Experimentally finding the right aggressiveness for retransmissions in thin TCP streams

116 Pages·2014·1.67 MB·English

by  Jonas Markussen

UNIVERSITY OF OSLO
Department of Informatics
Experimentally
finding the right
aggressiveness for
retransmissions in
thin TCP streams
How hard can it be?
Master thesis
Jonas Sæther
Markussen
18. August 2014
i
Experimentally finding the right aggressiveness for
retransmissions in thin TCP streams
JonasSætherMarkussen
18. August2014
ii
iii
Abstract
ThetraditionalconceptoffairnessinTCPisbasedonbeinglimitedbycongestioncontrol. Today,
however,weseethatTCPisbeingusedastransportforinteractiveapplicationsthathavelatencyre-
quirements.Theseapplicationscreateapplication-limited,thinstreamswhereretransmissions,rather
thancongestioncontrol,isthefactorcontrollingtheperformanceoftheflow. Keepingthemaximum
delayaslowaspossibleiscrucialtoimprovingtheQualityofExperienceforinteractive,thin-stream
applications. As there is little existing work and no clear consensus on how time-dependent, thin-
streamsshouldbetreated,weattempttoassesshowaggressivethesethinTCPstreamscanandshould
beonretransmissioninordertoreducetheirretransmissionlatencywhenlossoccurs. Inthisthesis,
wediscusshowwehavecreatedaLinuxnetworkingenvironmentandconductedourexperimentsin
ordertofindareasonabletrade-offbetweenaggressivenessandfairness. Ourfindingsindicatethat
an increased aggressiveness can be justified in competition with greedy streams, and we highlight
someissuessurroundingthinstreambehaviourneedstobefurtherstudied.
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v
Acknowledgements
What seemed like a relatively simple requests from my supervisors – set up a networking testbed
and run some tests in order to assess how modifications to the congestion control for thin streams
affectothertraffic–turnedouttobeenoughtears,latenightsandoutrightdesperationtobeatopic
for a thesis on its own. After much frustration and many dead-ends, accompanied by unexpected
and, sometimes, weird results, as well as a recurring reminder that I did not understand network
behaviouraswellasIthoughtIdidtobeginwith1,wasIfinallyabletoturnmyworkintosomething
thatresemblesamaster’sthesis.
First,Iwouldliketothankmysupervisors,Dr. AndreasPetlundandDr. CarstenGriwodz,fortheir
guidanceandfeedbackonmywork,patientlyansweringmyquestions,andtakingtimetoreviewand
discussmyobservationsandtestresults.Bothofthemhaveprovidedmewithalotinformationabout
newfindingsandresultsfromtherestofthethin-streamgrouphereatSimulaaswellaskeepingme
updated about research going on both at the networking group at the Department of Informatics at
UniversityofOsloandbyothersworkingontheRITEproject.
IwouldalsoliketothankDr. PålHalvorsenforhisvaluableinputonmytestresults,andespecially
for helping me concretise my thesis and narrowing down my test scenarios to something that was
manageable. HeandDr. PetlundreallyhelpedmetietogetherallthelooseendsIhad.
AspecialthankstoBendikRønningOpstad. Withouthispatchingofstreamzeroandourcom-
binedeffortstofixandimproveanalyseTCP,myworksimplywouldn’thavebeenpossible.Ihave
alsolearnedagreatdealfromourdiscussionsonvarioustopicsandproblems;everythingfromhow
theLinuxkernelworks,howTCPbehavesandwhatisactuallypassedtothenetworkdriverandput
"onthewire",tootherissuessuchasbestpracticesinsoftwaredevelopmentandsourcecontrol.
Additionally,IwouldliketothankMarkusFuchsandDr.DavidRosfortheirindividualcomparisons
oftheirsimulationresultsandmyownobservationsfromemulation, andespeciallyforconfirming
myobservations.Ialsowanttothankthembothforourdiscussionsaboutdifferencesandsimilarities
intheobservationsaswellashypothesisingaboutpossibleexplanations,whichgavemealotofnew
ideastotryout.
MythanksalsogoestoØysteinGylandforkick-startingmyLinuxnetworkingskills,andgettingme
startedonsettingupmytestbed,PrebenNensethOlsenforalotoftipsonhowtowriteathesis,and
to Tor Ivar Johannesen for getting me started with LATEX as well as spending the entire summer in
solitudewithmeouthereonFornebu,wheneverythingwascloseddownforthesummerholidays.
MygratitudeisalsoduetoAndersMoeandChristianTrytiforourfriendlytalksandfrequenttripsto
thestore. ThankstoMortenØdegaard,AndersEllefsen,KarolineKlever,NoraRaaum,AxelSanner,
HeleneCederstolpeAndresen,ChrisCarlmarandStåleKristoffersen,aswellasallmyfriendsand
familyforbelievinginmeandconstantlynaggingmetogetmythesisdone.
1...andIprobablystilldon’t!
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Contents
ExperimentallyfindingtherightaggressivenessforretransmissionsinthinTCPstreams i
Abstract iv
Acknowledgements vi
Contents viii
ListofFigures xii
ListofTables xiv
ListofAbbreviations xv
1 Introduction 1
1.1 Backgroundandrelatedwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Problemstatement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Scopeandlimitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.4 Researchmethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.5 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.6 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Thinstreams 6
2.1 Trafficcharacteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
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2.2 Latencyrequirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1 Onlinegames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2 Real-timemultimediaapplications . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.3 Administratingremotesystems . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.4 High-frequencyalgorithmictrading . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3 Thin-streamtransportprotocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1 UserDatagramProtocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.2 Real-TimeTransportProtocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4 TransmissionControlProtocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4.1 Headerlayout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4.2 Connectionmanagement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.4.3 Datatransfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4.4 Flowcontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5 CongestioncontrolinTCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5.1 Nagle’salgorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5.2 Delayedacknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5.3 Congestionwindow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5.4 Windowalgorithmvariants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.5.5 Retransmissiontime-outcalculation . . . . . . . . . . . . . . . . . . . . . . . . 27
2.6 Thin-streammodificationstoTCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.6.1 TCPsmartframing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.6.2 Earlyretransmit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.6.3 Modifiedfastretransmit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.4 Linearretransmissiontime-out . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.5 Taillossprobe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.6 Redundantdatabundling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.7 TCPfairness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
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