Conlusion

small but steady insider population is present. Naturally, a lower

c i

^{generally results in more}

insiders (although, therearesome utuations).

Again,ifwelookatthedown-saledplot,aninterestingtrendemerges: evenatomparatively

high entry osts, there is a steady-rate growth in tehnology penetration. This implies that

wireless ommunities networks are in an expanding phase today. This shows the possibilityof

theemergene ofa global wireless village.

high-relevane users drive thetehnology diusion. Moreover, theurrent real-world

pen-etration level and struture of global ommunity networksould imply steady growth for

the future.

While relaxing assumptions and using more omplex analyti tehniques might be hard in

this senario, there areplentyof pratial researhhallenges to takle inthefuture.

Mediator payo and business models. While thebusiness model we used inthis work

is based on the FON onept, we annot yet determine if it is optimal in any (soial welfare,

maximummediatorprot,et.) aspet. Further researhapplyingmehanismdesigntehniques

to theproblemofreating andevaluatingeientbusiness modelsforuser-providednetworking

is highly relevant. Additionally, more sophistiated user ategories an be introdued to rene

revenueow. Ontheother hand,the existeneofmultiplemediators indueompetition among

them,whihanbealsotakenintoonsideration. Last,theimpatofaglobalwirelessommunity

network on 3G/4Gmobile operators shouldalso be studied (see [64℄for guidelines).

Mobility patterns. Forour simulationswe onstrutedmobilitygraphsasrealisti as

pos-sibleintermsof usersloations, mobilitypatternsand roamingintensities. Howeverwe believe,

thatsuh anetworkevolvesaordingto theurrent stateoftheuser, ISPandmediator games,

whih eventuates a dynami struture, withtemporally hanging harateristis. Although the

understanding and modeling of the dynami properties of networks and ommunities is still in

heavydevelopment[76℄[77℄[78℄,itwouldbeinterestingtoseehowthedenedgamerulesinterat

withtheunderlying roaming struture.

Trameasurements. ModelingISPostsproportionaltotra isfar fromtrivialinthis

ontext. Foranadvanedostmodeloneshouldtakeintoaounttraandusageharateristis

as measured in the very network. This would inlude dierent tra proles for users (e.g.,

heavy hitters vs. e-mail and browsing). Also,the utility ofusers is greatly dependent on tra

prioritizationtehniquesandqualityofexperiene. Whiletherearesomeexistingsurveysdealing

withtheFONsystem[79℄,we still laka large-salemeasurement study to build upon.

Regulatoryaspets. Aountability(tyingthetratoagivenuser)andthefeasibilityof

lawful intereption are hallenging issues: these features may be neededif/when a truly global

wireless villageemerges. Boththelegalframeworkanditsenabling implementationarefar from

trivial: these arepossible diretionsfor a dierent branh ofhighlyinteresting futurework.

Congestion Control in Dynami Mobile

Environments

Limitations of urrent TCP protools and espeially their performane in high-speed

environ-ments are demonstrated in numerous studies [95℄ [96℄ [97℄ [98℄. As one of themain dierenes

to today's networks, it is expeted that future protools will have to operate on muh higher

speeds. For this end, high-speed variants of TCP are being developed and even deployed to

performbetterandbemoreadaptive tourrentandfuturetehnologies withhighbandwidthor

high bandwidth-delay produt (BDP). Suh links are beoming widespread as both wired and

wireless networks oer higher aess speeds, and the number of users and bandwidth-hungry

appliations inreases (e.g.,peer-to-peer lesharing networks).

Onthe other hand, wireless networks have always been hallenging for TCP, espeially for

ongestion ontrol algorithms. However, the evaluation of the topi is ambiguous; some

re-searhers argue that wireless links do not impose serious problems to TCP [85℄ [99℄ [86℄ [101℄,

while others deal with TCP's limitations in wireless environments in detail [102℄ [103℄. Sine

wireless tehnology approahestransmissionrates previously unheardof,futureongestion

on-trolprotoolshavetodealwithbothwirelessand high-speednetworkharateristisatthesame

time.

Further ompliating the issue, heterogeneity is emerging as a key harateristi of urrent

and future wireless networks. With the rih diversity of wireless networks already deployed

(WLAN, GPRS,EDGE,UMTS, HSPA,CDMA2000)or likely tobedeployedinthenearfuture

(3GPP-LTE/SAE, WiMAX),thedemandforommuniationmehanismswhihan operate

ef-iently overthe physial satteredness ofaesstehnologies issubstantial. Even a singledata

owanexperieneawidevarietyofunderlyingnetworktehnologies, thereforeswiftadaptation

to hanged onditions is essential to provide a high-quality servieto end-users. An espeially

hallengingtaskforTCPistoopewithhandovers: theseareinherentlypresentinmobileellular

Figure4.1: Measuredround-trip timesduring a handover

systems. Sine thedeployment of new wireless tehnologies are done inan inremental manner

thusseveraltehnologieso-exist,inter-tehnologyhandoversour. Theseinter-tehnology

han-dovers an result in a sudden inrease inlink apaity(referred to asan up-swith) as seenby

TCP. E.g.,ifausermovesfroma HSPA ellto an LTEell theresulting up-swith anbe tens

of Mb/s.

Theontribution of thishapter istwofold. InSetion 4.2, we investigate whether proposed

high-speed TCPvariantsperformwellinurrentandfuturemobilenetworkswithspeialregard

to handovers. Sine motivation for high-speed TCPs was rst neessitated by wired systems,

peuliaritiesofmobilenetworkshavenotalwaysbeen onsideredduringprotooldesign. Onthe

other hand,existing studies fous on today's networks, and do not try to predit what impat

would future hanges have on the same protools. Our goal is to provide measurement results

of high-speed TCP proposals in emulated future mobile environments and also to suggest the

inlusion of mobile-spei issues in the IRTF TMRG draft [104℄, and other douments. In

Setion 4.3 we propose SpeedDetet, a TCP extension for handling apaity up-swithes. The

keyidea ofour proposalis toimprove apaityutilization atup-swithes byexpliitly deteting

freeapaitythentriggeringtheunderlyingongestionontrolalgorithmtoanaggressiveprobing

regime. We argue that this an be done by monitoring measured round-trip times: a sudden

derease of RTT means that the bottlenek buer is empty as a result of inreased serviing

apaity(seeFigure4.1). Notethataseond,thoughmuhlesslikelyreasonforRTTdereaseis

thatthepaketsarere-routedthroughanewpathwithsmallerpropagationRTT.Theourrene

of this event isvery rareinthelifetimeof agiven ow, hene we omitits analysis.

4.1 Bakground

4.1.1 TCP variants for high-speed networks

In our work we examined four high-speed TCP variants and NewReno. The latter is inluded

for referene asthe most widely usedTCP avor today. Inthefollowing we shortlysummarize

themain featuresof the ongestionontrol algorithms inthese protools.

TCP's ongestion ontrol is window-based; that is, the amount of outstanding data in the

network is diretly ontrolled by the protool. The instantaneous sending rate at time

t

^an

be estimated as

λ(t) = W (t)/T (t)

^where

W (t)

^{is the ongestion window size and}

T (t)

^{is the}

round-trip time.

NewRenoTCP.ThisisthelatestversionofthelassialTCP[105℄. Theongestionontrol

isomposedoftwo phases,probing andderease. Probing inturnismadeup ofan exponential

inrease and a linearphase, slow start and ongestion avoidane, respetively. Whena timeout

or three dupliate ACK-s noties the sender of a loss event, the probing phase ends, and the

windowis eitherhalvedor dereased to 1. Formally,

W ← W + a(W )

on ACKreeption, and

W ← W · (1 − b(W ))

after thethirddupliate ACK,with

a(W ) = 1/W

^and

b(W ) = 1/2.

Thisalgorithm is oftenreferredto asAIMD(Additive Inrease, Multipliative Derease).

HighSpeed TCP. ThisalgorithmmodiesNewReno's inreaseanddereaseparameters to

ahievelargerongestionwindows[81℄. Funtions

a(W )

^and

b(W )

^{arederivedfromtheresponse}

funtion

W

HS

(p)

^{thatdesribesthe}relationshipof

W

^andthesteady-statepaketlossprobability

p

^{. The response funtion for HighSpeed is a modied version of NewReno's response funtion}

W

NR

(p) = 1.2/ √ p

^{. Inpartiular,}

W

HS

(p) =







W

^NR

(p)

^if

W 6 W

^low

, 0.12/p ^0.835

^if

W > W

low

,

thatleads to

b(W ) = f (log W )

^and

a(W ) = f

W ² b(W ) 2 − b(W )

.

Salable TCP. SalableTCP hanges theoriginal algorithm toupdatetheongestion

win-dow independently of

W

^{, suh that}

a(W ) = 0.01

^and

b(W ) = 0.125

^{[82℄. This strategy is}

essentiallyMIMD (Multipliative Inrease,Multipliative Derease).

BIC TCP. BIC stands for Binary Inrease Congestion, as ongestion window setting is

based on a binarysearh [83℄. On apaket loss, BIC rst reords

W

max

= W

^{,then redues}

W

aordingtoNewReno'salgorithmwith

b(W ) = 0.2

^{,andnallyreordsthenewwindowas}

W

^min

(eetively

W

min

= (1 − b)W

max

). Then, using thetwo extremes

W

min

and

W

max

itperformsa

binary searh by jumping to their mean value, and swithing to a linear inrease phase. When

W = W

max

is reahed BIC starts an aggressive searh for thenew

W

max

(max probing) until a

new lossours.

FAST TCP. FAST TCP omplements loss-based ongestion ontrol with queuing delay

estimation[84℄. Inpratie,

W

^isupdatedperiodiallyusing

T ₀

^{astheminimumRTT(onsidered}

aspropagation-only omponent),and

T

^{asthelast measured RTT.}

W ← (1 − γ )W + γ T ₀

T W + α

(4.1)

The aggressiveness ofthealgorithm is determined byonstants

α

^{,whihrepresents thenumber}

of thepakets to be heldinthebuer ofthebottleneklink at theequilibriumstate,and

γ

^,an

exponential smoothing parameter(

0 < γ 6 1

^).

4.1.2 TCP performane in wireless mobile networks

There isasigniant bodyofresearhrelatedto theinteration ofmobilewirelessenvironments

and TCP ongestion ontrol. A number of papersdeal with theperformane of standard TCP

in wireless networks [86℄ [99℄ [101℄. Others proposemodiations to standard TCP [85℄ [89℄ or

totally new ongestionontrol protools [87℄ [88℄[100℄ to dealwithproblems induedby mobile

networks.

The behaviorof TCP andother TCP-friendly ongestion ontrol shemes during handovers

arepresentedin[106℄ and[107℄. Although thesepapersdealwithhandovers theyassumeonlya

moderatelyhangingenvironment. AuthorsofTCP-Ho[108℄dealwithTCPperformaneduring

link outages, whih an be onsidered as a simple intra-tehnology handoverfrom TCP's point

of view. Furthermore, there is a paper assessing the performane of high speed TCP versions

during mobilehandovers[C7℄, thatwe buildon. Oneof themainresults presentedthere isthat

urrenthigh-speedTCP versionshave troubledealingwithsuddenapaityinreasesanddrops.

Withregard to non-ongestion detetion, an existing solution is Persistent Non-Congestion

Detetion (PNCD)part of TCP Westwood (TCP-W)whih is based on ontinuous rate

es-timation [88℄. PNCD raises a notiation whenever the ahieved sending rate is signiantly

lower than a possible estimated rate. The reation to the notiation generated by PNCD in

ssthresh values,thatontinuesuntila paketlossevent. A shortomingofAgileProbing isthat

oneit starts, it lls upthe buerompletely ausing unneessarypaket drops. Also, TCP-W

bases its rate estimation on minimum RTT measurements, that an be misleading in ertain

situations [109℄.

TCP-Hando is proposed in [92℄ to expliitly deal with handovers in mobile networks. A

shortoming ofthis solution is theuseof dediatedhandoversignaling between TCP-HOlient

andserver. SinethehandovermaynotbeseenbythehostOSatall,adediatedontrolmethod

is neessary to inform the host TCP about a handover. This solution introdues additional

overheadand omplexity to ongestionontrol.

In[102℄ and[104℄authors introduesguidelines foradequateevaluationofTCP performane

inwirelessandwiredenvironments. Weadheretotheseguidelines inourperformaneevaluation

presentedinSetions4.2.3, 4.3.3 and4.3.4.

In document B daeUiveiyfTe h (Pldal 93-99)