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

Host-IMP IMP-IMP

HOST Host-Host (NCP)

IMP

IMP

IMP

IMP

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


G

G

G G

SUBNET

SUBNET

SUBNET

SUBNET A

B

TCP

TCP

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

IP

ARP NetworkAccess

Media

ICMP IGMP

TransportLayer

TCP UDP


IP:Thewaistofthehourglass•  IPisthewaistofthehourglassofthe

Internetprotocolarchitecture

•  MulAplehigher-layerprotocols•  MulAplelower-layerprotocols

•  Onlyoneprotocolatthenetworklayer.

•  MinimumfuncAonalitytoconstructaninternet


Applications

HTTP FTP SMTP

TCP UDP

IP

Data link layer protocols

Physical layer protocols

HighestLayerHop-by-HopProtocol•  IPisthehighestlayerprotocolwhichisimplementedatbothroutersand

hosts(hop-by-hop)


Application

TCP

IP

Data Link

Application

TCP

IP

NetworkAccess

Application protocol

TCP protocol

IP protocol IP protocol

DataLink

DataLink

IP

DataLink

DataLink

IP

DataLink

DataLink

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

0 MF

DF

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.


IP

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.





0 MF

DF

FragmentaAon-How?

•  Totallength:Totallengthofthecurrentfragment.•  ConstraintsoffragmentaAon

–  FragmentaAoncanbedoneatthesenderoratintermediaterouters–  ThesamedatagramcanbefragmentedseveralAmes.–  ReassemblyoforiginaldatagramisonlydoneatdesAnaAonhosts!!

•  Why?

•  Howdetermineifadatagramisafragment?–  FO≠0or…–  MFflagisset





0 MF

DF

•  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


H1

R1 R2

R3 R4

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


H1

R1 R2

R3 R4

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

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

H1

R1 R2

R3 R4

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


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

Yes

Senddatagram

IP forwardingenabled?

No

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

IP

ARP NetworkAccess RARP

Media

ICMP IGMP

TransportLayer

TCP UDP

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

MAClayeraddresses•  WewilldiscussARPforbroadcastLANs,parAcularlyEthernetLANs


RARP

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

No

Yes

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

6

ARP Request or ARP Reply

28

Sourceaddress

6 2

CRC

4

Type0x8060 Padding

10

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


Host

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

REPLY

ICMPRouterSolicitaAon/AdverAsement

•  Aperbootstrapping,ahostbroadcastsanICMProutersolicitaAon.

•  Inresponse,routerssendanICMProuteradverAsementmessage

•  Also,routersperiodicallybroadcastICMProuteradverAsement

ThisissomeAmescalledtheRouterDiscoveryProtocol


Ethernet

H1

R1 R2

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


Host

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 …

R2

Destination Next Hop 10.1.0.0/24 …

R1

Ethernet

H1

R1 R2

(1) IP datagram

R1

(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

Port

Unreacha

ble

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)


DHCP

•  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





•  (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


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