IPAddressingandForwardingCOS461:ComputerNetworks

Spring2010(MW3:00‐4:20inCOS105)

MichaelFreedman

hIp://www.cs.princeton.edu/courses/archive/spring10/cos461/

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GoalsofToday’sLecture•  IPaddresses

– DoIed‐quadnotaUon–  IPprefixesforaggregaUon

•  AddressallocaUon–  Classfuladdresses–  ClasslessInterDomainRouUng(CIDR)– GrowthinthenumberofprefixesoverUme

•  Packetforwarding–  Forwardingtables–  Longest‐prefixmatchforwarding– Whereforwardingtablescomefrom

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IPAddress(IPv4)•  Aunique32‐bitnumber•  IdenUfiesaninterface(onahost,onarouter,…)•  RepresentedindoIed‐quadnotaUon

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00001100 00100010 10011110 00000101

12 34 158 5

GroupingRelatedHosts•  TheInternetisan“inter‐network”

– Usedtoconnectnetworkstogether,nothosts– Needswaytoaddressanetwork(i.e.,groupofhosts)

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host host host

LAN 1

... host host host

LAN 2

...

router router router WAN WAN

LAN = Local Area Network

WAN = Wide Area Network

ScalabilityChallenge•  Supposehostshadarbitraryaddresses

– TheneveryrouterwouldneedalotofinformaUon

– …toknowhowtodirectpacketstowardeveryhost

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host host host ... host host host ...

router router router

1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9

1.2.3.4

1.2.3.5

forwarding table

LAN 1 LAN 2 WAN WAN

a.k.a. FIB (forwarding information base)

ScalabilityChallenge•  Supposehostshadarbitraryaddresses

– TheneveryrouterwouldneedalotofinformaUon

– …toknowhowtodirectpacketstowardeveryhost

•  BackofenvelopcalculaUons– 32‐bitIPaddress:4.29billion(232)possibiliUes– Howmuchstorage?

•  Minimum:4Baddress+2Bforwardinginfoperline

•  Total:24.58GBjustforforwardingtable– Whathappensifanetworklinkgetscut?

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StandardCSTrick

Haveascalabilityproblem?

Introducehierarchy…

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HierarchicalAddressinginU.S.Mail•  AddressingintheU.S.mail

– Zipcode:08540– Street:OldenStreet– Buildingonstreet:35– Roominbuilding:308 – Nameofoccupant:MikeFreedman

•  ForwardingtheU.S.mail– DeliverleIertothepostofficeinthezipcode– AssignleIertomailmancoveringthestreet– DropleIerintomailboxforthebuilding/room– GiveleIertotheappropriateperson

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???

HierarchicalAddressing:IPPrefixes•  IPaddressescanbedividedintotwoporUons

– Network(lei)andhost(right)•  12.34.158.0/24isa24‐bitprefix

– Whichcovers28addresses(e.g.,upto255hosts)

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00001100 00100010 10011110 00000101

Network (24 bits) Host (8 bits)

12 34 158 5

ExpressingIPprefixes

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00001100 00100010 10011110 00000101

12 34 158 5

11111111 11111111 11111111 00000000

255 255 255 0

Address

Mask

IPprefix=IPaddress(AND)subnetmask

ScalabilityImproved•  Numberrelatedhostsfromacommonsubnet

–  1.2.3.0/24ontheleiLAN–  5.6.7.0/24ontherightLAN

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host host host

LAN 1

... host host host

LAN 2

...

router router router WAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

1.2.3.0/24

5.6.7.0/24

forwarding table

EasytoAddNewHosts•  Noneedtoupdatetherouters

–  E.g.,addinganewhost5.6.7.213ontheright– Doesn’trequireaddinganewforwarding‐tableentry

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host host host

LAN 1

... host host host

LAN 2

...

router router router WAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

1.2.3.0/24

5.6.7.0/24

forwarding table

host

5.6.7.213

AddressAllocaUon

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ClassfulAddressing•  Intheoldendays,onlyfixedallocaUonsizes

– ClassA:0*•  Verylarge/8blocks(e.g.,MIThas18.0.0.0/8)

– ClassB:10*•  Large/16blocks(e.g,.Princetonhas128.112.0.0/16)

– ClassC:110*•  Small/24blocks(e.g.,AT&TLabshas192.20.225.0/24)

– ClassD:1110*•  MulUcastgroups

– ClassE:11110*•  Reservedforfutureuse

•  ThisiswhyfolksusedoIed‐quadnotaUon!14

ClasslessInter‐DomainRouUng(CIDR)

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IP Address : 12.4.0.0 IP Mask: 255.254.0.0

00001100 00000100 00000000 00000000

11111111 11111110 00000000 00000000

Address

Mask

for hosts Network Prefix

Use two 32-bit numbers to represent a network. Network number = IP address + Mask

Written as 12.4.0.0/15 Introducedin1993RFC1518‐1519

CIDR:HierarchalAddressAllocaUon

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12.0.0.0/8

12.0.0.0/16

12.254.0.0/16

12.1.0.0/16 12.2.0.0/16 12.3.0.0/16

: : :

12.3.0.0/24 12.3.1.0/24

: :

12.3.254.0/24

12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 12.253.96.0/19 12.253.128.0/19 12.253.160.0/19

: : :

• Prefixes are key to Internet scalability – Address allocated in contiguous chunks (prefixes) – Routing protocols and packet forwarding based on prefixes – Today, routing tables contain ~200,000 prefixes (vs. 4B)

Scalability:AddressAggregaUon

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Provider is given 201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider

RoutersinrestofInternetjustneedtoknowhowtoreach201.10.0.0/21.ProvidercandirectIPpackets

toappropriatecustomer.

But,AggregaUonNotAlwaysPossible

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201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

Mul0‐homedcustomer(201.10.6.0/23)hastwoproviders.OtherpartsoftheInternetneedtoknow

howtoreachthesedesUnaUonsthroughbothproviders.

ScalabilityThroughHierarchy•  Hierarchicaladdressing

–  CriUcalforscalablesystem– Don’trequireeveryonetoknoweveryoneelse–  ReducesamountofupdaUngwhensomethingchanges

•  Non‐uniformhierarchy– Usefulforheterogeneousnetworksofdifferentsizes–  IniUalclass‐basedaddressingwasfartoocoarse–  ClasslessInterDomainRouUng(CIDR)helps

•  Nextfewslides– Historyofthenumberofglobally‐visibleprefixes–  Plotsare#ofprefixesvs.Ume

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Pre‐CIDR(1988‐1994):SteepGrowth

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Growthfasterthanimprovementsinequipmentcapability

CIDRDeployed(1994‐1996):MuchFlaIer

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Effortstoaggregate(evendecreasesaierIETFmeeUngs!)

CIDRGrowth(1996‐1998):RoughlyLinear

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GooduseofaggregaUon,andpeerpressureinCIDRreport

BoomPeriod(1998‐2001):SteepGrowth

23 InternetboomandincreasedmulU‐homing

Long‐TermView(1989‐2005):Post‐Boom

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ObtainingaBlockofAddresses•  SeparaUonofcontrol

–  Prefix:assignedtoaninsUtuUon– Addresses:assignedbytheinsUtuUontotheirnodes

•  Whoassignsprefixes?–  InternetCorp.forAssignedNamesandNumbers(IANA)

•  AllocateslargeaddressblockstoRegionalInternetRegistries–  RegionalInternetRegistries(RIRs)

•  E.g.,ARIN(AmericanRegistryforInternetNumbers)•  Allocatesaddressblockswithintheirregions•  AllocatedtoInternetServiceProvidersandlargeinsUtuUons

–  InternetServiceProviders(ISPs)•  Allocateaddressblockstotheircustomers•  Whomay,inturn,allocatetotheircustomers…

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FiguringOutWhoOwnsanAddress•  Addressregistries

– PublicrecordofaddressallocaUons–  InternetServiceProviders(ISPs)shouldupdatewhengivingaddressestocustomers

– However,recordsarenotoriouslyout‐of‐date•  Waystoquery

– UNIX:“whois–hwhois.arin.net128.112.136.35”– hIp://www.arin.net/whois/– hIp://www.geektools.com/whois.php– …

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ExampleOutputfor128.112.136.35OrgName: PrincetonUniversityOrgID: PRNUAddress: OfficeofInformaUonTechnologyAddress: 87ProspectAvenueCity: PrincetonStateProv: NJPostalCode: 08540Country: US

NetRange: 128.112.0.0‐128.112.255.255CIDR: 128.112.0.0/16NetName: PRINCETONNetHandle: NET‐128‐112‐0‐0‐1Parent: NET‐128‐0‐0‐0‐0NetType: DirectAllocaUonNameServer:DNS.PRINCETON.EDUNameServer:NS1.FAST.NETNameServer:NS2.FAST.NETNameServer:NS1.UCSC.EDUNameServer:ARIZONA.EDUNameServer:NS3.NIC.FR

Comment:RegDate: 1986‐02‐24Updated: 2007‐02‐27

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Are32‐bitAddressesEnough?•  Notallthatmanyuniqueaddresses

–  232=4,294,967,296(justoverfourbillion)–  Plus,somearereservedforspecialpurposes– And,addressesareallocatedinlargerblocks

•  Myfraternity/dormatMIThadasmanyIPaddrsasPrinceton!

•  And,manydevicesneedIPaddresses–  Computers,PDAs,routers,tanks,toasters,…

•  Long‐termsoluUon:alargeraddressspace–  IPv6has128‐bitaddresses(2128=3.403×1038)

•  Short‐termsoluUons:limpingalongwithIPv4–  Privateaddresses(RFC1918):

•  10.0.0.0/8,172.16.0.0/12,192.168.0.0/16– NetworkaddresstranslaUon(NAT)– Dynamically‐assignedaddresses(DHCP)

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HardPolicyQuesUons•  Howmuchaddressspacepergeographicregion?

–  Equalamountpercountry?–  ProporUonaltothepopulaUon?– Whataboutaddressesalreadyallocated?

•  MITsUllhas>>IPaddressesthanmostcountries?

•  Addressspaceportability?–  Keepyouraddressblockwhenyouchangeproviders?–  Pro:avoidhavingtorenumberyourequipment–  Con:reducestheeffecUvenessofaddressaggregaUon

•  Keepingtheaddressregistriesuptodate?– WhataboutmergersandacquisiUons?– DelegaUonofaddressblockstocustomers?– Asaresult,theregistriesarehorriblyoutofdate

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PacketForwarding

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Hop‐by‐HopPacketForwarding•  Eachrouterhasaforwardingtable

– MapsdesUnaUonaddresses…

– …tooutgoinginterfaces•  Uponreceivingapacket

–  InspectthedesUnaUonIPaddressintheheader–  Indexintothetable– Determinetheoutgoinginterface– Forwardthepacketoutthatinterface

•  Then,thenextrouterinthepathrepeats– AndthepackettravelsalongthepathtodesUnaUon

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SeparateTableEntriesPerAddress•  IfarouterhadaforwardingentryperIPaddr

–  Matchdes0na0onaddressofincomingpacket–  …totheforwarding‐tableentry–  …todeterminetheoutgoinginterface

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host host host

LAN 1

... host host host

LAN 2

...

router router router WAN WAN

1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9

1.2.3.4

1.2.3.5

forwarding table

SeparateEntryPer24‐bitPrefix•  Iftherouterhadanentryper24‐bitprefix

–  Lookonlyatthetop24bitsofthedesUnaUonaddress–  Indexintothetabletodeterminethenext‐hopinterface

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host host host

LAN 1

... host host host

LAN

...

router router router WAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

1.2.3.0/24

5.6.7.0/24

forwarding table

SeparateEntryClassfulAddress•  Iftherouterhadanentryperclassfulprefix

– MixtureofClassA,B,andCaddresses– DependsonthefirstcoupleofbitsofthedesUnaUon

•  IdenUfythemaskautomaUcallyfromtheaddress–  Firstbitof0:classAaddress(/8)–  Firsttwobitsof10:classBaddress(/16)–  Firstthreebitsof110:classCaddress(/24)

•  Then,lookintheforwardingtableforthematch–  E.g.,1.2.3.4mapsto1.2.3.0/24–  Then,lookuptheentryfor1.2.3.0/24– …toidenUfytheoutgoinginterface

•  Sofar,everythingisexactmatching34

CIDRMakesPacketForwardingHarder•  There’snosuchthingasafreelunch

–  CIDRallowsefficientuseoflimitedaddressspace–  But,CIDRmakespacketforwardingmuchharder

•  Forwardingtablemayhavemanymatches–  E.g.,entriesfor201.10.0.0/21and201.10.6.0/23–  TheIPaddress201.10.6.17wouldmatchboth!

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201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

LongestPrefixMatchForwarding•  ForwardingtablesinIProuters

– MapseachIPprefixtonext‐hoplink(s)•  DesUnaUon‐basedforwarding

– PackethasadesUnaUonaddress– RouteridenUfieslongest‐matchingprefix– Cutealgorithmicproblem:veryfastlookups

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4.0.0.0/8 4.83.128.0/17 201.10.0.0/21 201.10.6.0/23 126.255.103.0/24

201.10.6.17 destination

forwarding table

Serial0/0.1 outgoing link

AnotherreasonFIBsgetlarge

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201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

•  Ifcustomer201.10.6.0/23preferstoreceivetrafficfromProvider1(itmaybecheaper),thenP1needstoannounce201.10.6.0/23,not201.10.0.0/21

•  Can’talwaysaggregate![See“GeographicLocalityofIPPrefixes”M.Freedman,M.Vutukuru,N.Feamster,andH.Balakrishnan.InternetMeasurementConference(IMC),2005

SimplestAlgorithmisTooSlow•  ScantheforwardingtableoneentryataUme

–  SeeifthedesUnaUonmatchestheentry–  Ifso,checkthesizeofthemaskfortheprefix–  Keeptrackoftheentrywithlongest‐matchingprefix

•  Overheadislinearinsizeoftheforwardingtable–  Today,thatmeans200,000entries!– HowmuchUmedoyouhavetoprocess?

•  Consider10Gbpsroutersand64Bpackets•  10,000,000,000/8/64:19,531,250packetspersecond•  51nanosecondsperpacket

•  Needgreaterefficiencytokeepupwithlinerate–  BeIeralgorithms– HardwareimplementaUons

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PatriciaTree(1968)•  Storetheprefixesasatree

– Onebitforeachlevelofthetree–  Somenodescorrespondtovalidprefixes–  ...whichhavenext‐hopinterfacesinatable

•  Whenapacketarrives–  TraversethetreebasedonthedesUnaUonaddress–  Stopuponreachingthelongestmatchingprefix

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0 1

00 10 11

100 101 00*

0*

11*

EvenFasterLookups•  Patriciatreeisfasterthanlinearscan

–  ProporUonaltonumberofbitsintheaddress•  Patriciatreecanbemadefaster

–  Canmakeak‐arytree•  E.g.,4‐arytreewithfourchildren(00,01,10,and11)

–  Fasterlookup,thoughrequiresmorespace•  Canusespecialhardware

–  ContentAddressableMemories(CAMs)– Allowslook‐upsonakeyratherthanflataddress

•  HugeinnovaUonsinthemid‐to‐late1990s– AierCIDRwasintroduced(in1994)– …andlongest‐prefixmatchwasamajorboIleneck

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WheredoForwardingTablesComeFrom?

•  Routershaveforwardingtables– Mapprefixtooutgoinglink(s)

•  EntriescanbestaUcallyconfigured–  E.g.,“map12.34.158.0/24toSerial0/0.1”

•  But,thisdoesn’tadapt–  Tofailures–  Tonewequipment–  Totheneedtobalanceload– …

•  Thatiswhereothertechnologiescomein…–  RouUngprotocols,DHCP,andARP(laterincourse)

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HowDoEndHostsForwardPackets?

•  Endhostwithsinglenetworkinterface–  PCwithanEthernetlink–  Laptopwithawirelesslink

•  Don’tneedtorunarouUngprotocol–  Packetstothehostitself(e.g.,1.2.3.4/32)

•  Deliveredlocally–  PacketstootherhostsontheLAN(e.g.,1.2.3.0/24)

•  Sentouttheinterface:Broadcastmedium!–  Packetstoexternalhosts(e.g.,0.0.0.0/0)

•  Sentoutinterfacetolocalgateway•  HowthisinformaUonislearned

–  StaUcse|ngofaddress,subnetmask,andgateway– DynamicHostConfiguraUonProtocol(DHCP)

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WhatAboutReachingtheEndHosts?•  HowdoesthelastrouterreachthedesUnaUon?

•  Eachinterfacehasapersistent,globalidenUfier– MAC(MediaAccessControl)address–  BurnedintotheadaptorsRead‐OnlyMemory(ROM)–  Flataddressstructure(i.e.,nohierarchy)

•  ConstrucUnganaddressresoluUontable– MappingMACaddressto/fromIPaddress– AddressResoluUonProtocol(ARP)

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host host host

LAN

...

router

1.2.3.4 1.2.3.7 1.2.3.156

Conclusions•  IPaddress

– A32‐bitnumber– Allocatedinprefixes– Non‐uniformhierarchyforscalabilityandflexibility

•  Packetforwarding–  BasedonIPprefixes–  Longest‐prefix‐matchforwarding

•  Nextlecture–  TransmissionControlProtocol(TCP)

•  We’llcoversometopicslater–  RouUngprotocols,DHCP,andARP

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