Congestion A voidane Algorithms

A _s

P b

P r

P _b

A _r

Receiver Sender

Figure 4.1: Sliding Window FlowControl

4.2.1 Slow Start

TCP is a sliding window protool. The sliding window represents the amount of

data that may be unaknowledged and on ight in the network at any given time

in the ourse of aommuniation.

Slow-Startisanalgorithmforslidingwindowmanagement(termed inTCP

onges-tion window)forthe purpose ofrapidlyreahingthe stateofequilibrium. Given the

unknown apaity of all intermediate links in the ommuniation path, Slow-Start

initialisesthe ongestion windowto a minimalvalue.

Rapid inrease inthe ongestionwindow ismanaged by imitatingthe rate atwhih

aknowledgements are returned wheninjeting new pakets intothe network. This

aspires in reahing quikly the network apaity.

Wheneveranewonnetionisestablished,Slow-Startinitialisesthe ongestion

win-dowtothevalue ofonesegment. Eahtime anew aknowledgementisreeived, the

value of the ongestion window variable is inreased by the value of one additional

segment. This guaranties an almostexponential growth of the ongestion window.

At any given time the sender's usable window is dened as the minimum between

the reeiver's buer (advertised window) and the ongestion window [39℄. In this

manner, the sender obviates transmitting more data than an be handled by the

network orthe reeiver.

As the ongestion window surpasses the apaity of the network in size it auses

extended buering and overow. The realisation of data loss (e.g. by means of a

timeout)funtionsas anindiationtothe senderthat the ongestionwindowistoo

high.

Although Slow-Start is an independent algorithm, it is never implemented alone.

That is, after reahing equilibrium a ommuniation has to maintain equilibrium

while remaining apable of reating to alterations in network apaity. That is

the purpose of the Congestion Avoidane algorithm. The individual properties of

this algorithm and how it interats with Slow-Start are presented in the following

setions.

4.2.2 Congestion Avoidane

As desribed in the previous setion, Slow-Start aims to lead a ommuniation

to-wards equilibrium. However, it does not provide any mehanisms for preserving

equilibriumor adapting the usable window tohangingnetwork onditions.

The purpose of the Congestion Avoidane algorithmis to reognise the possibility

of large-saleongestion andtake appropriatestepstoprevent it. Meanwhile,when

sensing the potential for expansion, Congestion Avoidane has to make use of this

opportunity. Just like Slow-Start, the Congestion Avoidane algorithm fouses on

the management of the ongestion window to ahieve the aforementioned tasks.

This setionis devoted todesribing how existing researh has analytiallydened

the Congestion Avoidane algorithmand to show the parameters that ne-tune its

Aording to [69℄ the tra ona network an be modelledas:

L i = N + γ · L i − 1

^(4.1)

where

L i

^{is the load at interval}

i

^,

N

^{is aonstant that denotes the onstant tra}

injeted regularly to the network and

γ

^{is a perentage gure that represents a}

fration of tra, left overfrom the previous interval. If noongestion takesplae,

then

γ

^{islose tozero,otherwise}

L

^{willwitnessan}exponentialinrease. Inorderto ounteragainstnetworkongestionthesourehastomaththe exponentialinrease

in the network load with a similar derease in the ongestion window. This should

takeplaebeforeongestionmanagestoaet theommuniation'sthroughputbut

not tooearly tostarve the network for tra.

TCP is based on the fundamental assumption that on most network paths loss

due todamageis rare (

<<

^{1%) [69℄. As suh allpaket loss isalways aredited to}

networkongestion. TCPdeterminespaketlosswiththehelp ofthe retransmission

timeoutorthereeiptof several subsequent dupliate aknowledgements(dupaks).

Suhareregularaknowledgementsthatontaininthesequene numbereldof the

TCP header, the sequene numberof the last paket that was suessfully reeived

in-sequene.

The eventof aretransmissiontimeoutorthe reeiptofdupliate aknowledgements

is interpreted by TCP as the indiation of ongestion in the ommuniation path.

[24, 14℄ have shown that even though this assumption may be valid for wire-lined

networks it doesnot hold for wireless networks and mobileommuniations.

The loadinjetedbythesoureisregulatedwiththe helpoftheongestionwindow.

Network ongestion an be prevented by pursuing an exponential derease in the

oered load. This has been modelled in[69℄ as:

W i = d · W i − 1

^(4.2)

where

W

^{is the window size at interval}

i

^and

d

^{is the rate by whih the window}

should be redued inthe event of ongestion (

d <

^{1). The ongestion window may}

have tobe redued for several onsequent intervalsuntilongestion is eradiated.

The apaity of Congestion Avoidane to take advantage of available network

re-soures without ompromising its autious stane depends on the rate of inrease

in the ongestion window. In [69℄it is presented that Congestion Avoidaneshould

pursue a linear and not an exponential inrease in order to obviate overestimates

that are harateristi of Slow-Start. As suh, in the absene of ongestion, the

progress of the ongestion window ismodelledas:

W i = W i − 1 + u

^(4.3)

where

u

^{isaonstantthat denotesthe amountbywhihthe windowshouldinrease}

(

u << W max )

^.

The main reason behind network ongestion is the inrease in oered load aused

that ase, Congestion Avoidane should retreat and redue its ongestion window

size, sharing resoures with other ommuniations. For this [69℄ ditates that the

onstant

d

^{shouldtakeonthe value}

¹ / 2

^{indiatingthatinthe eventof ongestionthe}

ongestion window should be redued toone halfof its previous size.

In order tomaintain autiousnesswhen inreasing the ongestion window, the

on-stant

u

^{is set to the value}

1

^{. This means that the inrease per new ongestion}

window is going to be the size of one additional data unit (paket). It is stated in

[69℄ that initiallyit was aimed for a1% inrease rate but it was observed that this

rate eventuallyyielded

1

^{paket inrementspernew ongestion window.}

Although Slow-Start and Congestion Avoidane are two dierent algorithms that

serve a dierent purpose (the one to inrease the usable window until the state of

equilibrium and the other to adapt the ongestion windowto hanging onditions)

they are always implemented together [133, 143℄. The following setion is devoted

to desribing their interation.

4.2.3 The Combined Slow-Start with Congestion Avoidane Algorithm

For the ombination of the Congestion Avoidane and Slow-Start algorithms two

variables are introdued to eah TCP ommuniation. The rst is the ongestion

window(wnd) and the other is aSlow-Startthreshold (ssthresh).

The ongestionwindowrepresentsthe amountofdatathatmaybeunaknowledged

on ight in the network at any given time in the ourse of a ommuniation while

the Slow-Start threshold indiates when Slow-Start is terminated and Congestion

Avoidane takes over.

The usable window [39℄ is dened as the minimum between the reeiver's buer

(advertised window) and the ongestionwindow. In this manner, the sender avoids

transmitting more data than an be handled by the network or the reeiver. The

usable windowisthe value utilisedbythe TCP outputroutinewhilethe ongestion

windowis onlyused internallyto determine the usable window.

The ombined algorithmoperates asfollows:

1. During initialisationthe wnd is set to the size of one data unit whilessthresh

takes onthe maximum possible value.

2. Intheevent ofnetworkongestion(timeoutorreeiptofdupaks),thessthresh

is set to one half the usable window but at least equal to two data units. If

ongestionwasindiated by atimeout,thenwnd isset tothe size ofone data

unit.

3. When new data is aknowledged, wnd is inreased. The amount by whih it

is inreased depends on whether TCP is performing Slow-Startor Congestion

Avoidane.

In the rst ase wnd inreases exponential (one data unit per aknowledgement

4.2.4 Fast Retransmit

The original TCP [120℄ speiation did not inlude any information about the

protool'sreationtothereeiptbythereeiverofoutofsequene data(pakets). In

[31, 29℄itisstated that aTCP reeiver mayonly aknowledgesuessfully reeived

and in-sequene data. Consequently, when reeiving out of sequene data, TCP

responds with dupliate aknowledgements. These are regular aknowledgements

that ontain inthe sequene numbereld of theTCP header, thesequene number

of the last paket that was suessfully reeived in-sequene /iteJaobson1990.

An out of bound paketwould result inthe return of adupliate aknowledgement

while several of them would result in several dupliate aknowledgements. If the

numberof interepted dupliate aknowledgements supersedes apredened

thresh-old (usually three), then the sender pereives this as an indiation of data loss.

In that ase Fast Retransmit [68℄ ditates that TCP should perform an

immedi-ate retransmissionof what appears tobethe missingpaket, withoutwaiting for a

retransmission timer toexpire.

Paket loss is not the only reason for triggering the transmission of dupliate

a-knowledgements. The dynami harater of IP routingmay ause TCP pakets to

bedeliveredoutofsequene. Inthisase dupliateaknowledgements willbe

trans-mitted but it is expeted that the amount of dupliate aknowledgements will not

supersede the predened threshold and therefore willnot invokeFastRetransmit.

AswithSlow-StartandCongestionAvoidane,FastRetransmitisanalgorithmthat

is seldom used on itsown rather in onjuntion with the Fast Reovery Algorithm

presented inthe following setion.

4.2.5 Fast Reovery

Aordingtosetion4.2.3,upondeterminingnetworkongestionTCPisrequiredto

proeedwithimmediateredutionoftheongestionwindow. However, thefatthat

some of the transmitted data have managed to get to the reeiver and trigger the

transmissionofdupliateaknowledgementsisanindiationthatnetworkongestion

may be temporaryorof limited eet and Slow-Startshouldbeprevented.

Aording to Fast Reovery [68℄, after Fast Retransmit has identied and

retrans-mittedthe missingpaket, the ommuniationshouldavoidSlow-Startand proeed

with the invoation of Congestion Avoidane. This approah attempts to prevent

the pipefromstarving for tra following the disoveryof data loss.

4.2.6 Combining Fast Retransmit with Fast Reovery

The FastRetransmit algorithmappearedrstinthe4.3BSD Tahoeimplementation

and was followedby the entry ofthe FastReovery inthe 4.3BSD Reno

implemen-tation[134,39℄. Theoperationof theombinedFastRetransmitand FastReovery

•

^{Whenthe numberof dupliate} aknowledgements isgreaterthanthree (prede-ned threshold), ssthresh should be set to one half of the ongestion window,

but not less than two segments.

•

^{Lost segmentshouldbe}retransmittedand wnd shouldbe set tossthresh plus the size of three segments (the three reeived dupliate aknowledgements).

•

^{Foreverydupliate}aknowledgementreeivedthewnd shouldbeinremented byafullpaketsize. Ifpermittedbythenewvalueofwnd newpakets should

betransmitted.

•

^{The rst non dupliate} aknowledgement to arrive will aknowledge all out-standingdatapakets (theretransmittedandthosethattriggeredthedupaks).

Atthat pointthe wnd must beset tothevalueof ssthresh and theTCP

om-muniation must resume with Congestion Avoidane[134, 133℄.

Im Dokument Performance Evaluation and Improvement of the Mobile Internet Protocol: A Study of Hand-offs, Transport Layer Performance and Flow Mobility (Seite 75-80)