CS6250 Computer Networks Exam 1
Questions And Answers
●kDescribekthekrelationshipskbetweenkISPs,kIXPs,kandkCDNs.k-kCORRECTkkANSWER-
ThekInternetkiskakcomplexkecosystem.kItkiskbuiltkwithkaknetworkkofknetworks.kTheseknetwork
skincludek(ISPs)kInternetkServicekProviders,k(IXPs)kInternetkExchangekPoints,k(CDNs)kCon
tentkDeliverykNetworks.kInk2019,ktherekwerekapx.k500kIXPskaroundkthekworld.
largekscalekTier-
1kISPskthatkoperatekatkakglobalkscale,kandkessentiallyktheykformkthek"backbone"knetworkkov
erkwhichksmallerknetworkskcankconnect
IXPskarekinterconnectionkinfrastructures,kwhichkprovidekthekphysicalkinfrastructure,kwherek
multipleknetworksk(egkISPskandkCDNs)kcankinterconnectkandkexchangektrafficklocally.
CDNskareknetworkskthatkarekcreatedkbykcontentkproviderskwithkthekgoalkofkhavingkgreaterkc
ontrolkofkhowkthekcontentkiskdeliveredktokthekend-
users,kandkalsoktokreducekconnectivitykcosts.kSomekexamplekCDNskincludekGooglekandkN
etflix.kTheseknetworkskhavekmultiplekdatakcentersk-
kandkeachkonekofkthemkmaykbekhousingkhundredskofkserversk-
kthatkarekdistributedkacrosskthekworld.
●kExplainkAdditivekIncrease/MultiplicativekDecreasek(AIMD)kinkthekcontextkofkTCP.k-
kCORRECTkkANSWER-
TCPkdecreaseskthekwindowkwhenktheklevelkofkcongestionkgoeskbykhalvingkthekwindowksize,
kandkitkincreaseskthekwindowkwhenktheklevelkofkcongestionkgoeskdownkbykaddingktokthekwin
dowksize.kThiskcauseskconvergencektokthekoptimalkbandwidthkbykquicklykcuttingkusekinktime
skofkcongestionkwhilekslowlykincreasingkutilizationkwhenkthekcongestionkclears.
ThekideakbehindkadditivekincreasekisktokincreasekthekwindowkbykonekpacketkeverykRTTk(Ro
undkTripkTime).
OncekTCPkRenokdetectskcongestion,kitkreduceskthekratekatkwhichktheksenderktransmits.kSo,
kwhenkthekTCPksenderkdetectskthatkaktimeoutkoccurred,kthenkitksetskthekCongestionWindow
k(cwnd)ktokhalfkofkitskpreviouskvalue.
●kExplainkhowkTCPkCUBICkworks.k-kCORRECTkkANSWER-
CUBICkuseskakcubedkpolynomialkaskitskgrowthkfunction.kTokmaintainkthekTCPkfairnesskitkus
eskakmultiplicativekdecreasekandkreduceskthekwindowktokhalf.
,TCPkCUBICkiskfairkregardlesskofkRTTkbecausekthekcalculationkthatkiskusedkdependskonkthek
elapsedktimekfromktwokcongestionkevents,kratherkthankbeingkdependentkonkthekRTTkofkakco
nnection.
●kExplainkTCPkthroughputkcalculation.k-kCORRECTkkANSWER-Thekcalculationkis:
Pk=kProbabilitykofkpacketkloss
MSSk=kMaximumkSegmentkSize
RTTk=kRound-TripkTimes
BWk=kdatakperkcyclek/ktimekperkcycle
BWk=kMSS/RTTk*kCk/ksqrt(P)
●kHowkdoeskakhostkinferkcongestion?k-kCORRECTkkANSWER-
Thekhostkinferkcongestionkfromktheknetworkkbehaviorkmainlykthroughk2ksignals:
Firstkiskthekpacketkdelay.kAsktheknetworkkgetskcongested,kthekqueueskinkthekrouterkbufferskb
uildkup.kThiskleadsktokincreasedkpacketkdelays.kThus,kankincreasekinkthekround-
tripktime,kwhichkcankbekestimatedkbasedkonkACKs,kcankbekankindicatorkofkcongestionkinkthek
network.kHowever,kitkturnskoutkthatkpacketkdelaykinkaknetworkktendsktokbekvariable,kmakingk
delay-basedkcongestionkinferencekquitektricky.
Anotherksignalkforkcongestionkiskpacketkloss.kAsktheknetworkkgetskcongested,krouterskstartk
droppingkpackets.kNotekthatkpacketskcankalsokbeklostkduektokotherkreasonsksuchkaskroutingk
errors,khardwarekfailure,kTTLkexpiry,kerrorkinktheklinks,korkflowkcontrolkproblems,kalthoughkitk
iskrare.
●kHowkdoeskakroutekserverkwork?k-kCORRECTkkANSWER-Akroutekserver:
●kCollectskandkshareskroutingkinformationkfromkitskpeerskorkparticipantskthatkconnectskwithk
(i.e.kIXPkmemberskthatkconnectktokthekRS).
●kExecuteskitskownkBGPkdecisionkprocesskandkalsokre-
advertisekthekresultingkinformationk(I.e.kbestkroutekselection)ktokallkRS'skpeerkrouters.
●kAlsokknownkaskMulti-lateralkBGPkpeerksessions.
●kHowkdoeskakrouterkusekthekBGPkdecisionkprocessktokchoosekwhichkroutesktokimport?k-
kCORRECTkkANSWER-
Thekrouterkcompareskrouteskbykgoingkthroughkaklistkofkattributeskandkchooseskthekpathkwithk
theklowestkamountkofkhops,kinktheksimplestkscenario.kThekrouterkuseskakdecisionktoolkcalledk
LocalPrefkattributekwhichkiskthekpreferredkrouteklearnedkthroughkakspecifickASkoverkotherkA
Ses.kAnkASkrankskrouteskbykfirstkpreferringkthekroutesklearnedkfromkitskcustomers,kthenkthek
routesklearnedkfromkitskpeers,kandkfinallykfromkthekroutesklearnedkfromkitskproviders.kAnoth
erkattributekthatkcankaffectkroutingkdecisionskiskthekMEDk(Multi-
ExitkDiscriminator)kattribute.kThiskiskusedkbykASeskconnectedkbykmultipleklinksktokdesignate
kwhichkofkthoseklinkskarekpreferredkforkinboundktraffic.
●kHowkdoeskakTCPksenderklimitktheksendingkrate?k-kCORRECTkkANSWER-
TCPkuseskakcongestionkwindowkwhichkisksimilarktokthekreceivekwindowkusedkforkflowkcontro
l.kItkrepresentskthekmaximumknumberkofkunacknowledgedkdatakthatkaksendingkhostkcankhav
ekinktransitk(sentkbutknotkyetkacknowledged).
, TCPkuseskakprobe-and-
adaptkapproachkinkadaptingkthekcongestionkwindow.kUnderkregularkconditions,kTCPkincrea
seskthekcongestionkwindowktryingktokachievekthekavailablekthroughput.kOncekitkdetectskcon
gestionkthenkthekcongestionkwindowkiskdecreased.
Inkthekend,ktheknumberkofkunacknowledgedkdatakthatkaksenderkcankhavekiskthekminimumkofk
thekcongestionkwindowkandkthekreceivekwindow.
●kHowkdoeskankASkdeterminekwhatkrulesktokimport/export?k-kCORRECTkkANSWER-
ASkbusinesskrelationshipskdrivekankAS'kroutingkpolicieskandkinfluencekwhichkrouteskankASk
needsktokimportkorkexport.
Therekarek3kselectivektransitkroutes:kTransitkcustomerkroutes,kTransitkProviderkRoutes,kPe
erkRoutes.kThekmainkrulekiskfinancialkincentive.kTransitkcustomerkroutesk>kpeerkroutesk>ktra
nsitkproviderkroutes.
Similarlykaskexporting,kASeskarekselectivekaboutkwhichkroutesktokimportkbased,kprimarily,ko
nkwhichkneighboringkASkadvertiseskthemkandkwhatktypekofkbusinesskrelationshipkiskestablis
hed.kAnkASkreceiveskroutekadvertisementskfromkitskcustomers,kproviderskandkpeers.
WhenkankASkreceiveskmultiplekroutekadvertisementsktowardsktheksamekdestination,kfromk
multiplekASes,kthenkitkneedsktokrankkthekrouteskbeforekselectingkwhichkonektokimport.kThekr
outeskthatkarekpreferredkfirstkarekthekcustomerkroutes,kthenkthekpeerkrouteskandkfinallykthekp
roviderkroutes.kThekreasoningkbehindkthiskrankingkiskthatkankAS...
1.kwantsktokensurekthatkroutesktowardskitskcustomerskdoknotktraversekotherkASeskunnecess
arilykgeneratingkcosts,
2.kuseskroutesklearnedkfromkpeersksincekthesekarekusuallyk"free"k(underkthekpeeringkagree
ment),
3.kandkfinallykresortsktokimportkroutesklearnedkfromkproviderskaskthesekwillkaddktokcosts.
●kHowkdoeskpoisonkreverseksolvekthekcount-to-infinitykproblem?k-kCORRECTkkANSWER-
Calledkpoisonkreverse:ksincekzkreacheskxkthroughky,kzkwillkadvertisektokykthatkthekdistancekto
kxkiskinfinityk(Dz(x)=infinity).kHoweverkzkknowskthatkthiskisknotktruekandkDz(x)=5.kzktellskthiskli
ektoky,kasklongkaskitkknowskthatkitkcankreachktokxkviaky.kSincekykassumeskthatkzkhasknokpathkto
kxkexceptkviaky,kitkwillkneverksendkpacketsktokxkviakz.
Sokzkpoisonskthekpathkfromkzktoky.
●kIskTCPkfairkinkthekcasekwherektwokconnectionskhavektheksamekRTT?kExplain.kDifferentkR
TT?k-kCORRECTkkANSWER-InkTCP,kfairnesskmeans:kforkk-
connectionskpassingkthroughkonekcommonklinkkwithkcapacitykRkbps,keachkconnectionkgetsk
ankaveragekthroughputkofkR/k
IfktwokconnectionskhavektheksamekRTTkthenkthekthroughputkforkeachkshouldksumkupktokR.
SincekTCPkrelieskonkacknowledgementskofkreceivedkpackets,kRTTk(roundktripktime)kaffects
kthekconnectionkspeed.kInkAIMD,kakconnectionkwithkakfasterkRTTkwouldkbekablektokrampkfast
erkthankanotherkconnectionkwithkslowerkRTT.
(altkanswer)
Questions And Answers
●kDescribekthekrelationshipskbetweenkISPs,kIXPs,kandkCDNs.k-kCORRECTkkANSWER-
ThekInternetkiskakcomplexkecosystem.kItkiskbuiltkwithkaknetworkkofknetworks.kTheseknetwork
skincludek(ISPs)kInternetkServicekProviders,k(IXPs)kInternetkExchangekPoints,k(CDNs)kCon
tentkDeliverykNetworks.kInk2019,ktherekwerekapx.k500kIXPskaroundkthekworld.
largekscalekTier-
1kISPskthatkoperatekatkakglobalkscale,kandkessentiallyktheykformkthek"backbone"knetworkkov
erkwhichksmallerknetworkskcankconnect
IXPskarekinterconnectionkinfrastructures,kwhichkprovidekthekphysicalkinfrastructure,kwherek
multipleknetworksk(egkISPskandkCDNs)kcankinterconnectkandkexchangektrafficklocally.
CDNskareknetworkskthatkarekcreatedkbykcontentkproviderskwithkthekgoalkofkhavingkgreaterkc
ontrolkofkhowkthekcontentkiskdeliveredktokthekend-
users,kandkalsoktokreducekconnectivitykcosts.kSomekexamplekCDNskincludekGooglekandkN
etflix.kTheseknetworkskhavekmultiplekdatakcentersk-
kandkeachkonekofkthemkmaykbekhousingkhundredskofkserversk-
kthatkarekdistributedkacrosskthekworld.
●kExplainkAdditivekIncrease/MultiplicativekDecreasek(AIMD)kinkthekcontextkofkTCP.k-
kCORRECTkkANSWER-
TCPkdecreaseskthekwindowkwhenktheklevelkofkcongestionkgoeskbykhalvingkthekwindowksize,
kandkitkincreaseskthekwindowkwhenktheklevelkofkcongestionkgoeskdownkbykaddingktokthekwin
dowksize.kThiskcauseskconvergencektokthekoptimalkbandwidthkbykquicklykcuttingkusekinktime
skofkcongestionkwhilekslowlykincreasingkutilizationkwhenkthekcongestionkclears.
ThekideakbehindkadditivekincreasekisktokincreasekthekwindowkbykonekpacketkeverykRTTk(Ro
undkTripkTime).
OncekTCPkRenokdetectskcongestion,kitkreduceskthekratekatkwhichktheksenderktransmits.kSo,
kwhenkthekTCPksenderkdetectskthatkaktimeoutkoccurred,kthenkitksetskthekCongestionWindow
k(cwnd)ktokhalfkofkitskpreviouskvalue.
●kExplainkhowkTCPkCUBICkworks.k-kCORRECTkkANSWER-
CUBICkuseskakcubedkpolynomialkaskitskgrowthkfunction.kTokmaintainkthekTCPkfairnesskitkus
eskakmultiplicativekdecreasekandkreduceskthekwindowktokhalf.
,TCPkCUBICkiskfairkregardlesskofkRTTkbecausekthekcalculationkthatkiskusedkdependskonkthek
elapsedktimekfromktwokcongestionkevents,kratherkthankbeingkdependentkonkthekRTTkofkakco
nnection.
●kExplainkTCPkthroughputkcalculation.k-kCORRECTkkANSWER-Thekcalculationkis:
Pk=kProbabilitykofkpacketkloss
MSSk=kMaximumkSegmentkSize
RTTk=kRound-TripkTimes
BWk=kdatakperkcyclek/ktimekperkcycle
BWk=kMSS/RTTk*kCk/ksqrt(P)
●kHowkdoeskakhostkinferkcongestion?k-kCORRECTkkANSWER-
Thekhostkinferkcongestionkfromktheknetworkkbehaviorkmainlykthroughk2ksignals:
Firstkiskthekpacketkdelay.kAsktheknetworkkgetskcongested,kthekqueueskinkthekrouterkbufferskb
uildkup.kThiskleadsktokincreasedkpacketkdelays.kThus,kankincreasekinkthekround-
tripktime,kwhichkcankbekestimatedkbasedkonkACKs,kcankbekankindicatorkofkcongestionkinkthek
network.kHowever,kitkturnskoutkthatkpacketkdelaykinkaknetworkktendsktokbekvariable,kmakingk
delay-basedkcongestionkinferencekquitektricky.
Anotherksignalkforkcongestionkiskpacketkloss.kAsktheknetworkkgetskcongested,krouterskstartk
droppingkpackets.kNotekthatkpacketskcankalsokbeklostkduektokotherkreasonsksuchkaskroutingk
errors,khardwarekfailure,kTTLkexpiry,kerrorkinktheklinks,korkflowkcontrolkproblems,kalthoughkitk
iskrare.
●kHowkdoeskakroutekserverkwork?k-kCORRECTkkANSWER-Akroutekserver:
●kCollectskandkshareskroutingkinformationkfromkitskpeerskorkparticipantskthatkconnectskwithk
(i.e.kIXPkmemberskthatkconnectktokthekRS).
●kExecuteskitskownkBGPkdecisionkprocesskandkalsokre-
advertisekthekresultingkinformationk(I.e.kbestkroutekselection)ktokallkRS'skpeerkrouters.
●kAlsokknownkaskMulti-lateralkBGPkpeerksessions.
●kHowkdoeskakrouterkusekthekBGPkdecisionkprocessktokchoosekwhichkroutesktokimport?k-
kCORRECTkkANSWER-
Thekrouterkcompareskrouteskbykgoingkthroughkaklistkofkattributeskandkchooseskthekpathkwithk
theklowestkamountkofkhops,kinktheksimplestkscenario.kThekrouterkuseskakdecisionktoolkcalledk
LocalPrefkattributekwhichkiskthekpreferredkrouteklearnedkthroughkakspecifickASkoverkotherkA
Ses.kAnkASkrankskrouteskbykfirstkpreferringkthekroutesklearnedkfromkitskcustomers,kthenkthek
routesklearnedkfromkitskpeers,kandkfinallykfromkthekroutesklearnedkfromkitskproviders.kAnoth
erkattributekthatkcankaffectkroutingkdecisionskiskthekMEDk(Multi-
ExitkDiscriminator)kattribute.kThiskiskusedkbykASeskconnectedkbykmultipleklinksktokdesignate
kwhichkofkthoseklinkskarekpreferredkforkinboundktraffic.
●kHowkdoeskakTCPksenderklimitktheksendingkrate?k-kCORRECTkkANSWER-
TCPkuseskakcongestionkwindowkwhichkisksimilarktokthekreceivekwindowkusedkforkflowkcontro
l.kItkrepresentskthekmaximumknumberkofkunacknowledgedkdatakthatkaksendingkhostkcankhav
ekinktransitk(sentkbutknotkyetkacknowledged).
, TCPkuseskakprobe-and-
adaptkapproachkinkadaptingkthekcongestionkwindow.kUnderkregularkconditions,kTCPkincrea
seskthekcongestionkwindowktryingktokachievekthekavailablekthroughput.kOncekitkdetectskcon
gestionkthenkthekcongestionkwindowkiskdecreased.
Inkthekend,ktheknumberkofkunacknowledgedkdatakthatkaksenderkcankhavekiskthekminimumkofk
thekcongestionkwindowkandkthekreceivekwindow.
●kHowkdoeskankASkdeterminekwhatkrulesktokimport/export?k-kCORRECTkkANSWER-
ASkbusinesskrelationshipskdrivekankAS'kroutingkpolicieskandkinfluencekwhichkrouteskankASk
needsktokimportkorkexport.
Therekarek3kselectivektransitkroutes:kTransitkcustomerkroutes,kTransitkProviderkRoutes,kPe
erkRoutes.kThekmainkrulekiskfinancialkincentive.kTransitkcustomerkroutesk>kpeerkroutesk>ktra
nsitkproviderkroutes.
Similarlykaskexporting,kASeskarekselectivekaboutkwhichkroutesktokimportkbased,kprimarily,ko
nkwhichkneighboringkASkadvertiseskthemkandkwhatktypekofkbusinesskrelationshipkiskestablis
hed.kAnkASkreceiveskroutekadvertisementskfromkitskcustomers,kproviderskandkpeers.
WhenkankASkreceiveskmultiplekroutekadvertisementsktowardsktheksamekdestination,kfromk
multiplekASes,kthenkitkneedsktokrankkthekrouteskbeforekselectingkwhichkonektokimport.kThekr
outeskthatkarekpreferredkfirstkarekthekcustomerkroutes,kthenkthekpeerkrouteskandkfinallykthekp
roviderkroutes.kThekreasoningkbehindkthiskrankingkiskthatkankAS...
1.kwantsktokensurekthatkroutesktowardskitskcustomerskdoknotktraversekotherkASeskunnecess
arilykgeneratingkcosts,
2.kuseskroutesklearnedkfromkpeersksincekthesekarekusuallyk"free"k(underkthekpeeringkagree
ment),
3.kandkfinallykresortsktokimportkroutesklearnedkfromkproviderskaskthesekwillkaddktokcosts.
●kHowkdoeskpoisonkreverseksolvekthekcount-to-infinitykproblem?k-kCORRECTkkANSWER-
Calledkpoisonkreverse:ksincekzkreacheskxkthroughky,kzkwillkadvertisektokykthatkthekdistancekto
kxkiskinfinityk(Dz(x)=infinity).kHoweverkzkknowskthatkthiskisknotktruekandkDz(x)=5.kzktellskthiskli
ektoky,kasklongkaskitkknowskthatkitkcankreachktokxkviaky.kSincekykassumeskthatkzkhasknokpathkto
kxkexceptkviaky,kitkwillkneverksendkpacketsktokxkviakz.
Sokzkpoisonskthekpathkfromkzktoky.
●kIskTCPkfairkinkthekcasekwherektwokconnectionskhavektheksamekRTT?kExplain.kDifferentkR
TT?k-kCORRECTkkANSWER-InkTCP,kfairnesskmeans:kforkk-
connectionskpassingkthroughkonekcommonklinkkwithkcapacitykRkbps,keachkconnectionkgetsk
ankaveragekthroughputkofkR/k
IfktwokconnectionskhavektheksamekRTTkthenkthekthroughputkforkeachkshouldksumkupktokR.
SincekTCPkrelieskonkacknowledgementskofkreceivedkpackets,kRTTk(roundktripktime)kaffects
kthekconnectionkspeed.kInkAIMD,kakconnectionkwithkakfasterkRTTkwouldkbekablektokrampkfast
erkthankanotherkconnectionkwithkslowerkRTT.
(altkanswer)
