A Stable and Flexible TCP-friendly congestion control protocol for layered multicast transmission

transmission

IbtissamElKhayatandGuyLedu

Resear hUnitinNetworking

UniversityofLiege

InstitutMonteore-B28 -SartTilman

Liege4000-Belgique

Abstra t. We propose an improvement of our RLS (Re eiver-driven

Layered multi ast with Syn hronization points) proto ol, alled CIFL

for \Coding-Independent FairLayered mulati ast", along twoaxes. In

CIFL,ea hre eiverofalayeredmulti asttransmissionwilltryandnd

the adequate numberof layers to subs ribe to, so that the asso iated

throughput is fair towards TCP and stable in steady-state. The rst

improvementis that CIFLisnot spe i toany oding s heme. It an

workas wellwith anexponentially distributedsetof layers(wherethe

throughputofea hlayeriequalsthesumofthethroughputsofalllayers

belowi),orwithlayersofequalthroughputs,oranyothers heme.The

se ondimprovementistheex ellentstabilityoftheproto olwhi havoids

uselessjoinattemptsbylearningfromitsunsu essfulpreviousattempts

inthesame(orbetter)network onditions.Moreover,theproto oltries

and rea hes its ideal TCP-friendly as soon as possible by omputing

its target throughputin a lever way when an in ipient ongestion is

onrmed.

1 Introdu tion

Contrary to the urrent ompression standards (e.g. JPEG, MPEG-x,

H.26x), wavelet-based ompression te hniques (e.g. JPEG 2000) allow

for exibleandhighlys alable(inresolution,timeandquality)formats.

Althoughinter-framewaveletvideo odingisstillanopenresear harea,

it will enable very s alable video transmission where the data stream

anbesplitintoseveralhierar hi allayerswhosebit ontents(andthus

throughputs) an be dened in a very exible manner. Therefore, we

believe that any ongestion ontrol proto ol dedi ated to video

trans-missiontoanheterogenoussetofre eiversshouldbeindependentfrom

therelativeandabsolutethroughputsofea hlayer.Itshouldbehaveas

wellwithanexponentiallydistributedsetoflayers(wherethe

through-putofea hlayeriequalsthesumofthethroughputsofalllayersbelow

i),orwithlayersofequalthroughputs,oranyothers heme.

shareoftheavailablebandwidth.We onsiderintra-sessionfairness(i.e.

amongre eiversof thesame session) andinter-session fairness (i.e.

to-wardsothersessions oftheproto olortowards TCP onne tions).

Are eiver-drivenlayeredmulti ast(RLM)approa htosolvethe

hetero-geneity problem was rst proposed by M anne in [8℄. InRLM, every

layerrepresentsanIP multi ast groupand subs ription toa layer

im-plies subs ription to all the lower layers. There eiveradds and drops

layersa ording tothe network state.This re eiver-driven approa his

probably the most elegant way to solve the multi ast problem. It was

laterusedinRLC[13℄, MLDA [12℄ andPLM[4℄. Themain on ernof

RLMwas theintra-sessionfairness.Toa hieveit,a oordination

me h-anism betweenre eivers hasbeen designed. RLMwas not designed to

beTCP-friendly(i.e.fair towards TCP),norto guaranteeinter-session

fairness.RLCwasdesigned to befair towards TCP onne tionswhose

roundtriptime(RTT)was losetoonese ond,butnotingeneral.RLC

andMLDAsupportsomeformofinter-sessionfairness,inthesensethat

two ompetingRLC(MLDA)sessionswillgetthesamenumberoflayers

insteady-state,whi hmeansthatbothsessionsgetthesamethroughput

onlyin aseswherethetwosessionshavepartitionedtheirlayerssothat

theyhavethesamethroughputsinalllayers.This annotbethe asein

general.

In anearlier work, we have proposed a proto ol, alled RLS [3℄, that

providesintra-sessionandinter-sessionfairnessguarantees.Forexample,

for a largerange of RTTs, the ratio of throughputsbetween RLSand

TCPremainsintheinterval[ 1

3

,3℄,whi hisex ellent omparedtoRLM

andRLC.However,we notedthatRLS, though stable,still performed

toomanyunsu essfuljoinexperiments.Moreover,RLSwasdesignedto

workwithexponentiallydistributedlayersonly.Inthispaper,wepropose

abetterproto ol, alledCIFL,whi himprovesRLSalongthefollowing

lines:

{ Wemakenohypothesisonthethroughputsof thelayers,they an

haveanyvalue.

{ There eiversrea htheoptimallevelqui kly.

{ The stability is better, be ause the re eivers learnfrom their past

failures tojoinsome layersundersome onditions. Thismakesthe

re eivedthroughputverysmooth,andimprovesfairnesstoo.

Thepaperisorganized asfollows.Werstremindsomebasi on epts

inse tion 2.We explainthe prin iplesof CIFLinse tion 3,and show

itssimulatedperforman eresultsinse tion4.

2 Basi on epts

2.1 TCP-Friendly

TCPisthemostwidespreadtraÆ intheinternetandanynew

onges-tion ontrolled proto ol has to be designed to be TCP-friendly,whi h

meansthat itgets anaverage shareof thebandwidth(approximately)

equalto the average shareTCP would get inthe same onditions. As

TCPisuni astandweare onsideringmulti astproto ols,thedenition

share aTCP onne tion, betweenthatsour eand that re eiver, would

get.

InBest-eort networks,thereisnoreasontofavourvideotransmission

overTCPgiventheimportan eofthelatter.InIntegratedServi es

net-works where re eivers an reserve some (minimum)bandwidthfor the

videostream,one ouldletre eiversgetmorebandwidthprovidedthat

this extra share is fairly allo ated. InDierentiated Servi es networks

wherevideo stream anbe aggregated withothers andmay,notbe in

thesame lassasTCP ows,inter-sessionsfairnesswillbea hievedifall

video owsadoptthe samedenitionoffairness(andTCP-friendliness

may be a good andidate for that). So in all ases, TCP friendliness

seemsagoodrequirementtofulll.

ThethroughputofTCP(inbps)insteady-state,whentheloss ratiois

below16%,isroughlygivenbythefollowingformula[6℄:

Bt p C :s p pR TT withC= r 3 2 =1;22 (1)

wheresisthepa ketsize(inbits),R TTisthemeanroundtriptime(in

se )andpthepa ketlossratio.Amorepre iseformulathattakesTCP

timersintoa ount anbefoundin[9℄.

TheTCP y leistheaveragedelaybetweentwopa ketlossesin

steady-state. So we have one pa ket loss per y le, whi h an be formulated

as: p= s B t p :Cy l e ; (2)

where s is the pa ketsize in bits and Cy l e the duration of the TCP

y leasdes ribedabove.From(1)and(2), wederive:

Cy l e= Bt p:R TT 2 C 2 s (3) 2.2 Coordinationof re eivers

It was pointed outin [8℄, that a multi ast ongestion ontrol proto ol

annotbeee tiveifthesubsetofre eiversbehindthesameroutera t

without oordination. Indeed,if are eiver reates ongestion onalink

byrequestinganewlayer,anotherre eiver(re eivinglesslayers)might

interpretitsresulting losses as a onsequen e of its(too high)levelof

subs ription and may end up dropping its highest layer unne essarily

(be ause this layer will ontinueto be re eived by otherre eivers). So

oordinationisne essary,RLMhasproposedtouseannoun ement

mes-sages, and RLC to use syn hronization points (SPs). SPs are spe ial

pa ketsinthedatastream.Re eivers anonlyjoinanewlayerjustafter

re eiving anSP.InRLC, ea h layerhas itsown SPs, andthe re eiver

anonlyjoinlayeri+1,whenit re eives anSP inlayeri. [10℄ shows

thatthepresen eofSPsleadstoalowredundan yandgivesbetter

fair-ness.Thatis thereason whyRLSandCIFLbuildtheir oordinationof

3 The CIFL proto ol

Our goal is to reate a layered multi ast ongestion ontrol proto ol

whi his:

1. TCP-friendly.

2. Stable:asfewunsu essfuljoinexperimentsaspossible.

3. Generi :independentfromthethroughputofea hlayer.Toa hieve

thatCIFLwillestimatetheidealthroughput,andwilljoin,orleave,

oneorseverallayersaton etorea hathroughputwhi his loseto

the omputedtarget,basedonestimationsoftheRTTandtheloss

ratio.

4. Careful before adding layers at SPs, but qui k at removinglayers

whenanin ipient ongestion is onrmed. Thisis tobe ompared

withtheAdditiveIn reaseMultipli ativeDe rease(AIMD)s heme

ofTCP.

3.1 Estimationof the Round Trip Time

Ea hre eiverhastoestimateitsRTTtothesour e.The lassi als heme

istopingthesenderfromtimetotime,e.g.ea htimethere eiverjoinsor

leavealayer,ormorefrequently.However,for largesessions,thesender

anbe oodedbypingrequests.Ifroutersarea tive,asolutionbasedon

[1℄ anbeused,butweare lookingforasolutionthatdoesnotinvolve

routers.If thesenderknows thenumberr ofre eiversandthe number

pofpingrequestsit anpro essbetweentwoSPs,it anprovidethese

numbersintheSPs.

Knowingthesevalues,re eivers anpingthesenderwithprobability p

r .

Wedonotrequirethatpingrequestsbeimmediatelyfollowedbyaping

responsefromthesender.Toa hievethat,weimplementas hemesimilar

toRTCP[11℄.Supposeare eiversendsapingrequestattimeRs whi h

isre eivedbythesenderattimeSr.Thesenderstampsthepingrequest

atitsarrival,and whenit isable tosendapingresponse,say atS

s ,it

stampstheresponsewiththattimevalue.Ifthesenderisqui k,Ss will

be(almost)equaltoS

r

,butinany asethetimespentatthesender an

be omputedas Ss Sr.At Rr there eiverwill getthe pingresponse

andperformthefollowingoperations:

R e Send=(1 g)R e Send+g(Sr Rs)

SendR e =(1 g)SendR e +g(Rr Ss)

R TT =R e Send+SendR e

If all the data pa kets are timestamped,the re eiver an ontinously

estimatetheSendR e valuebyusingallthepa ketsitre eives.Between

twopings,theR e Send an hangewithoutbeingnoti edthough,whi h

3.2 The join

Syn hronization points. As said before, we use the SPs to

o-ordinate the re eivers. Contrary to RLC, SPs are only present in the

rst(base)layerand notinall of them.WhenaSP is re eived andif

thede isiontojoinisnottaken,there eiverremainsdeafto ongestion

duringa deafperiodTd.Thisis ne essarybe ause this ongestion an

beindu edbyanotherre eiverthathasusedthatSPtogetmorelayers.

Inpra ti ethedistan ebetweenSPsisatleast4se onds.Thisdistan e

isenoughtobegreaterthanany ommondeafperiod(seenextse tion).

However, to avoid all kinds of syn hronizations, the distan e between

SPsisrandomized.Itwillvarybetween4and16se onds.

In reaseof the throughput. There eivertriesandestimatesthe

bandwidthTCP would get ina similarsituation. To doso, it will use

formula (1) whi hrequires to knowits loss ratio. Butthe latter hasa

meaningonlywhenitis omputedoveraduration losetoaTCP y le.

Indeed,rememberthatformula(1)isonlyvalidinsteady-state.So,the

re eiver will refrain from using anSP to get more layers if it did not

stayatleastoneTCP y leatthe urrentlevel.Whenitisthe ase,the

re eiver omputesthebandwidthit angetasfollows:

Bnext= Cs R TT p P y le :

withP y lethelossratio omputedoverthelastTCP y le(seeformula

(3)).Iftherewerenoloss,thethroughput anbedoubled.Thatis

smi-larto TCP whi hwould havedoubled itswindow aftera y le. When

there eiverhas omputed itsoptimal bandwidth, it joins thesuitable

numberof layers to get the losest possible to the omputed

through-put.Todoso,itisne essarythattheSPs ontaininformationaboutthe

throughputsofallthelayers.

Stabilization. Whenthere eiverhasnogoodestimationofitsRTT,

e.g.be ausethereisalargenumberofre eiversandthepingsaredone

lessfrequently,the estimatedbandwidth anbe overestimated.Inthis

ase,there eiverwouldjoinlayersthatitwouldleavesoonafter.These

unsu essful join experiments anbe avoided if the re eiver anlearn

somethingfrompast failures.Tothisend,theCIFLre eiverwillre ord

thenetworkstate 1

asitwasjustbeforeanyunsu essfuljoinexperiment.

To dothis, everyre eivermaintainsasquarematrixQD withonerow

(andone olumn)perlayer.Ea helementQDi;jofthematrixrepresents

theminimumqueuingdelaythere eiverhasevermonitoredbeforeany

unsu essfuljoinexperimentfromlevelitolevelj.

When a re eiver at level urrentwants to join level target, it he ks

itsmatrixto seeifit has alreadyfailed tojoinanylayerbelow target

1

equiv-withaqueuing delay thatwasbelowthe urrent queuingdelay.It an

be omputedasfollows:

tested = urrent + 1

while (tested <= target &&

urrent_queuing_delay < QD( urrent,tested))

{ join

in r tested }

Whenthere eiverhasrea hedastablelevelandissubje tedtoveryfew

(orno)losses,itbasi allyspendsitstime omputingestimationsofthe

RTT,thequeuingdelayandthelossrate,refrainingfromjoiningatSPs.

3.3 The leave

Ifthe de isionto joinlayers anbe done at(not so frequent)SPsand

aftera y le haselapsed at the urrent level,the de ision to abandon

layerswhen ongestionappearsshouldbetakenmorequi kly.So,when

there eiverdete ts apotentialin ipient ongestionbyapa ketloss,it

will start monitoringthe loss ratio P

Tm

overashortinterval(denoted

T

m

),andthenthere eiverwill omputethenumberoflayersitde ides

toabandon. We willdis ussthe valueof Tm later, butwe knowithas

tobeshort,sayveryfew RTTstox ideas.Theproblem isthatT

m is

ingeneralshort omparedtoaTCP y le,whi hmakesitimpossibleto

useequation [6℄to omputeanew(lower) targetthroughput. Inorder

toproposeanotherformulato omputethatthroughput,wewillrequire

that, when the suitable number of layers are abandoned, the re eiver

willnotjoinanylayerbeforeaminimumamount oftime(denotedT

)

haselapsed.Clearly,T shouldbelargerthananequivalentTCP y le,

asbeforeany joinexperiment,and should endat anSP.However, the

distan ebetweenSPsbeing random,the futureo urren es ofSPsare

unknown.Inthe al ulationhowever, wewill onsiderthat all SPsare

equallyspa edoutof10se ,whi histheaveragespa ingbetweenSPs.

Toderiveourformula,wedene

{ Tmisthemonitoringperiodstartingataprobablein ipient

onges-tiondete tedbyapa ketloss insteady-state(orindu edbyajoin

experimentofthere eiver).

{ T ( for ompensation) is the minimal period during whi h the

re eiverwillhavetostayatitsnewlevelbeforejoininganylayer.It

is omputedasdes ribedabove.

{ B urrent isthe urrentthroughput,whi hwillremainsoduringTm,

{ Btarget is the unknownthroughput the re eiver will request after

leavingsomelayers,andwillkeepduringatleastT .

To ompute Btarget, we require that CIFL should get a TCP-friendly

throughputovertheTm+T interval.

Figure1showstheparametersweuse,andillustratesalsothatT nishes

atanSParrivingaftertheexpirationofthe y le.

Let=

B ur r ent

Btar g et

,themeanthroughputofthere eiveris:

B= TmB urrent+T Btarget T +T = Tm+T (T +T ) B urrent

Packet lost

Normal packet received

SP

Cycle

time

target

B

c

T

Fig.1.Amonitoringperiodfollowedbyits ompensationperiod

Wesupposethat thereis noloss duringT

,whi hmeansthat theloss

ratiopoverTm+T is:

p=

pa ketsl ostduring(Tm+T )

pa ketssentduring(Tm+T ) =

pa ketsl ostduringTm

pa ketssentduring(Tm+T )

Whereas: pa ketssentduring(Tm+T )=(Tm+T )B

and: pa ketsl ostduringTm=PTmTmB urrent=PTmTm:

(Tm+T ) T m +T B

Asimplerepla ementgives: p=PT

m T

m

Tm+T ;

TCP,whi hhasalossratioequaltop,re eivesinaverage:

B t p = sC R TT q PT m Tm T m +T

Ifweequateboththroughputs,i.e.B=Bt p,wederivethat:

= PT m TmT (Tm+T ) 2 C 2 s 2 RTT 2 B 2 ur r ent Tm 2 PT m

Re apitulation. Whenthe re eiverdete ts anin ipient ongestion,

orjustafterjoiningalayer,itmonitorsthelossratioduringTmandthen

omputes.

{ If itis greater than1 2

, there eiverleavesthe suitablenumberof

layerstogetathroughput loseto

B ur r ent

.Thenthere eiverignores

lossesduringthedeafperiodTd,whi hisne essarytoletthenetwork

rea hitsnewstateandmonitorit.Initially,thedeafperiodisequal

to1R TT,butitisupdatedea htimealayerisaddedorremovedas

follows.Knowingtime

drop

,thetimeatwhi hlayerswereabandoned,

andtimelast,there eptiontimeofthelastpa ketbelongingtoone

ofthedroppedlayers,there eivermakesanexponentialsmoothing

ofT

d

withthenewvalue\time

last time

drop ".

{ Else,itdoesnothingasittreatsthelossesasresultingfromasmall

transient ongestion.

Su has hemewouldnotbeeasilyadaptedtoTCPitselfbe auseTCP

may notre eiveenough segmentsduring anRTT(when itswindow is

small)to omputea urately.

Notealsothatifare eiverisinaleaveevaluationwhenanSPisre eived,

andadeafperiodisstarted,theleaveevaluationis an elled.Otherwise,

there eivermaybefalsely onfusedbyatransient ongestion duetoa

joinexperimentbyanotherre eiver.

The T

m

value. In this se tion, we brie y dis ussthe hoi eof T

m . Weknowthat: Bt p= T m B urrent +T B target Tm+T ;

So,whenTm in reases,Btarget de reases and B tar g et B t p de reases too.If B target B t p

the re eiverat the endofT

will normally in reaseits

bandwidthtorea hBt p.Toavoidthisos illation,weneedBtarget'Bt p.

AsT andarexed,

B target !B t p implies T m !0 SoT m

hastobeshort.However,asTCPtakesde isionsateveryR TT,

iftheCIFLre eiverevaluatesitslossratiooveradurationshorterthan

R TT,itwillgetabadestimation.Forthisreason,Tmhasbeenxedto

1R TT.

3.4 Start-up phase

WehaveexplainedhowtheCIFLre eiverbehavesinsteady-state.

How-ever,thisbehaviourisunsuitableattheverybeginning,be auseittends

tomimi TCP in ongestionavoidan e, insteadof aTCP inthe

slow-startphase.Therefore,whenTCPandCIFLstarttogether,CIFLwould

notgetitsfairshare,oronlyafteramu hlongerperiod.

Inthis se tion, we des ribe thestart-upphase ofCIFL. Inthis phase,

the re eiver uses all the SPs to join newlayers, so that it doubles its

throughputat every SP. For a set of exponentially distributed layers,

thiswouldmeanaddingalayer(butonly)ateverySP.Inothers hemes,

there eivermayjoinseverallayersaton e.Thismimi stheexponential

takeoofTCP,whi h ontinuesuntilthethroughputofsubs ribedlayers

is greaterthan thepeak throughputa tually re eived.On ethis state

is rea hed, the re eiver drops all layers above the maximum re eived

throughputandexitsthestart-upphase.

Moreover,thismoreaggressive phaseisusedtoestimatethebottlene k

apa itybymeasuringthesmallest delaybetweentwore eivedpa kets.

Knowingthis bottlene k apa ity, there eiverwill notattemptto join

layersthatwouldleadtoathroughputabovethisvalue.Thiswillredu e

thenumberofunsu essful joins, omparedtootherproto olslike[13℄,

[8℄,[12℄.

If,laterduring the session, pa ketshappen to transitthroughanother

pathwithmore bandwidth, or ifthe network issimply less ongested,

the re eiver will dis over it, be ause it will ontinue to estimate the

bottlene k apa ityasfollows:

estimatebwi=max(estimatebwi

1 ; pktsize tre vi tre vi 1 )

Onthe otherhand, if the traÆ is routed to less provisionedor more

ongestedlinks,itisnot areal problem,be ausethis estimated

bottle-ne k apa itywilljustbe omeoverestimated,andthusabitlessuseful

toavoidunsu essfuljoinexperiments.

3.5 S alability

B

S

S

R

1

2

2

(a)2sour es2re eivers

R

1

S

_N

_N

2

1

1 Mb

520 Kb

4 Mb

4 Mb

(b) 2 re eivers and one

sour e

Fig.2.Topologies

Notehowever,thattheRTTsarestilladjustedateveryre eivedpa ket

byusingthetimestampingapproa h,butregularresyn hronizationsare

alsousefulto ompensate lo kdrift anddelayvariationsonthereturn

pathasexplainedinse tion3.1.

Notealsothatalessa urateestimationoftheRTTwillsimplyleadtoa

bandwidthtargetwhi hislessTCP-Friendly(seeformula(1)).Moreover,

someexperien esperformedoverinternethave shownus thatduring1

hourtheratio betweenthe longest RTTand theshortestone israrely

greaterthan2.

4 Simulations

ForoursimulationsweusethenetworksimulatorNS([7℄).Wewillstart

by showing that there eiver, on e at itsoptimal level,doesnot make

unsu essfuljoinexperiments.Thenwewillshowthatintra-sessionand

inter-sessionfairnessarefullled,bothtowardsTCPandtowardsother

CIFLsessions. 0 5 10 15 20 25 30 35 0 50 100 150 200

Levels

Time (sec)

Receiver 1

Receiver 2

Throughput (bps)

Time (sec)

Receiver 1

Receiver 2

Fig.3.TwodierentCIFLre eiversbeginningatdierenttimes

4.1 Several re eivers

In this se tion, we will show the importan e of the deaf periods. To

this end,we use thetopologyof Figure2(b). We seeonFigure 3that

when re eiver R2 starts, re eiver R1 has already rea hed its optimal

throughput.Thenew omerwill reate ongestionontheN

1 N

2

linkand

R1 willbesubje tedto this ongestion. Withoutdeafperiods,re eiver

R

1

would rea tbyleavingsomelayers.Thisde reasewould beuseless

be ausethelayersitwouldleavewould ontinuetotransitthroughthe

bottlene k. In se tion 4.4 of [3℄, we showedthat without deaf periods

and whenthe delay to R

1

is larger thanthe delay to R

2 , R

1

loses its

fairshare.Withdeafperiods,re eiverR1 doesnotrea ttolosses aused

byR

2

.We anseebythesameo asionthatthere eiversrea hqui kly

their optimal level thanksto the qui k start-up phase.They also nd

0

200000

400000

600000

800000

1e+06

0

20

40

60

80

100 120 140 160 180 200

Throughput (bps)

Time (sec)

CIFL

0

200000

400000

600000

800000

1e+06

0

100

200

300

400

500

600

700

800

Throughput (bps)

Time (sec)

TCP

Fig.4.Bottlene ksharingbetweenTCPandCIFL

4.2 TCP versus CIFL

Reminder. Previous studies([5℄ and [2℄) have shownthat the most

severesituations togetfairnesstowardsTCParethefollowing:

{ EitherTCPbeginsbeforethemulti astproto ol,whentheRTTis

short,

{ OrTCPbeginsafterthemulti astproto ol,for longRTTs.

In the rst ase, TCP is so aggressive that it prevents the multi ast

proto ol(RLM,RLC) toget areasonableshareofthe apa ity.Inthe

se ond ase,TCPissoslowandsovulnerablethatit annotgeta

reason-ablethroughput. Thishappensbe ause TCP's lo k isitsRTT,whi h

isnotthe ase fortheothermulti astproto ols.WewillseethatCIFL

performsmu hbetterintheextreme ases.

0

1

2

3

4

5

0

200

400

600

800

1000

1200

1400

Ratio tcp / other traffic

one-way delay in milliseconds

RLC

RLM

CIFL

Fig.5.RLM,RLCandCIFLa ordingtotheonewaydelay

Simulations. WeusethetopologyofFigure2(a)withoneTCPsour e

andoneCIFLre eivertotestthebehaviourofbothtraÆ s.The

param-etervaluesare:B0 =64kbpsandBi+1=Bi+B0 i.e.8iLi=L0=B0.

Figure4(a)showsthesharingintherstextreme ase,i.e.ashortRTT

(1ms)andTCPbeginsbeforeCIFL.WeseethatCIFLissoaggressive

inthebeginningthatit antakemorebandwidththanTCP.Afterthis,

itde reasesitsthroughputtogetexa tlywhatTCPgets.Inthemedium

term,thesharingratio, rate

TCP

rate

CIFL

,is1.Forthese ondextreme ase,the

RTTisapproximativelyequalto2se . Figure4(b)illustratesthis ase

andshowsthatwhenTCPbegins,CIFLhalvesitsthroughputbyleaving

8 layers, and doesnot performany future attempt. TCP pays for its

greediness.Infa t,everytimeTCPwantstohavemorebandwidththan

theoptimum,it reates ongestionand hastode reaseitsthroughput.

Andsin e theRTTis large,TCP needs along timeto rea hagain its

fairshareofthebandwidth.Thatiswhytheratioisaround0.6forlong

RTTs,whileitremainsslightlybelow1forshortRTTs.

0

200000

400000

600000

800000

1e+06

0

50

100

150

200

250

300

0

5

10

15

20

25

Throughput (bps)

Levels

Time (sec)

CIFL

TCP

Levels

Fig.6. Thebehaviourofthere eiverwhenTCPdisappears

propagation delay. We have done it for RLM and RLC too (for more

results on erning thesetwoproto ols, refer to[5℄). Theresults are

il-lustratedonFigure5. TheCIFL urve remains lose to 1, ontrary to

RLMandRLCwhoseratios onvergetozeroforlongRTTsandarefar

above1forshortRTTs.

NowweshowthataCIFLre eiver anndabetteroptimallevel,whena

ompetingTCP onne tionstops.We onsidertopology2(a) withTCP

andCIFLbeginningsimultaneously.Afterbothproto olshaverea hed

theirfairbandwidth,TCPstops.Therearetwopossibilities:

1. IfthenextSParrivesmorethata y leafterTCPhasdisappeared,

the CIFL re eiver is unlikely to have suered loss during the last

y le. Ifso, itwill doubleitsbandwidth(limitedtothe bottlene k

apa ity ithad omputed).

2. IfthenextSParriveslessthana y leafterTCPhasdisappeared,

theCIFLre eiver anstillhavenoti edsomelossesduringthelast

y le. This ase is illustrated inFigure6.The CIFLre eiver

om-putesitsnewestimatedfairshareandde idestojoina ertain

num-beroflayers(inour ase, 2layers).WhenthenextSParrives,the

CIFLre eiverbehavesasintheprevious ase.

4.3 Fairness towardsanother CIFL session

Wewill showthat CIFLfullls thisrequirement.This issobe ause all

CIFL sessions try to be TCP-friendly. We use two dierent sessions,

onewithB 1

0

equalto64KbpsandtheotheronewithB 2

0

equalto 124

Kbps.TobeinadiÆ ult ase,the sessionwith thesmallestbase layer

beginswhentheotheronehasalreadyrea heditsoptimallevel.Figure7

illustratestheresult.Weseethatbothsessionsgetthesamebandwidth,

althoughthesebandwidths orrespond to dierent numberoflayers in

thetwosessions.

5 Con lusion

We have proposed a ongestion ontrol proto ol for layered multi ast

transmission, alledCIFL,thatensures:

{ intra-sessionfairness,

{ fairnesstowardsTCP,theratio TCP

CIFL

is loseto1

{ fairnessbetweensessionsintermsofthroughput(insteadoflevels),

{ stability, the re eiver uses its past failures to performs a sort of

reinfor ementlearning.

Wehavesimulatedourproto olindierentsituations,andtheobtained

0

200000

400000

600000

800000

1e+06

0

20

40

60

80 100 120 140 160 180 200

Throughput (bps)

Time (sec)

Session 2 bw

0

5

10

15

20

25

30

0

50

100

150

200

Levels

Time (sec)

Session 2 levels

Fig.7.Fairnessbetween2dierentCIFLsessions

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