congestion.ppt

8/17/2019 congestion.ppt

1/27

Spring 2002 CS 461 1

Congestion Control

Outline

Queuing Discipline

Reacting to Congestion

Avoiding Congestion


2/27


Issues• To sides o! t"e sa#e coin

$ pre%allocate resources so at to avoid congestion

$ control congestion i! &and "en' is occurs

• To points o! i#ple#entation $ "osts at t"e edges o! t"e netor( &transport protocol'

$ routers inside t"e netor( &)ueuing discipline'

• *nderl+ing service #odel $ ,est%e!!ort &assu#e !or no'

$ #ultiple qualities of service &later'

Destinatio

1.5-Mbps T1 link

Router


3/27

Spring 2002 CS 461 -

.ra#eor( • Connectionless !los

$ se)uence o! pac(ets sent ,eteen source/destination pair $ #aintain soft state at t"e routers

• Taono#+ $ router%centric versus "ost%centric

$ reservation%,ased versus !eed,ac(%,ased $ indo%,ased versus rate%,ased

Router

Source

2

Source

1

Source

3

Router

Router

Destinatio

2

Destinatio

1


4/27


valuation

• .airness• oer &ratio o! t"roug"put to dela+'

Optimalload

Load


5/27


Queuing Discipline

• .irst%In%.irst%Out &.I.O' $ does not discri#inate ,eteen tra!!ic sources

• .air Queuing &.Q' $ eplicitl+ segregates tra!!ic ,ased on !los

$ ensures no !lo captures #ore t"an its s"are o! capacit+

$ variation eig"ted !air )ueuing &5.Q'

• ro,le#Flo 1

Flo 2

Flo 3

Flo !

Round-robi

ser"ice


6/27


.Q Algorit"#

• Suppose cloc( tic(s eac" ti#e a ,it is trans#itted

• 7et P i denote t"e lengt" o! pac(et i

• 7et S i denote t"e ti#e "en start to trans#it pac(et i

• 7et F i denote t"e ti#e "en !inis" trans#itting pac(et i

• F i 8 S i 9 P i

• 5"en does router start trans#itting pac(et i

$ i! ,e!ore router !inis"ed pac(et i % 1 !ro# t"is !lo: t"eni##ediatel+ a!ter last ,it o! i % 1 & F i-1'

$ i! no current pac(ets !or t"is !lo: t"en start trans#itting"en arrives &call t"is Ai'

• T"us F i 8 MAX & F i % 1: Ai' 9 P i


7/27

Spring 2002 CS 461 ;

.Q Algorit"# &cont'

• .or #ultiple !los $ calculate F i !or eac" pac(et t"at arrives on eac" !lo

$ treat all F i


8/27

Spring 2002 CS 461 >

TC Congestion Control

• Idea

$ assu#es ,est%e!!ort netor( &.I.O or .Q routers' eac"

source deter#ines netor( capacit+ !or itsel! $ uses i#plicit !eed,ac(

$ AC?s pace trans#ission & self-clocking '

• C"allenge

$ deter#ining t"e availa,le capacit+ in t"e !irst place

$ ad@usting to c"anges in t"e availa,le capacit+


9/27

Spring 2002 CS 461

Additive Increase/Bultiplicative

Decrease• O,@ective ad@ust to c"anges in t"e availa,le capacit+• =e state varia,le per connection CongestionWindow

$ li#its "o #uc" data source "as in transit

MaxWin = MIN(CongestionWindow, AdvertisedWindow)

EffWin = MaxWin - (LastByteSent -LastByteA!ed)

• Idea $ increase CongestionWindow "en congestion goes don $ decrease CongestionWindow "en congestion goes up


10/27

Spring 2002 CS 461 10

AIBD &cont'

• Question "o does t"e source deter#ine "et"er ornot t"e netor( is congested

• Anser a ti#eout occurs $ ti#eout signals t"at a pac(et as lost

$ pac(ets are seldo# lost due to trans#ission error

$ lost pac(et i#plies congestion


11/27

Spring 2002 CS 461 11

AIBD &cont'

• In practice incre#ent a little !or eac" AC?

Inre"ent = (MSS # MSS)$CongestionWindow

CongestionWindow %= Inre"ent

• Algorit"#

$ incre#ent CongestionWindow ,+

one pac(et per RTT &linear increase'

$ divide CongestionWindow ,+ to

"enever a ti#eout occurs

&multiplicative decrease'

Source Destination


12/27

Spring 2002 CS 461 12

AIBD &cont'

• Trace satoot" ,e"avior

)'

2'

1.' 2.' 3.' !.' 5.' ).' *.' &.' +.' Time #seconds$

*'

3'

!'

5'

1'

1'.'


13/27

Spring 2002 CS 461 1-

Slo Start

• O,@ective deter#ine t"e availa,lecapacit+ in t"e !irst

• Idea

$ ,egin it" CongestionWindow 8 1 pac(et

$ dou,le CongestionWindow eac" RTT&incre#ent ,+ 1 pac(et !or eac" AC?'

Source Destination


14/27

Spring 2002 CS 461 14

Slo Start &cont'• ponential grot": ,ut sloer t"an all at once

• *sed $ "en !irst starting connection

$ "en connection goes dead aiting !or ti#eout

• Trace

• ro,le# lose up to "al! a CongestionWindow


15/27

Spring 2002 CS 461 13

.ast Retrans#it and .ast Recover+

• ro,le# coarse%grain TC

ti#eouts lead to idle periods

• .ast retrans#it use

duplicate AC?s to triggerretrans#ission

,acket 1

,acket 2

,acket 3

,acket !

,acket 5

,acket )

Retransmitpacket 3

/ 1

/ 2

/ 2

/ 2

/ )

/ 2

Sender Recei"er


16/27

Spring 2002 CS 461 16

Results

• .ast recover+ $ s(ip t"e slo start p"ase

$ go directl+ to "al! t"e last success!ulCongestionWindow (sst&res&)

)'

2'

1.' 2.' 3.' !.' 5.' ).' *.'

Time #seconds$

*'

3'

!'

5'

1'


17/27

Spring 2002 CS 461 1;

Congestion Avoidance

• TC


18/27

Spring 2002 CS 461 1>

DC,it• Add ,inar+ congestion ,it to eac" pac(et "eader

• Router $ #onitors average )ueue lengt" over last ,us+9idle c+cle

$ set congestion ,it i! average )ueue lengt" F 1 $ atte#pts to ,alance t"roug"out against dela+

0ueue len(t1

.urrenttime

Tim.urrent

c2cle

,re"ious

c2cle-"era(in(inter"al


19/27


nd Gosts

• Destination ec"oes ,it ,ac( to source

• Source records "o #an+ pac(ets resulted in set ,it

• I! less t"an 30H o! last indo


20/27

Spring 2002 CS 461 20

Rando# arl+ Detection &RD'

• =oti!ication is i#plicit $

@ust drop t"e pac(et &TC ill ti#eout' $ could #a(e eplicit ,+ #ar(ing t"e pac(et

• arl+ rando# drop $ rat"er t"an ait !or )ueue to ,eco#e !ull: drop eac"

arriving pac(et it" so#e drop probability "enevert"e )ueue lengt" eceeds so#e drop level


21/27

Spring 2002 CS 461 21

RD Details

• Co#pute average )ueue lengt" AvgLen = (' - Weig&t) # AvgLen %

Weig&t # Sa"eLen

0 J Weig&t J 1 &usuall+ 0002'

Sa"eLen is )ueue lengt" eac" ti#e a pac(et arrivesMaTresold MinTresold

"(Len


22/27

Spring 2002 CS 461 22

RD Details &cont'

• To )ueue lengt" t"res"olds

if AvgLen *= Min+&res&od t&en

enee t&e a!et

if Min+&res&od * AvgLen * Max+&res&od t&en

aate ro.a.iity /

dro arriving a!et wit& ro.a.iity /

if Man+&res&od *= AvgLen t&endro arriving a!et


23/27

Spring 2002 CS 461 2-

RD Details &cont'

• Co#puting pro,a,ilit+

+e"/ = Max/ # (AvgLen - Min+&res&od)$(Max+&res&od - Min+&res&od)

/ = +e"/$(' - ont # +e"/)

• Drop ro,a,ilit+ Curve,#drop$

1.'

Ma,

MinTres MaTres

"(Len


24/27

Spring 2002 CS 461 24

Tuning RD• ro,a,ilit+ o! dropping a particular !lo


25/27

Spring 2002 CS 461 23

TC Eegas• Idea source atc"es !or so#e sign t"at router


26/27

Spring 2002 CS 461 26

Algorit"#•

7etBase0++

,e t"e #ini#u# o! all #easured RTTs &co##onl+ t"eRTT o! t"e !irst pac(et'• I! not over!loing t"e connection: t"en

Exet0ate = CongestionWindow$Base0++

• Source calculates sending rate & Ata0ate' once per RTT

• Source co#pares Ata0ate it" Exet0ate

1iff = Exeted0ate - Ata0ate

if 1iff *

inrease CongestionWindow ineary

ese if 1iff 2

derease CongestionWindow ineary

ese

eave CongestionWindow n&anged


27/27

Spring 2002 CS 461 2;

Algorit"# &cont'

• ara#eters − 8 1 pac(et

− 8 - pac(ets

• ven !aster retrans#it $ (eep !ine%grained ti#esta#ps !or eac" pac(et

$ c"ec( !or ti#eout on !irst duplicate AC?

*')'5'!'3'2'1'

Time #seconds$

'.5 1.' 1.5 2.' 2.5 3.' 3.5 !.' !.5 5.' 5.5 ).' ).5 *.' *.5 &.'

'.5 1.' 1.5 2.' 2.5 3.' 3.5 !.' !.5 5.' 5.5 ).' ).5 *.' *.5 &.'

2!'

2''

1)'

12'

&'

!'

Time #seconds$

Date post:	06-Jul-2018
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