第 5 章运输层

54321 A B 1 2AP1LAN2WANAP2AP3AP4IP LAN1AP1AP2AP454321IP TCP UDP AP3

IP TCP UDP

TCP UDP

OSI A BTSAPNSAP

TCP UDP

5.2 TCP/IP 5.2.1 TCP/IP(1) UDP (User Datagram Protocol)(2) TCP (Transmission Control Protocol)

TCP UDP TPDU (Transport Protocol Data Unit)TCP TCP (segment) UDP UDP

TCP UDP UDP UDP UDP UDP TCP TCP TCP

5.2.2 TSAP

TCP UDPTCP IP UDP TCP UDPTCP UDP IP

16 bit

0~1023

(socket) TCP () TCP (socket)IP

Berkeley Sockets

5.3 UDP 5.3.1 UDP UDP IP UDP UDP UDP UDP 8

UDP 51000UDP 69TFTP TFTP UDP

5.3.2 UDP UDP IP IP 017IP 44112122222 UDP

UDP IP IP 017IP 44112122222 UDP UDP 8 4

UDP IP IP 017IP 44112122222 UDP UDP

5.4 TCP 5.4.1 TCP TCP TCP

TCP

TCP20 FIN32 bitSYNRSTPSHACKURG 0 8 16 24 31 TCP TCP TCP IP IP

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 2

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 4 TCP

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 4 bit TCP TCP 32 bit 4

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 6 bit 0

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 URG URG 1 ()

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 ACK ACK 1 ACK 0

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 PSH (PuSH) TCP 1

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 RST (ReSeT) RST 1 TCP

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 SYN SYN 1

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 FIN (FINal) FIN 1

2 TCP TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 2 TCP 12

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 16 bit

TCP20 FINSYNRSTPSHACKURG 0 8 16 24 31 TCP MSS (Maximum Segment Size)MSS TCP MSS MSS TCP TCP TCP

5.4.3 TCP TCP TCP TCP TCP 1

5.5 5.5.1

H1 H2 R1 R2 R3 H1 H2

( H1 H2 R1 R2 R3 H1 H2 R1R2R3H1H2

5.6 5.6.1 A BAP2AP1

:Fig. 3-9

5.6.2 A B

(1) (2) (3) (4) (5) ( ) (2)

(1) (2) (3) (4) (5) (1)

ABDATADATADATADATA B B B BABDATA BDATA B

A Simplex Stop-and-Wait ProtocolFig. 3-10

5.6.3 ABACKACK(a)

A(timeout timer) tout B A

1 B B B A ACK B A ACK

N(S) 0 1

5.6.4 nACKn n 1 n ACK1 0 1 ACK0 1 0

(1) (2) V(S)0 (3) N(S)V(S)(4) (5) (6) {(7)(8)}(7) ACKnn n = 1 V(s) V(S)[1 V(S)] (3) (6) (8) (4)

(1)V(R)0(2) (3) N(S) = V(R)(4) (6)(4) (5) V(R)[1 V(R)](6) nV(R) ACKnn(2)

ACKn V(S) N(S) V(R) ACKn V(R) n

CRC CRC ARQ (Automatic Repeat reQuest)

A Simplex Protocol for a Noisy Channel 101

PARPositive Acknowledgement with RetransmissionARQAutomatic Repeat reQuestFig. 3-11

5.6.5 tf tf lf (bit) C (bit/s) tf = lf /C = lf /C (s) 3-1 tf B tp B tpr ACK ta

ABDATADATAACK tp tpr ta tp tprtftout

tout = tp + tpr+ ta + tp + tpr (3-2) tpr ta tp tout = 2tp (3-3)

tT = tf + tout = tf + 2tp (3-4) () p tav tav = tT ( 1 + )

= {1 P[ 1] + 2 P[ 2] + 3 P[ 3] +} = {1 P[ 1 ] P[ 2 ] + 2 P[ 1, 2 ] P[ 3 ] + 3 P[ 1, 2, 3 ] P[ 4 ] +} = p(1 p) + 2p2(1 p) + 3p3(1 p) + P[X] X

tav p = 0, tav = tT

(3-5)

max max = 1/tav = (1 p) / tT (3-6) max (1 p) / tT (3-7) tf tf (1 p) / < 1 (3-8) tT tT / tf 1 (3-9) 1 1 p (3-10)

Td , Tf, CL Tf = L/C Cr = / = Tf / (Tf + 2Td) = 1/(1+2Td *C/L)L

Td = 250ms,C=50kb/s,L=1kbit,Cr=1/(1+2TdC/L)=1/(1+2*0.250*50*103/1*103)=3.8

ARQ ARQ ARQ

5.7 ARQ 5.7.1 ARQ

ARQ ACK1 DATA0ACK2 DATA1DATA2 DATA3 ACK2DATA4 ACK2DATA5 ACK2ACK3 DATA2ACK4 DATA3ABtout??

(1) 2 3 2 (2) ACK1 0 DATA0 1 ACK2 1 DATA1 2

(3) A 2 2 A 5 2 ARQ Go-back-N ARQ N

(4) ARQ N

5.7.2 WT

01234567012WT 5 (a)

ARQ WR = 1

01234567012WR 0 (a)

n 1 WT 2n 1 ARQ 3 bit 7 8

A One Bit Sliding Window ProtocolN = 101piggybackingFig. 3-13

nEnable_Network_LayerDisable_Network_Layer < MaxSeq + 1MaxSeq = 71 0 ~ 72 7 3 8 0 ~ 74 7 nFig. 3-16

P5Network_layer_readyFrame_arrival Cksum_err timeout ntimeout

5.7.3 ()() 100 % ()

5.8 ARQ ARQ ARQ n WR 2n/2 (3-18)

selective repeat1Fig. 3-15(b)

A Protocol Using Selective RepeatMaxSeq(MaxSeq+1)/2 MaxSeq = 7, = 7 0 ~ 67 ~ 506 7 ~ 5 0 0Fig. 3-19

Ack_ExpectedNextFrame_ToSendFrame_ExpectedTooFarNAKFig. 3-18

P6Network_layer_readyFrame_arrival NAKNAKCksum_err NAKtimeout Ack_timeout ACK

timeoutAckNAKack_timeout

Td , Tf, CL,WsCr = / = Ws*Tf / (Tf + 2Td) = Ws*/(1+2Td *C/L)Ws

Td = 250ms,C=50kb/s,L=1kbit,100%Ws=2 *Td *C/ L+1 =2*0.25*50*103/103+1=26

5.9 5.9.1 (congestion) > (5-1)

(reassembly deadlock)A4A2A1B3C1B1B2C3B4A3C2 P Q R H AB C PQ R H 4 4 R A 4 A3 H A3 P Q Q

5.9.3 TCP 1. TCP TCP ()

900 9 100 500 TCP 1002003004005006007008009001012013014015016017018011

400 200 300 10020030040050060070080090010120130140150160170180111002003004005006007008009001012013014015016017018011

10020030040050060070080090010120130140150160170180111002003004005006007008009001012013014015016017018011 400 400 400

400 A B A 300 201 500A 200 A 200 301 500A 300 A 100 401 500A 500 A 200 501 700A 100 501 700 A 600

2. TCP rwnd (receiver window) (advertised window) cwnd (congestion window)

rwnd cwnd (1) rwnd TCP (2) cwnd (congestion window)

rwnd cwnd Min [rwnd, cwnd] (7-1) rwnd < cwnd cwnd < rwnd

cwnd MSS MSS cwnd

TCP 1 16 ssthresh = 16246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

cwnd rwnd 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

cwnd 1 M0 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12 ACK1 M0 M1 cwnd 1 2 M1 M2

ACK2 ACK3 ACK 1 cwnd 2 4 M4 ~ M6 4 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

ACK 1 cwnd 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

cwnd ssthresh cwnd = 16 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

24 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

ssthresh 12 24 1 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

cwnd = 12 MSS 246810121416182022004812162024 cwndssthresh = 16 ssthresh = 12

(multiplicative decrease) ssthresh 0.5ssthresh

(additive increase) cwnd MSS

3. ACK

A BB M1 M2A M1 M2A ACK3 M3A M3 A M4A M5B ACK3A M6B ACK3B M2( M3

(1) ACK ssthresh(2) cwnd 1 ssthresh + 3 MSS (3) ACK n n > 3 cwnd ssthresh + n MSS(4) (5) ACK cwnd ssthresh

5.9.4 TCP TCP TCP

TCP IP T1T2T3

5.9.5 TCP 1.

TCP (client)(server)

TCP B A

TCP A B A A B A B

ABTCPSlow Start32K(1K = 1024), MSS1KA->B, B, B, MSSA01., A1ACK10240, A2.ACK = 32768A3.32K40K, ,

110, 11232, , (32 + 1 / 32) * 1024 = 328003 1 MSS = 1KB 40 / 2 = 20 KB

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第 5 章 运输层

第 5 章运输层