tcp: overview computer networks lecture 17 full duplex data: … · 2016-11-08 · data transfer...

1

Computer Networks

LECTURE 17

Transport Layer

Tamal Biswas

Department of Computer Science University of Rochester

Computer Networks (Transport Layer) 3-1 Computer Networks (Transport Layer) 3-2

TCP Overview RFCs 79311221323 2018 2581

full duplex databull bi-directional data flow

in same connection

bull MSS maximum segment size

connection-orientedbull handshaking (exchange

of control msgs) inits sender receiver state before data exchange

flow controlledbull sender will not

overwhelm receiver

point-to-pointbull one sender one receiver

reliable in-order byte steambull no ldquomessage

boundariesrdquo

pipelinedbull TCP congestion and

flow control set window size

Send and Receive Window

Send and receive buffer sizes can be set on individual sockets by using the SO_SNDBUF and SO_RCVBUF socket options with the setsockopt(2) call

Ref httpman7orglinuxman-pagesman7tcp7html

Computer Networks (Transport Layer) 3-3

SO_RCVBUF

Sets or gets the maximum socket receive buffer in bytes

The kernel doubles this value (to allow space for bookkeeping overhead) when it is set using setsockopt(2) and this doubled value is returned by getsockopt(2)

The default value is set by the procsysnetcorermem_default file and the maximum allowed value is set by the procsysnetcorermem_max file

The minimum (doubled) value for this option is 256


2

SO_SNDBUF

Sets or gets the maximum socket send buffer in bytes


The default value is set by the procsysnetcorewmem_default file and the maximum allowed value is set by the procsysnetcorewmem_maxfile



Example

int getsockopt(int sockfd int level intoptname void optval socklen_t optlen)

int setsockopt(int sockfd int level intoptname const void optval socklen_toptlen)

Example int n = 1024 3

setsockopt(socket SOL_SOCKET SO_RCVBUF

ampn sizeof(n))



TCP segment structure

source port dest port

32 bits

application

data

(variable length)

sequence number

acknowledgement number

receive window

Urg data pointerchecksum

FSRPAUhead

len

not

used

options (variable length)

URG urgent data

(generally not used)

ACK ACK

valid

PSH push data now


RST SYN FIN

connection estab

(setup teardown

commands)

bytes

rcvr willing

to accept

counting

by bytes

of data

(not segments)

Internet

checksum

(as in UDP)


TCP seq numbers ACKs

sequence numbers

bull byte stream ldquonumberrdquo of first byte in segmentrsquos data

acknowledgements

bull seq of next byte expected from other side

bull cumulative ACK

Q how receiver handles out-of-order segments

bullA TCP spec doesnrsquot say - up to implementor source port dest port

sequence number


checksum

rwnd

urg pointer

incoming segment to sender

A

sent ACKed

sent not-yet ACKed(ldquoin-flightrdquo)

usablebut not yet sent

not usable

window sizeN

sender sequence number space


sequence number


checksum

rwnd

urg pointer

outgoing segment from sender

3



UsertypeslsquoCrsquo

host ACKsreceipt

of echoedlsquoCrsquo

host ACKsreceipt oflsquoCrsquo echoesback lsquoCrsquo

simple telnet scenario

Host BHost A

Seq=42 ACK=79 data = lsquoCrsquo


Seq=43 ACK=80


TCP round trip time timeout

Q how to set TCP timeout value

longer than RTT

bull but RTT varies

too short premature timeout unnecessary retransmissions

too long slow reaction to segment loss

Q how to estimate RTT SampleRTT measured

time from segment transmission until ACK receipt

bull ignore retransmissions

SampleRTT will vary want estimated RTT ldquosmootherrdquobull average several recent

measurements not just current SampleRTT


RTT gaiacsumassedu to fantasiaeurecomfr

100

150

200

250

300

350

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

time (seconnds)

RT

T (

mil

lise

con

ds)

SampleRTT Estimated RTT

EstimatedRTT = (1- )EstimatedRTT + SampleRTT

exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125


RTT (

mill

iseco

nds)


sampleRTT

EstimatedRTT

time (seconds) Computer Networks (Transport Layer) 3-12

timeout interval EstimatedRTT plus ldquosafety marginrdquobull large variation in EstimatedRTT -gt larger safety margin

estimate SampleRTT deviation from EstimatedRTT

DevRTT = (1-)DevRTT +

|SampleRTT-EstimatedRTT|


(typically = 025)

TimeoutInterval = EstimatedRTT + 4DevRTT

estimated RTT ldquosafety marginrdquo

Check out the online interactive exercises for more

examples httpgaiacsumassedukurose_rossinteractive

4


Chapter 3 outline

31 transport-layer services

32 multiplexing and demultiplexing

33 connectionless transport UDP

34 principles of reliable data transfer

35 connection-oriented transport TCPbull segment structure

bull reliable data transfer

bull flow control

bull connection management

36 principles of congestion control

37 TCP congestion control


TCP reliable data transfer

TCP creates rdt service on top of IPrsquos unreliable servicebull pipelined segments

bull cumulative acks

bull single retransmission timer

retransmissions triggered bybull timeout events

bull duplicate acks

letrsquos initially consider simplified TCP senderbull ignore duplicate acks

bull ignore flow control congestion control


TCP sender events

data rcvd from app

create segment with seq

seq is byte-stream number of first data byte in segment

start timer if not already running bull think of timer as for

oldest unacked segment

bull expiration interval TimeOutInterval

timeout

retransmit segment that caused timeout

restart timer

ack rcvd

if ack acknowledges previously unacked segmentsbull update what is known

to be ACKed

bull start timer if there are still unacked segments


TCP sender (simplified)

wait

for

event

NextSeqNum = InitialSeqNum

SendBase = InitialSeqNum

L

create segment seq NextSeqNum

pass segment to IP (ie ldquosendrdquo)

NextSeqNum = NextSeqNum + length(data)

if (timer currently not running)

start timer

data received from application above

retransmit not-yet-acked segment with smallest seq

start timer

timeout

if (y gt SendBase)

SendBase = y

SendBasendash1 last cumulatively ACKed byte

if (there are currently not-yet-acked segments)

start timer

else stop timer

ACK received with ACK field value y

5


TCP retransmission scenarios

lost ACK scenario

Host BHost A

Seq=92 8 bytes of data

ACK=100


Xtim

eout

ACK=100

premature timeout

Host BHost A


ACK=100

Seq=92 8bytes of data

tim

eout

ACK=120


ACK=120

SendBase=100

SendBase=120

SendBase=120

SendBase=92



X

cumulative ACK

Host BHost A


ACK=100


tim

eout


ACK=120


TCP ACK generation [RFC 1122 RFC 2581]

event at receiver

arrival of in-order segment with

expected seq All data up to

expected seq already ACKed


expected seq One other

segment has ACK pending

arrival of out-of-order segment

higher-than-expect seq

Gap detected

arrival of segment that

partially or completely fills gap

TCP receiver action

delayed ACK Wait up to 500ms

for next segment If no next segment

send 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 that

segment starts at lower end of gap


TCP fast retransmit

time-out period often relatively longbull long delay before

resending lost packet

detect lost segments via duplicate ACKsbull sender often sends

many segments back-to-back

bull if segment is lost there will likely be many duplicate ACKs

if sender receives 3 ACKs for same data

(ldquotriple duplicate ACKsrdquo)

resend unacked segment with smallest seq likely that unacked

segment lost so donrsquot wait for timeout

TCP fast retransmit


6


X

fast retransmit after sender receipt of triple duplicate ACK

Host BHost A


ACK=100

tim

eout

ACK=100

ACK=100

ACK=100

TCP fast retransmit




Chapter 3 outline







bull flow control





TCP flow controlapplication

process

TCP socketreceiver buffers

TCPcode

IPcode

application

OS

receiver protocol stack

application may remove data from

TCP socket buffers hellip

hellip slower than TCP receiver is delivering(sender is sending)

from sender

receiver controls sender so

sender wonrsquot overflow

receiverrsquos buffer by transmitting

too much too fast

flow control


TCP flow control

buffered data

free buffer spacerwnd

RcvBuffer

TCP segment payloads

to application process

receiver ldquoadvertisesrdquo free buffer space by including rwnd value in TCP header of receiver-to-sender segmentsbull RcvBuffer size set via

socket options (typical default is 4096 bytes)

bull many operating systems autoadjust RcvBuffer

sender limits amount of unacked (ldquoin-flightrdquo) data to receiverrsquos rwnd value

guarantees receive buffer will not overflow

receiver-side buffering

7


Chapter 3 outline







bull flow control





Connection Management

before exchanging data senderreceiver ldquohandshakerdquo agree to establish connection (each knowing the other willing

to establish connection)

agree on connection parameters

connection state ESTABconnection variables

seq client-to-serverserver-to-client

rcvBuffer size

at serverclient

application

network

connection state ESTABconnection Variables


rcvBuffer size

at serverclient

application

network

Socket clientSocket =

newSocket(hostnameport

number)

Socket connectionSocket =

welcomeSocketaccept()


Q will 2-way handshake always work in network

variable delays

retransmitted messages (eg req_conn(x)) due to message loss

message reordering

canrsquot ldquoseerdquo other side

2-way handshake

Letrsquos talk

OKESTAB

ESTAB

choose xreq_conn(x)

ESTAB

ESTABacc_conn(x)

Agreeing to establish a connection



2-way handshake failure scenarios

retransmitreq_conn(x)

ESTAB

req_conn(x)

half open connection(no client)

client terminates

serverforgets x

connection x completes


ESTAB

req_conn(x)

data(x+1)

retransmitdata(x+1)

acceptdata(x+1)

choose xreq_conn(x)

ESTAB

ESTAB

acc_conn(x)

client terminates

ESTAB

choose xreq_conn(x)

ESTAB

acc_conn(x)

data(x+1) acceptdata(x+1)

connection x completes server

forgets x

8


TCP 3-way handshake

SYNbit=1 Seq=x

choose init seq num xsend TCP SYN msg

ESTAB

SYNbit=1 Seq=yACKbit=1 ACKnum=x+1

choose init seq num ysend TCP SYNACKmsg acking SYN

ACKbit=1 ACKnum=y+1

received SYNACK(x) indicates server is livesend ACK for SYNACK

this segment may contain client-to-server data

received ACK(y) indicates client is live

SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN


TCP 3-way handshake FSM

closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)

SYNACK(seq=yACKnum=x+1)create new socket for

communication back to client

SYNACK(seq=yACKnum=x+1)

ACK(ACKnum=y+1)ACK(ACKnum=y+1)

L


TCP closing a connection

client server each close their side of connectionbull send TCP segment with FIN bit = 1

respond to received FIN with ACKbull on receiving FIN ACK can be combined with own FIN

simultaneous FIN exchanges can be handled


FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1

wait for serverclose

can stillsend data

can no longersend data

LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED


FIN_WAIT_1 FINbit=1 seq=xcan no longersend but canreceive data

clientSocketclose()

client state server state

ESTABESTAB

9


Chapter 3 outline







bull flow control





congestion informally ldquotoo many sources sending too much

data too fast for network to handlerdquo different from flow control

manifestations

bull lost packets (buffer overflow at routers)

bull long delays (queueing in router buffers)

a top-10 problem

Principles of congestion control


Causescosts of congestion scenario 1

two senders two receivers

one router infinite buffers

output link capacity R

no retransmission

maximum per-connection throughput R2

unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin

large delays as arrival rate lin approaches capacity


one router finite buffers

sender retransmission of timed-out packetbull application-layer input = application-layer output lin = lout

bull transport-layer input includes retransmissions lin lin

finite shared output

link buffers

Host A

lin original data

Host B

loutlin original data plus

retransmitted data

lsquo


10


idealization perfect knowledge

sender sends only when router buffers available


link buffers

lin original dataloutlin original data plus

retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space

Idealization known losspackets can be lost dropped at router due to full buffers

sender only resends if packet known to be lost


A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions

including duplicated

that are delivered

Host B

Realistic duplicates packets can be lost dropped at

router due to full buffers

sender times out prematurely sending two copies both of which are delivered


11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered

ldquocostsrdquo of congestion more work (retrans) for given ldquogoodputrdquo unneeded retransmissions link carries multiple copies of pkt

bull decreasing goodput

R2lin






four senders

multihop paths

timeoutretransmit

Q what happens as lin and linrsquo

increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data

lin original data plus

retransmitted data

A as red linrsquo increases all arriving

blue pkts at upper queue are dropped blue throughput g 0


another ldquocostrdquo of congestion

when packet dropped any ldquoupstream transmission capacity used for that packet was wasted


C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12


TCP congestion control additive increase multiplicative decrease

approach sender increases transmission rate (window size) probing for usable bandwidth until loss occurs

bull additive increase increase cwnd by 1 MSS every RTT until loss detected

bull multiplicative decrease cut cwnd in half after loss cwnd

TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth

additively increase window size helliphellip until loss occurs (then cut window in half)

time


TCP Congestion Control details

sender limits transmission

cwnd is dynamic function of perceived network congestion

TCP sending rate

roughly send cwnd bytes wait RTT for ACKS then send more bytes

last byteACKed sent not-

yet ACKed(ldquoin-flightrdquo)

last byte sent

cwnd

LastByteSent-

LastByteAckedlt cwnd


rate ~~cwnd

RTTbytessec


TCP Slow Start

when connection begins increase rate exponentially until first loss eventbull initially cwnd = 1 MSS

bull double cwnd every RTT

bull done by incrementing cwnd for every ACK received

summary initial rate is slow but ramps up exponentially fast

Host A

RT

T

Host B

time


TCP detecting reacting to loss

loss indicated by timeoutbull cwnd set to 1 MSS

bull window then grows exponentially (as in slow start) to threshold then grows linearly

loss indicated by 3 duplicate ACKs TCP RENO

bull dup ACKs indicate network capable of delivering some segments

bull cwnd is cut in half window then grows linearly

TCP Tahoe always sets cwnd to 1 (timeout or 3 duplicate acks)

13


Q when should the exponential increase switch to linear

A when cwnd gets to 12 of its value before timeout

Implementation variable ssthresh

on loss event ssthreshis set to 12 of cwnd just before loss event

TCP switching from slow start to CA


examples httpgaiacsumassedukurose_rossinteractive Computer Networks (Transport Layer) 3-50

Summary TCP Congestion Control

timeout

ssthresh = cwnd2cwnd = 1 MSS

dupACKcount = 0retransmit missing segment

L

cwnd gt ssthresh

congestion

avoidance

cwnd = cwnd + MSS (MSScwnd)dupACKcount = 0

transmit new segment(s) as allowed

new ACK

dupACKcount++

duplicate ACK

fast

recovery

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeout

ssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3cwnd = ssthreshdupACKcount = 0

New ACK

slow

start

timeout

ssthresh = cwnd2 cwnd = 1 MSS


cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s) as allowed

new ACKdupACKcount++

duplicate ACK

L

cwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

NewACK

NewACK

NewACK


TCP throughput

avg TCP thruput as function of window size RTTbull ignore slow start assume always data to send

W window size (measured in bytes) where loss occursbull avg window size ( in-flight bytes) is frac34 W

bull avg thruput is 34W per RTT

W

W2

avg TCP thruput = 34

WRTT

bytessec


TCP Futures TCP over ldquolong fat pipesrdquo

example 1500 byte segments 100ms RTT want 10 Gbps throughput

requires W = 83333 in-flight segments

throughput in terms of segment loss probability L [Mathis 1997]

to achieve 10 Gbps throughput need a loss rate of L = 210-10 ndash a very small loss rate

new versions of TCP for high-speed

Ref httpccrsigcommorgarchive1997jul97ccr-9707-mathispdf

TCP throughput = 122 MSS

RTT L

14


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

TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair

two competing sessions additive increase gives slope of 1 as throughout increases

multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughput

congestion avoidance additive increase

loss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2


Fairness (more)

Fairness and UDP

multimedia apps often do not use TCPbull do not want rate

throttled by congestion control

instead use UDPbull send audiovideo at

constant rate tolerate packet loss

Fairness parallel TCP connections

application can open multiple parallel connections between two hosts

web browsers do this

eg link of rate R with 9 existing connectionsbull new app asks for 1 TCP gets

rate R10

bull new app asks for 11 TCPs gets R2


Summary

principles behind transport layer services

bull multiplexing demultiplexing


bull flow control

bull congestion control

instantiation implementation in the Internetbull UDP

bull TCP

15

Disclaimer

Parts of the lecture slides contain original work of James Kurose and Keith Ross The slides are intended for the sole purpose of instruction of computer networks at the University of Rochester All copyrighted materials belong to their original owner(s)


2

SO_SNDBUF

Sets or gets the maximum socket send buffer in bytes


The default value is set by the procsysnetcorewmem_default file and the maximum allowed value is set by the procsysnetcorewmem_maxfile



Example

int getsockopt(int sockfd int level intoptname void optval socklen_t optlen)

int setsockopt(int sockfd int level intoptname const void optval socklen_toptlen)

Example int n = 1024 3

setsockopt(socket SOL_SOCKET SO_RCVBUF

ampn sizeof(n))



TCP segment structure


32 bits

application

data

(variable length)

sequence number


receive window

Urg data pointerchecksum

FSRPAUhead

len

not

used

options (variable length)

URG urgent data


ACK ACK

valid

PSH push data now


RST SYN FIN

connection estab

(setup teardown

commands)

bytes

rcvr willing

to accept

counting

by bytes

of data

(not segments)

Internet

checksum

(as in UDP)



sequence numbers

bull byte stream ldquonumberrdquo of first byte in segmentrsquos data

acknowledgements

bull seq of next byte expected from other side

bull cumulative ACK

Q how receiver handles out-of-order segments

bullA TCP spec doesnrsquot say - up to implementor source port dest port

sequence number


checksum

rwnd

urg pointer

incoming segment to sender

A

sent ACKed

sent not-yet ACKed(ldquoin-flightrdquo)

usablebut not yet sent

not usable

window sizeN



sequence number


checksum

rwnd

urg pointer

outgoing segment from sender

3




host ACKsreceipt




Host BHost A



Seq=43 ACK=80




longer than RTT

bull but RTT varies










100

150

200

250

300

350

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

time (seconnds)

RT

T (

mil

lise

con

ds)





RTT (

mill

iseco

nds)


sampleRTT

EstimatedRTT







(typically = 025)





4


Chapter 3 outline







bull flow control










bull duplicate acks




TCP sender events

data rcvd from app






timeout


restart timer

ack rcvd


to be ACKed




wait

for

event



L





start timer



start timer

timeout

if (y gt SendBase)

SendBase = y



start timer

else stop timer


5



lost ACK scenario

Host BHost A


ACK=100


Xtim

eout

ACK=100

premature timeout

Host BHost A


ACK=100


tim

eout

ACK=120


ACK=120

SendBase=100

SendBase=120

SendBase=120

SendBase=92



X

cumulative ACK

Host BHost A


ACK=100


tim

eout


ACK=120



event at receiver









Gap detected



TCP receiver action



send ACK








TCP fast retransmit










TCP fast retransmit


6


X


Host BHost A


ACK=100

tim

eout

ACK=100

ACK=100

ACK=100

TCP fast retransmit




Chapter 3 outline







bull flow control






process


TCPcode

IPcode

application

OS





from sender




too much too fast

flow control


TCP flow control

buffered data


RcvBuffer









7


Chapter 3 outline







bull flow control











rcvBuffer size

at serverclient

application

network



rcvBuffer size

at serverclient

application

network



number)





variable delays


message reordering


2-way handshake

Letrsquos talk

OKESTAB

ESTAB

choose xreq_conn(x)

ESTAB

ESTABacc_conn(x)






ESTAB

req_conn(x)


client terminates

serverforgets x



ESTAB

req_conn(x)

data(x+1)

retransmitdata(x+1)

acceptdata(x+1)

choose xreq_conn(x)

ESTAB

ESTAB

acc_conn(x)

client terminates

ESTAB

choose xreq_conn(x)

ESTAB

acc_conn(x)



forgets x

8


TCP 3-way handshake

SYNbit=1 Seq=x


ESTAB



ACKbit=1 ACKnum=y+1




SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN



closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)





L







FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1


can stillsend data


LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED



clientSocketclose()


ESTABESTAB

9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



3




host ACKsreceipt




Host BHost A



Seq=43 ACK=80




longer than RTT

bull but RTT varies










100

150

200

250

300

350

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

time (seconnds)

RT

T (

mil

lise

con

ds)





RTT (

mill

iseco

nds)


sampleRTT

EstimatedRTT







(typically = 025)





4


Chapter 3 outline







bull flow control










bull duplicate acks




TCP sender events

data rcvd from app






timeout


restart timer

ack rcvd


to be ACKed




wait

for

event



L





start timer



start timer

timeout

if (y gt SendBase)

SendBase = y



start timer

else stop timer


5



lost ACK scenario

Host BHost A


ACK=100


Xtim

eout

ACK=100

premature timeout

Host BHost A


ACK=100


tim

eout

ACK=120


ACK=120

SendBase=100

SendBase=120

SendBase=120

SendBase=92



X

cumulative ACK

Host BHost A


ACK=100


tim

eout


ACK=120



event at receiver









Gap detected



TCP receiver action



send ACK








TCP fast retransmit










TCP fast retransmit


6


X


Host BHost A


ACK=100

tim

eout

ACK=100

ACK=100

ACK=100

TCP fast retransmit




Chapter 3 outline







bull flow control






process


TCPcode

IPcode

application

OS





from sender




too much too fast

flow control


TCP flow control

buffered data


RcvBuffer









7


Chapter 3 outline







bull flow control











rcvBuffer size

at serverclient

application

network



rcvBuffer size

at serverclient

application

network



number)





variable delays


message reordering


2-way handshake

Letrsquos talk

OKESTAB

ESTAB

choose xreq_conn(x)

ESTAB

ESTABacc_conn(x)






ESTAB

req_conn(x)


client terminates

serverforgets x



ESTAB

req_conn(x)

data(x+1)

retransmitdata(x+1)

acceptdata(x+1)

choose xreq_conn(x)

ESTAB

ESTAB

acc_conn(x)

client terminates

ESTAB

choose xreq_conn(x)

ESTAB

acc_conn(x)



forgets x

8


TCP 3-way handshake

SYNbit=1 Seq=x


ESTAB



ACKbit=1 ACKnum=y+1




SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN



closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)





L







FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1


can stillsend data


LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED



clientSocketclose()


ESTABESTAB

9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



4


Chapter 3 outline







bull flow control










bull duplicate acks




TCP sender events

data rcvd from app






timeout


restart timer

ack rcvd


to be ACKed




wait

for

event



L





start timer



start timer

timeout

if (y gt SendBase)

SendBase = y



start timer

else stop timer


5



lost ACK scenario

Host BHost A


ACK=100


Xtim

eout

ACK=100

premature timeout

Host BHost A


ACK=100


tim

eout

ACK=120


ACK=120

SendBase=100

SendBase=120

SendBase=120

SendBase=92



X

cumulative ACK

Host BHost A


ACK=100


tim

eout


ACK=120



event at receiver









Gap detected



TCP receiver action



send ACK








TCP fast retransmit










TCP fast retransmit


6


X


Host BHost A


ACK=100

tim

eout

ACK=100

ACK=100

ACK=100

TCP fast retransmit




Chapter 3 outline







bull flow control






process


TCPcode

IPcode

application

OS





from sender




too much too fast

flow control


TCP flow control

buffered data


RcvBuffer









7


Chapter 3 outline







bull flow control











rcvBuffer size

at serverclient

application

network



rcvBuffer size

at serverclient

application

network



number)





variable delays


message reordering


2-way handshake

Letrsquos talk

OKESTAB

ESTAB

choose xreq_conn(x)

ESTAB

ESTABacc_conn(x)






ESTAB

req_conn(x)


client terminates

serverforgets x



ESTAB

req_conn(x)

data(x+1)

retransmitdata(x+1)

acceptdata(x+1)

choose xreq_conn(x)

ESTAB

ESTAB

acc_conn(x)

client terminates

ESTAB

choose xreq_conn(x)

ESTAB

acc_conn(x)



forgets x

8


TCP 3-way handshake

SYNbit=1 Seq=x


ESTAB



ACKbit=1 ACKnum=y+1




SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN



closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)





L







FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1


can stillsend data


LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED



clientSocketclose()


ESTABESTAB

9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



5



lost ACK scenario

Host BHost A


ACK=100


Xtim

eout

ACK=100

premature timeout

Host BHost A


ACK=100


tim

eout

ACK=120


ACK=120

SendBase=100

SendBase=120

SendBase=120

SendBase=92



X

cumulative ACK

Host BHost A


ACK=100


tim

eout


ACK=120



event at receiver









Gap detected



TCP receiver action



send ACK








TCP fast retransmit










TCP fast retransmit


6


X


Host BHost A


ACK=100

tim

eout

ACK=100

ACK=100

ACK=100

TCP fast retransmit




Chapter 3 outline







bull flow control






process


TCPcode

IPcode

application

OS





from sender




too much too fast

flow control


TCP flow control

buffered data


RcvBuffer









7


Chapter 3 outline







bull flow control











rcvBuffer size

at serverclient

application

network



rcvBuffer size

at serverclient

application

network



number)





variable delays


message reordering


2-way handshake

Letrsquos talk

OKESTAB

ESTAB

choose xreq_conn(x)

ESTAB

ESTABacc_conn(x)






ESTAB

req_conn(x)


client terminates

serverforgets x



ESTAB

req_conn(x)

data(x+1)

retransmitdata(x+1)

acceptdata(x+1)

choose xreq_conn(x)

ESTAB

ESTAB

acc_conn(x)

client terminates

ESTAB

choose xreq_conn(x)

ESTAB

acc_conn(x)



forgets x

8


TCP 3-way handshake

SYNbit=1 Seq=x


ESTAB



ACKbit=1 ACKnum=y+1




SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN



closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)





L







FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1


can stillsend data


LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED



clientSocketclose()


ESTABESTAB

9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



6


X


Host BHost A


ACK=100

tim

eout

ACK=100

ACK=100

ACK=100

TCP fast retransmit




Chapter 3 outline







bull flow control






process


TCPcode

IPcode

application

OS





from sender




too much too fast

flow control


TCP flow control

buffered data


RcvBuffer









7


Chapter 3 outline







bull flow control











rcvBuffer size

at serverclient

application

network



rcvBuffer size

at serverclient

application

network



number)





variable delays


message reordering


2-way handshake

Letrsquos talk

OKESTAB

ESTAB

choose xreq_conn(x)

ESTAB

ESTABacc_conn(x)






ESTAB

req_conn(x)


client terminates

serverforgets x



ESTAB

req_conn(x)

data(x+1)

retransmitdata(x+1)

acceptdata(x+1)

choose xreq_conn(x)

ESTAB

ESTAB

acc_conn(x)

client terminates

ESTAB

choose xreq_conn(x)

ESTAB

acc_conn(x)



forgets x

8


TCP 3-way handshake

SYNbit=1 Seq=x


ESTAB



ACKbit=1 ACKnum=y+1




SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN



closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)





L







FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1


can stillsend data


LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED



clientSocketclose()


ESTABESTAB

9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



7


Chapter 3 outline







bull flow control











rcvBuffer size

at serverclient

application

network



rcvBuffer size

at serverclient

application

network



number)





variable delays


message reordering


2-way handshake

Letrsquos talk

OKESTAB

ESTAB

choose xreq_conn(x)

ESTAB

ESTABacc_conn(x)






ESTAB

req_conn(x)


client terminates

serverforgets x



ESTAB

req_conn(x)

data(x+1)

retransmitdata(x+1)

acceptdata(x+1)

choose xreq_conn(x)

ESTAB

ESTAB

acc_conn(x)

client terminates

ESTAB

choose xreq_conn(x)

ESTAB

acc_conn(x)



forgets x

8


TCP 3-way handshake

SYNbit=1 Seq=x


ESTAB



ACKbit=1 ACKnum=y+1




SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN



closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)





L







FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1


can stillsend data


LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED



clientSocketclose()


ESTABESTAB

9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



8


TCP 3-way handshake

SYNbit=1 Seq=x


ESTAB



ACKbit=1 ACKnum=y+1




SYNSENT

ESTAB

SYN RCVD

client state

LISTEN

server state

LISTEN



closed

L

listen

SYNrcvd

SYNsent

ESTAB



number)

SYN(seq=x)



SYN(x)





L







FIN_WAIT_2

CLOSE_WAIT

FINbit=1 seq=y

ACKbit=1 ACKnum=y+1

ACKbit=1 ACKnum=x+1


can stillsend data


LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2max

segment lifetime

CLOSED



clientSocketclose()


ESTABESTAB

9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



9


Chapter 3 outline







bull flow control







manifestations



a top-10 problem







no retransmission


unlimited shared

output link buffers

Host A

original data lin

Host B

throughput lout

R2

R2

lout

lin R2

dela

y

lin







link buffers

Host A

lin original data

Host B


retransmitted data

lsquo


10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



10





link buffers


retransmitted data

copy

free buffer space

R2

R2

lout

lin


Host B

A



retransmitted data

copy

no buffer space




A

Host B



retransmitted data

free buffer space




R2

R2lin

lout

when sending at R2

some packets are

retransmissions but

asymptotic goodput

is still R2 (why)

A

Host B


A

linloutlin

copy

free buffer space

timeout

R2

R2lin

lout

when sending at R2

some packets are

retransmissions


that are delivered

Host B





11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



11


R2

lout

when sending at R2

some packets are

retransmissions


that are delivered



R2lin






four senders

multihop paths

timeoutretransmit


increase


link buffers

Host A lout


Host B

Host C

Host D

lin original data


retransmitted data







C2

C2

lo

ut

linrsquo


Chapter 3 outline







bull flow control




12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



12






TC

P s

ender

cong

estion w

indow

siz

e

AIMD saw tooth

behavior probing

for bandwidth


time





TCP sending rate




last byte sent

cwnd

LastByteSent-



rate ~~cwnd

RTTbytessec


TCP Slow Start





Host A

RT

T

Host B

time









13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



13










timeout



L

cwnd gt ssthresh

congestion

avoidance



new ACK

dupACKcount++

duplicate ACK

fast

recovery


duplicate ACK



dupACKcount == 3

timeout




New ACK

slow

start

timeout





duplicate ACK

L


NewACK

NewACK

NewACK


TCP throughput




W

W2


WRTT

bytessec










RTT L

14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



14



TCP connection 1

bottleneck

router

capacity R

TCP Fairness

TCP connection 2


Why is TCP fair



R

R







Fairness (more)

Fairness and UDP









rate R10



Summary




bull flow control



bull TCP

15

Disclaimer



15

Disclaimer



tcp: overview computer networks lecture 17 full duplex data: … · 2016-11-08 · data transfer...

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