Livello Rete ARP – ICMP - DHCP - Home page | …disi.unitn.it/locigno/didattica/reti/15-16/05.2_ARP-ICMP-DHCP.pdf · Reti (già “Reti di ... – a pool of dynamic addresses to

Reti(già “Reti di Calcolatori”)

Livello ReteARP – ICMP - DHCP

RenatoLoCigno – MicheleSegata

http://disi.unitn.it/locigno/index.php/teaching-duties/computer-networks

Acknowledgement

• Credits– Partof thematerialis based onslides provided by thefollowing authors• JimKurose,KeithRoss,“ComputerNetworking:ATopDownApproach,”4thedition,Addison-Wesley,July2007

• DouglasComer,“ComputerNetworksandInternets,”5thedition,PrenticeHall

• Behrouz A.Forouzan,Sophia Chung Fegan,“TCP/IPProtocol Suite,”McGraw-Hill,January 2005

• Latraduzione,sepresente,èingeneraleopera(eresponsabilità)deldocente

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Contenuto e temi

• Spaziodiindirizzamento

• IndirizziIPelorouso

• Consegnadeipacchetti

• ConfigurazionedeiPCedellereti

• InstradamentoeRouting

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ARP: ADDRESS RESOLUTION PROTOCOL

ProtocollodisupportoaIPpermapparegliindirizziIPsulleinterfaccefisiche,ovverosugliindirizziMAC(Ethernet)


Address Resolution

• Acrucialstepoftheforwardingprocessrequiresatranslation:– forwardingusesIPaddresses– aframetransmittedmustcontaintheMACaddressofthenexthop

– IPmusttranslatethenext-hopIPaddresstoaMACaddress• Theprincipleis:

– IPaddressesareabstractions• providedbyprotocolsoftware

– TheData-LinkdoesnotknowhowtolocateacomputerfromitsIPaddress• thenext-hopaddressmustbetranslatedtoanequivalentMACaddress


Address Resolution

• Translationfromacomputer'sIPaddresstoanequivalenthardwareaddressisknownasaddressresolution– AndanIPaddressissaidtoberesolvedtothecorrectMACaddress

• Addressresolutionislocaltoanetwork– simpleforPoint-to-Pointconnections– needaprotocolinthegeneralcase ofsharedaccessmedium

• Aserver-basedsolutionintroducesdelaysandaweakpoint• Localcommunicationsarecheapandoftenthemediumis

broadcast• A“broadcastandselect”solutionistheonechosenbyIETF


Address Resolution

• Onecomputercanresolvetheaddressofanothercomputeronlyifbothcomputersattachtothesamephysicalnetwork– Directdelivery– Acomputerneverresolvestheaddressofacomputeronaremotenetwork

– Addressresolutionisalwaysrestrictedtoasinglenetwork


Address Resolution

• Howcanahostknowiftheaddresstoresolveislocal?– ifitislocal,thedest.IPaddressshouldhavethesameNetID(prefix)ofthesourceIPaddress

• Whathappensiftheaddressisnotlocal?– Indirectdelivery– Givethepackettoamachinerouterthatisonthewaytothedestinationè nexttopic

– MustinanycasetranslatetheIPoftheRouterintoitsMACaddress


Address Resolution Protocol (ARP)

• SupposeBneedstoresolvetheIPaddressofC• Bbroadcastsarequestthatsays:

– “I'mlookingfortheMACaddressofacomputerthathasIPaddressC”• Thebroadcastonlytravelsacrossonenetwork• AnARPrequestmessagereachesallcomputersonanetwork• WhenCreceivesacopyoftherequestitsendsadirectedreplybacktoBthatsays:

– “I'mthecomputerwithIPaddressC,andmyMACaddressisM”


ARP Message Format

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ARP Message Format

• HARDWAREADDRESSTYPE– 16-bitfieldthatspecifiesthetypeofhardwareaddress– thevalueis1forEthernet

• PROTOCOLADDRESSTYPE– 16-bitfieldthatspecifiesthetypeofprotocoladdress– thevalueis0x0800forIPv4

• HADDRLEN– 8-bitintegerthatspecifiesthesizeofahardwareaddressinbytes

• PADDRLEN– 8-bitintegerthatspecifiesthesizeofaprotocoladdressinbytes


ARP Message Format

• OPERATION– 16-bitfieldthatspecifieswhetherthemessage

• “request”(1)or“response”(2)• SENDERHADDR

– HADDRLENbytesforthesender'shardwareaddress• SENDERPADDR

– PADDRLENbytesforthesender'sprotocoladdress• TARGETHADDR

– HADDRLENbytesforthetarget'shardwareaddress• TARGETPADDR

– PADDRLENbytesforthetarget'sprotocoladdress


ARP Message Format

• AnARPmessagecontainsfieldsfortwoaddressbindings– onebindingtothesender– othertotheintendedrecipient,ARPcallsittarget

• Whenarequestissent– thesenderdoesnotknowthetarget'shardwareaddress(thatistheinformationbeingrequested)• fieldTARGETHADDRinanARPrequestisfilledwith“0”

• Inaresponse– thetargetbindingreferstotheinitialcomputerthatsenttherequest


ARP Encapsulation

• WhenittravelsacrossaphysicalnetworkanARPmessageisencapsulatedinahardwareframe– e.g.,Ethernet

• AnARPmessageistreatedasdatabeingtransported– thenetworkdoesnotparsetheARPmessageorinterpretfields


ARP Encapsulation

• Thetype fieldintheframeheaderspecifiesthattheframecontainsanARPmessage

• Asendermustassigntheappropriatevaluetothetypefieldbeforetransmittingtheframe

• Areceivermustexaminethetypefieldineachincomingframe• Ethernetusestypefield0x806 todenoteanARPmessage• ThesamevalueisusedforbothARPrequests/responses

– FrametypedoesnotdistinguishbetweentypesofARPmessages

– AreceivermustexaminetheOPERATIONfieldinthemessagetodeterminewhetheranincomingmessageisarequestoraresponse


ARP Caching and Message Processing

• SendinganARPrequestforeachdatagramisinefficient– Threeframestraversethenetworkforeachdatagram

• anARPrequest,ARPresponse,andthedatadatagramitself• Mostcommunicationsinvolveasequenceofpackets

– asenderislikelytorepeattheexchangemanytimes• Toreducenetworktraffic

– ARPsoftwareextractsandsavestheinformationfromaresponse• soitcanbeusedforsubsequentpackets

– Thesoftwaredoesnotkeeptheinformationindefinitely• Instead,ARPmaintainsasmalltableofbindingsinmemory



• ARPmanagesthetableasacache– anentryisreplacedwhenaresponsearrives– theoldestentryisremovedwheneverthetablerunsoutofspaceorafteranentryhasnotbeenupdatedforalongperiodoftime

– ARPstartsbysearchingthecachewhenitneedstobindanaddress

• ARPentriesexpireafter~30stoavoidsendingpacketstothewrongdestinationifthemappingIP-MACchanges



• Ifthebindingispresentinthecache– ARPusesthebindingwithouttransmittingarequest

• Ifthebindingisnotpresentinthecache– ARPbroadcastsarequest– waitsforaresponse– updatesthecache– sendthepacket

• ThecacheisupdatedwhenanARPmessagearrives– eitherarequestoraresponse– sincetrafficisnormallytwo-wayupdatingthecacheonrequestsreducesoverhead


ICMP: INTERNET CONTROL MESSAGE PROTOCOL

Messaggidicontrollo,segnalazione,erroreallivelloIP


Internet Control Message Protocol

• IPincludesacompanionprotocol,ICMP– Itisusedtoreporterrorsbacktotheoriginalsource

• IPandICMPareco-dependent– IPdependsonICMPtoreporterrors– andICMPusesIPtocarryerrormessages

• ICMPcanbeseenasasignalingprotocolfornetworkmanagementandmaintenance

• ManyICMPmessageshavebeendefined





• ICMPcontainstwomessagetypes:– messagesusedtoreporterrors

• e.g.,TimeExceeded andDestinationUnreachable– messagesusedtoobtaininformation

• e.g.,EchoRequest andEchoReply• EchoRequest/Replyareusedbythepingapplicationtotest

connectivity– Whenahostreceivesanechorequestmessage

• ICMPsoftwareonahostorroutersendsanechoreplythatcarriesthesamedataastherequest


ICMP Format and Encapsulation

• ICMPusesIPtotransportmessages:– whenarouterhasanICMPmessagetosend

• createsanIPdatagramandencapsulatestheICMPmessageinit

– theICMPmessageisthepayloadareaoftheIPdatagram– thedatagramis forwardedasusual


ICMP handling

• ICMPmessagesdonothavespecialpriority– Theyareforwardedlikeanyotherdatagram,withoneminorexception

• IfanICMPerrormessagecausesanerror– noerrormessageissent

• Thereasonshouldbeclear:– thedesignerswantedtoavoidtheInternetbecomingcongestedcarryingerrormessagesabouterrormessages


Uso di ICMP

• Comando“ping”– Echo Request +Echo Replay

• Comandotraceroute– IlmittenteinvianormalipacchettiIPconTTLsettatoa1,2,3,...– ConTTL=1,ilprimorouterdecrementaTTLchearrivaa0,quindiilpacchettovienescartatoeilroutermanda(dovrebbemandare)unmessaggioICMLTimeExceeded

– ConTTL=2ilprimorouterdecrementaeinoltra,ilsecondo...– Ecosì via

• Esempi“live”– Conping misuroRTT,conTraceroute capiscochestradafailmiopacchetto


DHCP: DYNAMIC HOSTCONFIGURATION PROTOCOL

Comebootstrappare unaretesenzadoverconfigurareisingolihost


Protocol Parameters and Configuration

• Onceahostorrouterhasbeenpoweredon,OSisstartedandthenetworksoftwareisinitialized

• Howdoesthenetworksoftwareinahostorrouterbeginoperation?

• Forarouter,theconfigurationmanagermustspecifyinitialvaluesforitemssuchas– theIPaddressforeachnetworkinterface– theprotocolsoftwaretorun– andinitialvaluesforaforwardingtable– theconfigurationissaved,andarouterloadsthevaluesduringstartup

• Hostconfigurationusuallyusesatwo-stepprocess,knownasbootstrapping– DHCPisusedtotakecareofmostconfigurationneeds


4-28

Scenario client-server DHCP

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

DHCP server

Il client DHCP in arrivo su questa reteha bisogno diun indirizzo

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Dynamic Host Conf. Protocol (DHCP)

• Whenacomputerboots– theDHCPclientbroadcastsaDHCPRequest– theserver(s)sendaDHCPReply

• aserverreplyiscalledoffer• theserverisofferinganaddresstotheclient

• WecanconfigureaDHCPservertosupplytwotypesofaddresses:– permanentlyassignedaddresses– apoolofdynamicaddressestobeallocatedondemand

• Typically,apermanentaddressisassignedtoaserver,andadynamicaddressisassignedtoanarbitraryhost

• Addressesassignedondemandarenotgivenoutforanarbitrarylengthoftime


serverDHCP:223.1.2.5 Nuovohost

tempo

Identificazione DHCP

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

Offerta DHCPsrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

Richiesta DHCPsrc: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

Conferma DHCPsrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

Scenario client-server DHCP


Dynamic Host Conf Protocol (DHCP)

• DHCPissuesaleaseontheaddressforafiniteperiod– TheuseofleasesallowsaDHCPservertoreclaimaddresses

• Whentheleaseexpires– theserverplacestheaddresstothepoolofavailableaddresses

• Whenaleaseexpires,ahostcanchoosetorelinquishtheaddressorrenegotiatewithDHCPtoextendthelease– Negotiationoccursconcurrentwithotheractivity

• Normally,DHCPapproveseachleaseextension– Acomputercontinuestooperatewithoutanyinterruption– However,aservermaybeconfiguredtodenyleaseextensionforadministrativeortechnicalreasons

– DHCPgrantsabsolutecontrolofleasingtoaserver– Ifaserverdeniesanextensionrequest

• thehostmuststopusingtheaddress


DHCP Protocol Operation

• Recoveryfromlossorduplication– DHCPisdesignedtoinsurethatmissingorduplicatepacketsdonotresultinmisconfiguration

– Ifnoresponseisreceived• ahostretransmitsitsrequest

– Ifaduplicateresponsearrives• ahostignorestheextracopy

• Cachingofaserveraddress– onceahostfindsaDHCPserver

• thehostcachestheserver'saddress• Avoidanceofsynchronizedflooding

– DHCPtakesstepstopreventsynchronizedrequests


DHCP Message Format


DHCP Message Format

• OPspecifieswhetherthemessageisaRequestoraResponse• HTYPEandHLENfieldsspecifythenetworkhardwaretypeand

thelengthofahardwareaddress• FLAGSspecifieswhetheritcanreceivebroadcastordirected

replies• HOPSspecifieshowmanyserversforwardedtherequest• TRANSACTIONIDENTIFIERprovidesavaluethataclientcanuse

todetermineifanincomingresponsematchesitsrequest• SECONDSELAPSEDspecifieshowmanysecondshaveelapsed

sincethehostbegantoboot• ExceptforOPTIONS(OP),eachfieldinaDHCPmessagehasa

fixedsize


DHCP Message Format

• Laterfieldsinthemessageareusedinaresponsetocarryinformationbacktothehostthatsentarequest– ifahostdoesnotknowitsIPaddress,theserverusesfieldYOURIPADDRESStosupplythevalue

– serverusesfieldsSERVERIPADDRESSandSERVERHOSTNAMEtogivethehostinformationaboutthelocationofaserver

– ROUTERIPADDRESScontainstheIPaddressofadefaultrouter• DHCPallowsacomputertonegotiatetofindabootimage

– Todoso,thehostfillsinfieldBOOTFILENAMEwitharequest– TheDHCPserverdoesnotsendanimage


Documents

Livello Rete ARP – ICMP - DHCP - Home page | …disi.unitn.it/locigno/didattica/reti/15-16/05.2_ARP-ICMP-DHCP.pdf · Reti (già “Reti di ... – a pool of dynamic addresses to