Computer Networks: Transmission Control Protocol (TCP)

Computer Networks:Transmission Control Protocol (TCP)

Ivan Marsic

Rutgers University

Chapter 2 - TCP

Transmission Control Protocol (TCP)

Chapter 2

UDP Header

UDP datagram data (if any)

16-bit source port number 16-bit destination port number

0 15 16 31

16-bit datagram length 16-bit datagram checksum

8bytes

UD

P p

ayl

oad

UD

P h

ead

er

Topic:TCP Protocol

TCP Packet (“Segment”) Format

Setting Retransmission Timers

Flow Control

TCP in the Protocol Stack

• End-to-end protocol

• Most popular protocolin the Internet(together with IP,“TCP/IP protocol suite”

TCP (TransmissionControl Protocol)

Layer 2:

Network

Layer 1:

Link

Layer 3:

End-to-End

Transmission Control Protocol (TCP)

• Notion of a “session” or “connection”, so TCP is said to be a connection-oriented protocol

• TCP “Quality of Service”: Assured delivery

• Congestion avoidance helps improve throughput

TCP Session

• Continuous stream of bytes

becomes available for application to use

when a connection is established

and disappears when the connection is closed

Encapsulation of TCP Datain IP Datagrams

Application

OSI Layer 4/5: TCP

OSI Layer 3: IP

From application: stream of bytes

Slice into TCP segments

Packetize into IP packets

TCP TCPhdr payload

TCP TCPhdr payload

TCP TCPhdr payload

TCP TCPhdr payload

IPhdr

TCP TCPhdr payload

IPhdrIP

hdrTCP TCPhdr payload

IPhdr

TCP TCPhdr payload

IPhdrIP

hdrTCP TCPhdr payload

NetworkNetwork

To application: stream of bytes

Unwrap TCP segments

Concatenate to a byte stream

TCP Header

16-bit source port number 16-bit destination port number

32-bit sequence number

flags

options (if any)

TCP segment data (if any)

0 15 16 31

16-bit TCP checksum 16-bit urgent data pointer

16-bit advertised receive window size4-bit

headerlength

unused(6 bits)

URG

ACK

PSH

RST

SYN

FIN

20bytes

32-bitacknowledgement number

TC

P p

ayl

oad

TC

P h

eade

r

Recall the IPv4 Packet Header

8-bit differentiatedservices (DS)

16-bit datagram length(in bytes)

16-bit datagram identification

options (if any)

data

0 15 16 31

8-bit time to live(TTL)

16-bit header checksum

13-bit fragment offset

4-bitversionnumber

20bytes

4-bitheaderlength

32-bit destination IP address

32-bit source IP address

8-bit user protocol

unused

DF

MF

7 8

flags

Urgent Data Pointer

• Last byte of urgent data (LBUD) = sequenceNumber+ urgentPointer

• First byte of urgent data never explicitly defined

• Any data in Receive buffer up to LBUD may be considered urgent

urgent data pointer

sequence number

Received byte stream, stored in Receive Buffer

seq. num + urg. ptr = Last Byte of Urgent Data

urgent data

Received TCP segment

non-urgent data

Retransmission Timer CalculationSearch the Web for RTT distribution measurement

0

Measured RTT value

Occ

urr

ence

fre

qu

ency

of

mea

sure

d R

TT

va

lue

s

0 1

1

2

2

RTT distribution measuredduring quiet periods

RTT distribution measuredduring peak usage periods

0

TimeoutInterval

Cases for which theRTO timer will expiretoo soon

(a)(b)

(c)

Measured RTT value

Retransmission Timer Calculation

Exponential Weighted Moving Average (EWMA)

• Timeout Interval, set as +4 :TimeoutInterval(t) = EstimatedRTT(t) + 4 DevRTT(t)

• Estimated mean value of RTT:EstimatedRTT(t) = (1) EstimatedRTT(t 1) + SampleRTT(t)

• Estimated current standard deviation of RTT:DevRTT(t) = (1) DevRTT(t 1) + SampleRTT(t) EstimatedRTT(t 1)

• The initial value is set as:DevRTT(0) = 1/2 SampleRTT(0) for the first RTT measurement

• Recommended values of the control parameters are = 0.125 and = 0.25

Retransmission Timer Management

(from Listing 2-1)• In method TCPSender.send(), Listing 2-1 // called by the application layer from

above• in Line 24:• if (RTO timer not already running) {• set the RTO timer to the current value• as calculated in methods handle() and RTOtimeout();• start the timer;• }

• In method TCPSender.handle(), Listing 2-1 // called by IP layer when ACK arrives• in Lines 36 - 38:• if ((lastByteAcked < lastByteSent) {• calculate the new value of the RTO timer using Eq. (2.2);• re-start the RTO timer;• }

• In method TCPSender.RTOtimeout(), Listing 2-1 // called when RTO timeout• in Line 43:• double the TimeoutInterval; // exponential backoff , Eq. (2.1)• start the timer;

TCP Connection Establishment

ClientClient ServerServer

#1

#2

#2

LISTEN(passive open)

SYN_SENT(active open)

SYN_RCVD

ESTABLISHED

ESTABLISHED

SYN 122750000:122750000(0)win 2048, <mss 1024>

ack 2363371521, win 2048

SYN 2363371521:2363371521(0)

ack 122750000, win 4096, <mss 512>

Time

Visit http://en.wikipedia.org/wiki/Network_socket to learn about network sockets

http://en.wikipedia.org/wiki/Network_socket

http://en.wikipedia.org/wiki/Network_socket

Example TCP SessionClientClient ServerServer

CongWin = 1 MSS = 512 bytesRcvWindow = 4096 bytes

CongWin = 1 + 1 = 2

CongWin = 2 +1+1 = 4

CongWin = 5 +1+1+1 = 8

#6

#7

#10

#10

#10

(TCP Sender) (TCP Receiver)

#3

#4

#4

Da

ta Tra

ns

po

rt(In

itial ph

ase

: “Slo

w S

tart”)

Gap in sequence!(buffer 512 bytes)

Gap in sequence!(buffer 1024 bytes)

#14

#10

#5

#6

#7

#8

#9

#11

#12

#13

#14

1024:1535(512), ack 0, win 2048

1536:2047(512), ack 0, win 2048

ack 1536, win 4096

2560:3071(512)

ack 2048, win 3584

ack 3584, win 4096

4096:4607(512)

4608:5119(512), ack 0, win 2048

512:1023(512), ack 0, win 2048

ack 512, win 4096

2048:2559(512), ack 0, win 2048

3072:3583(512), ack 0, win 2048

0:511(512), ack 0, win 2048

3584:4097(512), ack 0, win 2048

5120:5631(512), ack 0, win 2048

ack 5632, win 4096

ack 2048, win 4096 #7

CongWin = 4 +1 = 5

CongWin = 8 +1+1+1+1 = 12

#7

2560:3071(512)

2048:2559(512), ack 0, win 2048

3072:3583(512), ack 0, win 2048

#7

#8

#9

ack 2048, win 4096 #7

ack 2048, win 3584 (duplicate)

detail:

detail

Time

ack 3584, win 3072ack 3584, win 3584

#6

Flow vs. Congestion ControlFlow control Congestion control

Feedback:“Receiveroverflowing”

Feedback:“Not muchgetting through”

Sender

Receiver

Sender

Receiver

Sender

Receiver

Sender

Receiver

Topic:TCP Congestion Control

TCP Tahoe

TCP Reno

TCP NewReno

The Problem

• TCP is a window-based protocol

– Window size represents the amount of resources available for a communication session

– It limits the number of outstanding (unacknowledged) packets

• However, unlike a simple ARQ (Chapter 1), “resources” include both receiver resources and network resources

• How to know the available network resources?

Network Conceptual Model

We don’t know when sources will start/end their sessions; also their datarates are variable

Many sources and many receivers …

NetworkNetworkNetworkNetwork

(a)

RcvWin

CongWin

RcvWin

CongWin

Simplified Network Model

The entire network is abstracted as a single router – “black box”

(b)Network resourcesRepresented by “Congestion Window Size”

Receiver resourcesRepresented by “Receive Window Size”

TCP Buffer Parameters

[ Sending Application ] [ Receiving Application ]

LastByteAcked LastByteSent

FlightSize(Buffered in send buffer)

TCPsender’s

byte stream

Sent &acked

Allowed tosend

NextByteExpected LastByteRecvd

Buffered in RcvBuffer

TCPreceiver’sbyte stream

Delivered toapplication

Gap inrecv’d data

Increasingsequence num.

Increasingsequence num.

TCP Congestion Control Summary

1. Sender starts with a small window size 2. Send a “burst” of packets (size of the current

sender window) into the network3. Wait for a feedback about success rate

(acknowledgements from the receiver end)4. When the (positive/negative) feedback arrives:

a) + If the success rate is greater than zero, increase the sender window size and go to Step 2

b) – If loss is detected, decrease the sender window size and go to Step 2

TCP Congestion Control Param’s

Variable Definition

MSS

The maximum segment size that the sender can transmit. MSS does not include the TCP/IP headers and options—only the data payload. This value can be based on the maximum transmission unit (MTU) of the first link, the path MTU discovery algorithm, or other factors. By setting MSS to the path MTU, the sender may avoid packet fragmentation (Section 1.4.1), although this is difficult to achieve because routers change routes dynamically. [Note that RFC-2581 distinguishes the sender maximum segment size (SMSS) and the receiver maximum segment size (RMSS).]

RcvWindow The size of the most recently advertised receiver window.CongWindow Sender’s current estimate of the available buffer space in the bottleneck router.LastByteAcked The highest sequence number currently acknowledged.LastByteSent The sequence number of the last byte the sender sent.FlightSize The amount of data that the sender has sent, but not yet had acknowledged.

EffectiveWindow

The maximum amount of data that the sender is currently allowed to send. At any given time, the sender must not send data with a sequence number higher than the sum of the highest acknowledged sequence number and the minimum of CongWindow and RcvWindow.

SSThresh

The slow start threshold used by the sender to decide whether to employ the slow-start or congestion-avoidance algorithm to control data transmission. The slow start algorithm is used when CongWindow < SSThresh, while the congestion avoidance algorithm is used when CongWindow > SSThresh. When CongWindow and SSThresh are equal the sender may use either slow start or congestion avoidance.

TCP Sender State Diagram

1. Set CongWin & SSThresh2. Send EfctWin of data, if any3. Re-start RTO timer

“Send data” 1. Set CongWin & SSThresh2. Re-send oldest outstanding segment3. Double the RTO timer & re-start it

“Retransmit”

duplicate ACK (less than 3) /new ACK /“Send data”

timeout /“Retransmit”

Start /new ACK

& (CongWin SSThresh) /“Send data”

TCP Sender


3 duplicate ACKs /“Retransmit”

(Fast retransmit)

3 duplicate ACKs /“Retransmit”

(Fast retransmit)

duplicate ACK (less than 3) /new ACK /“Send data”

Slow Start

CongWin SSThresh

for every new ACK, set:

CongWin CongWin + 1

Congestion Avoidance

MSSCongWin

CongWin SSThresh

for every new ACK, set:

CongWin CongWin + MSS

[Tahoe]

[Reno]after “fast recovery”

TCP Receiver State Diagram

In-order segment /

500 msec elapsed /Send ACK

In-order seg. /Send ACK

Out-of-order segment /Buffer it & Send dupACK

Out-of-order segment /Buffer it & Send dupACK

In-order segment, completely fills gaps /Send ACK

In-order segment,partially fills gaps /

Send ACK

Start /

Delayed acknowledging

Segments arriving in-order

TCP Receiver

(NextByteExpected = LastByteRecvd + 1)

(LastByteRecvd > NextByteExpected)

Out-of-order segments

Immediate acknowledging

How Much Sender Can Send

• The sender must always ensure that:LastByteSent LastByteAcked min{CongWindow, RcvWindow}

the amount of unacknowledged data should never exceed:FlightSize = LastByteSent LastByteAcked min{CongWindow, RcvWindow}

• TCP session invariant (“allowed to send”):EffectiveWindow = min{CongWindow, RcvWindow} FlightSize

How TCP Sender DetectsSegment Loss

• Retransmission timer (RTO) expiration

• Reception of three duplicate ACKs(four identical ACKs without the arrival of any other intervening packets)

Why three dupACKs:

– If there is just a reordering of the segments, there will be only one or two dupACKs before the reordered segment is processed, which will then generate a regular ACK.

– If three or more dupACKs are received in a row, it is a strong indication that a segment has been lost.

How TCP Sender DetectsSegment Loss

EventTCP

VersionTCP Sender’s Action

TimeoutTahoe

Set CongWindow = 1MSS Reno

3dup ACKs

Tahoe Set CongWindow = 1MSS

Reno

Set CongWindow =max {½ FlightSize, 2MSS} + 3MSS

Slow Start Threshold

• Determines when the “slow start” phase ends and the “congestion avoidance” phase begins

• SSThresh = max {½ FlightSize, 2MSS}

TCP Tahoe Sender

SSThresh(t) = max {½ CongWindow(t1), 2MSS} (2.4)

)1(CongWin

MSSMSS)1(CongWin)(CongWin

ttt [bytes] (2.5)

Congestion Window computation under Congestion Avoidance:Congestion Window computation under Congestion Avoidance:

(Effective Window computed as before …)

When packet loss detected via 3 dupACKs:

TCP-Tahoe Sender State Diagram

1. Set CongWin & SSThresh2. Send EfctWin of data, if any3. Re-start RTO timer

“Send data” 1. Set CongWin & SSThresh2. Re-send oldest outstanding segment3. Double the RTO timer & re-start it

“Retransmit”


Start / Slow Start



dupACK /

new ACK /“Send data”


Fast retransmitdupACK & (count 3) /

“Retransmit”

dupACKscount = 0

new ACK & (CongWin SSThresh) /“Send data”Tahoe Sender

dupACK /

dupACK /new ACK /“Send data”



“Retransmit”

0 dupACKscount 3

dupACKscount = 0

dupACK /

0 dupACKscount 3

TCP-Reno Sender State Diagram




dupACK /




“Retransmit”

Reno Sender

dupACK /

dupACK /



0 dupACKscount 3

dupACKscount = 0

dupACK /

Slow Start


new ACK & (CongWin SSThresh) /“Send data”

recovery ACK /CongWin SSThresh& “Send data”

Fas

t re

tran

smit

dupA

CK

& (

coun

t

3)/

“Ret

rans

mit”

Fast Recovery

On entry, set:

SSThresh ½ CongWinCongWin SSThresh + 3MSS

& “Send data”

0 dupACKscount 3

dupACKscount = 0

Start /

dupACK /CongWin CongWin + 1 & “Send data”

Example 2.1: TCP Bottleneck Link

10 Mbps

1 Mbps

6+1 packets

Sender Receiver

Router

Router Architecture

CPU

Memory

6 slots for packets

I/O Transmit/Receive

1 packet in transmission

Detail of Transmission Round #5

181920212227 181920212227

Sender

6 packets buffered:

171819202122 171819202122

Router Receiver

#16#17#18#19#20#21#22#23#24#25#26#27#28#29

#16#16

#17#17

#18#18

Transmission time on link_2equals 10 (Tx time on link_1)

Segment #16received

Segment #17received

Segmentstransmitted:

Time = 4 RTT

Time

Droppedpackets

LastByteA

ckedLastB

yteSent

#14#15

#29#30

FlightS

ize

LastByteR

ecvdN

extByteE

xpected

#14#15

packet #27 occupies the slot emptied by packet #17

SenderSender ReceiverReceiver

#1

#2,3

#4,5,6,7

#8,9, …,14,15

#16,17, …,22,23,…,27,28,29

#15

CongWin = 1 MSS

CongWin = 1 + 1

CongWin = 2 +1+1

CongWin = 4 +1+1+1+1

CongWin = 8 +1+…+1

7

CongWin = 1

CongWin = 1 + 1

#2

#4

#15

#23

EfctWin = 14

EfctWin = 8

8 segments sent

14 segments sent

EfctWin = 0

#8

#15,15,…,15

#15

1 RTT

2 RTT

3 RTT

4 RTT

5 RTT

6 RTT

7 RTT

8 RTT

9 RTT

1024 bytesto application

2 KBto appl

4 KBto appl

7 KBto appl

[ 1 segment lost ]

(buffer 8 KB) [ 5 segments missing ]

8 KBto appl

[ 4 segments missing ][ segment #27 buffered ]

Gap in sequence!(buffer 7 KB)

[ segment #15 missing ]

7+1 dupACKs

seg 27

seg 15 (retransmission)

ack 15

ack 23

seg 1

ack 2

dup ack 15

seg 15

7 dup ack 15

(loss)

EfctWin = 4

EfctWin = 2

EfctWin = 1

#23

#24

CongWin = 1

EfctWin = 1

CongWin = 1 + 1

EfctWin = 0

CongWin = 1 + 1

EfctWin = 0

RTO timeoutseg 23 (retransmission)

ack 24

1024 bytesto appl

[ 3 segments missing ][ segment #27 buffered ]Time [RTT]

RTO Timer Timeout Patternack 236 RTT

7 RTT

16 RTT

17 RTT

18 RTT

19 RTT

20 RTT

21 RTT

22 RTT

23 RTT


ack 26

24 RTT


ack 249 RTT

10 RTT

12 RTT

13 RTT

14 RTT

15 RTT


ack 25

RTO t-out

8 RTT

11 RTT

RTO t-out

RTO t-out

icon indicates whenRTO timer is started

TCP Tahoe (2)MSS = 1024 bytes, RcvWindow = 64 KB

12

4

8

14

10242048

4096

8192

[MSS][bytes]

EffctWin#1

#2,3

#4,5

,6,7

#8,9

, …,1

4,15

#16,

17, …

,22,

23,…

,29

#15

#23

#40,

41,4

2,43

,44

#31,

32

#45,

46,4

7,48

,49,

50

#66,

67,6

8, …

,72

#65

#73,

74

#75,

76,7

7,78

#79,

80,8

1,82

,83

1 2 3 4 5 7 8 109 11 140139 141

144143 145

148149

150

Seg

men

ts s

ent

Time

[RTT]

65535

137#2

9138

14336

1MSS

15MSS

2MSS

#33,

34,3

5

#36,

37,3

8,39

142

#51,

52,5

3, …

,57

#58,

59,6

0, …

,64,

65

6 147146

CongWin

EffctWinFlightSize

SSThresh9

6 3 dupACKs

RTO timeout

TCP Tahoe (3)

12

4

8

15

Co

ng

Win

[MS

S]

0 1 2 3 4 5 6 7 8 9 10 2423 2625 2827 3029 31 32 33 34 35 36 37 3811 2221

Multiplicative decreaseTCP Tahoe: CongWin 1 MSSTCP Reno: CongWin 1/2 FlightSize Additive increase

(Congestion avoidance)When CongWin SSThresh, for each ACK:CongWin(t) CongWin(t–1) + MSS2 / CongWin(t–1)

Slow startFor each ACK:CongWin CongWin + 1

Slow startFor each ACK:CongWin CongWin + 1

SSThresh

Time [RTT]

39

Fast retransmitRetransmit oldest un-acknowledged segment

after 3 dupACKs without waiting for RTO timeout

Retransmit based on RTO timeout(“slow retransmit”)

TCP Sender States / Phases

Defined by: Congestion Window (CW) SSThreshCongestion Window calculation:

CW(t) = CW(t – 1) + #ACKMSS

Slow Start:

Fast Retransmit:Defined as:When 3 dupACKs received,retransmit the oldest outstanding packet without waiting for RTO timeout

Defined by: Congestion Window (CW) > SSThreshCongestion Window calculation:

CW(t) = CW(t – 1) + MSS

Congestion Avoidance:

#ACKMSS

CW(t – 1)

Reno-Specific State / Phase

Fast Recovery:Defined as:When 3 dupACKs received,Congestion Window calculation:

CW(t) = max {½ FlightSize(t), 2MSS} + 3MSS

Same as for Tahoe:Slow Start, Fast Retransmit, Congestion Avoidance

( Unlike TCP Tahoe sender, which sets CW(t) = 1MSS when 3 dupACKs received )

TCP Reno Sender

SSThresh(t) = max {½ FlightSize(t1), 2MSS} (2.6)

CongWindow(t) = SSThresh(t) + 3MSS (2.7)

CongWindow = SSThresh (2.8)

(Effective Window computed as before …)

Enter Fast Recovery: count each “old ACK” as +MSS to CongWin.

When packet loss detected via 3 dupACKs:

When “new ACK” received, exit Fast Recovery and enter Congestion Avoidance.

TCP Reno (1)

2424

12

4

8

15

10242048

4096

8192

16384

[MSS][bytes]

EffctWin#1

#2,3

#4,5

,6,7

#8,9

, …,1

4,15

#16,

17, …

,22,

23,…

,29

#15,

30,…

,35,

36,3

7

#23

#38

(1 b

yte)

#24

#50

(1 b

yte)

#28

#74,

82,8

3

SSThresh

#29

#53,

54,5

5,56

#36,

57

#58

#59

(1 b

yte)

#37

#60,

61,

…,6

6

#67,

68, …

,73,

74#7

5,76

, …

,81

0 1 2 3 4 5 6 7 8 9 10 2120 2322 2524 2726 28 29 30 31 32 33 34 35

Seg

men

ts s

ent

Time

[RTT]

65535

#84,

85,…

,90,

91,…

,94,

95

36

1122

11 19

#26

18

36363737

59596666

7373

44

2323

29292828

7+1 dupACKs

881515

6 dupACKs

#39

(1 b

yte)

#40

(1 b

yte)

CongWin

EffctWin#4

1 (1

byt

e)

3 dupACKs

#47

(1 b

yte)

#48

(1 b

yte)

#49

(1 b

yte)

#51

(1 b

yte)

#52

(1 b

yte)

FlightSize

4 dupACKs

7 dupACKs

TCP Reno (2)

CongWin =

CongWin =

CongWin =

CongWin =EfctWin = 2

7168 bytesto application7168 bytes

to application

1024 bytesto appl

1024 bytesto appl

7.6 MSSEfctWin = 0

EfctWin = 1

EfctWin = 43.6

13.6


3.7


3.7



6.7

3.61327.6

6.6323.7

SenderSender ReceiverReceiver

#52 (1 byte)

#2926 RTT

#29

#3627 RTT

#36,36,36,3628 RTT

(buffer 1 byte)

Gap in sequence!(buffer 4 KB)

4 dupACKs

Time [RTT]

3 dupACKs

#53,54,55,56

#36,57

#3729 RTT

#58

#3730 RTT

#59 (1 byte)

#3731 RTT

#37

#6032 RTT

1024 + 16 + 6144bytes to application

(buffer 1024 bytes)

(buffer 1 KB)

(buffer 1 byte)3 dupACKs

#37

#60,61, …,66

seg 52 (1 byte)

ack 29

seg 29

ack 36

ack 37

seg 58

ack 37

seg 59 (1 byte)

ack 37

seg 37

ack 60

seg 36

ack 37

ack 36

Topic:TCP NewReno Protocol

Partial ACKs for Pkt Loss Detection

Example

How TCP Sender Discovers Packet Loss

• Retransmission timer (RTO) expired

3 dupACKs received

• Partial ACK received

TCP Tahoe &TCP Reno

TCP NewReno

TCP NewReno – Partial ACKs

• Defined at the time tloss when the sender discovers a packet loss (by 3 dupACKs):

• Full acknowledgment: If the sender will receive an ACK for all packets that were outstanding at the time tloss

• Partial acknowledgment: If the sender will receive an ACK for some of the packets that were outstanding at the time tloss

TCP NewReno

LastByteAcked =LastByteSent

#1#1

EffectiveWindow

#2#2 #3#3 #4#4 #5#5 #6#6

NextByteExpected =LastByteRecvd + 1

LastByteRecvd

TCP Sender TCP Receiver

(loss)

(loss)

(loss)


FlightSize

#2#2 #4#4 #6#6

NextByteExpected

Ack#1 (duplicate)

Ack#1 (duplicate)

Ack#1 (duplicate)

Seg#1 (retransmission)

#2#2 #4#4 #6#6

NextByteExpectedLastByteRecvd

#1#1

LastByteRecvd

LOSS DETECTED(3dupACKs)

Ack#3 (partial)LOSS DETECTED(partial ACK)


Time

Receive bufferLastByteAcked =LastByteSent

#1#1

EffectiveWindow

#2#2 #3#3 #4#4 #5#5 #6#6

NextByteExpected =LastByteRecvd + 1

LastByteRecvd

TCP Sender TCP Receiver

(loss)

(loss)

(loss)


FlightSize

#2#2 #4#4 #6#6

NextByteExpected

Ack#1 (duplicate)

Ack#1 (duplicate)

Ack#1 (duplicate)


#2#2 #4#4 #6#6

NextByteExpectedLastByteRecvd

#1#1

LastByteRecvd

LOSS DETECTED(3dupACKs)

Ack#3 (partial)LOSS DETECTED(partial ACK)


Time

Receive bufferReceive buffer

tloss

At this time define:Full acknowledgment = ACK for all 6 outstanding pkts

TCP NewReno Fast Recovery

• NewReno sender enters Fast Recovery when a loss is detected (by 3 dupACKs)

• NewReno sender exits Fast Recovery when it receives the full acknowledgement(defined at time tloss)

• After Fast Recovery, NewReno sender enters Congestion Avoidance

Congestion Window Calculationduring Fast Recovery

• NewlyAcked =LastByteAcked(t) LastByteAcked(t 1)

• If (NewlyAcked < MSS), thenCongWindow(t) =CongWindow(t 1) NewlyAcked

• Else if (NewlyAcked MSS), thenCongWindow(t) =CongWindow(t 1) NewlyAcked + MSS

TCP NewReno: Example (1)

Example 2.2

txmit (Link 1) << txmit (Link 2)

tprop (Link 2) = 6 txmit (Link 2)

tprop (Link 1) << tprop (Link 2)

Sender Receiver

Link 1 4+1 packets Link 2

Router

TCP NewReno: Example (2)Example 2.2

txmit(Link 1)

txmit(Link 2)

tprop(Link 1)

Receiver

Sender/Router

1 2 3

100

pkt ack

transmission on Link 1

propagation on Link 1

transmission on Link 2

propagation on Link 2

Sender

Router

1 2 3

Receiver

Sender/Router

100

pkt ack

txmit(Link 1)

tprop(Link 1)

txmit(Link 2)

11 22 33 44 55 66 77 88 99 101011111212131314141515

Receiver

Sender/Router

10 200 30 40 50 60

16161717181920212224262830323436 42

pkt ack

3840 442323

Congestion window [segments]

Packets in flight on 2nd link

3dupACK

Packets in Router buffer

0

10

20

0

5

6

0

2525

Time [in packet transmission slots]

SSThresh = 65535 bytes

SSThresh = 11 MSS

Lost packets:23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45 (12 total)

4646

CongWin

98

11109

13121110

15141312111

32 3

54

765

76 7

15141312

151413

1514

171615

19181716

2120191817

2221201918

2422212019

2624222120

2826242221

3028262422

3230282624

3432302826

3634323028

3836343230

4038363432

4240383634

4442403836

44424038

444240

234442

234423

462546

1 232

54

6543 4

7654

765

876

987

1098

111098

12111098

14131211109

1312111098

151413121110

161514131211

171615141312

181716151413

191817161514

201918171615

212019181716

222120191817

242221201918

262422212019

282624222120

302826242221

323028262422

343230282624

363432302826

383634323028

403836343230

424038363432

444240383634

4442403836

2344424038

23444240

234442

234423 25

4625

Slow Start Fast RecoveryFast Retransmit

2727 29294949 3131 53535454 333356565757585859596060

Receiver

Sender/Router

70 8060 90 100 110 120

3535616162626363646465656666 676768686969707071717272 7474

pktack

73733939 75757676

Congestion window [segments]

Packets in flight on 2nd link

Packets in Router buffer

20

30

40

0

5

6

0

8282

Time [in packet transmission slots]

83838484

CongWin

5633

585751

4929

5231

5352

5958

605960

6135

6261

6362

6463

6564

666566

6737

6867

6968

7069

7170

7271

7372

743973

757439

767574

777675

787776

85848382

86858483

87868584

274727

57563355

58575633

605958575633

5958575633

6059585756

3560595857

6135605958

6261356059

6362613560

6463626135

656463626135

666564636261

376665646362

673766656463

686737666564

696867376665

706968676637

717069686766

727170696867

737271706968

397372717069

743973727170

757439737271

767574397372

777675743973

787776757439

797877767574

807978777675

818079787776

824181807978

838241818079

484727

2948

47474848

484748

4727

5049

5150

4929

504929

51504929

315150

50505151

5453

555455

515049

523151

535231

54535231

5554535231

5756

462546

5252

4847

5555 3737

55545352

33555453

56335554

777778787979808081814141 858586868787

797877

807978

818079

82418180

83824181

84838241

88878685

89888786

90898887

418180797877

848382418180

858483824181

868584838241

SSThresh = 11 MSS

CW = 51

Fast Recovery (cont’d)


Topic:TCP Over Wireless

Reducing Link-Layer Overheads

TCP Over Wireless

• Slow start until reaches the SSThresh size;set timer for every burst

• If no ACK congestion;halve the window size; slow start again

Wireless Link-layer OverheadTCP layer:

TCP data segment[1024 KB + 40 bytes headers]

TCP ACK segment[0 KB + 40 bytes headers]

Link layer:

Link layer overhead: backoff delay, interframe spaces, link-layer control frames (RTS, CTS, ACK)

Computer Networks: Transmission Control Protocol (TCP)

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