Chapter3 Redes de computadoras

8/9/2019 Chapter3 Redes de computadoras

1/108

Transport Layer3-1

Chapter 3

Transport Layer

Computer Networking:A Top Down Approach5thedition.Jim Kurose Keith !ossAddison"#es$ey Apri$%&&'.

A note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers).

Theyre in o!eroint form so you can add, modify, and delete slides

(including this one) and slide content to suit your needs. They obviously

represent a lotof !ork on our part. "n return for use, !e only ask the

follo!ing:

"f you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, !ed like people to use our book#)"f you post any slides in substantially unaltered form on a !!! site, that

you note that they are adapted from (or perhaps identical to) our slides, and

note our copyright of this material.

Thanks and en$oy# %&''W

All material copyright *++-/*/

%.& 'urose and '.W. oss, All ights eserved


2/108

Transport Layer3-2

Chapter 3: Transport Layer

(ur goa$s: understand princip$es

)ehind transport$ayer ser*ices:

mu$tip$e+ing,demu$tip$e+ing

re$ia)$e data trans-er -$ow contro$ congestion contro$

$earn a)out transport$ayer protoco$s in thenternet:

/D0: connection$esstransport

TC0: connection"orientedtransport

TC0 congestion contro$


3/108

Transport Layer3-3

Chapter 3 out$ine

3.1 Transport"$ayerser*ices

3.% 2u$tip$e+ing and

demu$tip$e+ing3.3 Connection$esstransport: /D0

3. 0rincip$es o- re$ia)$e

data trans-er

3.5 Connection"orientedtransport: TC0 segment structure re$ia)$e data trans-er -$ow contro$ connection management

3.4 0rincip$es o-congestion contro$

3. TC0 congestion contro$


4/108

Transport Layer3-4

Transport ser*ices and protoco$s

pro*ide$ogica$ communication)etween app processesrunning on di--erent hosts

transport protoco$s run in endsystems

send side: )reaks appmessages into segmentspasses to network $ayer

rc* side: reassem)$essegments into messages

passes to app $ayer more than one transport

protoco$ a*ai$a)$e to apps nternet: TC0 and /D0

app$icationtransport

networkdata $inkphysica$

app$icationtransportnetworkdata $ink

physica$

$ogica$end"end

transport


5/108

Transport Layer3-5

Transport *s. network $ayer

network $ayer:$ogica$communication)etween hosts

transport $ayer:$ogica$communication)etween processes re$ies on enhances

network $ayer ser*ices

6ouseho$d ana$ogy:

1% kids sending $etters to1% kids

processes 7 kids app messages 7 $etters

in en*e$opes hosts 7 houses

transport protoco$ 7 Annand 8i$$ who demu+ toin"house si)$ings

network"$ayer protoco$ 7posta$ ser*ice


6/108

Transport Layer3-6

nternet transport"$ayer protoco$s

re$ia)$e in"orderde$i*ery 9TC0 congestion contro$ -$ow contro$

connection setup unre$ia)$e unordered

de$i*ery: /D0 no"-ri$$s e+tension o-

;)est"e--ort< 0

ser*ices not a*ai$a)$e: de$ay guarantees )andwidth guarantees

app$icationtransportnetworkdata $inkphysica$


network

data $inkphysica$





app$icationtransport


$ogica$end

"endtransport


7/108Transport Layer3-7

Chapter 3 out$ine


3.% 2u$tip$e+ing and

demu$tip$e+ing3.3 Connection$esstransport: /D0

3. 0rincip$es o- re$ia)$e

data trans-er

3.5 Connection"orientedtransport: TC0 segment structure re$ia)$e data trans-er -$ow contro$ connection management





2u$tip$e+ing,demu$tip$e+ing

app$ication

transport

network$ink

physica$

01 app$ication

transport

network

$ink

physica$

app$ication

transport

network$ink

physica$

0%03 001

host 1 host % host 3

7 process7 socket

de$i*ering recei*ed segmentsto correct socket

Demu$tip$e+ing at rc* host: gathering data -rom mu$tip$esockets en*e$oping data withheader 9$ater used -ordemu$tip$e+ing

2u$tip$e+ing at send host:



6ow demu$tip$e+ing works host recei*es 0 datagrams

each datagram has source 0address destination 0 address

each datagram carries 1 transport"$ayer segment

each segment has source destinationport num)er

host uses 0 addresses = portnum)ers to direct segment toappropriate socket

source port > dest port >

3% )its

app$icationdata

9message

other header -ie$ds

TC0,/D0 segment -ormat


10/108Transport Layer 3-

Connection$ess demu$tip$e+ing

reca$$:create sockets withhost"$oca$ port num)ers:

DatagramSocket mySocket1 = new

DatagramSocket(12534);

DatagramSocket mySocket2 = newDatagramSocket(12535);

reca$$:when creatingdatagram to send into /D0socket must speci-y

9dest 0 address dest port num)er

when host recei*es /D0segment: checks destination port

num)er in segment

directs /D0 segment tosocket with that portnum)er

0 datagrams withdi--erent source 0

addresses and,or sourceport num)ers directedto same socket


11/108Transport Layer 3-

Connection$ess demu+ 9cont

DatagramSocket serverSocket = new DatagramSocket(6428);

C$ient0:8

0%

c$ient0: A

010103

ser*er0: C

?0: 4%@

D0: '15

?0: '15

D0: 4%@

?0: 4%@

D0: 55

?0: 55

D0: 4%@

?0 pro*ides ;return address o- pkt )eing ACKed

dup$icate ACK at sender resu$ts in same action asNAK: retransmit current pkt


37/108

Transport Layer 3-

rdt%.%: sender recei*er -ragments

Wait for

call / from

above

sndpkt 0 make1pkt(/, data, checksum)udt1send(sndpkt)

rdt1send(data)

udt_send(sndpkt)

rdt1rcv(rcvpkt) 44

( corrupt(rcvpkt) 66

isACK(rcvpkt,1))

rdt1rcv(rcvpkt)

44 notcorrupt(rcvpkt)

44 isACK(rcvpkt,0)

Wait for

A3'

/

sender ?2-ragment

rdt1rcv(rcvpkt) 44 notcorrupt(rcvpkt)

44 has1se7*(rcvpkt)

e2tract(rcvpkt,data)

deliver1data(data)

sndpkt = make_pkt(ACK1, chksum)

udt1send(sndpkt)

Wait for

/ from

belo!

rdt1rcv(rcvpkt) 44

(corrupt(rcvpkt) 66

has_seq1(rcvpkt))

udt_send(sndpkt)

recei*er ?2

-ragment


38/108

Transport Layer 3-

rdt3.&: channe$s with errors and$oss

New assumption:under$ying channe$ cana$so $ose packets 9dataor ACKs checksum se. > ACKs

retransmissions wi$$ )eo- he$p )ut not enough

Approach:sender waits;reasona)$e< amount o-time -or ACK

retransmits i- no ACK

recei*ed in this time i- pkt 9or ACK Must de$ayed

9not $ost: retransmission wi$$ )e

dup$icate )ut use o- se.

>Gs a$ready hand$es this recei*er must speci-y se

> o- pkt )eing ACKed reuires countdown timer


39/108

Transport Layer 3-

rdt3.& sender

sndpkt 0 make1pkt(/, data, checksum)

udt1send(sndpkt)

start1timer

rdt1send(data)

Wait

for

A3'/

rdt1rcv(rcvpkt) 44


isA3'(rcvpkt,*) )

Wait for

call * from

above

sndpkt 0 make1pkt(*, data, checksum)

udt1send(sndpkt)

start1timer

rdt1send(data)

rdt1rcv(rcvpkt)44 notcorrupt(rcvpkt)

44 isA3'(rcvpkt,/)

rdt1rcv(rcvpkt) 44


isA3'(rcvpkt,/) )

rdt1rcv(rcvpkt)

44 notcorrupt(rcvpkt)44 isA3'(rcvpkt,*)

stop1timer

stop1timer

udt1send(sndpkt)

start1timer

timeout

udt1send(sndpkt)

start1timer

timeout

rdt1rcv(rcvpkt)

Wait for

call /from

above

Wait

for

A3'*

rdt1rcv(rcvpkt)


40/108

Transport Layer 3-

rdt3.& in action


41/108

Transport Layer 3-

rdt3.& in action


42/108

Transport Layer 3-

0er-ormance o- rdt3.&

rdt3.& works )ut per-ormance stinks e+: 1 )ps $ink 15 ms prop. de$ay @&&& )it packet:

/ sender: uti$iationO -raction o- time sender )usy sending

/sender

=.&&@

3&.&&@

= &.&&&%5L , !

!TT P L , !

=

i- !TT73& msec 1K8 pkt e*ery 3& msec " 33k8,sec thruput o*er 1 )ps $ink network protoco$ $imits use o- physica$ resourcesF

dsmicrosecon8bps10

bits8000

9

R

Ldtrans


43/108

Transport Layer 3-

rdt3.&: stop"and"wait operation

first packet bit transmitted, t 0 /sender receiver

TT

last packet bit transmitted, t 0 8

first packet bit arrives

last packet bit arrives, send A3'

A3' arrives, send ne2t

packet, t 0 TT 9 8

/sender

=.&&@

3&.&&@= &.&&&%5

L , !

!TT P L , !=


44/108

Transport Layer 3-

0ipe$ined protoco$s

pipe$ining:sender a$$ows mu$tip$e ;in"-$ight


45/108

Transport Layer 3-

0ipe$ining: increased uti$iation

first packet bit transmitted, t 0 /

sender receiver

TT

last bit transmitted, t 0 8

first packet bit arrives

last packet bit arrives, send A3'

A3' arrives, send ne2t

packet, t 0 TT 9 8

last bit of nd

packet arrives, send A3'last bit of rdpacket arrives, send A3'

/sender

=.&%

3&.&&@= &.&&&@

3 Q L , !

!TT P L , !=

ncrease uti$iation

)y a -actor o- 3F


46/108

Transport Layer 3-

0ipe$ined 0rotoco$s

o")ack"N: )ig picture: sender can ha*e up to

N unacked packets inpipe$ine

rc*r on$y sendscumu$ati*e acks doesnGt ack packet i-

thereGs a gap sender has timer -or

o$dest unacked packet i- timer e+pires

retransmit a$$ unackGedpackets

?e$ecti*e !epeat: )ig pic sender can ha*e up to

N unackGed packets inpipe$ine

rc*r sends indi*idua$ack-or each packet

sender maintains timer-or each unacked

packet when timer e+piresretransmit on$yunackGed packet


47/108

Transport Layer 3-

o"8ack"N

?ender: k")it se > in pkt header ;window< o- up to N consecuti*e unackGed pkts a$$owed

ACK9n: ACKs a$$ pkts up to inc$uding se > n " ;cumu$ati*e ACK< may recei*e dup$icate ACKs 9see recei*er

timer -or each in"-$ight pkt

timeout9n:retransmit pkt n and a$$ higher se > pkts in window


48/108

Transport Layer 3-

8N: sender e+tended ?2

Waitstart1timer

udt1send(sndpkt;base


49/108

Transport Layer 3-

8N: recei*er e+tended ?2

ACK"on$y: a$ways send ACK -or correct$y"recei*ed pktwith highest in"orderse > may generate dup$icate ACKs

need on$y remem)er expectedseqnum

out"o-"order pkt: discard 9donGt )u--er " no recei*er )u--eringF !e"ACK pkt with highest in"order se >

Wait

udt1send(sndpkt)

default

rdt1rcv(rcvpkt) 44 notcurrupt(rcvpkt)

44 hasse7num(rcvpkt,e2pectedse7num)

e2tract(rcvpkt,data)

deliver1data(data)

sndpkt 0 make1pkt(e2pectedse7num,A3',chksum)

udt1send(sndpkt)

e2pectedse7num99

e2pectedse7num0*

sndpkt 0

make1pkt(e2pectedse7num,A3',chksum)


50/108

Transport Layer 3-

8N inaction


51/108

Transport Layer 3-

?e$ecti*e !epeat

recei*er indi*idua$$yacknow$edges a$$ correct$yrecei*ed pkts )u--ers pkts as needed -or e*entua$ in"order de$i*ery

to upper $ayer

sender on$y resends pkts -or which ACK notrecei*ed sender timer -or each unACKed pkt

sender window

N consecuti*e se >Gs again $imits se >s o- sent unACKGed pkts


52/108

Transport Layer 3-

?e$ecti*e repeat: sender recei*er windows


53/108

Transport Layer 3-

?e$ecti*e repeat

data -rom a)o*e : i- ne+t a*ai$a)$e se > in

window send pkt

timeout9n: resend pkt n restart timerACK9n in Bsend)asesend)asePN: mark pkt n as recei*ed i- n sma$$est unACKed pkt

ad*ance window )ase tone+t unACKed se >

senderpkt n in Brc*)ase rc*)asePN"1 send ACK9n out"o-"order: )u--er in"order: de$i*er 9a$so

de$i*er )u--ered in"orderpkts ad*ance window tone+t not"yet"recei*ed pkt

pkt n in Brc*)ase"Nrc*)ase"1 ACK9n

otherwise: ignore

recei*er


54/108

Transport Layer 3-

?e$ecti*e repeat in action

? $ ti t


55/108

Transport Layer 3-

?e$ecti*e repeat:di$emma

I+amp$e: se >Gs: & 1 % 3 window sie73

recei*er sees nodi--erence in twoscenariosF

incorrect$y passesdup$icate data as new in9a

R:what re$ationship)etween se > sie andwindow sieE


56/108

Transport Layer 3-

Chapter 3 out$ine


3.% 2u$tip$e+ing anddemu$tip$e+ing

3.3 Connection$esstransport: /D0

3. 0rincip$es o- re$ia)$edata trans-er

3.5 Connection"orientedtransport: TC0 segment structure re$ia)$e data trans-er

-$ow contro$ connection management




57/108

Transport Layer 3-

TC0: (*er*iew !Cs: '3 11%% 13%3 %&1@ %5@1

-u$$ dup$e+ data: )i"directiona$ data -$ow

in same connection 2??: ma+imum segment

sie connection"oriented:

handshaking 9e+changeo- contro$ msgs initssender recei*er state

)e-ore data e+change -$ow contro$$ed:

sender wi$$ noto*erwhe$m recei*er

point"to"point: one sender one recei*er

re$ia)$e in"order )ytesteam:

no ;message )oundaries< pipe$ined:

TC0 congestion and -$owcontro$ set window sie

send = recei*e )u--ers

s o c k e t

d o o r T 3

s e n d b u f f e r

T 3

r e c e i v e b u f f e r

s o c k e t

d o o r

s e g m e n t

a p p l i c a t i o n

! r i t e s d a t aa p p l i c a t i o n

r e a d s d a t a

TC0 t t t


58/108

Transport Layer 3-

TC0 segment structure

source port > dest port >

3% )its

app$icationdata9*aria)$e $ength

seuence num)er

acknow$edgement num)er

!ecei*e window

/rg data pnterchecksum

?!0A/head$en

notused

(ptions 9*aria)$e $ength

/!: urgent data9genera$$y not used

ACK: ACK >*a$id

0?6: push data now

9genera$$y not used

!?T ?HN N:connection esta)9setup teardown

commands

> )ytesrc*r wi$$ingto accept

counting)y )yteso- data9not segmentsF

nternetchecksum

9as in /D0


59/108

Transport Layer 3-

TC0 se. >Gs and ACKs?e. >Gs:

)yte stream ;num)er o- ne+t )yte

e+pected -rom otherside

cumu$ati*e ACK

R:how recei*er hand$esout"o-"order segments

A: TC0 spec doesnGtsay " up toimp$ementor

6ost A 6ost 8

e70B.,A3'0C+,data0D3

e70C+,A

3'0B:,data0

D3

e70B:,A3'0E/

/sertypes

SCG

host ACKsreceipt

o- echoed

SCG

host ACKsreceipt o-

SCG echoes)ack SCG

timesimp$e te$net scenario


60/108

Transport Layer 3-

TC0 !ound Trip Time and Timeout

R:how to set TC0timeout *a$ueE

$onger than !TT )ut !TT *aries

too short:premature timeout unnecessary

retransmissions

too $ong: s$owreaction to segment$oss

R:how to estimate !TTE SampleRTT:measured time -rom

segment transmission unti$ ACKreceipt ignore retransmissions

SampleRTTwi$$ *ary wantestimated !TT ;smoother< a*erage se*era$ recent

measurements not Mustcurrent SampleRTT


61/108

Transport Layer 3-


EstimatedRTT = (1- )EstimatedRTT ! SampleRTT

I+ponentia$ weighted mo*ing a*erage in-$uence o- past samp$e decreases e+ponentia$$y -ast typica$ *a$ue: =&.1%5


62/108

Transport Layer 3-

I+amp$e !TT estimation:


63/108

Transport Layer 3-


?etting the timeout EstimatedRTTp$us ;sa-ety margin< $arge *ariation in EstimatedRTT -"$arger sa-ety margin

-irst estimate o- how much ?amp$e!TT de*iates -rom Istimated!TT:

Time#ut$nterval = EstimatedRTT ! %&evRTT

&evRTT = (1-

)&evRTT !

'SampleRTT-EstimatedRTT'

(tpicall = 0.2*)

Then set timeout inter*a$:


64/108

Transport Layer 3-

Chapter 3 out$ine










65/108

Transport Layer 3-

TC0 re$ia)$e data trans-er

TC0 creates rdtser*ice on top o- 0Gsunre$ia)$e ser*ice

pipe$ined segments cumu$ati*e acks TC0 uses sing$e

retransmission timer

retransmissions aretriggered )y: timeout e*ents dup$icate acks

initia$$y considersimp$i-ied TC0 sender: ignore dup$icate acks ignore -$ow contro$

congestion contro$

TC0 sender e*ents:


66/108

Transport Layer 3-

TC0 sender e*ents:data rc*d -rom app:

Create segment withse >

se > is )yte"streamnum)er o- -irst data

)yte in segment start timer i- not

a$ready running 9thinko- timer as -or o$destunacked segment

e+piration inter*a$:TimeOt!nterva"

timeout:

retransmit segmentthat caused timeout

restart timer

Ack rc*d: - acknow$edges

pre*ious$y unackedsegments update what is known to

)e acked start timer i- there are

outstanding segments

5e2te75um 0 "nitiale75um

endFase 0 "nitiale75um


67/108

Transport Layer 3-

TC0

sender9simp$i-ied

endFase 0 "nitiale75um

loop (forever) ?

s!itch(event)

event:data received from application abovecreate T3 segment !ith se7uence number 5e2te75um

if (timer currently not running)

start timer

pass segment to "

5e2te75um 0 5e2te75um 9 length(data)

event:timer timeout

retransmit not-yet-ackno!ledged segment !ith

smallest se7uence number

start timer

event:A3' received, !ith A3' field value of y

if (y G endFase) ?endFase 0 y

if (there are currently not-yet-ackno!ledged segments)

start timer

@

@ H end of loop forever H

Comment: ?end8ase"1: $astcumu$ati*e$yacked )yteI+amp$e:?end8ase"1 7 1U

y7 3 so the rc*rwants 3P U

y ?end8ase sothat new data isacked

TC0 t i i i


68/108

Transport Layer 3-

TC0: retransmission scenarios

6ost A

Ae70*//,./bytesdata

A3'

0*//

timepremature timeout

6ost 8

e70+.,Ebytesdata

A3'0*./

e70+.,Ebytesdata

?eA

7'%timeout

A3'

0*./

6ost A

e70+.,Ebytesdata

A3'0*/

/

$osstimeout

$ost ACK scenario

6ost 8

Ie70+.,Ebytesdata

A3'0*//

time

?eA7'%timeout

?end8ase7 1&&

?end8ase7 1%&

?end8ase7 1%&

?end8ase7 1&&


69/108

Transport Layer 3-

TC0 retransmission scenarios 9more

6ost A

e70+.,Ebytesdata

A3'0*/

/

$osstimeout

Cumu$ati*e ACK scenario

6ost 8

I

e70*//,./bytesdata

A3'0*.

/

time

?end8ase7 1%&


70/108

Transport Layer 3-

TC0 ACK generationB!C 11%% !C %5@1

Jvent at eceiver

Arrival of in-order segment !ith

e2pected se7 K. All data up to

e2pected se7 K already A3'ed

Arrival of in-order segment !ith

e2pected se7 K. Lne other

segment has A3' pending

Arrival of out-of-order segment

higher-than-e2pect se7. K .Map detected

Arrival of segment that

partially or completely fills gap

T3 eceiver action

Nelayed A3'. Wait up to O//ms

for ne2t segment. "f no ne2t segment,

send A3'

"mmediately send single cumulative

A3', A3'ing both in-order segments

"mmediately send duplicate ACK,

indicating se7. K of ne2t e2pected byte

"mmediate send A3', provided that

segment starts at lo!er end of gap


71/108

Transport Layer 3-

ast !etransmit

time"out period o-tenre$ati*e$y $ong: $ong de$ay )e-ore

resending $ost packet

detect $ost segments*ia dup$icate ACKs. sender o-ten sends

many segments )ack"to")ack

i- segment is $ost therewi$$ $ike$y )e manydup$icate ACKs.

i- sender recei*es 3ACKs -or the samedata it supposes thatsegment a-ter ACKed

data was $ost: -ast retransmit: resend

segment )e-ore timere+pires

6ost A 6ost 8


72/108

Transport Layer 3-

6ost A

timeout

6ost 8

time

I

resend.ndsegment

&igure .C esending a segment after triple duplicate A3'

$ h


73/108

Transport Layer 3-

event:A3' received, !ith A3' field value of y

if (y G endFase) ?

endFase 0 y

if (there are currently not-yet-ackno!ledged segments)

start timer

@else ?

increment count of dup A3's received for y

if (count of dup A3's received for y 0 ) ?

resend segment !ith se7uence number y

@

ast retransmit a$gorithm:

a dup$icate ACK -ora$ready ACKed segment

-ast retransmit

h $


74/108

Transport Layer 3-

Chapter 3 out$ine









C0 $ C $


75/108

Transport Layer 3-

TC0 $ow Contro$

recei*e side o- TC0connection has arecei*e )u--er:

speed"matchingser*ice: matching thesend rate to therecei*ing appGs drain

rate app process may )es$ow at reading -rom)u--er

sender wonGt o*er-$owrecei*erGs )u--er )ytransmitting too

muchtoo -ast

-$ow contro$

TC0 $ $ h i k


76/108

Transport Layer 3-

TC0 $ow contro$: how it works

9suppose TC0 recei*erdiscards out"o-"ordersegments

spare room in )u--er= Rcv+ind#,

= Rcvuer-/astteRcvd -

astteRead

rc*r ad*ertises spareroom )y inc$uding *a$ueo- Rcv+ind#,insegments

sender $imits unACKeddata to Rcv+ind#, guarantees recei*e

)u--er doesnGt o*er-$ow

Ch 3 $i


77/108

Transport Layer 3-

Chapter 3 out$ine










78/108


79/108

Transport Layer 3-

TC0 Connection 2anagement 9cont.

C$osing a connection:

c$ient c$oses socket:clientS#cet.cl#se()6

?tep 1:c$ientend systemsends TC0 N contro$segment to ser*er

?tep %:ser*er recei*es Nrep$ies with ACK. C$oses

connection sends N.

c$ient

&"5

ser*er

A3'

A3'

&"5

c$ose

c$ose

c$osedtimed

wait


80/108

Transport Layer 3-

TC0 Connection 2anagement 9cont.

?tep 3:c$ientrecei*es Nrep$ies with ACK.

Inters ;timed wait< " wi$$respond with ACK torecei*ed Ns

?tep :ser*er recei*es ACK.Connection c$osed.

Note:with sma$$ modi-icationcan hand$e simu$taneous Ns.

c$ient

&"5

ser*er

A3'

A3'

&"5

c$osing

c$osing

c$osedtimed

wait

c$osed

TC0 Connection 2anagement 9cont


81/108

Transport Layer 3-

TC0 Connection 2anagement 9cont

TC0 c$ient$i-ecyc$e

TC0 ser*er

$i-ecyc$e

Ch t 3 t$i


82/108

Transport Layer 3-

Chapter 3 out$ine







3.40rincip$es o-congestion contro$


0 i i $ - C ti C t $


83/108

Transport Layer 3-

0rincip$es o- Congestion Contro$

Congestion: in-orma$$y: ;too many sources sending too much

data too -ast -or networkto hand$e< di--erent -rom -$ow contro$F mani-estations:

$ost packets 9)u--er o*er-$ow at routers $ong de$ays 9ueueing in router )u--ers

a top"1& pro)$emF

C , - 1


84/108

Transport Layer 3-

Causes,costs o- congestion: scenario 1

two senders tworecei*ers one router

in-inite )u--ers

no retransmission

$arge de$ayswhen congested

ma+imumachie*a)$ethroughput

unlimited shared

output link buffers

Post Ain : original data

Post F

out

C , - i i %


85/108

Transport Layer 3-

Causes,costs o- congestion: scenario %

one router -inite)u--ers sender retransmission o- timed"out packet

app$ication"$ayer input 7 app$ication"$ayer output:in 0 out transport"$ayer input inc$udes retransmissions :in in

finite shared output

link buffers

Post A

in: original data

Post F

outQin:original data,plusretransmitted data

S

Congestion scenario %a: idea$ case


86/108

Transport Layer 3-

Congestion scenario %a: idea$ case

sender sendson$y when router)u--ers a*ai$a)$e


link buffers

Post A

in: original data

Post F


copy

in

out

free buffer space!

Congestion scenario %): known $oss


87/108

Transport Layer 3-

Post A

in: original data

Post F


copy

no buffer space!

packets may get dropped atrouter due to -u$$ )u--ers

sometimes $ost sender on$y resends i-

packet knownto )e $ost

9admitted$y idea$ied

Congestion scenario %): known$oss

Congestion scenario %): known $oss


88/108

Transport Layer 3-

Congestion scenario %) known$oss

Post A

in: original data

Post F


free buffer space!


sometimes not $ost sender on$y resends i-

packet knownto )e $ost

9admitted$y idea$ied

in

out

when sending at!,% some packetsare retransmissions)ut asymptoticgoodput is sti$$ !,%9whyE

Congestion scenario %c: dup$icates


89/108

Transport Layer 3-


sender times outpremature$y sending twocopies )oth o- which arede$i*ered

Post A

in

Post F

outQincopy

free buffer space!

Congestion scenario %c dup$icates

timeout

in

out

when sending at!,% some packetsare retransmissionsinc$uding dup$icatedthat are de$i*eredF

Congestion scenario %c: dup$icates


90/108

Transport Layer 3-


sender times outpremature$y sending twocopies )oth o- which arede$i*ered

Congestion scenario %c dup$icates

out

when sending at!,% some packetsare retransmissionsinc$uding dup$icatedthat are de$i*eredF

;costs< o- congestion: more work 9retrans -or gi*en ;goodput< unneeded retransmissions: $ink carries mu$tip$e copies o- pkt

decreasing goodput

in

C , t - ti i 3


91/108

Transport Layer 3-

Causes,costs o- congestion: scenario 3 -our senders mu$tihop paths timeout,retransmit

in

R:what happens asand increaseE

in


link buffers

Post Ain : original data

Post F

out

Qin : original data, plus

retransmitted data

C , t - ti i 3


92/108

Transport Layer 3-

Causes,costs o- congestion: scenario 3

another ;cost< o- congestion: when packet dropped any ;upstream transmission capacity used

-or that packet was wastedF

P

os

t

A

P

o

s

t

F

o

u

t

Approaches towards congestion contro$


93/108

Transport Layer 3-

Approaches towards congestion contro$

end"end congestioncontro$:

no e+p$icit -eed)ack -romnetwork

congestion in-erred -romend"system o)ser*ed $ossde$ay

approach taken )y TC0

network"assistedcongestion contro$:

routers pro*ide -eed)ack

to end systems sing$e )it indicating

congestion 9?NADIC)it TC0,0 ICNAT2

e+p$icit rate sendershou$d send at

Two )road approaches towards congestion contro$:

Case study: AT2 A8! congestion contro$


94/108

Transport Layer 3-


A8!: a*ai$a)$e )it rate: ;e$astic ser*ice< i- senderGs path

;under$oaded


95/108

Transport Layer 3-


two")yte I! 9e+p$icit rate -ie$d in !2 ce$$ congested switch may $ower I! *a$ue in ce$$

senderG send rate thus ma+imum supporta)$e rate on path IC )it in data ce$$s: set to 1 in congested switch

i- data ce$$ preceding !2 ce$$ has IC set sender sets C)it in returned !2 ce$$

Chapter 3 out$ine


96/108

Transport Layer 3-

Chapter 3 out$ine









TC0 congestion contro$: additi*e increase


97/108

Transport Layer 3-

g mu$tip$icati*e decrease

E ' b y t e s

* , ' b y t e s

. B ' b y t e s

t im e

c o n g e s t i o n

! i n d o !

approach:increase transmission rate 9window sie pro)ing -or usa)$e

)andwidth unti$ $oss occurs additi*e increase:increase c,nd)y 1 2?? e*ery !TT unti$ $oss detected mu$tip$icati*e decrease:cut c,nd in ha$- a-ter $oss

time

cwn#:congestion!indo!

siRe

saw tooth)eha*ior: pro)ing

-or )andwidth

TC0 Congestion Contro$: detai$s


98/108

Transport Layer 3-

TC0 Congestion Contro$: detai$s

sender $imits transmission: astteSent-astte7ced

c,nd

rough$y

c,ndis dynamic -unction o-percei*ed network congestion

6ow does senderpercei*e congestionE

$oss e*ent 7 timeout or3 dup$icate acks

TC0 sender reducesrate 9c,nd a-ter $osse*ent

three mechanisms:

A2D s$ow start conser*ati*e a-ter

timeout e*ents

rate 7 cwnd

!TT8ytes,sec

TC0 ?$ow ?tart


99/108

Transport Layer 3-

TC0 ?$ow ?tart

when connection)egins increase ratee+ponentia$$y unti$-irst $oss e*ent:

initia$$y c,nd7 1 2?? dou)$e c,nde*ery !TT done )y incrementingc,nd-or e*ery ACKrecei*ed

summary:initia$ rate iss$ow )ut ramps upe+ponentia$$y -ast

6ost A

onesegment

!TT

6ost 8

time

t!osegments

foursegments

!e-inement: in-erring $oss


100/108

Transport Layer3-

!e-inement: in-erring $oss

a-ter 3 dup ACKs: c,ndis cut in ha$- window then grows $inear$y

)ut a-ter timeout e*ent: c,ndinstead set to 1 2??U

window then grows e+ponentia$$y to a thresho$d then grows $inear$y

3 dup ACKs indicates

network capa)$e o-de$i*ering some segmentstimeout indicates a;more a$arming

Chapter3 Redes de computadoras

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Transcript of Chapter3 Redes de computadoras