Performance of rdt3.0 rdt3.0: stop-and-wait operationartoo/CN2009/slides/Chapter3-2.pdf · rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 sender receiver RTT

1

Transport Layer 3-1

Performance of rdt30

rdt30 works but performance stinksexample 1 Gbps link 15 ms e-e prop delay 1KB packet

Ttransmit = 8kbpkt109 bsec = 8 microsec

U sender utilization ndash fraction of time sender busy sending

U sender =

008

30008 = 000027 L R

RTT + L R =

L (packet length in bits)R (transmission rate bps) =

1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps linknetwork protocol limits use of physical resources

Transport Layer 3-2

rdt30 stop-and-wait operation

first packet bit transmitted t = 0

sender receiver

RTT

last packet bit transmitted t = L R

first packet bit arriveslast packet bit arrives send ACK

ACK arrives send next packet t = RTT + L R

U sender =

008

30008 = 000027 L R

RTT + L R =

Transport Layer 3-3

Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo yet-to-

be-acknowledged pktsrange of sequence numbers must be increasedbuffering at sender andor receiver

Two generic forms of pipelined protocols go-Back-N selective repeat

Transport Layer 3-4

Pipelining increased utilization

first packet bit transmitted t = 0

sender receiver

RTT

last bit transmitted t = L R

first packet bit arriveslast packet bit arrives send ACK

ACK arrives send next packet t = RTT + L R

last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK

U sender =

024

30008 = 00008 3 L R

RTT + L R =

Increase utilizationby a factor of 3

2

Transport Layer 3-5

Go-Back-NSender

k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed

ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)

timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window

Transport Layer 3-6

GBN sender extended FSM

Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])

timeout

rdt_send(data)

if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)

start_timernextseqnum++

elserefuse_data(data)

base = getacknum(rcvpkt)+1If (base == nextseqnum)

stop_timerelsestart_timer

rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)

base=1nextseqnum=1

rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)

Λ

Transport Layer 3-7

GBN receiver extended FSM

ACK-only always send ACK for correctly-received pktwith highest in-order seq

may generate duplicate ACKsneed only remember expectedseqnum

out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq

Wait

udt_send(sndpkt)default

rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)

extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++

expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)

Λ

Transport Layer 3-8

GBN inaction

3

Transport Layer 3-9

Selective Repeat

receiver individually acknowledges all correctly received pkts

buffers pkts as needed for eventual in-order delivery to upper layer

sender only resends pkts for which ACK not received

sender timer for each unACKed pktsender window

N consecutive seq rsquosagain limits seq s of sent unACKed pkts

Transport Layer 3-10

Selective repeat sender receiver windows


Selective repeat

data from above if next available seq in window send pkt

timeout(n)resend pkt n restart timer

ACK(n) in [sendbasesendbase+N]

mark pkt n as receivedif n smallest unACKed pkt advance window base to next unACKed seq

senderpkt n in [rcvbase rcvbase+N-1]

send ACK(n)out-of-order bufferin-order deliver (also deliver buffered in-order pkts) advance window to next not-yet-received pkt

pkt n in [rcvbase-Nrcvbase-1]

ACK(n)otherwise

ignore

receiver


Selective repeat in action

4


Selective repeatdilemma

Example seq rsquos 0 1 2 3window size=3

receiver sees no difference in two scenariosincorrectly passes duplicate data as new in (a)

Q what relationship between seq size and window size


Chapter 3 outline

31 Transport-layer services32 Multiplexing and demultiplexing33 Connectionless transport UDP34 Principles of reliable data transfer

35 Connection-oriented transport TCP

segment structurereliable data transferflow controlconnection management

36 Principles of congestion control37 TCP congestion control


TCP Overview RFCs 793 1122 1323 2018 2581

full duplex databi-directional data flow in same connectionMSS maximum segment size

connection-orientedhandshaking (exchange of control msgs) initrsquossender receiver state before data exchange

flow controlledsender will not overwhelm receiver

point-to-pointone sender one receiver

reliable in-order byte steam

no ldquomessage boundariesrdquopipelined

TCP congestion and flow control set window size

send amp receive buffers

socketdoor

TCPsend buffer

TCPreceive buffer

socketdoor

segment

applicationwrites data

applicationreads data


TCP segment structure

source port dest port

32 bits

applicationdata

(variable length)

sequence numberacknowledgement number

Receive windowUrg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG urgent data (generally not used)

ACK ACK valid

PSH push data now(generally not used)

RST SYN FINconnection estab(setup teardown

commands)

bytes rcvr willingto accept

countingby bytes of data(not segments)

Internetchecksum

(as in UDP)

5


TCP seq rsquos and ACKsSeq rsquos

byte stream ldquonumberrdquo of first byte in segmentrsquos data

ACKsseq of next byte expected from other sidecumulative ACK

Q how receiver handles out-of-order segments

A TCP spec doesnrsquot say - up to implementor

Host A Host B

Seq=42 ACK=79 data = lsquoCrsquo


Seq=43 ACK=80

Usertypes

lsquoCrsquo

host ACKsreceipt

of echoedlsquoCrsquo

host ACKsreceipt oflsquoCrsquo echoes

back lsquoCrsquo

timesimple telnet scenario


TCP Round Trip Time and Timeout

Q how to set TCP timeout valuelonger than RTT

but RTT variestoo short premature timeout

unnecessary retransmissions

too long slow reaction to segment loss

Q how to estimate RTTSampleRTT measured time from segment transmission until ACK receipt

ignore retransmissionsSampleRTT will vary want estimated RTT ldquosmootherrdquo

average several recent measurements not just current SampleRTT



EstimatedRTT = (1- α)EstimatedRTT + αSampleRTT

Exponential weighted moving averageinfluence of past sample decreases exponentially fasttypical value α = 0125


Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr

100

150

200

250

300

350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106time (seconnds)

RTT

(milli

seco

nds)

SampleRTT Estimated RTT

6



Setting the timeoutEstimtedRTT plus ldquosafety marginrdquo

large variation in EstimatedRTT -gt larger safety marginfirst estimate of how much SampleRTT deviates from EstimatedRTT

TimeoutInterval = EstimatedRTT + 4DevRTT

DevRTT = (1-β)DevRTT +β|SampleRTT-EstimatedRTT|

(typically β = 025)

Then set timeout interval


Chapter 3 outline






TCP reliable data transfer

TCP creates rdtservice on top of IPrsquos unreliable servicePipelined segmentsCumulative acksTCP uses single retransmission timer

Retransmissions are triggered by

timeout eventsduplicate acks

Initially consider simplified TCP sender

ignore duplicate acksignore flow control congestion control


TCP sender eventsdata rcvd from app

Create segment with seq seq is byte-stream number of first data byte in segmentstart timer if not already running (think of timer as for oldest unacked segment)expiration interval TimeOutInterval

timeoutretransmit segment that caused timeoutrestart timer

Ack rcvdIf acknowledges previously unackedsegments

update what is known to be ackedstart timer if there are outstanding segments

7


TCP sender(simplified)

NextSeqNum = InitialSeqNumSendBase = InitialSeqNum

loop (forever) switch(event)

event data received from application above create TCP segment with sequence number NextSeqNumif (timer currently not running)

start timerpass segment to IP NextSeqNum = NextSeqNum + length(data)

event timer timeoutretransmit not-yet-acknowledged segment with

smallest sequence numberstart timer

event ACK received with ACK field value of y if (y gt SendBase)

SendBase = yif (there are currently not-yet-acknowledged segments)

start timer

end of loop forever

Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked


TCP retransmission scenariosHost A

Seq=100 20 bytes data

ACK=100

timepremature timeout

Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 t

imeo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 t

imeo

ut

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100


TCP retransmission scenarios (more)Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

Cumulative ACK scenario

Host B

X


ACK=120

time

SendBase= 120


TCP ACK generation [RFC 1122 RFC 2581]

Event at Receiver

Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed

Arrival of in-order segment withexpected seq One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK

Immediately send single cumulative ACK ACKing both in-order segments

Immediately send duplicate ACK indicating seq of next expected byte

Immediate send ACK provided thatsegment starts at lower end of gap

8


Fast Retransmit

Time-out period often relatively long

long delay before resending lost packet

Detect lost segments via duplicate ACKs

Sender often sends many segments back-to-backIf segment is lost there will likely be many duplicate ACKs

If sender receives 3 ACKs for the same data it supposes that segment after ACKeddata was lost

fast retransmit resend segment before timer expires




start timer

else increment count of dup ACKs received for yif (count of dup ACKs received for y = 3)

resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit


Chapter 3 outline






TCP Flow Control

receive side of TCP connection has a receive buffer

speed-matching service matching the send rate to the receiving apprsquos drain rateapp process may be

slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer by

transmitting too muchtoo fast

flow control

9


TCP Flow control how it works

(Suppose TCP receiver discards out-of-order segments)spare room in buffer

= RcvWindow= RcvBuffer-[LastByteRcvd -

LastByteRead]

Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKeddata to RcvWindow

guarantees receive buffer doesnrsquot overflow


Chapter 3 outline






TCP Connection ManagementRecall TCP sender receiver

establish ldquoconnectionrdquobefore exchanging data segmentsinitialize TCP variables

seq sbuffers flow control info (eg RcvWindow)

client connection initiatorSocket clientSocket = new Socket(hostnameport

number)

server contacted by clientSocket connectionSocket = welcomeSocketaccept()

Three way handshakeStep 1 client host sends TCP

SYN segment to serverspecifies initial seq no data

Step 2 server host receives SYN replies with SYNACK segment

server allocates buffersspecifies server initial seq

Step 3 client receives SYNACK replies with ACK segment which may contain data


TCP Connection Management (cont)

Closing a connection

client closes socketclientSocketclose()

Step 1 client end system sends TCP FIN control segment to server

Step 2 server receives FIN replies with ACK Closes connection sends FIN

client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10



Step 3 client receives FIN replies with ACK

Enters ldquotimed waitrdquo -will respond with ACK to received FINs

Step 4 server receives ACK Connection closed

Note with small modification can handle simultaneous FINs

client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline






Principles of Congestion Control

Congestioninformally ldquotoo many sources sending too much data too fast for network to handlerdquodifferent from flow controlmanifestations

lost packets (buffer overflow at routers)long delays (queueing in router buffers)

a top-10 problem

11


Causescosts of congestion scenario 1

two senders two receiversone router infinite buffers no retransmission

large delays when congestedmaximum achievable throughput

unlimited shared output link buffers

Host Aλin original data

Host B

λout



one router finite buffers sender retransmission of lost packet

finite shared output link buffers

Host A λin original data

Host B

λout

λin original data plus retransmitted data


Causescosts of congestion scenario 2always (goodput)ldquoperfectrdquo retransmission only when loss

retransmission of delayed (not lost) packet makes larger(than perfect case) for same

λin

λout=

λin

λoutgtλ

inλout

ldquocostsrdquo of congestionmore work (retrans) for given ldquogoodputrdquounneeded retransmissions link carries multiple copies of pkt

R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3


Causescosts of congestion scenario 3four sendersmultihop pathstimeoutretransmit

λin

Q what happens as and increase λ

in



Host B

λout


12



Another ldquocostrdquo of congestionwhen packet dropped any ldquoupstream transmission capacity used for that packet was wasted

Host A

Host B

λou

t


Approaches towards congestion control

End-end congestion controlno explicit feedback from networkcongestion inferred from end-system observed loss delayapproach taken by TCP

Network-assisted congestion controlrouters provide feedback to end systems

single bit indicating congestion (SNA DECbit TCPIP ECN ATM)explicit rate sender should send at

Two broad approaches towards congestion control


Case study ATM ABR congestion control

ABR available bit rateldquoelastic servicerdquoif senderrsquos path ldquounderloadedrdquo

sender should use available bandwidth

if senderrsquos path congested

sender throttled to minimum guaranteed rate

RM (resource management) cellssent by sender interspersed with data cellsbits in RM cell set by switches (ldquonetwork-assistedrdquo)

NI bit no increase in rate (mild congestion)CI bit congestion indication

RM cells returned to sender by receiver with bits intact



two-byte ER (explicit rate) field in RM cellcongested switch may lower ER value in cellsenderrsquo send rate thus maximum supportable rate on path

EFCI bit in data cells set to 1 in congested switchif data cell preceding RM cell has EFCI set sender sets CI bit in returned RM cell

13


Chapter 3 outline






TCP congestion control additive increase multiplicative decrease

8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow

Approach increase transmission rate (window size) probing for usable bandwidth until loss occurs

additive increase increase CongWin by 1 MSS every RTT until loss detectedmultiplicative decrease cut CongWin in half after loss

timecong

estio

n w

indo

w s

ize

Saw toothbehavior probing

for bandwidth


TCP Congestion Control details

sender limits transmissionLastByteSent-LastByteAcked

le CongWin

Roughly

CongWin is dynamic function of perceived network congestion

How does sender perceive congestionloss event = timeout or3 duplicate acksTCP sender reduces rate (CongWin) after loss event

three mechanismsAIMDslow startconservative after timeout events

rate = CongWinRTT Bytessec


TCP Slow Start

When connection begins CongWin = 1 MSS

Example MSS = 500 bytes amp RTT = 200 msecinitial rate = 20 kbps

available bandwidth may be gtgt MSSRTT

desirable to quickly ramp up to respectable rate

When connection begins increase rate exponentially fast until first loss event

14


TCP Slow Start (more)

When connection begins increase rate exponentially until first loss event

double CongWin every RTTdone by incrementing CongWin for every ACK received

Summary initial rate is slow but ramps up exponentially fast

Host A

one segment

RTT

Host B

time

two segments

four segments


RefinementQ When should the

exponential increase switch to linear

A When CongWingets to 12 of its value before timeout

ImplementationVariable Threshold At loss event Threshold is set to 12 of CongWin just before loss event


Refinement inferring lossAfter 3 dup ACKs

CongWin is cut in halfwindow then grows linearly

But after timeout eventCongWin instead set to 1 MSS window then grows exponentiallyto a threshold then grows linearly

3 dup ACKs indicates network capable of delivering some segments

timeout indicates a ldquomore alarmingrdquocongestion scenario

Philosophy


Summary TCP Congestion Control

When CongWin is below Threshold sender in slow-start phase window grows exponentially

When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly

When a triple duplicate ACK occurs Thresholdset to CongWin2 and CongWin set to Threshold

When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS

15


TCP sender congestion control

SS or CA

SS or CA

SS or CA

CongestionAvoidance (CA)

Slow Start (SS)

State

CongWin and Threshold not changed

Increment duplicate ACK count for segment being acked

Duplicate ACK

Enter slow startThreshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo

Timeout

Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS

Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo

Loss event detected by triple duplicate ACK

Additive increase resulting in increase of CongWin by 1 MSS every RTT

CongWin = CongWin+MSS (MSSCongWin)

ACK receipt for previously unackeddata

Resulting in a doubling of CongWin every RTT

CongWin = CongWin + MSS If (CongWin gt Threshold)

set state to ldquoCongestion Avoidancerdquo


CommentaryTCP Sender Action Event


TCP throughput

Whatrsquos the average throughout of TCP as a function of window size and RTT

Ignore slow startLet W be the window size when loss occursWhen window is W throughput is WRTTJust after loss window drops to W2 throughput to W2RTT Average throughout 75 WRTT


TCP Futures TCP over ldquolong fat pipesrdquo

Example 1500 byte segments 100ms RTT want 10 Gbps throughputRequires window size W = 83333 in-flight segmentsThroughput in terms of loss rate

L = 210-10 WowNew versions of TCP for high-speed

LRTTMSSsdot221


Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16


Why is TCP fairTwo competing sessions

Additive increase gives slope of 1 as throughout increasesmultiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughput

Conn

e cti

on 2

th r

ough

put

congestion avoidance additive increaseloss decrease window by factor of 2



Fairness (more)Fairness and UDP

Multimedia apps often do not use TCP

do not want rate throttled by congestion control

Instead use UDPpump audiovideo at constant rate tolerate packet loss

Research area TCP friendly

Fairness and parallel TCP connectionsnothing prevents app from opening parallel connections between 2 hostsWeb browsers do this Example link of rate R supporting 9 connections

new app asks for 1 TCP gets rate R10new app asks for 11 TCPs gets R2


Chapter 3 Summaryprinciples behind transport layer services

multiplexing demultiplexingreliable data transferflow controlcongestion control

instantiation and implementation in the Internet

UDPTCP

Nextleaving the network ldquoedgerdquo (application transport layers)into the network ldquocorerdquo

2

Transport Layer 3-5

Go-Back-NSender

k-bit seq in pkt headerldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed

ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquomay receive duplicate ACKs (see receiver)

timer for each in-flight pkttimeout(n) retransmit pkt n and all higher seq pkts in window

Transport Layer 3-6

GBN sender extended FSM

Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])

timeout

rdt_send(data)

if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum)udt_send(sndpkt[nextseqnum])if (base == nextseqnum)

start_timernextseqnum++

elserefuse_data(data)

base = getacknum(rcvpkt)+1If (base == nextseqnum)

stop_timerelsestart_timer

rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)

base=1nextseqnum=1

rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)

Λ

Transport Layer 3-7

GBN receiver extended FSM

ACK-only always send ACK for correctly-received pktwith highest in-order seq

may generate duplicate ACKsneed only remember expectedseqnum

out-of-order pkt discard (donrsquot buffer) -gt no receiver bufferingRe-ACK pkt with highest in-order seq

Wait

udt_send(sndpkt)default

rdt_rcv(rcvpkt)ampamp notcurrupt(rcvpkt)ampamp hasseqnum(rcvpktexpectedseqnum)

extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++

expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)

Λ

Transport Layer 3-8

GBN inaction

3

Transport Layer 3-9

Selective Repeat









Selective repeat








ACK(n)otherwise

ignore

receiver



4







Chapter 3 outline















socketdoor

TCPsend buffer

TCPreceive buffer

socketdoor

segment






32 bits

applicationdata

(variable length)



FSRPAUheadlen

notused



ACK ACK valid



commands)



Internetchecksum

(as in UDP)

5







Host A Host B



Seq=43 ACK=80

Usertypes

lsquoCrsquo

host ACKsreceipt



back lsquoCrsquo

















100

150

200

250

300

350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106time (seconnds)

RTT

(milli

seco

nds)


6










Chapter 3 outline


















7











start timer

end of loop forever





ACK=100


Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 t

imeo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 t

imeo

ut

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100



Seq=92 8 bytes data

ACK=100

loss

tim

eout


Host B

X


ACK=120

time

SendBase= 120



Event at Receiver





TCP Receiver action





8


Fast Retransmit










start timer





fast retransmit


Chapter 3 outline






TCP Flow Control






flow control

9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


3

Transport Layer 3-9

Selective Repeat









Selective repeat








ACK(n)otherwise

ignore

receiver



4







Chapter 3 outline















socketdoor

TCPsend buffer

TCPreceive buffer

socketdoor

segment






32 bits

applicationdata

(variable length)



FSRPAUheadlen

notused



ACK ACK valid



commands)



Internetchecksum

(as in UDP)

5







Host A Host B



Seq=43 ACK=80

Usertypes

lsquoCrsquo

host ACKsreceipt



back lsquoCrsquo

















100

150

200

250

300

350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106time (seconnds)

RTT

(milli

seco

nds)


6










Chapter 3 outline


















7











start timer

end of loop forever





ACK=100


Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 t

imeo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 t

imeo

ut

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100



Seq=92 8 bytes data

ACK=100

loss

tim

eout


Host B

X


ACK=120

time

SendBase= 120



Event at Receiver





TCP Receiver action





8


Fast Retransmit










start timer





fast retransmit


Chapter 3 outline






TCP Flow Control






flow control

9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


4







Chapter 3 outline















socketdoor

TCPsend buffer

TCPreceive buffer

socketdoor

segment






32 bits

applicationdata

(variable length)



FSRPAUheadlen

notused



ACK ACK valid



commands)



Internetchecksum

(as in UDP)

5







Host A Host B



Seq=43 ACK=80

Usertypes

lsquoCrsquo

host ACKsreceipt



back lsquoCrsquo

















100

150

200

250

300

350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106time (seconnds)

RTT

(milli

seco

nds)


6










Chapter 3 outline


















7











start timer

end of loop forever





ACK=100


Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 t

imeo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 t

imeo

ut

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100



Seq=92 8 bytes data

ACK=100

loss

tim

eout


Host B

X


ACK=120

time

SendBase= 120



Event at Receiver





TCP Receiver action





8


Fast Retransmit










start timer





fast retransmit


Chapter 3 outline






TCP Flow Control






flow control

9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


5







Host A Host B



Seq=43 ACK=80

Usertypes

lsquoCrsquo

host ACKsreceipt



back lsquoCrsquo

















100

150

200

250

300

350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106time (seconnds)

RTT

(milli

seco

nds)


6










Chapter 3 outline


















7











start timer

end of loop forever





ACK=100


Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 t

imeo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 t

imeo

ut

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100



Seq=92 8 bytes data

ACK=100

loss

tim

eout


Host B

X


ACK=120

time

SendBase= 120



Event at Receiver





TCP Receiver action





8


Fast Retransmit










start timer





fast retransmit


Chapter 3 outline






TCP Flow Control






flow control

9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


6










Chapter 3 outline


















7











start timer

end of loop forever





ACK=100


Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 t

imeo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 t

imeo

ut

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100



Seq=92 8 bytes data

ACK=100

loss

tim

eout


Host B

X


ACK=120

time

SendBase= 120



Event at Receiver





TCP Receiver action





8


Fast Retransmit










start timer





fast retransmit


Chapter 3 outline






TCP Flow Control






flow control

9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


7











start timer

end of loop forever





ACK=100


Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 t

imeo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 t

imeo

ut

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100



Seq=92 8 bytes data

ACK=100

loss

tim

eout


Host B

X


ACK=120

time

SendBase= 120



Event at Receiver





TCP Receiver action





8


Fast Retransmit










start timer





fast retransmit


Chapter 3 outline






TCP Flow Control






flow control

9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


8


Fast Retransmit










start timer





fast retransmit


Chapter 3 outline






TCP Flow Control






flow control

9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


9





LastByteRead]




Chapter 3 outline










number)













client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


10







client

FIN

server

ACK

ACK

FIN

closing

closing

closedti

med

wai

t

closed



TCP clientlifecycle

TCP serverlifecycle


Chapter 3 outline









a top-10 problem

11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


11







Host B

λout






Host B

λout





λin

λout=

λin

λoutgtλ

inλout


R2

R2λin

λ out

b

R2

R2λin

λ out

a

R2

R2λin

λ out

c

R4

R3



λin


in



Host B

λout


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


12




Host A

Host B

λou

t




















13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


13


Chapter 3 outline







8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow



timecong

estio

n w

indo

w s

ize


for bandwidth




le CongWin

Roughly






TCP Slow Start






14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


14






Host A

one segment

RTT

Host B

time

two segments

four segments












Philosophy







15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


15



SS or CA

SS or CA

SS or CA


Slow Start (SS)

State



Duplicate ACK


Timeout













TCP throughput







LRTTMSSsdot221



TCP connection 1

bottleneckrouter

capacity R

TCP connection 2

TCP Fairness

16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


16




R

R



Conn

e cti

on 2

th r

ough

put















UDPTCP


Documents

Performance of rdt3.0 rdt3.0: stop-and-wait operationartoo/CN2009/slides/Chapter3-2.pdf · rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 sender receiver RTT