• Datalink(L2):Localdelivery(forwarding)
• Physical(L1):Bitsonwire
FiveLayers(Top-Down)
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• Akindofmodularity
• Functionalityseparatedintolayers• Layerninterfaceswithonlylayern-1andlayern+1
• Hidescomplexityofsurroundinglayers
Layering
Builtontopofreliabledelivery
Builtontopofbest-effortforwarding
Builtontopofbest-effortrouting
Builtontopofphysicalbittransfer
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Anend-to-endviewofthelayers
• Application:Providingnetworksupportforapps• Transport(L4):(Reliable)end-to-enddelivery• Network(L3):Routing• Datalink(L2):Localdelivery(forwarding)• Physical(L1):Bitsonwire
Whydoesthepacketgoallthewaytonetworklayerateachhop?
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Questions?
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• Howtobreaksystemintomodules?
• Layering
• Wherearemodulesimplemented?
• End-to-EndPrinciple
• Whereisstatestored?
• Fate-Sharing
ThreeInternetDesignPrinciples
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• Layersaresimpleifonlyonasinglemachine
• Juststackofmodulesinteractingwiththoseabove/below
• Butweneedtoimplementlayersacrossmachines
• Hosts• Routers/switches
• Whatgetsimplementedwhere?Andwhy?
DistributingLayersacrossNetwork
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• Bitsarriveonwire,mustmakeituptoapplication
• Therefore,alllayersmustexistathost!
WhatgetsimplementedonHost?
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• Bitsarriveonwire• Physicallayernecessary
• Packetsmustbeforwardedtonextrouter/switch
• Datalinklayernecessary
• Routersparticipateinglobaldelivery• Networklayernecessary
• Routersdonotsupportreliabledelivery• Transportlayer(andabove)notsupported• Why?
WhatgetsimplementedonRouter?
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• Lowerthreelayersimplementedeverywhere
• Toptwolayersonlyimplementedathosts
Visualizingwhatgetsimplementedwhere
Endhost
Router/switch
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• Layeringdoesn'ttellyouwhatserviceseachlayershouldprovide
• Whatisaneffectivedivisionofresponsibilitybetweenvariouslayers?
Butwhyimplementedthisway?
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Ifafuncdoncancompletelyandcorrectlybeimplementedonlywiththeknowledgeandhelpoftheapplicadonstandingattheendpointsofthecommunicadonsystem,
thenprovidingthatfuncdonasafeatureofthecommunicadonsystemitselfisnotpossible.
Somedmesprovidinganincompleteversionofthatfuncdonasafeatureofthecommunicadonsystemitselfmaybeusefulasaperformanceenhancement.
End-to-endPrinciple
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End-to-endPrinciple:anexample
• Supposeeachlinklayertransmissionisreliable
• Doesthatensureend-to-end(application-to-application)reliability?
• Supposenetworklayerisreliable• Doesthatensureend-to-end(application-to-application)reliability?
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Ifafuncdoncancompletelyandcorrectlybeimplementedonlywiththeknowledgeandhelpoftheapplicadonstandingattheendpointsofthecommunicadonsystem,
thenprovidingthatfuncdonasafeatureofthecommunicadonsystemitselfisnotpossible.
Somedmesprovidinganincompleteversionofthatfuncdonasafeatureofthecommunicadonsystemitselfmaybeusefulasaperformanceenhancement.
End-to-endPrinciple:letsreadagain
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Assumethecondidon(IF)holds.Then,
• End-to-endimplementation• Correct• Generalized,andsimplifieslowerlayers
• In-networkimplementation• Insufficient• Mayhelp—orhurt—performance
End-to-endPrinciple(Interpretation)
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Whatdoestheendmean?
End-to-endPrinciple(Interpretation)
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• Everyoneknowswhatitis• So,youmust!
• Everyonebelievesit• So,youmust!
• Nobodyknowswhatitmeans
• Wearealldoomedanyways.
End-to-endPrinciple(Threethingstoknow)
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Questions?
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• Howtobreaksystemintomodules?
• Layering
• Wherearemodulesimplemented?
• End-to-EndPrinciple
• Whereisthestatestored?
• Fate-sharing
ThreeInternetDesignPrinciples
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• Notethattheend-to-endprinciplereliedon“fate-sharing”• Invariantsonlybreakwhenendpointsthemselvesbreak
• Minimizethedependenceonothernetworkelements
• Thisshoulddictateplacementofstate
Fate-Sharing
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• Whenstoringstateinadistributedsystem,colocateitwithentitiesthatrelyonthatstate
• Onlywayfailurecancauselossofthecriticalstateisiftheentitythatcaresaboutitalsofails…• …inwhichcaseitdoesn’tmatter
• Oftenarguesforkeepingnetworkstateatendhostsratherthaninsiderouters• E.g.,packetswitchingratherthancircuitswitching
GeneralPrinciple:Fate-Sharing
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Questions?
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• Howtobreaksystemintomodules
• Dictatedbylayering
• Wheremodulesareimplemented
• DictatedbyEnd-to-EndPrinciple
• Wherestateisstored
• DictatedbyFateSharing
DecisionsandtheirPrinciples
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FromArchitecturetoDesign:
DesignGoals
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• Wroteapaperin1988thattriedtocapturewhytheInternetturnedoutasitdid
• Itdescribedanorderedlistofprioritiesthatinformedthedecision
• Whatdoyouthinkthoseprioritieswere?
DavidClark
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• Connectexistingnetworks
• Robustinfaceoffailures
• Supportmultipletypesofdeliveryservices
• Accommodateavarietyofnetworks
• Allowdistributedmanagement
• Easyhostattachment
• Costeffective
• Allowresourceaccountability
InternetDesignGoals(Clark’88)
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Wantoneprotocolthatcouldbeusedtoconnectanypairof(existing)networks
• Differentnetworksmayhavedifferentneeds
• Forsome:reliabledeliverymoreimportant
• Forothers:performancemoreimportant
• Butthereisoneneedthateverynetworkhas:connectivity
• TheInternetProtocol(IP)isthatunifyingprotocol• All(existing)networksmustbeabletoimplementit
#1:ConnectExistingNetworks
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Aslongasnetworkisnotpartitioned,twohostsshouldbeabletocommunicate(eventually)
• Musteventuallyrecoverfromfailures
• Verysuccessfulinthepast;unclearhowrelevantnow• Availabilityisbecomingincreasinglyimportantthanrecovery
#2:RobustinFaceofFailures
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Differentdeliveryservices(applications)shouldbeabletoco-exist
• Alreadyimpliesanapplication-neutralframework
• Buildlowestcommondenominatorservice
• Again:connectivity• Applicationsthatneedreliabilitymayuseit
• Applicationsthatdonotneedreliabilitycanignoreit
• Thisisn’tasobviousasitseems…
• Whatwouldapplicationsin2050need?
#3:SupportMultipleTypesofDeliveryServices
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Questions?
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Mustbeabletosupportdifferentnetworkswithdifferenthardware
• Incrediblysuccessful!• Minimalrequirementsonnetworks
• Noneedforreliability,in-order,fixedsizepackets,etc.• Aresultofaimingforlowestcommondenominator
• Again:Focusonconnectivity• Letnetworksdospecificimplementationsforotherfunctionalities
• Automaticallyadapt:WiFi,LTE,3G,4G,5G….
#4:VarietyofNetworks
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Noneedtohaveasingle“vantage”pointtomanagenetworks
• Bothacurseandablessing• Importantforeasydeployment
• Makesmanagementhardtoday
• Recenteffortshaveimprovedmanagementofindividualnetworks
• ButnoattempttomanagetheInternetasawhole…
• Whatmightmakethiscomplex?
#5:DecentralizedManagement
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Themechanismthatallowshoststoattachtonetworksmustbemadeaseasyaspossible,butnoeasier
• Clarkobservesthatcostofhostattachmentmaybehigherbecausehostshadtobesmart
• Buttheadministrativecostofaddinghostsisverylow,whichisprobablymoreimportant
• Plug-and-playkindofbehavior…
• Andnowmosthostsaresmartforotherreasons• Sothecostisactuallyminimal…
#6:EasyHostAttachment
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Makenetworksascheapaspossible,butnocheaper
• Cheaperthancircuitswitchingatlowend
• Moreexpensivethancircuitswitchingathighend
• Notabadcompromise:
• Cheapwhereitcounts(low-end)• Moreexpensiveforthosewhocanpay…
#7:CostEffective
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Eachnetworkelementmustbemadeaccountableforitsresourceusage
• Failure!
#8:ResourceAccountability
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“Werejectkings,presidentsandvoting.Webelieveinroughconsensusandrunningcode.”
--DavidClark
InternetMotto
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• Buildsomethingthatworks
• Connectexistingnetworks
• Robustinfaceoffailures
• Supportmultipletypesofdeliveryservice
• Accommodateavarietyofnetworks
• Allowdistributedmanagement
• Easyhostattachment
• Costeffective
• Allowresourceaccountability
RealGoals
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• Whatgoalsaremissingfromthislist?
• Suggestions?
• Whatwouldtheresultingdesignlooklike?
Questionstothinkabout
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• Performance
• Security• Resiliencetoattacks(denial-of-service)• Endpointsecurity• Trackingdownmisbehavingusers
• Privacy
• Availability
• Resourcesharing(fairness,etc.)
• ISP-levelconcerns• Economicissuesofinterconnection
Someofthemissingissues
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Questions?
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• Beginningof“Designofcomputernetworks”
• StartwithLayer1andLayer2• Physicalbits(verylittle)• Localbest-effortforwarding• Lotofinterestingaspects• Lotofgroupactivities• …
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