The Transport Layer: TCP and UDP - - PDF document

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The Transport Layer: TCP and UDP - - PDF document

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COLE POLYTECHNIQUE FDRALE DE LAUSANNE The Transport Layer: TCP and UDP Contents

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SLIDE 1

1

  • The Transport Layer: TCP and UDP
  • ÉCOLE POLYTECHNIQUE

FÉDÉRALE DE LAUSANNE

  • Contents

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slide-2
SLIDE 2

2

  • 1. Error Recovery
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  • The Philosophy of Errors in a Layered Model

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SLIDE 3

3

  • The Layered Model Transforms Errors into

Packet Losses

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solution A R1 R2 B * * * * * * *$ *' *' *' *$!. *$ *$ A R1 R2 B * * * * * * *$ *$ *$ *$ *$!. *' *' *$ *'

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The Case for End-to-end Error Recovery

R2 B A R3 R4 R1 1 4 2 3 5 6 7

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SLIDE 4

4

  • The Case for Hop-By-Hop Error Recovery

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  • The Capacity of the End-to-End Path

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? ? ? !#%%# # A R1 R1 R1 R1 R1 R1 B Loss probability links solution

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SLIDE 5

5

  • End-to-end Error Recovery is Inefficient when

Packet Error Rate is high

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  • solution
  • Conclusion: Where is Error Recovery located in

the TCP/IP architecture ?

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SLIDE 6

6

  • 2. Mechanisms for Error Recovery
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  • ARQ Protocols

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SLIDE 7

7

  • Why Sliding Window ?

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  • How Sliding Window Works.

Usable Window

P = 1 A = 0 P = 0 A =2 P = 2 P = 3 P = 4 A =1 P = 5 P = 6 P = 7 P = 8 P = 9 A =3 P = 10 A =4 A =5 A =6 A =7 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12

Legend Maximum Send Window = Offered Window ( = 4 here)

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SLIDE 8

8

  • @>!,!,,,

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  • An Example of ARQ Protocol with Selective

Repeat

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SLIDE 9

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SLIDE 10

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and Negative Acks

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SLIDE 11

11

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  • Where are ARQ Protocols Used ?

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slide-12
SLIDE 12

12

  • Are There Alternatives to ARQ ?

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SLIDE 13

13

  • 3. Flow Control

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SLIDE 14

14

  • Can we use Sliding Window for Flow Control ?

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  • Credit Flow Control

0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 P = 1 A = -1, credit = 2 P = 0 P = 2 P = 3 P = 4 A = 0, credit = 2 P = 5 P = 6 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 A = 2, credit = 4 0 1 2 3 4 5 6 7 8 9 10 11 12 A = 0, credit = 4 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 A = 4, credit = 2 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 A = 6, credit = 0 0 1 2 3 4 5 6 7 8 9 10 11 12 A = 6, credit = 4 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 P = 7

slide-15
SLIDE 15

15

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slide-16
SLIDE 16

16

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  • 4. The Transport Layer

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slide-17
SLIDE 17

17

  • Why both TCP and UDP ?

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  • UDP Uses Port Numbers

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SLIDE 18

18

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  • The UDP service

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slide-19
SLIDE 19

19

  • UDP is used via a Socket Library
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  • How the Operating System views UDP

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slide-20
SLIDE 20

20

  • 5. TCP basics

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SLIDE 21

21

  • TCP views data as a stream of bytes

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slide-22
SLIDE 22

22

  • TCP Basic Operation

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slide-23
SLIDE 23

23

  • Losses are Also Detected by “Fast Retransmit”

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slide-24
SLIDE 24

24

  • TCP uses Connections

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  • TCP Connection Phases

SYN, seq=x syn_sent SYN seq=y, ack=x+1 ack=y+1 established established snc_rcvd listen FIN, seq=u ack=v+1 ack=u+1 FIN seq=v fin_wait_2 time_wait close_wait last_ack closed application active open passive open application close: active close fin_wait_1 Connection Setup Data Transfer Connection Release

slide-25
SLIDE 25

25

  • padding
  • ptions (if any)

srce port dest port sequence number ack number hlen window

code bits

rsvd urgent pointer checksum segment data (if any) TCP header (20 Bytes +

  • ptions)

IP header (20 B + options) <= MSS bytes

code bit meaning urg urgent ptr is valid ack ack field is valid psh this seg requests a push rst reset the connection syn connection setup fin sender has reached end of byte stream

  • TCP Segment Format

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slide-26
SLIDE 26

26

  • TCP is used via a Socket Library

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03 !09"3"" 03! % 03!-* !% ;B 03" ! 03!".H ;B "!- %03 " %% 03" #! !!.! "%!##!# %-!" #""# "

% ./tcpClient <destAddr> bonjour les amis % % ./tcpServ & % client socket(); server socket(); bind(); connect(); send(); close(); bind(); listen(); accept(); receive(); close();

  • How the Operating System views TCP Sockets

program TCP IP id=3 id=4 incoming connection queue buffer port=32456 address=128.178.151.84 id=5 buffer socket socket socket

slide-27
SLIDE 27

27

  • Test Your Understanding

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  • 6. TCP, advanced

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slide-28
SLIDE 28

28

  • When to send an ACK

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  • Nagle’s Algorithm

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(data written by upper layer) or (new ack received) -> if full segment ready then send segment else if there is no unacknowledged data then send segment else start override timer; leave

  • verride timer expires -> create segment and send

(data written by upper layer) or (new ack received) -> if full segment ready then send segment else if there is no unacknowledged data then send segment else start override timer; leave

  • verride timer expires -> create segment and send
slide-29
SLIDE 29

29

  • Example: Nagle’algorithm

8000:8001(1) ack 101 win 6000 1

A B

a -> b -> c -> d -> e -> f -> 101:102(1) ack 8001 win 14000 2 8001:8003(2) ack 102 win 6000 3 102:102(0) ack 8003 win 13998 4 8003:8005(2) ack 102 win 6000 102:102(0) ack 8003 win 14000 5 6 102:102(0) ack 8005 win 13998 7 8005:8006(1) ack 102 win 6000 8

  • Silly Window Syndrome Avoidance: Why ?

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%) %) %) 4; 4; 4; 4;77777 77777 77777 77777 < < < <77777 77777 77777 77777= = = =

slide-30
SLIDE 30

30

  • Silly Window Syndrome Avoidance

;;

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6% ;; %

!%% #! . ;; 8%-

%- %- %-< < < <

keep nextByteExpected + offeredWindow fixed until: reserve · min (MSS, 1/2 receiveBuffer) keep nextByteExpected + offeredWindow fixed until: reserve · min (MSS, 1/2 receiveBuffer)

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  • 77

77 77 77=; =; =; =;7 7 7 7 -

  • 7

7 7 7= = = = ; ; ; ;77777777777777 77777777777777 77777777777777 77777777777777 %- %- %- %- 7777777777777777 7777777777777777 7777777777777777 7777777777777777= = = =

  • SWS Avoidance Example

ack 0 win 2000 <----- 0:1000 -----> bufferSize= 2000B, freebuf = 1000B, reserve = 0B 1000:2000 -----> freebuf= 0B, reserve = 0B ack 2000, win 0 <----- application reads 1 Byte: freeBuf=reserve=1B, .... application has read 500 B: reserve = 500 persistTimer expires window probe packet sent 2000:2001 -----> data is not accepted (out of window) ack 2000, win 0 <----- .... application has read 1000 B: reserve = 1000 ack 2000, win 1000 <----- 2000:3000 ----->

slide-31
SLIDE 31

31

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  • Round Trip Estimation

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sampleRTT = last measured round trip time estimatedRTT = last estimated average round trip time deviation = last estimated round trip deviation initialization (first sample): estimatedRTT = sampleRTT + 0.5s; deviation = estimatedRTT/2 new value of sampleRTT available -> Err = sampleRTT - estimatedRTT estimatedRTT = estimatedRTT + 0.125 * Err deviation = deviation + 0.250 * (|Err|- deviation) RTO = estimatedRTT + 4*deviation sampleRTT = last measured round trip time estimatedRTT = last estimated average round trip time deviation = last estimated round trip deviation initialization (first sample): estimatedRTT = sampleRTT + 0.5s; deviation = estimatedRTT/2 new value of sampleRTT available -> Err = sampleRTT - estimatedRTT estimatedRTT = estimatedRTT + 0.125 * Err deviation = deviation + 0.250 * (|Err|- deviation) RTO = estimatedRTT + 4*deviation

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SLIDE 32

32

  • Sample RTO

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  • Conclusions

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SLIDE 33

33

  • Solutions
  • The Philosophy of Errors in a Layered Model

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SLIDE 34

34

  • The Layered Model Transforms Errors into

Packet Losses

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SLIDE 35

35

  • End-to-end Error Recovery is Inefficient when

Packet Error Rate is high

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SLIDE 36

36

  • Why Sliding Window ?

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SLIDE 37

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SLIDE 38

38

  • Are There Alternatives to ARQ ?

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SLIDE 39

39

  • Can we use Sliding Window for Flow Control ?

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SLIDE 40

40

  • Losses are Also Detected by “Fast Retransmit”

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SLIDE 41

41

  • Sws avoid.

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