Network Layer - Courses · • Link-layer (layer 2) ... • Only one protocol at the network layer....

NetworkLayer

BradSmith

Spring2017 CE151-AdvancedNetworks 1

LogisAcs•  SignupforPiazza(piazza.com)

•  Sunday–  Firstlabdue.–  FirstlabsecAonishalfofthelabgrade.–  85pointsforthelab–  85pointsforaMendingfirstlabsecAon–  VMProblems

•  Tuesday–  Linklayerlecture

•  Thursday–  Read“End-to-EndArgument”paper–  IPv4/layerexercise–  IPv4quiz…revieworangeslides

netref.soe.ucsc.edu

InternetBackground•  TheARPAnetwasthefirstpacket-switchednetwork.•  It’ssuccessledtotheInternet•  TheARPAnetarchitecture

–  Homogeneousswitches•  InterfaceMessageProcessors(IMPs)

–  Heterogeneoushosts–  TheNetworkControlProtocol(NCP)implementedconservaAveerror,flow,

andcongesAoncontrol

•  Newtechnicalchallenges–  MoresophisAcatedeBrror,flow,andcongesAoncontrolacrossanetwork.

•  Link-layer(layer2)networkfromtoday’sperspec>ve.

TheARPAnet

Host-IMP IMP-IMP

HOST Host-Host (NCP)

application application

ARPAnetArchitecture•  Host-to-Hostprotocol

–  ImplementedintheNetworkControlProgram(NCP)–  PrimaryfuncAons:connecAonmanagement,processnaming(“sockets”),flow

control(memorymanagement)

•  Host-IMPprotocol–  PrimaryfuncAon:“bidirecAonalcommunicaAonbetweenthehostandIMP”…

basicallythehost’snetworkinterface.

•  IMP-IMPprotocol–  PrimaryfuncAons:(de-)packeAzingdata,flowcontrol(end-pointIMPs),error

detecAon,reliablydelivery(ingresstoegressIMPs),rouAngthroughIMPtopology.

GoalsoftheInternet•  New,diversenetworktechnologies

–  LANs(e.gEthernet)–  DARPApacketradionetwork(PRNET)–  DARPAsatellitenetwork(SATNET)

•  Internetgoal–  InterconnecAonofdiversenetworks,whichwecallsubnets

•  Minimal-servicenetworkmodel

•  VintCerfandRobertKahn-“AProtocolforPacketNetworkInterconnecAon”(May,1974)

Minimal-ServiceNetworkModel•  Newconcepts

–  “Gateway”interconnectsdissimilarnetworks.–  Internet-levelhostaddressing

•  NewfuncAonality–  NCPupgraded

•  End-to-end,slidingwindowerror,flow,andcongesAoncontrol•  CalledTransmissionControlProtocol(TCP)

TheInternetArchitecture

SUBNET

SUBNET A

EvoluAonofTCP/IP•  TCPoriginallymeanttoreplaceNCP

•  Problems–  MonolithicfuncAonality–  Decidedtoseparate“addressinglayer”from“streamlayer”–  Allowforfuturedatagramand/ornon-reliable(e.g.voice)protocols

•  Movefromsmartnetworktosmarthosts…“End-to-endPrinciple”

•  TCPsplitintoTCP/IP–  InternetProtocol(IP)

•  Best-effortrouAng•  Internetleveladdressing

–  TransmissionControlProtocol(TCP)•  New,end-to-enderror,flow,andcongesAoncontrol

•  WhatistheminimalsetoffuncAonalityneededtoconstructaninternet?

End-to-EndPrinciple

�If a function can completely and correctly be implemented only with the knowledge and help of the application end-points of a

communication system, then the function should not be implemented in the communication system itself (although

sometimes it may be useful to implement an incomplete version of the function in the communication system as a performance

enhancement).�

“End-to-End Arguments in System Design” by Saltzer, Reed, and Clark (‘84)

End-to-EndPrinciple•  AvoidsimpacAngapplicaAonsthatdon’tneedthisfuncAon.

•  Reducescomplexityofthenetwork…–  Dumbnetwork,smartend-points…reverseofthetelephonesystem!

•  Overall,improvesefficiencyandreliabilityofthenetwork.

•  E.g.TransportfuncAons…–  End-pointsmustbeinvolvedtoensuredatamakesittotheprocess

(corrupAonintheend-system)–  ThereforenotransportfuncAonalityinthenetwork.

•  ReasonableinterpretaAon…forfuncAonsrequiringend-pointinvolvement–  ThinktwicebeforeimplemenAnginthenetwork–  JusAfyasaperformanceenhancement–  Don’timposeaburdenonapplicaAonsthatdon’trequireit–  E.g.verylossy(wireless)links–  Doesn’tneedtobeheavy-weight…candependonend-to-endmechanisms

•  Problems–  MonolithicfuncAonality–  Reliabledeliveryahost-hostissue

InternetArchitectureFollowingposAngfromDaveCrocker(earlyInternetparAcipant)tothe“end2end-interest”mailinglist(4/11/13):

Here'sacommentthatIsentearliertoday,toanon-technicalpersonwhoisawareoftheoverallInternetAmeline,butIbelievedoesnotunderstandwhatisdisAncAveaboutInternet'architecture'.I'mcuriousaboutreacAonsonthislist,andanypossibleimprovements--includingcompletereplacement--butmoreimportantlyI'minterestedinfillinginthedetails:

TheoriginaluseofthetermInternetwastodescribeadis;nc;vetechnicaldesignforadistributed,scalabledataexchangefabric.Itsdesigncharacteris;csdifferdrama;callyfromthoseofitspredecessor,theArpanet,andfromotherrelatedefforts.

That'swhatIsent.Toprimethepumpforthedetail:

Bysaying'fabric'Imeanttodis;nguishthemechanismformovingrawdatafromtheapplica;onsthatusedit.WhatI'dclassasdis;nc;vewere•  theTCP/IPsepara>on,•  theremarkablymodestfunc>onalityofIP,eventothepointofmovingit'scontrolplanetothe

nextlevelupwithICMP,and•  con>nuingwithmodestexpecta>onsthelayerbelow(whichmadeitpossibletooperateover

anymediumincludingbirds.)Thisisusuallycharacterizedasmovingrobustnesstotheedges.

Review•  IPgoals

–  interconnectdiversenetworktechnologies,makingminimalassumpAonsoftheunderlyingnetworks

–  implementtheminimalsetoffuncAonalityneededtoconstructaninternet…inthissenseIPisthewaistofthenetworkprotocolstackhourglass

•  IPimplementsadatagram,packet-switchedmodelofcommunicaAons.

•  Packet-switchcommunicaAoninvolvestransmissionofdigitaldata–  inpackets–  noresourcereservaAon...usestaAsAcalmulAplexingtoshareachannel

Review•  End-to-EndPrinciple

–  IfafuncAonrequiresend-pointinvolvementtoimplementthefuncAoncompletelyandcorrectly,thenonlyimplementintheend-points!

•  AvoidsimpacAngnetworkapplicaAonsthatdon’tneedthisfuncAon•  Reducescomplexityofthenetwork...dumbnetwork,smartend-points.Reverseof

thetelephonesystem!•  Overall,improvesefficiencyandreliabilityofthenetwork.

–  ReasonableinterpretaAon...ifafuncAonrequiresinvolvementofend-points...•  ThinktwicebeforeimplemenAngitinthenetwork•  OnlyjusAficaAonisasaperformanceenhancement•  DonotimposeaburdenonapplicaAonsthatdon’trequireit•  E.g.verylossy(wireless)links

ReadingReview

•  “End-to-EndArgumentsinSystemDesign”–Salzer,Reed,Clark–  HowdoesthereliablefiletransferproblemmoAvatetheend-to-end

principle?

–  WhatistheprimaryreasonforexcepAonstotheend-to-endprinciple?

–  WhatisacommonproblemwithimplemenAngaperformance-enhancementinthecommunicaAonchannel?

–  Whyisitopenacceptabletoimplementaweakversionofend-to-endservicesasanexcepAontotheend-to-endprinciple(i.e.outsidetheend-nodes)?

•  Problems–  MonolithicfuncAonality–  Reliabledeliveryahost-hostissue

ReceiveanIPdatagram

IPDatagramProcessing1.  IPheadervalidaAon2.  ProcessopAonsinIPheader3.  ParsingthedesAnaAonIP

address

4.  RouAngtablelookup5.  DecrementTTL6.  PerformfragmentaAon(if

necessary)

7.  Calculatechecksum8.  Transmittonexthop9.  SendICMPpacket(ifnecessary)

AMAZING!!!!

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

RouAng

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

RouAng

AddressResoluAonProtocol

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

RouAng

InternetControlMessageProtocol

IP-OrientaAon•  IP(InternetProtocol)isaNetworkLayerProtocol.•  IP’scurrentversionisVersion4(IPv4).•  ItisspecifiedinRFC791.•  IPv6isbeingdeployednow…

NetworkLayer

Link Layer

ARP NetworkAccess

ICMP IGMP

TransportLayer

TCP UDP

IP:Thewaistofthehourglass•  IPisthewaistofthehourglassofthe

Internetprotocolarchitecture

•  MulAplehigher-layerprotocols•  MulAplelower-layerprotocols

•  Onlyoneprotocolatthenetworklayer.

•  MinimumfuncAonalitytoconstructaninternet

Applications

HTTP FTP SMTP

TCP UDP

Data link layer protocols

Physical layer protocols

HighestLayerHop-by-HopProtocol•  IPisthehighestlayerprotocolwhichisimplementedatbothroutersand

hosts(hop-by-hop)

Application

Data Link

Application

NetworkAccess

Application protocol

TCP protocol

IP protocol IP protocol

DataLink

IP protocol

RouterRouter HostHost

IPService•  DeliveryserviceofIPisminimal…packet-switchedcommunicaAonmodel

–  Datasentinpackets–  Sta>s>calmul>plexing(best-effort…packetscanbedroppedordeliveredout

oforder)

•  IPimplementsdatagramflavorofpacket-switching

•  DisAnguishingcharacterisAcofdatagramisitisconnecAonless–  Routescomputedonanevent-drivenbasis(topologychanges)–  Forwardingdecisionsdoneperpacket–  Differentpacketsinthesameflowmayfollowdifferentpaths–  Noper-flowstateisrequired–  Thinktelegram

•  Whatistheotherformofpacket-switching?

IPService•  Virtual-circuit

•  Howisvirtual-circuitdifferentfromdatagram?

•  DisAnguishingcharacterisAcofvirtual-circuitisitisconnecAon-oriented–  RoutecomputaAonandforwardingdecisionsdoneonce/flow–  Requiresper-flowstate–  Thinktelephone-callwithoutbandwidthreservaAons

•  Consequencesofdatagrammodel–  Higherlayerprotocolshavetodealwithlossesorwithduplicatepackets

•  Hmmm…moreaccurately,Iwouldsaythisisaconsequenceofpacket-switching

–  Packetsmaybedeliveredout-of-sequence

IPService•  IPsupportsthefollowingservices:

–  one-to-one (unicast)–  one-to-all (broadcast)–  one-to-group (mulAcast)

•  IPmulAcastalsosupportsamany-to-manyservice.•  IPmulAcastrequiressupportofotherprotocols(IGMP,mulAcastrouAng)•  Anycast?

unicast broadcast multicast

Review•  DisAnguishingcharacterisAcsofpacket-switchedcommunicaAon

–  DatatransmiMedinpackets–  StaAsAcalmulAplexing(best-effort-packetscanbedroppedordeliveredoutoforder)

•  Packet-switchingcomesintwoflavors–  virtual-circuit-connecAonoriented

•  routeonce/flow•  per-flowforwardingstate

–  datagram–connecAonless•  routescomputedoneventdriven-basis•  per-desAnaAonforwardingstate

•  Consequencesofpacket-switching–  higherlayerprotocolshavetodealwithlossesorwithduplicatepackets–  withdatagrammodel,packetsmaybedeliveredoutofsequence

Review•  4classesofservices

–  Unicast–  Broadcast–  MulAcast–  Anycast

Review•  IPgoals

–  interconnectdiversenetworktechnologies,makingminimalassumpAonsoftheunderlyingnetworks

–  implementtheminimalsetoffuncAonalityneededtoconstructaninternet…inthissenseIPisthewaistofthenetworkprotocolstackhourglass

•  IPimplementsadatagram,packet-switchedmodelofcommunicaAons.

•  Packet-switchcommunicaAoninvolvestransmissionofdigitaldata–  inpackets–  noresourcereservaAon...usestaAsAcalmulAplexingtoshareachannel

•  best-effort-packetscanbedroppedordeliveredoutoforder

Review•  Consequencesofpacket-switching

–  higherlayerprotocolshavetodealwithlossesorwithduplicatepackets–  withdatagrammodel,packetsmaybedeliveredoutofsequence

•  End-to-endPrincipal

•  Packet-switchingcomesintwoflavors–  virtual-circuit-connecAonoriented

•  routeonce/flow•  per-flowforwardingstate

–  datagram–connecAonless•  routescomputedoneventdriven-basis•  per-desAnaAonforwardingstate

•  4classesofservices:Unicast,Broadcast,MulAcast,Anycast

ReceiveanIPdatagram

address

necessary)

AMAZING!!!!

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

RouAng

IPDatagramFormat

ECNversion headerlength DS total length (in bytes)

Identification Fragment offset

source IP address

destination IP address

options (0 to 40 bytes)

payload

4 bytes

time-to-live (TTL) protocol header checksum

bit # 0 15 23 248 317 16

IPHeaderFields•  Version(4bits):currentversionis4.•  Headerlength(4bits):

–  LengthofIPheader,inmulAplesof4bytes–  20bytes≤HeaderLength≤(24-1)*4=60bytes

•  Servicefield(1byte)–  Iffirstthreebitsare0,interpretedasoriginalType-of-Service(TOS).–  Otherwise

•  6bits:DifferenAatedService(DS)(RFC2474):•  2bits:ExplicitCongesAonNoAficaAon(ECN)(RFC3168):

IPHeaderFields•  Totallength(16bits):

–  TotallengthofIPv4datagram,inbytes.–  20bytes≤TotalLength≤216-1=65535bytes–  Lengthofdata=totallength-headerlength

•  IdenAficaAon(16bits):UniqueidenAficaAonofadatagramfromahost.IncrementedwheneveradatagramistransmiMed

•  Flags(3bits):–  Firstbitalwayssetto0–  DFbit(Donotfragment)–  MFbit(Morefragments)

ForFragmentaAon…willbeexplainedlater

IPHeaderFields•  TimeToLive(TTL)(1byte):

–  Specifieslongestpathsbeforedatagramisdropped–  RoleofTTLfield:EnsurethatpacketiseventuallydroppedwhenarouAng

loopoccurs

Usedasfollows:–  Sendersetsthevalue(e.g.,64)–  Eachrouterdecrementsthevalueby1–  Whenthevaluereaches0,thedatagramisdropped

•  NoTTLinEthernet…whydoyouthinktheydidn’tincludeone?

IPHeaderFields•  Protocol(1byte):

–  Specifiesthehigher-layerprotocol.–  UsedfordemulAplexingtohigherlayers.

1 = ICMP 2 = IGMP

6 = TCP 17 = UDP

4 = IP-in-IPencapsulation

IPHeaderFields•  Headerchecksum(2bytes):Simple16-

bitlongchecksumcoversonlyheader.•  Upperlayerprotocolscoverdata•  IPishighesthop-by-hopprotocol;need

tominimizeprocessing

IPHeaderFields•  OpAonType

–  Copyflag:IndicatesifopAontobecopiedtofragments.–  OpAonclass:0=Control,2=Debug/Measurement,rest“Reserved”.–  OpAonnumber:idenAfiesopAon

•  OpAonlength:notpresentforNoopandEndofOpAons•  OpAondata:notpresentforNOPandEndofOpAons

FragmentaAon•  MaximumsizeofIPdatagramis65535…

–  …butlink-layerpayloadlimitstypicallymuchsmaller•  CalledtheMaximumTransmissionUnit(MTU).•  ExampleMTUs:

•  FragmentIPdatagramslargerthanMTUofalink.•  Issues

–  HowcommunicatefragmentaAonamonghopsinapath?–  HowhandlepathscontainingnetworkswithdifferentMTUs?–  WhereisfragmentaAondone?

FragmentaAon-How?•  Involvesfollowingfields(pluschecksum)

•  IdenAficaAon:IDandsourceIPaddressuniquelyidenAfydatagram.•  Flags

–  DF:“Don’tfragment.”Discardandsenderror.–  MF:“Morefragments.”Morefragmentsfollow.

•  FragmentOffset:Offsetofcurrentpayloadinoriginaldatagram.–  Only13bitfield-givesoffsetinunitsof8bytes–  NumberoffirstbyteinpayloadisFO*8.–  Sizeofallfragments,butlast,mustbemulApleof8.

FragmentaAon-How?

•  Totallength:Totallengthofthecurrentfragment.•  ConstraintsoffragmentaAon

–  FragmentaAoncanbedoneatthesenderoratintermediaterouters–  ThesamedatagramcanbefragmentedseveralAmes.–  ReassemblyoforiginaldatagramisonlydoneatdesAnaAonhosts!!

•  Why?

•  Howdetermineifadatagramisafragment?–  FO≠0or…–  MFflagisset

•  ExplanaAon•  WhatdoesFO=0mean?Thisisthefirstfragment.•  WhatdoesMF=0mean?Thisisthelastfragment.•  Whatdoes((FO=0)and(MF=0))mean?Thisisboththefirstandlastfragment->

thisistheonlyfragment->thisistheoriginalpacket(i.e.notafragment).NOTE:thisistheonlywaytonotbeafragment.

•  Soapacketisafragmentif!((FO=0)and(MF=0))<->((FO≠0)or(MF≠0))

FragmentaAonExample

FragmentaAonExample•  ExamplewherelastfragmentisnotmulApleof8.

IP datagram

Router

Fragment 2Fragment 3

MTU: 1000MTU: 4000

Fragment 1

Header length: 20Total length: 2400

Identification: 0xa428DF flag: 0MF flag: 0

Fragment offset: 0

fragment offset: 0

Fragment offset: 122

Fragment offset: 244

Whycalculatechecksum@eachhop?

•  Fieldswillchange…–  TTL–  FragmentaAoninformaAon–  Headerlength–  Others..?

Review•  IPheaders…

–  Checksumonlycoversheadertominimizehop-by-hopprocessing•  Assumesupperlayerprotocolscoverdata

–  Checksummustberecalculatedateachhop•  IPheaderfieldschange…TTL,FragmentaAoninformaAon,Headerlength

–  FragmentaAonandreassembly•  UsedtomatchpacketstolinkMTUs•  FragmentaAoncanbedoneatanyhoponthepath•  ReassemblycanonlybedoneatthedesAnaAon

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

RouAng

Forwardingvs.RouAng•  TherearetwodisAnctprocessestodeliveringIPdatagrams:

–  Forwarding:deliverapacketonehopclosertodesAnaAon.–  RouAng:computeforwardingstate.

•  Forwarding,beingindataplane,mustbedoneasfastaspossible:–  Onrouters,isopendonewithsupportofhardware–  OnPCs,isdoneinkerneloftheoperaAngsystem

•  RouAng,beingoutsidedataplane,islessAme-criAcal–  OnaPC,rouAngisdoneasabackgroundprocess

Forwarding•  AninternetisacollecAonofsubnets•  IPimplementshop-by-hopdeliveryofpacketsbetweenhostsinan

internet•  Routers

–  Interconnectsubnets(broadcastdomains)–  Forwardpacketsacrossaninternet

H210.2.1.0/24

20.1.0.0/1610.1.2.0/24

10.1.0.0/24 10.3.0.0/16

20.2.1.0/28

Routers•  Routersimplement

–  Forwardingprocess–  RouAngprocess

•  RoutersconnectmulAplesubnets–  HaveaninterfaceonmulAplesubnets–  Forwardpacketsbetweensubnets

•  Receiveononeinterface•  ProcessIPheader•  Determinenexthop•  Sendoutnexthopinterface

H210.2.1.0/24

20.1.0.0/1610.1.2.0/24

10.1.0.0/24 10.3.0.0/16

20.2.1.0/28

RouAngandForwardingRouAngfuncAonsinclude:

–  routecalculaAon–  maintenanceoftherouAngtable–  execuAonofrouAngprotocols

•  Oncommercialroutershandledbyasinglegeneralpurposeprocessor,calledrouteprocessor

IPforwardingisper-packetprocessing•  Onhigh-endcommercialrouters,IPforwardingisdistributed•  Mostworkisdoneontheinterfacecards

ForwardingTable•  ForwardingtableistheinterfacebetweentherouAngandforwarding

processes–  Simplemechanism–  Implements(potenAally)complexpolicies

•  MapsdesAnaAonaddresstonexthoptowardsdesAnaAon

Control

Datapath: per-packet processing

routingtable

Routingfunctions

IPForwarding

routing tablelookup

routing tableupdates

incoming IPdatagrams

outgoing IPdatagrams

routingprotocol

ForwardingTables•  Eachrouterandeachhostmaintainsaforwardingtablewhichtellsthe

routerhowtoprocessanoutgoingpacket•  Maincolumns:

–  DesAnaAonaddress:whereistheIPdatagramgoingto?–  Nexthoporinterface:howtoforwardtheIPdatagram?

•  RouAngtablesaresetsothatadatagramgetsclosertotheitsdesAnaAoneveryhop

Destination Next Hop 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28

direct direct R4 direct R4 R4

Routing table of a host or router IP datagrams can be directly delivered (�direct�) or are sent to a router (�R4�)

ForwardingTableLookup•  Whenarouterorhostneedsto

transmitanIPdatagram,itperformsaforwardingtablelookup

•  Forwardingtablelookup:UsetheIPdesAnaAonaddressasakeytosearchtherouAngtable.

•  ResultofthelookupistheIPaddressofanexthoprouter,orthenameofanetworkinterface

ForwardingTableMatch•  Forwardingtableentriescomposedof:<IP Address>”/”<mask length>

–  E.g.128.114.48.128/26

•  Maskdefinesthenetworkpartofanaddress–  LogicallyANDtheaddresswithavalueofmask length ‘1’sfollowedby‘0’s –  128.114.48.128/26 = 10000000 01110010 00110000 10xxxxxx

–  ”x”=don’tcare(hostpartofaddress)

•  Forwardingtablematchoccurswhen…–  BoththerouAngentryandIPaddresseshavethesamenetworkpart…–  …giventheroute’snetworkmask

•  Example:is128.114.48.0/17amatchingroutefor128.114.122.5?–  128.114.48.0/17 =10000000 01110010 0xxxxxxx xxxxxxxx–  128.114.122.5/17 =10000000 01110010 01111010 00000101–  Yes.

ForwardingTableLookup•  Whatorderusedinconsideringforwardingtableentries?•  OnlymaMerswhenrouAngentriesoverlap

–  Use128.114.0.0/16 or128.114.48.0/17for128.114.128.5?

•  128.114.128.5matches128.114.0.0/16–  128.114.0.0/16 =10000000 01110010 xxxxxxxx xxxxxxxx–  128.114.128.5/16=10000000 01110010 10000000 00000101

•  128.114.128.5alsomatches128.114.0.0/17 –  128.114.128.0/17=10000000 01110010 0xxxxxxx xxxxxxxx–  128.114.128.5/17=10000000 01110010 00000000 00000101

•  Answerislongestprefixmatch–  Docomparisoninorderofincreasingmasklength–  Why?Becauseitisuseful…

•  Alsoshortestprefixmatchdoesn’tmakesense(never“see”longerprefix)

•  Howuse?CC-NIEexample

DeliverywithForwardingTables

direct R3 R3 R3 R3 R3

H210.2.1.0/24

20.1.0.0/1610.1.2.0/24

10.1.0.0/24 10.3.0.0/16

20.2.1.0/28

20.2.1.2/28

R3 R3 R2 direct direct R2

R3 direct direct R3 R2 R2

R1 R1 direct R4 direct direct

R2 R2 R2 R2 R2 direct

to: 20.2.1.2

RouteAggregaAon •  LongestprefixmatchalgorithmallowsaggregaAonofprefixeswith

idenAcalnexthopaddresstoasingleentry•  Benefits

–  ReducessizeofrouAngtables–  MoreefficientrouAngtablelookups

Destination Next Hop 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.0.0.0/8

R3 direct direct

R3 R2 R2

TypesofForwardingTableEntries•  Networkroute

–  DesAnaAonaddresswith0<prefixlength<32(e.g.,10.0.2.0/24)–  Mostentriesarenetworkroutes

•  Hostroute–  DesAnaAonaddresswithprefixlength=32(e.g.,10.0.1.2/32)–  Usedtospecifyaseparaterouteforcertainhosts

•  Defaultroute–  DesAnaAonaddresswithprefixlength=0(i.e.0.0.0.0/0)–  MatchesalldesAnaAons–  Commonlyusetoconnectacompany’sedgeroutertotheISPnetwork.

•  Specialcasesoflongestprefixmatch

ipcalc

•  IMO,subnetsarebestthoughtofasaddressranges•  ipcalccalculatestheserangesforyou…

% ipcalc 128.114.48.0/17 Address: 128.114.48.0 10000000.01110010.0 0110000.00000000

Netmask: 255.255.128.0 = 17 11111111.11111111.1 0000000.00000000 Wildcard: 0.0.127.255 00000000.00000000.0 1111111.11111111 =>

Network: 128.114.0.0/17 10000000.01110010.0 0000000.00000000 HostMin: 128.114.0.1 10000000.01110010.0 0000000.00000001 HostMax: 128.114.127.254 10000000.01110010.0 1111111.11111110 Broadcast: 128.114.127.255 10000000.01110010.0 1111111.11111111

Hosts/Net: 32766 Class B

•  …addressrangeof128.114.0.1–127.254•  hMp://jodies.de/ipcalc(availableascommandlinetool)

ProcessingofanIPdatagraminIP

UDP TCP

Inputqueue

Lookup nexthop

RoutingProtocol

Destinationaddress local?

Staticrouting

Senddatagram

IP forwardingenabled?

Discard

Yes No

Demultiplex

routingtable

IP module

Data Link Layer

IProuter:IPforwardingenabledHost:IPforwardingdisabled

DesAnaAon-BasedForwarding•  InternetrouAngusesasinglepathperdesAnaAon•  DesAnaAon-basedforwardingisarestrictedversionofsingle-path•  ApaththroughanodetoadesAnaAon…•  …mustbeanextensionofthepathfromthenodetothedes>na>on.•  Thisiscomingbacktohauntus…

Review•  AninternetisacollecAonofsubnets.•  AsubnetisdefinedbyanIPprefix(usingaddress/masknotaAon)•  IPimplementshop-by-hopdeliveryofpacketsbetweenhostsinan

internet•  Routersconnectsubnetsandforwardpacketsacrossaninternet

–  Forwarding:selecAonofpacket'snexthop,dataplane,fast(inhardware)–  RouAng:compuAngforwardingstate,signalingplane,notasAmecriAcal

•  TheforwardingtableistheinterfacebetweentherouAngandforwardingprocesses–  DesAnaAon–  Nexthop

•  ForwardingtablelookupsaredoneusingLongestPrefixMatch•  TheInternetusesdesAnaAon-basedforwarding,whichisarestricted

versionofsingle-pathforwarding.

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

RouAng

AddressResoluAonProtocol(ARP)

NetworkLayer

Link Layer

ARP NetworkAccess RARP

ICMP IGMP

TransportLayer

TCP UDP

AddressResoluAonProtocol(ARP)•  TheInternetisbasedonIPaddresses•  Datalinkprotocols(Ethernet,FDDI,ATM)mayhavedifferent(MAC)addresses•  TheARPandRARPprotocolsperformthetransla>onbetweenIPaddressesand

MAClayeraddresses•  WewilldiscussARPforbroadcastLANs,parAcularlyEthernetLANs

Ethernet MACaddress(48 bit)

ARPIP address(32 bit)

Driver-levelIPPacketProcessing

loopbackDriver

IP Input

Put on IPinput queue

ARPdemultiplex

Ethernet Frame

Ethernet

IP destination of packet= local IP address ?

IP destination = multicastor broadcast ?

IP Output

Put on IPinput queue

No: get MACaddress withARP

ARPPacket

IP datagram

YesEthernet

Driver

AddressTranslaAonwithARPARPRequest:

ArgonbroadcastsanARPrequesttoallstaAonsonthenetwork:“WhatisthehardwareaddressofRouter137?”

Argon128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Request:What is the MAC addressof 128.143.71.1?

AddressTranslaAonwithARPARPReply:

Router137unicastsanARPreplytowithitshardwareaddress.

Argon128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Reply:The MAC address of 128.143.71.1is 00:e0:f9:23:a8:20

ARPPacketFormat

Destinationaddress

ARP Request or ARP Reply

Sourceaddress

Type0x8060 Padding

Ethernet II header

Hardware type (2 bytes)

Hardware addresslength (1 byte)

Protocol addresslength (1 byte) Operation code (2 bytes)

Target hardware address*

Protocol type (2 bytes)

Source hardware address*

Source protocol address*

Target protocol address*

* Note: The length of the address fields is determined by the corresponding address length fields

Example•  ARPRequestfromArgon:

Sourcehardwareaddress:00:a0:24:71:e4:44Sourceprotocoladdress: 128.143.137.144Targethardwareaddress: 00:00:00:00:00:00Targetprotocoladdress: 128.143.137.1

•  ARPReplyfromRouter137: Sourcehardwareaddress:00:e0:f9:23:a8:20 Sourceprotocoladdress: 128.143.137.1Targethardwareaddress: 00:a0:24:71:e4:44 Targetprotocoladdress: 128.143.137.144

ARPCache•  SincesendinganARPrequest/replyforeachIPdatagramis

inefficient,hostsmaintainacache(ARPCache)ofcurrententries.Theentriesexpireaper20minutes.

•  ContentsoftheARPCache(“arp–a”):(128.143.71.37)at00:10:4B:C5:D1:15[ether]oneth0(128.143.71.36)at00:B0:D0:E1:17:D5[ether]oneth0(128.143.71.35)at00:B0:D0:DE:70:E6[ether]oneth0(128.143.136.90)at00:05:3C:06:27:35[ether]oneth1(128.143.71.34)at00:B0:D0:E1:17:DB[ether]oneth0(128.143.71.33)at00:B0:D0:E1:17:DF[ether]oneth0

OtherARPUses•  WhathappensifanARPRequestismadeforanon-exisAnghost?

SeveralARPrequestsaremadewithincreasingAmeintervalsbetweenrequests.Eventually,ARPgivesup.

•  WhatifahostsendsanARPRequestforitsownIPaddress?

Theothermachinesrespond(gratuitousARP)asifitwasanormalARPrequest.ThisisusefulfordetecAngifanIPaddresshasalreadybeenassigned.

•  Similarly,whatifahostsendsanARPReplywithitsownMACaddress?

TheothermachinesacceptthenewMACaddressforthehost(alsocalledagratuitousARP).ThisisusefulformovingIPaddressestonewNICs.

ProxyARP•  ProxyARP:HostorrouterrespondstoARPRequestthatarrivesfromone

ofitsconnectednetworksforahostthatisonanotherofitsconnectednetworks.

128.143.137.1/1600:e0:f9:23:a8:20 128.143.71.1/24

128.143.0.0/16Subnet

128.143.71.0/24Subnet

Router137

ARP Request:What is the MAC addressof 128.143.71.21?

128.143.137.144/16 128.143.171.21/2400:20:af:03:98:28

Argon Neon

ARP Reply:The MAC address of128.143.71.21 is00:e0:f9:23:a8:20

Review•  TheAddressResoluAonProtocoltranslatesfromIPtoMACaddresses.

•  ARPworksby–  BroadcasAngARPRequestsforanIPaddress–  UnicasAnganARPReplywiththeMACaddresstotherequestor.–  ARPRequestsarerepeatedunAlaReplyisreceivedorARPAmesout.

•  HostsmaintainanARPcachetolimittheneedforARPqueriesforeverypacketsent

•  GratuitousARPareARPRequest/RepliesthatareissuedforotherthanstandardARPpurposes–  GratuitousARPRequestsdetectifanIPaddressisinuse–  GratuitousARPRepliescanbeusedtomoveanIPaddresstoanewNIC

•  RouterscanbeconfiguredtoissueProxyARPRepliestoARPRequestsononeofitsinterfacesforhostsonanotherinterface

ReceiveanIPdatagram

address

necessary)

IPheaderprocessing

RouAng

Overview•  IPreliesonseveralotherprotocolstoperformnecessarycontroland

rouAngfuncAons:–  ControlfuncAons(ICMP)–  MulAcastsignaling(IGMP)–  Se}nguprouAngtables(RIP,OSPF,BGP,PIM,…)

Control

Routing

ICMP IGMP

RIP OSPF BGP PIM

Overview•  TheInternetControlMessageProtocol(ICMP)isahelperprotocolthat

supportsIPwithfacilityfor–  Simplequeries–  Errorrepor>ng

•  DefinedinRFC792.•  ConceptuallyICMPisapartofIP…•  …howeverisimplemented“ontop”ofIP•  ICMPmessagesareencapsulatedinIPdatagrams:

IP header ICMP message

IP payload

ICMPmessageformat4byteheader:•  Type(1byte):typeofICMPmessage•  Code(1byte):subtypeofICMPmessage•  Checksum(2bytes):similartoIPheaderchecksum.Checksumiscalculatedover

enAreICMPmessage•  EachICMPmessagesisatleast8byteslong

–  IfthereisnoaddiAonaldata,thereare4bytessettozero.

additional informationor

0x00000000

type code checksum

bit # 0 15 23 248 317 16

ICMPQuerymessagesICMPquery:•  Requestsentbyhosttoarouterorhost•  Replysentbacktoqueryinghost

ICMP Request

Host or router

ICMP Reply

ExampleofaQuery:“ping”•  EachPingistranslatedintoanICMPEchoRequest•  ThePing’edhostrespondswithanICMPEchoReply•  AddiAonalinformaAon:IdenAfier,Sequence#,Data•  SourceanddesAnaAonaddressesswapped,typecodechangedto0,

checksumrecomputed.

Host or

Router

ICMP ECHO REQUEST Host or

router

ICMP ECHO

ICMPRouterSolicitaAon/AdverAsement

•  Aperbootstrapping,ahostbroadcastsanICMProutersolicitaAon.

•  Inresponse,routerssendanICMProuteradverAsementmessage

•  Also,routersperiodicallybroadcastICMProuteradverAsement

ThisissomeAmescalledtheRouterDiscoveryProtocol

Ethernet

ICMP routeradvertisement

ExampleofICMPQueriesType/Code DescripAon

8/0 EchoRequest0/0 EchoReply

13/0 TimestampRequest14/0 TimestampReply

10/0 RouterSolicitaAon9/0 RouterAdverAsement

ThepingcommandusesEchoRequest/EchoReply

ICMPErrormessages•  ICMPerrormessagesreporterrorcondiAons•  Typicallysentwhenadatagramisdiscarded•  ErrormessageisopenpassedfromICMPtotheapplicaAonprogram

IP datagram

Host or router

ICMP ErrorMessage

IP datagramis discarded

ICMPErrormessages•  ICMPerrormessagesincludethecompleteIPheaderandthe

first8bytesofthepayload(typically:UDP,TCP)

Unused (0x00000000)

IP header ICMP header IP header 8 bytes of payload

ICMP Message

from IP datagram that triggered the error

type code checksum

ManipulateRouAngtablewithICMP

•  WhenarouterdetectsthatanIPdatagramshouldhavegonetoadifferentrouter,therouter(hereR2)–  forwardstheIPdatagramtothecorrectrouter–  sendsanICMPredirectmessagetothehost

•  HostusesICMPmessagetoupdateitsrouAngtable

Destination Next Hop 10.1.0.0/24 …

Ethernet

(1) IP datagram

(2) IP datagram(3) ICMP redirect

Example:ICMPPortUnreachable•  RFC792:If,inthedesAnaAonhost,theIPmodulecannotdeliverthe

datagrambecausetheindicatedprotocolmoduleorprocessportisnotacAve,thedesAnaAonhostmaysendadesAnaAonunreachablemessagetothesourcehost.

•  Scenario:

Client

Request a service at a port 80

Server

No process is waiting at port 80

Unreacha

ExampleofanError:traceroute•  SendUDPdatagramtodesAnaAonwithIPTTLof1.•  WaitforICMPTEmessagetogetIPaddressofrouter(source).•  IncreaseTTLandrepeat.•  DesAnaAonidenAfiedbyuseofhighUDPportresulAnginICMPPort

Unreachablemessage.•  AddiAonalinformaAon(forbothmessages):

–  InternetHeader–  64bitsoforiginaldatagram

•  Demo

%tcpdump–nvhostcas01.ucsc.eduoricmp%traceroute–ncas01.ucsc.edu

FrequentICMPErrormessage

Type Code Description 3

0–15 Destination

unreachable Notification that an IP datagram could not be forwarded and was dropped. The code field contains an explanation.

5 0–3 Redirect Informs about an alternative route for the datagram and should result in a routing table update. The code field explains the reason for the route change.

11 0, 1 Time exceeded

Sent when the TTL field has reached zero (Code 0) or when there is a timeout for the reassembly of segments (Code 1)

12 0, 1 Parameter problem

Sent when the IP header is invalid (Code 0) or when an IP header option is missing (Code 1)

Somesubtypesofthe“DesAnaAonUnreachable”

Code Description Reason for Sending 0 Network

Unreachable No routing table entry is available for the destination network.

1 Host Unreachable

Destination host should be directly reachable, but does not respond to ARP Requests.

2 Protocol Unreachable

The protocol in the protocol field of the IP header is not supported at the destination.

3 Port Unreachable

The transport protocol at the destination host cannot pass the datagram to an application.

4 Fragmentation Needed and DF Bit Set

IP datagram must be fragmented, but the DF bit in the IP header is set.

Review•  ICMPprovidestwobasicservices:

–  Networkqueries–  ErrorreporAng

•  FuncAonofanICMPmessagedeterminedbyTypeandCodefields.•  Fornetworkqueries

–  TypefielddefinesmatchingRequest/Replytypes–  Codefieldis0–  AddiAonalinformaAonfieldusedforparameters

•  ForerrorreporAng–  TypefieldidenAfiesgeneralclassoferrors–  CodefieldidenAfiesspecificerror–  Typicallysentwhenadatagramisdiscarded–  IncludeIPheaderandfirst8bytesofpayload(UDPorTCPdata)

•  AcronymforrememberingDHCPsequence?– DORK

•  Discover-broadcast•  Offer-unicast•  Request-broadcast•  acK-unicast

QuesAons?

ExtraSlides

LongestPrefixMatch•  Givenforwardingtable

•  (1)128.114.48.0/17•  (2)128.114.48.0/20•  (3)128.114.48.0/22•  (4)0/0

•  WhichentryischosenfordesAnaAon:128.114.122.5•  Answer:#1•  ExplanaAon(rememberbitvalues:1286432168421)

–  10000000 01110010 0xxxxxxx xxxxxxxx =128.114.48.0/17(0.0–127.255)–  10000000 01110010 0011xxxx xxxxxxxx =128.114.48.0/20(48.0–63.255)–  10000000 01110010 001100xx xxxxxxxx =128.114.48.0/22(48.0–51.255)–  10000000 01110010 01111010 00000101 =128.114.122.5

•  (1)128.114.48.0/17•  (2)128.114.48.0/20•  (3)128.114.48.0/22•  (4)0/0

•  WhichentryischosenfordesAnaAon:128.114.50.2•  Answer:??•  ExplanaAon(rememberbitvalues:1286432168421)

–  10000000 01110010 0xxxxxxx xxxxxxxx =128.114.48.0/17(0.0–127.255)–  10000000 01110010 0011xxxx xxxxxxxx =128.114.48.0/20(48.0–63.255)–  10000000 01110010 001100xx xxxxxxxx =128.114.48.0/22(48.0–51.255)

•  (1)128.114.48.0/17•  (2)128.114.48.0/20•  (3)128.114.48.0/22•  (4)0/0

•  Giveanexampleofanaddressthatwoulduse(4)…thedefaultroute.

ARPAnetArchitecture•  Host-to-HostprotocolimplementedintheNetworkControlProgram

(NCP)-TheprimaryfuncAonoftheNCPistoestablishconnecAons,breakconnecAons[withameanstoidenAfyprocessesusing“sockets”],andcontroldataflowovertheconnecAons.

•  Host-IMP–“responsibleforbidirecAonalcommunicaAonbetweenthehostandIMP”–thehost’sinterfacetotheARPAnet

•  IMP-IMP–“receivingdatapackets,conducAngrudimentaryerrordetecAon,determiningtheroutefortheforwardingofdatapacketsandtransmi}ngthistothenextIMP.”

•  SrcIMP-DstIMP–flowcontrol(managementofreceivermemory),reliabledelivery,fragmentaAon(intopackets)andreassembly.

•  Internetworking:TechnologicalFoundaAonsandApplicaAonsByChristophMeinel,HaraldSack

Network Layer - Courses · • Link-layer (layer 2) ... • Only one protocol at the network layer....

Documents

Protocols and Protocol Suit Review Lecture 13. Overview Network Access Layer Transport Layer Protocols Protocol Data Unit Protocol Architecture TCP/IP

Network Layer Internet Protocol: IP Addressing

Chapter 2 Internet Protocol DoD Model Four layers: – Process/Application layer – Host-to-Host layer – Internet layer – Network Access layer

Wireless Sensor Networks Akyildiz/Vuran 1 PROTOCOL STACK Application Layer Transport Layer Network Layer Link Layer Physical Layer Power Management Plane

Network Layer: Internet Protocol

chap6 network layer protocol

CSci4211: Network Layer: Part I1 Network Layer: Part I Network Layer and IP Protocol! Part I: Network Layer Functions and Service Models –Network Layer

Performance of Host Identity Protocol on Lightweight ... · Host Identity Protocol ... – NAT traversal. HIP Protocol Stack Physical Layer Link Layer Network Layer Transport Layer

Internet Network Layer: Overviewkwke/cn/nlp.pdf · ARP RARP ICMP IP IGMP Network Layer Packet (Daragram) TCP TP Layer UTP Segment or Datagram Based on Protocol number Based on protocol

Network Coding-Based Congestion Control at Network Layer: Protocol Design and Evaluation

1 Internet Protocol/ Network Layer. 2 Position of IP in TCP/IP protocol suite

Information Network I Network layer: the Internet Protocol · Information Network I Network layer: the Internet Protocol Youki Kadobayashi NAIST

Transmission Control Protocol / Internet Protocol (TCP/IP) Application Layer Transport Layer Internetwork Layer Network Interface Layer Parziale, Lydia,

1 Datagram Networks: Internet Protocol (IPv4). 2 The Internet Network layer: IP Internet Network Layer Components: –IP protocol (addressing, datagram

Simple Network Management Protocol · • The Simple Network Management Protocol (SNMP) Internet-standard protocol (application layer protocol) • Used for managing devices on IP

CCNA1-1 Chapter 5 OSI Network Layer. CCNA1-2 Chapter 5 OSI Network Layer Internet Protocol Version 4 (IPV4)

Datagram Networks: Internet Protocol (IPv4)eem.eskisehir.edu.tr/isan/EEM 482/icerik/Network Layer -2.pdf · Datagram Networks: Internet Protocol (IPv4) 2 The Internet Network layer:

The Network Layer and the Internet Protocol

COMPUTER NETWORK -- APPLICATION LAYER PROTOCOL

Connecting The Network Layer to Data Link Layer. ARP in the IP Layer The Address Resolution Protocol (ARP) The Address Resolution Protocol (ARP) Part