3. 第三章课后习题及答案

...第三章1.(Q1)Supposethenetworklayerprovidesthefollowingservice.Thenetworklayerinthesourcehostacceptsasegmentofmaximumsize1,200bytesandadestinationhostaddressfromthetransportlayer.Thenetworklayerthenguaranteestodeliverthesegmenttothetransportlayeratthedestinationhost.Supposemanynetworkapplicationprocessescanberunningatthedestinationhost.a.Designthesimplestpossibletransport-layerprotocolthatwillgetapplicationdatatothedesiredprocessatthedestinationhost.Assumetheoperatingsysteminthedestinationhosthasassigneda4-byteportnumbertoeachrunningapplicationprocess.b.Modifythisprotocolsothatitprovidesa“returnaddress”tothedestinationprocess.c.Inyourprotocols,doesthetransportlayer“havetodoanything”inthecoreofthecomputernetwork.Answer:a.CallthisprotocolSimpleTransportProtocol(STP).Atthesenderside,STPacceptsfromthesendingprocessachunkofdatanotexceeding1196bytes,adestinationhostaddress,andadestinationportnumber.STPaddsafour-byteheadertoeachchunkandputstheportnumberofthedestinationprocessinthisheader.STPthengivesthedestinationhostaddressandtheresultingsegmenttothenetworklayer.ThenetworklayerdeliversthesegmenttoSTPatthedestinationhost.STPthenexaminestheportnumberinthesegment,extractsthedatafromthesegment,andpassesthedatatotheprocessidentifiedbytheportnumber.b.Thesegmentnowhastwoheaderfields:asourceportfieldanddestinationportfield.Atthesenderside,STPacceptsachunkofdatanotexceeding1192bytes,adestinationhostaddress,asourceportnumber,andadestinationportnumber.STPcreatesasegmentwhichcontainstheapplicationdata,sourceportnumber,anddestinationportnumber.Itthengivesthesegmentandthedestinationhostaddresstothenetworklayer.Afterreceivingthesegment,STPatthereceivinghostgivestheapplicationprocesstheapplicationdataandthesourceportnumber.c.No,thetransportlayerdoesnothavetodoanythinginthecore;thetransportlayer“lives”intheendsystems.2.(Q2)Consideraplanetwhereeveryonebelongstoafamilyofsix,everyfamilylivesinitsownhouse,eachhousehasauniqueaddress,andeachpersoninagivenhousehasauniquename.Supposethisplanethasamailservicethatdeliverslettersformsourcehousetodestinationhouse.Themailservicerequiresthat(i)theletterbeinanenvelopeandthat(ii)theaddressofthedestinationhouse(andnothingmore)beclearlywrittenontheenvelope.Supposeeachfamilyhasadelegatefamilymemberwhocollectsanddistributeslettersfortheotherfamilymembers.Thelettersdonotnecessarilyprovideanyindicationoftherecipientsoftheletters.a.UsingthesolutiontoProblemQ1aboveasinspiration,describeaprotocolthatthedelegatescanusetodeliverlettersfromasendingfamilymembertoareceivingfamilymember.b.Inyourprotocol,doesthemailserviceeverhavetoopentheenvelopeandexaminetheletterinordertoprovideitsservice.Answer:a.Forsendingaletter,thefamilymemberisrequiredtogivethedelegatetheletteritself,theaddressofthedestinationhouse,andthenameoftherecipient.Thedelegateclearlywritestherecipient’snameonthetopoftheletter.Thedelegatethenputstheletterinanenvelopeandwritestheaddressofthedestinationhouseontheenvelope.Thedelegatethengivesthelettertotheplanet’smailservice.Atthereceivingside,thedelegatereceivestheletterfromthemailservice,takestheletteroutoftheenvelope,andtakesnoteoftherecipientnamewrittenatthetopoftheletter.Thedelegatethangivesthelettertothefamilymemberwiththisname.b.No,themailservicedoesnothavetoopentheenvelope;itonlyexaminestheaddressontheenvelope.3.(Q3)DescribewhyanapplicationdevelopermightchoosetorunanapplicationoverUDPratherthanTCP.Answer:AnapplicationdevelopermaynotwantitsapplicationtouseTCP’scongestioncontrol,whichcanthrottletheapplication’ssendingrateattimesofcongestion.Often,designersofIPtelephonyandIPvideoconferenceapplicationschoosetoruntheirapplicationsoverUDPbecausetheywanttoavoidTCP’scongestioncontrol.Also,someapplicationsdonotneedthereliabledatatransferprovidedbyTCP.4.(P1)SupposeClientAinitiatesaTelnetsessionwithServerS.Ataboutthesametime,ClientBalsoinitiatesaTelnetsessionwithServerS.Providepossiblesourceanddestinationportnumbersfora.ThesegmentsentfromAtoB.b.ThesegmentsentfromBtoS.c.ThesegmentsentfromStoA.d.ThesegmentsentfromStoB.e.IfAandBaredifferenthosts,isitpossiblethatthesourceportnumberinthesegmentfromAtoSisthesameasthatfromBtoS?f.Howaboutiftheyarethesamehost?Answer:sourceportnumbersdestinationportnumbersaA→S46723bB→S51323cS→A23467dS→B23513eYes.fNo.5.(P2)ConsiderFigure3.5Whatarethesourceanddestinationportvaluesinthesegmentsflowingformtheserverbacktotheclients’processes?WhataretheIPaddressesinthenetwork-layerdatagramscarryingthetransport-layersegments?Answer:SupposetheIPaddressesofthehostsA,B,andCarea,b,c,respectively.(Notethata,b,caredistinct.)TohostA:Sourceport=80,sourceIPaddress=b,destport=26145,destIPaddress=aTohostC,leftprocess:Sourceport=80,sourceIPaddress=b,destport=7532,destIPaddress=cTohostC,rightprocess:Sourceport=80,sourceIPaddress=b,destport=26145,destIPaddress=c6.(P3)UDPandTCPuse1scomplementfortheirchecksums.Supposeyouhavethefollowingthree8-bitbytes:01101010,01001111,01110011.Whatisthe1scomplementofthesumofthese8-bitbytes?(NotethatalthoughUDPandTCPuse16-bitwordsincomputingthechecksum,forthisproblemyouarebeingaskedtoconsider8-bitsums.)Showallwork.WhyisitthatUDPtakesthe1scomplementofthesum;thatis,whynotjustsuethesum?Withthe1scomplementscheme,howdoesthereceiverdetecterrors?Isitpossiblethata1-biterrorwillgoundetected?Howabouta2-biterror?Answer:01010101+011100001100010111000101+0100110000010001One'scomplement=11101110.Todetecterrors,thereceiveraddsthefourwords(thethreeoriginalwordsandthechecksum).Ifthesumcontainsazero,thereceiverknowstherehasbeenanerror.Allone-biterrorswillbedetected,buttwo-biterrorscanbeundetected(e.g.,ifthelastdigitofthefirstwordisconvertedtoa0andthelastdigitofthesecondwordisconvertedtoa1).7.(P4)SupposethattheUDPreceivercomputestheInternetchecksumforthereceivedUDPsegmentandfindsthatitmatchesthevaluecarriedinthechecksumfield.Canthereceiverbeabsolutelycertainthatnobiterrorshaveoccurred?Explain.Answer:No,thereceivercannotbeabsolutelycertainthatnobiterrorshaveoccurred.Thisisbecauseofthemannerinwhichthechecksumforthepacketiscalculated.Ifthecorrespondingbits(thatwouldbeaddedtogether)oftwo16-bitwordsinthepacketwere0and1thenevenifthesegetflippedto1and0respectively,thesumstillremainsthesame.Hence,the1scomplementthereceivercalculateswillalsobethesame.Thismeansthechecksumwillverifyeveniftherewastransmissionerror.8.(P5)a.Supposeyouhavethefollowing2bytes:01001010and01111001.Whatisthe1scomplementofsumofthese2bytes?b.Supposeyouhavethefollowing2bytes:11110101and01101110.Whatisthe1scomplementofsumofthese2bytes?c.Forthebytesinpart(a),giveanexamplewhereonebitisflippedineachofthe2bytesandyetthe1scomplementdoesn’tchange.Answer:a.Addingthetwobytesgives10011101.Takingtheone’scomplementgives01100010b.Addingthetwobytesgives00011110;theone’scomplementgives11100001.c.firstbyte=00110101;secondbyte=01101000.9.(P6)Considerourmotivationforcorrectingprotocolrdt2.1.Showthatthereceiver,showninthefigureonthefollowingpage,whenoperatingwiththesendershowinFigure3.11,canleadthesenderandreceivertoenterintoadeadlockstate,whereeachiswaitingforaneventthatwillneveroccur.Answer:Supposethesenderisinstate“Waitforcall1fromabove”andthereceiver(thereceivershowninthehomeworkproblem)isinstate“Waitfor1frombelow.”Thesendersendsapacketwithsequencenumber1,andtransitionsto“WaitforACKorNAK1,”waitingforanACKorNAK.Supposenowthereceiverreceivesthepacketwithsequencenumber1correctly,sendsanACK,andtransitionstostate“Waitfor0frombelow,”waitingforadatapacketwithsequencenumber0.However,theACKiscorrupted.Whentherdt2.1sendergetsthecorruptedACK,itresendsthepacketwithsequencenumber1.However,thereceiveriswaitingforapacketwithsequencenumber0and(asshowninthehomeworkproblem)alwayssendsaNAKwhenitdoesn'tgetapacketwithsequencenumber0.Hencethesenderwillalwaysbesendingapacketwithsequencenumber1,andthereceiverwillalwaysbeNAKingthatpacket.Neitherwillprogressforwardfromthatstate.10.(P7)DrawtheFSMforthereceiversideofprotocolrdt3.0Answer:Thesendersideofprotocolrdt3.0differsfromthesendersideofprotocol2.2inthattimeoutshavebeenadded.Wehaveseenthattheintroductionoftimeoutsaddsthepossibilityofduplicatepacketsintothesender-to-receiverdatastream.However,thereceiverinprotocolrdt.2.2canalreadyhandleduplicatepackets.(Receiver-sideduplicatesinrdt2.2wouldariseifthereceiversentanACKthatwaslost,andthesenderthenretransmittedtheolddata).Hencethereceiverinprotocolrdt2.2willalsoworkasthereceiverinprotocolrdt3.0.11.(P8)Inprotocolrdt3.0,theACKpacketsflowingfromthereceivertothesenderdonothavesequencenumbers(althoughtheydohaveanACKfieldthatcontainsthesequencenumberofthepackettheyareacknowledging).WhyisitthatourACKpacketsdonotrequiresequencenumbers?Answer:TobestAnswerthisquestion,considerwhyweneededsequencenumbersinthefirstplace.Wesawthatthesenderneedssequencenumberssothatthereceivercantellifadatapacketisaduplicateofanalreadyreceiveddatapacket.InthecaseofACKs,thesenderdoesnotneedthisinfo(i.e.,asequencenumberonanACK)totelldetectaduplicateACK.AduplicateACKisobvioustotherdt3.0receiver,sincewhenithasreceivedtheoriginalACKittransitionedtothenextstate.TheduplicateACKisnottheACKthatthesenderneedsandhenceisignoredbytherdt3.0sender.12.(P9)Giveatraceoftheoperationofprotocolrdt3.0whendatapacketsandacknowledgmentpacketsaregarbled.YourtraceshouldbesimilartothatusedinFigure3.16Answer:Supposetheprotocolhasbeeninoperationforsometime.Thesenderisinstate“Waitforcallfromabove”(toplefthandcorner)andthereceiverisinstate“Waitfor0frombelow”.ThescenariosforcorrupteddataandcorruptedACKareshowninFigure1.13.(P10)Considerachannelthatcanlosepacketsbuthasamaximumdelaythatisknown.Modifyprotocolrdt2.1toincludesendertimeoutandretransmit.Informallyarguewhyyourprotocolcancommunicatecorrectlyoverthischannel.Answer:Here,weaddatimer,whosevalueisgreaterthantheknownround-trippropagationdelay.Weaddatimeouteventtothe“WaitforACKorNAK0”and“WaitforACKorNAK1”states.Ifthetimeouteventoccurs,themostrecentlytransmittedpacketisretransmitted.Letusseewhythisprotocolwillstillworkwiththerdt2.1receiver.•Supposethetimeoutiscausedbyalostdatapacket,i.e.,apacketonthesenderto-receiverchannel.Inthiscase,thereceiverneverreceivedtheprevioustransmissionand,fromthereceiver'sviewpoint,ifthetimeoutretransmissionisreceived,itlookexactlythesameasiftheoriginaltransmissionisbeingreceived.•SupposenowthatanACKislost.Thereceiverwilleventuallyretransmitthepacketonatimeout.ButaretransmissionisexactlythesameactionthatistakeifanACKisgarbled.Thusthesender'sreactionisthesamewithaloss,aswithagarbledACK.Therdt2.1receivercanalreadyhandlethecaseofagarbledACK.14.(P11)Considertherdt3.0protocol.Drawadiagramshowingthatifthenetworkconnectionbetweenthesenderandreceivercanreordermessages(thatis,thattwomessagespropagatinginthemediumbetweenthesenderandreceivercanbereordered),thenthealternating-bitprotocolwillnotworkcorrectly(makesureyouclearlyidentifythesenseinwhichitwillnotworkcorrectly).Yourdiagramshouldhavethesenderontheleftandthereceiverontheright,withthetimeaxisrunningdownthepage,showingdata(D)andacknowledgement(A)messageexchange.Makesureyouindicatethesequencenumberassociatedwithanydataoracknowledgementsegment.Answer:15.(P12)Thesendersideofrdt3.0simplyignores(thatis,takesnoactionon)allreceivedpacketsthatareeitherinerrororhavethewrongvalueintheack-numfieldofanacknowledgementpacket.Supposethatinsuchcircumstances,rdt3.0weresimplytoretransmitthecurrentdatapacket.Wouldtheprotocolstillwork?(hint:Considerwhatwouldhappeniftherewereonlybiterrors;therearenopacketlossesbutprematuretimeoutcanoccur.Considerhowmanytimesthenthpacketissent,inthelimitasnapproachesinfinity.)Answer:Theprotocolwouldstillwork,sincearetransmissionwouldbewhatwouldhappenifthepacketreceivedwitherrorshasactuallybeenlost(andfromthereceiverstandpoint,itneverknowswhichoftheseevents,ifeither,willoccur).Togetatthemoresubtleissuebehindthisquestion,onehastoallowforprematuretimeoutstooccur.Inthiscase,ifeachextracopyofthepacketisACKedandeachreceivedextraACKcausesanotherextracopyofthecurrentpackettobesent,thenumberoftimespacketnissentwillincreasewithoutboundasnapproachesinfinity.16.(P13)Considerareliabledatatransferprotocolthatusesonlynegativeacknowledgements.Supposethesendersendsdataonlyinfrequently.WouldaNAK-onlyprotocolbepreferabletoaprotocolthatusesACKs?Why?Nowsupposethesenderhasalotofdatatosendandtheendtoendconnectionexperiencesfewlosses.Inthissecondcase,wouldaNAK-onlyprotocolbepreferabletoaprotocolthatusesACKs?Why?Answer:InaNAKonlyprotocol,thelossofpacketxisonlydetectedbythereceiverwhenpacketx+1isreceived.Thatis,thereceiversreceivesx-1andthenx+1,onlywhenx+1isreceiveddoesthereceiverrealizethatxwasmissed.Ifthereisalongdelaybetweenthetransmissionofxandthetransmissionofx+1,thenitwillbealongtimeuntilxcanberecovered,underaNAKonlyprotocol.Ontheotherhand,ifdataisbeingsentoften,thenrecoveryunderaNAK-onlyschemecouldhappenquickly.Moreover,iferrorsareinfrequent,thenNAKsareonlyoccasionallysent(whenneeded),andACKareneversent–asignificantreductioninfeedbackintheNAK-onlycaseovertheACK-onlycase.17.(P14)Considerthecross-countryexampleshowninFigure3.17.Howbigwouldthewindowsizehavetobeforthechannelutilizationtobegreaterthan80percent?Answer:Ittakes8microseconds(or0.008milliseconds)tosendapacket.inorderforthesendertobebusy90percentofthetime,wemusthaveutil=0.9=(0.008n)/30.016ornapproximately3377packets.18.(P15)ConsiderascenarioinwhichHostAwantstosimultaneouslysendpacketstoHostBandC.AisconnectedtoBandCviaabroadcastchannel—apacketsentbyAiscarriedbythechanneltobothBandC.SupposethatthebroadcastchannelconnectingA,B,andCcanindependentlyloseandcorruptpackets(andso,forexample,apacketsentfromAmightbecorrectlyreceivedbyB,butnotbyC).Designastop-and-wait-likeerror-controlprotocolforreliabletransferringpacketsfromAtoBandC,suchthatAwillnotgetnewdatafromtheupperlayeruntilitknowsthatBandChavecorrectlyreceivedthecurrentpacket.GiveFSMdescriptionsofAandC.(Hint:TheFSMforBshouldbeessentiallybesameasforC.)Also,giveadescriptionofthepacketformat(s)used.Answer:Inoursolution,thesenderwillwaituntilitreceivesanACKforapairofmessages(seqnumandseqnum+1)beforemovingontothenextpairofmessages.Datapacketshaveadatafieldandcarryatwo-bitsequencenumber.Thatis,thevalidsequencenumbersare0,1,2,and3.(Note:youshouldthinkaboutwhya1-bitsequencenumberspaceof0,1onlywouldnotworkinthesolutionbelow.)ACKmessagescarrythesequencenumberofthedatapackettheyareacknowledging.TheFSMforthesenderandreceiverareshowninFigure2.Notethatthesenderstaterecordswhether(i)noACKshavebeenreceivedforthecurrentpair,(ii)anACKforseqnum(only)hasbeenreceived,oranACKforseqnum+1(only)hasbeenreceived.Inthisfigure,weassumethattheseqnumisinitially0,andthatthesenderhassentthefirsttwodatamessages(togetthingsgoing).Atimelinetraceforthesenderandreceiverrecoveringfromalostpacketisshownbelow:SenderReceivermakepair(0,1)sendpacket0Packet0dropssendpacket1receivepacket1bufferpacket1sendACK1receiveACK1(timeout)resendpacket0receivepacket0deliverpair(0,1)sendACK0receiveACK019.(P16)ConsiderascenarioinwhichHostAandHostBwanttosendmessagestoHostC.HostsAandCareconnectedbyachannelthatcanloseandcorrupt(butnotreorder)message.HostsBandCareconnectedbyanotherchannel(independentofthechannelconnectingAandC)withthesameproperties.ThetransportlayeratHostCshouldalternateindeliveringmessagesfromAandBtothelayerabove(thatis,itshouldfirstdeliverthedatafromapacketfromA,thenthedatafromapacketfromB,andsoon).Designastop-and-wait-likeerror-controlprotocolforreliabletransferringpacketsfromAtoBandC,withalternatingdeliveryatCasdescribedabove.GiveFSMdescriptionsofAandC.(Hint:TheFSMforBshouldbeessentiallybesameasforA.)Also,giveadescriptionofthepacketformat(s)used.Answer:Thisproblemisavariationonthesimplestopandwaitprotocol(rdt3.0).Becausethechannelmaylosemessagesandbecausethesendermayresendamessagethatoneofthereceivershasalreadyreceived(eitherbecauseofaprematuretimeoutorbecausetheotherreceiverhasyettoreceivethedatacorrectly),sequencenumbersareneeded.Asinrdt3.0,a0-bitsequencenumberwillsufficehere.ThesenderandreceiverFSMareshowninFigure3.Inthisproblem,thesenderstateindicateswhetherthesenderhasreceivedanACKfromB(only),fromC(only)orfromneitherCnorB.Thereceiverstateindicateswhichsequencenumberthereceiveriswaitingfor.20.(P17)InthegenericSRprotocolthatwestudiedinSection3.4.4,thesendertransmitsamessageassoonasitisavailable(ifitisinthewindow)withoutwaitingforanacknowledgment.SupposenowthatwewantanSRprotocolthatsendsmessagestwoatatime.Thatis,thesenderwillsendapairofmessagesandwillsendthenextpairofmessagesonlywhenitknowsthatbothmessagesinthefirstpairhavebeenreceivercorrectly.Supposethatthechannelmaylosemessagesbutwillnotcorruptorreordermessages.Designanerror-controlprotocolfortheunidirectionalreliabletransferofmessages.GiveanFSMdescriptionofthesenderandreceiver.Describetheformatofthepacketssentbetweensenderandreceiver,andviceversa.IfyouuseanyprocedurecallsotherthanthoseinSection3.4(forexample,udt_send(),start_timer(),rdt_rcv(),andsoon),clearlystatetheiractions.Giveanexample(atimelinetraceofsenderandreceiver)showinghowyourprotocolrecoversfromalostpacket.Answer:21.(P18)ConsidertheGBNprotocolwithasenderwindowsizeof3andasequencenumberrangeof1024.Supposethatattimet,thenextin-orderpacketthatthereceiverisexpectinghasasequencenumberofk.Assumethatthemediumdoesnotreordermessages.Answerthefollowingquestions:a.Whatarethepossiblesetsofsequencenumberinsidethesender’swindowattimet?JustifyyourAnswer.b.WhatareallpossiblevaluesoftheACKfieldinallpossiblemessagescurrentlypropagatingbacktothesenderattimet?JustifyyourAnswer.Answer:a.HerewehaveawindowsizeofN=3.Supposethereceiverhasreceivedpacketk-1,andhasACKedthatandallotherpreceedingpackets.IfalloftheseACK'shavebeenreceivedbysender,thensender'swindowis[k,k+N-1].SupposenextthatnoneoftheACKshavebeenreceivedatthesender.Inthissecondcase,thesender'swindowcontainsk-1andtheNpacketsuptoandincludingk-1.Thesender'swindowisthus[k-N,k-1].Bythesearguments,thesenderswindowisofsize3andbeginssomewhereintherange[k-N,k].b.Ifthereceiveriswaitingforpacketk,thenithasreceived(andACKed)packetk-1andtheN-1packetsbeforethat.IfnoneofthoseNACKshavebeenyetreceivedbythesender,thenACKmessageswithvaluesof[k-N,k-1]maystillbepropagatingback.Becausethesenderhassentpackets[k-N,k-1],itmustbethecasethatthesenderhasalreadyreceivedanACKfork-N-1.OncethereceiverhassentanACKfork-N-1itwillneversendanACKthatislessthatk-N-1.Thustherangeofin-flightACKvaluescanrangefromk-N-1tok-1.22.(P19)AnswertrueorfalsetothefollowingquestionsandbrieflyjustifyyourAnswer.a.WiththeSRprotocol,itispossibleforthesendertoreceiveanACKforapacketthatfallsoutsideofitscurrentwindow.b.WithCBN,itispossibleforthesendertoreceiveranACKforapacketthatfallsoutsideofitscurrentwindow.c.Thealternating-bitprotocolisthesameastheSRprotocolwithasenderandreceiverwindowsizeof1.d.Thealternating-bitprotocolisthesameastheGBNprotocolwithasenderandreceiverwindowsizeof1.Answer:a.True.Supposethesenderhasawindowsizeof3andsendspackets1,2,3att0.Att1(t1>t0)thereceiverACKS1,2,3.Att2(t2>t1)thesendertimesoutandresends1,2,3.Att3thereceiverreceivestheduplicatesandre-acknowledges1,2,3.Att4thesenderreceivestheACKsthatthereceiversentatt1andadvancesitswindowto4,5,6.Att5thesenderreceivestheACKs1,2,3thereceiversentatt2.TheseACKsareoutsideitswindow.b.True.Byessentiallythesamescenarioasin(a).c.True.d.True.Notethatwithawindowsizeof1,SR,GBN,andthealternatingbitprotocolarefunctionallyequivalent.Thewindowsizeof1precludesthepossibilityofout-of-orderpackets(withinthewindow).AcumulativeACKisjustanordinaryACKinthissituation,sinceitcanonlyrefertothesinglepacketwithinthewindow.23.(Q4)WhyisitthatvoiceandvideotrafficisoftensentoverTCPratherthanUDPintoday’sInternet.(Hint:TheAnswerwearelookingforhasnothingtodowithTCP’scongestion-controlmechanism.)Answer:SincemostfirewallsareconfiguredtoblockUDPtraffic,usingTCPforvideoandvoicetrafficletsthetrafficthoughthefirewalls24.(Q5)IsitpossibleforanapplicationtoenjoyreliabledatatransferevenwhentheapplicationrunsoverUDP?Ifso,how?Answer:Yes.Theapplicationdevelopercanputreliabledatatransferintotheapplicationlayerprotocol.Thiswouldrequireasignificantamountofworkanddebugging,however.25.(Q6)ConsideraTCPconnectionbetweenHostAandHostB.SupposethattheTCPsegmentstravelingformHostAtoHostBhavesourceportnumberxanddestinationportnumbery.Whatarethesourceanddestinationp