tcpip-chapter p通信與信息工程學(xué)院網(wǎng)絡(luò)工程系

TCP/IP協(xié)議原理網(wǎng)絡(luò)工程系通信與信息:

wPhone:

61830521

(O)Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/232Introduction應(yīng)用的需求：reliability傳輸大量的數(shù)據(jù)，要求可靠的通信服務(wù)自身的可靠性機(jī)制弱底層網(wǎng)絡(luò)和IP網(wǎng)絡(luò)是不可靠、無連接投遞TCPProcess-to-process

comm.:

same

withUDPToadd

connection-oriented

and

reliabilityfeatures

to

the

services

of

IP?TCP/IP

Protocol

Suite2020/11/233理解TCP的功能提供process-to-process

communication在IP層上建立起：可靠的，順序的（reliable,sequential）分組傳輸服務(wù)面向連接的通信Error-control源端到目的端的流量控制（flow-control

）To

prevent

a

fast

source

of

packets

fromoverwhelming

a

slowsinkAdaptive

bandwidth

sharing

in

the

network→congestion

control?TCP/IP

Protocol

Suite2020/11/234TCP的功能ResponsibilitiesTo create

a

process-to-process

communication使用兩個端點(diǎn)地址（IP＋端

）通信To

providea

flow-control

and

error-controlmechanism

at

the

transport

layerError

control:

ARQ

→

Window

based

controlAcknowledgement

packetTime-outRetransmissionFlow

control:

sliding

window

protocol

→

Window

basedcontrolTo provide

a

congestion

control:

sliding

windowprotocolTo

provide

a

connection

mechanism

for

theprocesses?TCP/IP

Protocol

Suite2020/11/235OverviewIPLANsMANsWANsICMPIGMPARPRARPNetworkLayerNetworkAccess

LayerTransmission

Control

Protocol，TCPRFC

793，傳輸控制協(xié)議ApplicationLayerTCPUDP?TCP/IP

Protocol

Suite2020/11/236TransportLayerProcess-to-process

Communication端口、端點(diǎn)概念與方式與UDP完全一樣連接：TCP上通信雙方抽象的虛電路連接80Port:

80416250Endpoint:

(,

80)Connection:

(,

80)

and

(4,16250)?TCP/IP

Protocol

Suite2020/11/237Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/238TCP

ServicesStream

delivery

service（流交付服務(wù)）Full-duplex

service（全雙工服務(wù)）Connection-oriented

serviceReliable

service?TCP/IP

Protocol

Suite2020/11/239理解：Stream

Delivery與UDP交付區(qū)別：進(jìn)程把預(yù)先定義好邊界的一組報(bào)文（用戶數(shù)據(jù)報(bào)）發(fā)送給UDPUDP對這些報(bào)文中的每一個添加首部，傳遞給

IP傳輸U(kuò)DP和IP都無法識別這些數(shù)據(jù)報(bào)之間是否存在任何關(guān)系——獨(dú)立處理每一個數(shù)據(jù)報(bào)?TCP/IP

Protocol

Suite2020/11/2310TCP

Services：Stream

Delivery

ServiceStream

delivery

service

requires:Sending

process

candeliver

data

as

astream

ofbytesReceiving

process

canobtain

data

as

astreamofbytesstream

of

bytes

→進(jìn)程間使用自己認(rèn)為適宜的任何大小的數(shù)據(jù)片進(jìn)行發(fā)送或接收（最小1字節(jié)）發(fā)送進(jìn)程 接收進(jìn)程Stream

of

byteTCPTCPTCP

creates

a

environment

in

which

the

two

processes

seem

to

beconnected

by

an

“tube”

that

carriers

their

data

across

the

InternetSending

and

receiving

buffersTCP為什么需要buffer？原因：the

sending

and

the

receiving

processes

may

notnecessarily

write

or

read

data

at

the

samerateTCP要為進(jìn)程構(gòu)建”tube”——“可靠傳輸”流量控制發(fā)送進(jìn)程與接收進(jìn)程產(chǎn)生和消耗數(shù)據(jù)的速率不一致差錯控制2u13TCP

segmentsTCP是如何發(fā)送“字節(jié)流”的？按報(bào)文段（segment）傳輸——Why？報(bào)文段：若干字節(jié)構(gòu)成IP是按分組（Package）處理的（而非字節(jié)流），1個segment就封裝在1個Package上述過程對于接收進(jìn)程是透明的字節(jié)流按照報(bào)文段傳輸?shù)?/p>

：接收方TCP收到報(bào)文段可能失序、損傷、重復(fù)，或者丟失！TCP

ServicesStream

delivery

service（流交付服務(wù)）Full-duplex

service（全雙工服務(wù)）Connection-oriented

serviceReliable

service?TCP/IP

Protocol

Suite2020/11/2314TCP

Services：Full-Duplex

Service全雙工：數(shù)據(jù)可以在同一時間雙向流動每一個TCP都有發(fā)送緩存和接收緩存DataACKPiggybacking捎帶D

an

flow

inboth

direction

atthe

same

time?TCP/IP

Protocol

Suite2020/11/2315TCP

ServicesStream

delivery

service（流交付服務(wù)）Full-duplex

service（全雙工服務(wù)）Connection-oriented

serviceReliable

service?TCP/IP

Protocol

Suite2020/11/2316TCP

Services：Connection-OrientedService建立連接，使用連接，拆除連接建立的是虛連接（virtualconnection），而非物理連接（physical

connection）封裝成IP分組的TCP報(bào)文段可能走不同的路徑到達(dá)目的地接收到的TCP報(bào)文段可能：亂序、丟失、損壞、重復(fù)而TCP需要向上層按順序交付數(shù)據(jù)IP網(wǎng)絡(luò)?TCP/IP

Protocol

Suite2020/11/2317TCP

ServicesStream

delivery

service（流交付服務(wù)）Full-duplex

service（全雙工服務(wù)）Connection-oriented

serviceReliable

service?TCP/IP

Protocol

Suite2020/11/2318TCP

Services：Reliable

ServiceReliabilitySequential,

without

error,

and

without

anypart

lostorduplicated如何實(shí)現(xiàn)可靠傳輸？——ARQ（AutomaticRepeat

reQuest）基本思路：接收方：must

continuously

return

acknowledgments(ACK)for

successfully

received

data發(fā)送方：每一個發(fā)送的數(shù)據(jù)都需要接收方的確認(rèn)（ACK）發(fā)送每一個數(shù)據(jù)都需要緩存，并啟動定時器，超時重傳?TCP/IP

Protocol

Suite2020/11/2319Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/2320Window

based control

Protocol——基本協(xié)議技術(shù)序號（Sequence

number）確認(rèn)（Acknowledgment）超時重傳機(jī)制窗口滑動（sliding）擴(kuò)展（expanding）縮回（shrinking）關(guān)閉（closing）?TCP/IP

Protocol

Suite2020/11/2321TCPNumbering

Bytes（給字節(jié)

）To

number

all

data

bytes

transmittedin

aconnection

→字節(jié)號,而不是給每個報(bào)文段分配Numbering

is

independentin

each

directionThe

numbering

starts

randomly,

NOT

from

0選取范圍：0

~

232-1某個TCP連接上的某個報(bào)文段的序號（sequence

number）＝報(bào)文段中第一個數(shù)據(jù)字節(jié)的字節(jié)號TCPSendingSending

bufferReceiving

bufferReceivingDatestreamSegmentData?TCP/IP

Protocol

Suite2020/11/2322練習(xí)Imagine

a

TCP

connection

is

transferring

a

file

of6000

bytes.

The

byte

is

numbered

10010.What

are

the

sequence

numbers

for

each

segmentfourif

data

is

sent

in

five

segments

with

thesegments

carrying

1,000

bytes

and

the

lastsegment

carrying

2,000

bytes?The

following

shows

the

sequence

number

foreach

segment:Segment

1

→

10,010Segment

2

→

11,010Segment

3

→

12,010Segment

4

→

13,010Segment

5

→

14,010(10,010

to

11,009)(11,010

to

12,009)(12,010

to

13,009)(13,010

to

14,009)(14,010

to

16,009)Sender1301014010100101101012010Seg1

Seg2

Seg3

Seg4Seg5?TCP/IPProtocol

Suite2020/11/2323報(bào)文段的序號報(bào)文段的序號＝

報(bào)文段中第一個數(shù)據(jù)字節(jié)的字節(jié)號，那么如果報(bào)文段不攜帶數(shù)據(jù)，那么這個報(bào)文段是否需要

？如何

？原則：需要確認(rèn)的報(bào)文，才需要序號?TCP/IP

Protocol

Suite2020/11/2324Window

based control

Protocol——基本協(xié)議技術(shù)序號（Sequence

number）確認(rèn)（Acknowledgment）超時重傳機(jī)制窗口滑動（sliding）擴(kuò)展（expanding）縮回（shrinking）關(guān)閉（closing）?TCP/IP

Protocol

Suite2020/11/2325Acknowledgment

（確認(rèn)，ACK）Acknowledgment

number（確認(rèn)號）：對已經(jīng)收到的字節(jié)表示確認(rèn)Positive

ACK（肯定確認(rèn)）The

number

of

the

next

data

byte

apartyexpects

to

receiveCumulative

ACK（累計(jì)確認(rèn)）例如

TCP報(bào)文段中的確認(rèn)號是：1234，意味著：已經(jīng)收到了字節(jié)號＝1234的以前的所有字節(jié)希望收到了下一個TCP報(bào)文段的序號＝1234?TCP/IP

Protocol

Suite2020/11/2326Discussion例1:

a

segment

with

Seq#

=X, Data

Len=LThen the

Seq#

of

the

next

segment:例2:a

segment

with

Ack#

=XThis

means

all

bytes

from

the

beginning

up

toX-1

has

been

receivedFeatures報(bào)文的順序關(guān)系數(shù)據(jù)流的位置，更便于流的復(fù)原需較大的序號空間（32bit，232→4Gbyte）序號不連續(xù)，n1<n2<n3…X+L?TCP/IP

Protocol

Suite2020/11/2327Window

based control

Protocol——基本協(xié)議技術(shù)序號（Sequence

number）確認(rèn)（Acknowledgment）超時重傳機(jī)制窗口滑動（sliding）擴(kuò)展（expanding）縮回（shrinking）關(guān)閉（closing）?TCP/IP

Protocol

Suite2020/11/2328超時重傳機(jī)制發(fā)送方每發(fā)送一個報(bào)文段（Segment），就啟動一個定時器，如果在定時內(nèi)，沒有收到對該報(bào)文段的確認(rèn)（ACK），重傳該報(bào)文段發(fā)送方必須緩存已經(jīng)發(fā)送但未收到確認(rèn)的報(bào)文段發(fā)方在定時內(nèi)沒有收到確認(rèn)（ACK），該報(bào)文段損壞或者該報(bào)文段丟失或者ACK丟失?TCP/IP

Protocol

Suite2020/11/2329Window

based control

Protocol——基本協(xié)議技術(shù)序號（Sequence

number）確認(rèn)（Acknowledgment）超時重傳機(jī)制窗口滑動（sliding）擴(kuò)展（expanding）縮回（shrinking）關(guān)閉（closing）?TCP/IP

Protocol

Suite2020/11/2330滑動窗口The

size

of

the

sliding

windows

is

72020/11/23TCP/IP

Protocol

Suite312020/11/23TCP/IP

Protocol

Suite窗口：緩存中一組字節(jié)號（或者報(bào)文序號）的集合——窗口覆蓋了緩存中的一部分單元窗口有左、右兩沿，均可向右滑動落在發(fā)送窗口（sending

window）字節(jié)號可能存在兩部分?jǐn)?shù)據(jù)：發(fā)方已發(fā)送但還未收到確認(rèn)的字節(jié)號（或者報(bào)文號）集合發(fā)方可以立即發(fā)送的字節(jié)號（或者報(bào)文號）集合發(fā)送窗口大小Ws＝ 連續(xù)發(fā)送的最大字節(jié)數(shù)落在接收窗口（receiving

window

）的字節(jié)號：收方希望接收的字節(jié)號（或者報(bào)文號）集合接收窗口大小Wr＝允許 接收的最大字節(jié)數(shù)（即接收緩存大?。┳止?jié)號不在發(fā)送窗口的數(shù)據(jù)不能發(fā)送字節(jié)號不在接收窗口的數(shù)據(jù)不能接收通過設(shè)置窗滑動窗口：發(fā)送窗口與接收窗口口大小控制發(fā)送數(shù)據(jù)量發(fā)送窗口與接收窗口接收窗口，rwnd=12020/133發(fā)送窗口24發(fā)送窗口——滑動且大小變化！發(fā)送窗口大小=min(rwnd,

cwnd)rwnd：接收窗口cwnd：擁塞窗口滑動（sliding）——TCP允許隨時改變滑動窗口的大小！擴(kuò)展（expanding）：右沿右移允許發(fā)送 的字節(jié)縮回（shrinking）：右沿左移回退N協(xié)議關(guān)閉/合攏（closing）：左沿右移某些字節(jié)已被確認(rèn)，移出窗口2020/11/23TCP/IP

ProtocolSuite35TCP中的滑動窗口既實(shí)現(xiàn)高效的可靠傳輸，又提供了流量控制高效可靠傳輸：發(fā)方在等待確認(rèn)之前，可以發(fā)送多個分組流量控制：端到端：接收方控制發(fā)送窗口的大小→控制發(fā)送速率擁塞控制：路由器超載→擁塞→TCP減小發(fā)送窗口→控制發(fā)送速率6ExampleFigure

shows

an

unrealistic

example

of

a

sliding

window.

Thesender

has

sent

bytes

up

to

202.

We

assume

that

cwnd

is20

(in

reality

this

value

is

thousands

of

bytes).

The

receiverhas

sent

an

acknowledgment

number

of

200

with

anrwnd

of9

bytes

(in

reality

this

value

is

thousands

of

bytes).

The

sizeof

the

sender

window

is

the

minimum

of

rwnd

and

cwnd

or

9bytes.

Bytes

200

to

202

are

sent,

but

not

acknowledged.Bytes

203

to

208

can

be

sent

without

worrying

aboutacknowledgment.

Bytes

209

and

above

cannot

be

sent.Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/2337FlowControl（流量控制）TCP采用的流控方法：控制發(fā)送窗口的大小發(fā)送方根據(jù)接收方反饋的信息控制發(fā)送窗口的操作關(guān)鍵：發(fā)送窗口大小（WS）??

接收方 窗口(advertisementwindow)的大?。╮wnd）rwnd：接收方允許發(fā)送方連續(xù)發(fā)送的數(shù)據(jù)量，也就等于接收方空閑緩存的大小?TCP/IP

Protocol

Suite2020/11/2338Sender

Buffer

&

Sender

WindowSender

Windowcan

be

sentimmedia

ysent,

notacknowledgednext

byteto

be

sentWs?TCP/IP

Protocol

Suite2020/11/2339TCP/IP

ProtocolSuiteReceiver

WindowReceiver

bufferTo

store

TCP

segments

in

order

（緩存亂序到達(dá)的報(bào)文段，向上層提供順序數(shù)據(jù)）接收方

窗口的大小(rwnd)＝Wr

－

已占用的緩存字節(jié)數(shù)Consider:

Howtoprocess

an

out-of-order

TCP

segment? (

acceptornot)next

byteto

be

receivedRFC

1122:

section0and

1Receiver

WindowTo

store

TCP

segments

in

orderReceiverSeg(1901)

)

ACK(19)2)?TCP/IP

Protocol

Suite2020/11/2341Expanding

the

senderwindow（發(fā)送窗口的擴(kuò)展）The

receiving

process

consumes

datafaster

than it

receives→

rwnd

??→將rwnd通知發(fā)送方（反饋）→發(fā)送方擴(kuò)展Ws

（Ws

＝

rwnd）0Sender02

3

45Receiver

Wr＝10rwnd＝48ACK6，rwnd=801Ws＝＝84?TCP/IP

Protocol

Suite2020/11/2342Shrinking

the

sender

window（發(fā)送窗口的縮?。?Sender02

3

45The

receiving

process

consumes

data

slowerthan

it

receives→

rwnd

??→將rwnd通知發(fā)送方（反饋）→發(fā)送方收縮為Ws

＝

rwndReceiver

Wr＝10rwnd＝34ACK7，rwnd=301Ws＝＝346?TCP/IP

Protocol

Suite2020/11/2343Closing

thesenderwindow（發(fā)送窗口的關(guān)閉）The

receiver

buffer

is

totally

full→

rwnd＝0→將rwnd通知發(fā)送方（反饋）→發(fā)送方關(guān)閉窗口（窗口左邊＝右邊），停止數(shù)據(jù)的發(fā)送Q：When

to

star

ding??TCP/IP

Protocol

Suite2020/11/2344Window

Management1000

3000Buffer4000SenderReceiver?TCP/IP

Protocol

Suite2020/11/2345TCP的糊涂窗口綜合癥SillyWindowSyndrome，SWS（糊涂窗口綜合癥）RFC

813SymptomTosend data

in

very

small

segments,

whichreduces

the

efficiency

of

the

operationExample:

1

byte

of

data,40

bytes

of

TCPand

IPheadersCauseThe

sending

application

program

creates

dataslowlyThe

receiving

application

program

consumesdataslowlyor

both?TCP/IP

Protocol

Suite2020/11/2346發(fā)送方SWS解決方案發(fā)送方SWS：原因：發(fā)送進(jìn)程數(shù)據(jù)產(chǎn)生緩慢現(xiàn)象：發(fā)端TCP逐個字節(jié)地發(fā)送TCP報(bào)文段解決思路：避免TCP逐個字節(jié)發(fā)送發(fā)送方延遲發(fā)送，收集應(yīng)用程序的發(fā)送數(shù)據(jù)， 合理的數(shù)據(jù)量關(guān)鍵問題：延遲時間如何確定延遲長：通信延遲增加延遲短：產(chǎn)生短的TCP報(bào)文段延遲策略：Nagle’s

algorithm發(fā)送方TCP發(fā)送第一塊從應(yīng)用進(jìn)程收到的數(shù)據(jù)發(fā)送方等待ACK，并且累積數(shù)據(jù)，發(fā)送方發(fā)送條件：數(shù)據(jù)累積到可以封裝成最大長度的報(bào)文段或者收到上一報(bào)文段的ACK?TCP/IP

Protocol

Suite2020/11/2347接收方SWS解決方案接收方SWS：原因：接收進(jìn)程數(shù)據(jù)消耗緩慢現(xiàn)象：發(fā)端TCP逐個字節(jié)地發(fā)送TCP報(bào)文段解決思路：Clark解決方案：目前接收窗口大?。┙邮辗绞盏綀?bào)文段就進(jìn)行確認(rèn)（如果空閑緩存空間：小于1個報(bào)文段最大長度或者小于緩存空間的1/2接收方

：目前接收窗口＝0延遲確認(rèn)：發(fā)送方延遲發(fā)送ACK，直到有足夠空閑緩存空間?TCP/IP

Protocol

Suite2020/11/2348Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/2349Error

Control目標(biāo)：接收應(yīng)用程序從TCP收到的數(shù)據(jù)流是按序的、無差錯的、無丟失、無重復(fù)的差錯控制分兩步：Detecting

and

correctingCorrupted

segmentsLost

segmentsOut-of-order

segmentsDuplicatedsegmentsTCP中實(shí)現(xiàn)差錯控制的三種工具Checksum：用來檢查受損報(bào)文Acknowledgment:

positive

and

cumulativeTime-out:

retransmissionHow

to

detect

?How

to

correct??TCP/IP

Protocol

Suite2020/11/2350確認(rèn)ACK報(bào)文段不消耗序號，也不被確認(rèn)什么時候產(chǎn)生ACK報(bào)文段（規(guī)則1）捎帶確認(rèn)：發(fā)送數(shù)據(jù)報(bào)文段時——減少ACK報(bào)文段（規(guī)則2）推遲確認(rèn)：接收端無數(shù)據(jù)發(fā)送，按序收到數(shù)據(jù)，且前一個報(bào)文段已確認(rèn)——減少ACK報(bào)文段立即確認(rèn)：（規(guī)則3）接收端無數(shù)據(jù)發(fā)送，按序收到數(shù)據(jù)，且前一個報(bào)文段未確認(rèn)——避免不必要的重傳（規(guī)則4）當(dāng)收到失序報(bào)文段——快速重傳（規(guī)則5）當(dāng)丟失的報(bào)文段到達(dá)（規(guī)則6）重復(fù)報(bào)文段到達(dá)——解決ACK丟失?TCP/IP

Protocol

Suite2020/11/2351確認(rèn)類型累計(jì)確認(rèn)（ACK）表示希望收到的下一個字節(jié)的序號并對該字節(jié)以前的所有字節(jié)的累計(jì)確認(rèn)選擇確認(rèn)（SACK）報(bào)告失序，重復(fù)的報(bào)文段?TCP/IP

Protocol

Suite2020/11/2352重傳差錯控制的 ：重傳報(bào)文段當(dāng)報(bào)文段受損、丟失時，就需要重傳什么時候重傳？重傳超時（RTO）定時器超時發(fā)送端連續(xù)收到3個重復(fù)的ACK重傳超時定時器超時發(fā)送方就認(rèn)為：對應(yīng)報(bào)文段受損或者丟失收到3個重復(fù)的ACK發(fā)送方就認(rèn)為：某個報(bào)文丟失，且收方收到大量亂序報(bào)文快速重傳?TCP/IP

Protocol

Suite2020/11/2353Normal

operation?TCP/IP

Protocol

Suite2020/11/2354Corrupted

Segment（報(bào)文段受損）Sender

ReceiverSegment3corruptedOK

OKTime-outOK2020/11/23TimeTime?TCP/IP

Protocol

Suite55Lost

SegmentSenderReceiverSegment3

lostOK

OKTime-outOK2020/11/23TimeTime?TCP/IP

Protocol

Suite56Lost

Segment?TCP/IP

Protocol

Suite2020/11/2357Duplicate

Segment（重復(fù)報(bào)文段）CauseWhen

the

acknowledgment

does

not

arrive

beforethetime-out重復(fù)報(bào)文段解決方案：the

destination

TCP

（接收方TCP）檢測出重復(fù)報(bào)文段Sequence

numberSimply

discard

the

packetReceiver124524924假設(shè)，TCP報(bào)文長為400ByteSeg(524A)

CK(1324)?TCP/IP

Protocol

Suite582020/11/23Out-of-Order

Segment（失序報(bào)文段）CauseTCP

uses

the

services

of

IP,

an

unreliable,connectionless

network

layer

service解決方案：the

destination

TCP

（接收方TCP）檢測出失序的報(bào)文段:sequence

numberCorrectingTCP

does

NOT

acknowledge

an

out-of-order

segmentuntil

it

receives

all

of

the

segments

that

precede

itIf

the

acknowledgment

is

delay,

the

timer

of

the

out-of-order

segment

may

mature

at

the

source

TCP

andthe

segment

may

be

resent

——

duplicate

segmentSeg(1910)

)

ACK(19)2)Receiver?TCP/IP

Protocol

Suite2020/11/2359206Fast

retransmissionLost

AcknowledgmentSender

ReceiverTimeTimeAcknowledgmentlostOK

OK

OK?TCP/IPProtocol

Suite2020/11/2361Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/2362Congestion

Control（擁塞控制）Congestion：網(wǎng)絡(luò)中，路由器接收過多的分組，超過其處理能力時，發(fā)生擁塞

→

分組被丟棄Some

packets

could

be

dropped

by

the

router→

no

ack

is

sent

from

the

destination→TCP發(fā)送端會重傳超時定時器會超時→

the

sender

retransmits

the

lost

packetTo

create

more

congestion

and

more

drop→

more

retransmission

and

more

congestionFinally,

the

whole

system

collapses結(jié)論：擁塞引起的重傳，會使情況更糟?TCP/IP

Protocol

Suite2020/11/2363Capacity（網(wǎng)絡(luò)容量）：網(wǎng)絡(luò)能處理的分組數(shù)Throughput（吞吐率）：單位時間內(nèi)通過網(wǎng)絡(luò)的分組數(shù)Packet

delay,

ThroughputVS.networkload?TCP/IP

Protocol

Suite2020/11/2364Router

queues?TCP/IP

Protocol

Suite2020/11/2365Congestion

Control端點(diǎn)（主機(jī)）上的TCP是無法準(zhǔn)確知道網(wǎng)絡(luò)因何原因或者在何處發(fā)生擁塞擁塞對于端點(diǎn)而言，表現(xiàn)為：TCP傳輸延遲增加，導(dǎo)致超時重傳TCP應(yīng)該在網(wǎng)絡(luò)出現(xiàn)擁塞時，必須減慢發(fā)送速率（或停止傳輸）→

擁塞控制問題：TCP如何發(fā)現(xiàn)網(wǎng)絡(luò)的擁塞呢？TCP

assumes（關(guān)鍵假設(shè)）The

causeof

a

lost

segment

is

dueto

congestion

in

thenetwork問題：發(fā)現(xiàn)擁塞如何處理？——控制發(fā)送速率In

flowcontrol，發(fā)送窗口由接收方直接控制Sender

window

=

Receiver

window網(wǎng)絡(luò)中擁塞，發(fā)送窗口還要取決于網(wǎng)絡(luò)的擁塞情況?TCP/IP

Protocol

Suite2020/11/2366Congestion

Window（擁塞窗口）Sender

window

=

Min

{rwnd,

cwnd}rwnd:

receiver’s

advertised

window

（接收方 窗口）——areceiver-sidelimit——流量控制cwnd:congestion

window（擁塞窗口）

——asender-side

limit（反映了網(wǎng)絡(luò)的擁塞狀況）——擁塞控制?TCP/IP

Protocol

Suite2020/11/2367小結(jié)：TCP中的幾個窗口接收方

窗口（

rwnd

）：接收方通過反饋告訴發(fā)送方的允許傳輸?shù)臄?shù)據(jù)量（單位：可以是字節(jié)，也可以是報(bào)文段）擁塞窗口（

cwnd

）：發(fā)送方為了進(jìn)行擁塞控制而限制自己傳輸?shù)臄?shù)據(jù)量實(shí)際發(fā)送窗口的大?。組in

{rwnd,cwnd}哪個窗口值小，那種控制就起作用擁塞窗口如何確定——擁塞策略?TCP/IP

Protocol

Suite2020/11/2368Congestion

ControlAlgorithmsSlow

start（慢啟動）

congestion

avoidance（擁塞避免）以及擁塞檢測慢啟動：指數(shù)增大擁塞避免：加法增大擁塞檢測：乘法減小Fast

retransmit/Fast

recovery（快速重傳和快速恢復(fù)）To

detect

and

repairloss

based

on ing

duplicate

ACKsTCP

assumes3

or

more

duplicate

ACKs

→

a

segment

has

beenlost參考文獻(xiàn)Van

Jacobson，Congestion

Avoidance

and

Control，

M88RFC2001，TCP

Slow

Start,

Congestion

Avoidance,

Fast

Retransmit,and

Fast

Recovery

AlgorithmsRFC

2581，TCP

Congestion

ControlWilliam

Stallings，高速網(wǎng)絡(luò)與互聯(lián)網(wǎng)絡(luò)——性能與服務(wù)質(zhì)量（第二版），第12章TCP通信量控制，電子工業(yè)

，2003?TCP/IP

Protocol

Suite2020/11/2369Slow

start（慢啟動）建立一條新的TCP連接以后，執(zhí)行慢啟動：初始狀態(tài)cwnd＝1（1個最大報(bào)文段MSS）發(fā)送

允許發(fā)送1個TCP報(bào)文段（因?yàn)榘l(fā)送窗口＝1）等待ACK每收到1個ACK，發(fā)送窗口向右滑動1個報(bào)文段的長度cwnd＋1

→發(fā)送方連續(xù)發(fā)送2個報(bào)文段0直到發(fā)送窗口大?。╓s）＝ssthresh（慢啟動閾值）慢啟動過程實(shí)際上是發(fā)送TCP在TCP連接建立初期試探網(wǎng)絡(luò)的擁塞狀況，以避免因一次發(fā)送過多的報(bào)文段而引起網(wǎng)絡(luò)的擁塞sender0ACK1

ACK2

ACK3注：慢啟動實(shí)際并不慢，每過1個RTT，cwnd就翻倍，呈2的指數(shù)增長?TCP/IP

Protocol

Suite2020/11/2370Congestion

avoidance（擁塞避免）在慢啟動階段，擁塞窗口以指數(shù)形式增長，要避免發(fā)生擁塞，當(dāng)擁塞窗口達(dá)到某個值后，必須減慢擁塞窗口的增長規(guī)律當(dāng)發(fā)送窗口大小＝ssthresh時，發(fā)送方進(jìn)入擁塞避免階段在這個階段中，只有當(dāng)窗口中所有報(bào)確認(rèn)后，擁塞窗口＋1實(shí)質(zhì)上：每收到1個ACK，cwnd＝cwnd＋1/cwnd當(dāng)RTT較大時，可以認(rèn)為是每隔一個RTT，擁塞窗口cwnd＋1→加性增加（Addictive

Increase）擁塞避免直到出現(xiàn)重傳超時定時器超時，即發(fā)生擁塞?TCP/IP

Protocol

Suite2020/11/2371Congestion

avoidance?TCP/IP

Protocol

Suite2020/11/2372擁塞檢測推測擁塞：超時重傳ssthresh（慢啟動閾值）設(shè)置為當(dāng)前cwnd（擁塞窗口）的一半即ssthresh

＝

[cwnd

/2]cwnd（擁塞窗口）=1重新執(zhí)行慢啟動0sender0cwnd＝

16

ssthresh＝

83?TCP/IP

Protocol

Suite2020/11/2373快速重傳和快速恢復(fù)如果不是超時，而是連續(xù)收到3個ACK，處理方式是：ssthresh（慢啟動閾值）設(shè)置為當(dāng)前cwnd（擁塞窗口）的一半cwnd＝當(dāng)前閾值開始擁塞避免階段為什么這樣做？連續(xù)收到3個ACK，說明1個報(bào)文段可能丟失了，但后面的幾個報(bào)文段又安全到達(dá)?TCP/IP

Protocol

Suite2020/11/2374TCP擁塞策略小結(jié)?TCP/IP

Protocol

Suite2020/11/2375擁塞舉例Number

of

transmitted

segmentsCongestion

window

size(

insegments)SlowstartCongestionavoidance?TCP/IP

Protocol

Suite2020/11/2376Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/2377TCPTimerRetransmission

timer（重傳超時定時器）The

waiting

time

for

an

ack

of

a

segmentTo

control

a

lost

or

discarded

segmentPersistencetimer（持久）To

deal

with

the

zero

window-size

advertisementThe

waiting

time

for

an

ack

with

a

non-zero

window

sizeKeepalive

timer（保活）To

prevent

a

long

idle

connection

between

two

TCPThe

waiting

time

for

some

data

from

a

clientTime-waited

timer（時間等待）To

be

used

during

connection

termination?TCP/IP

Protocol

Suite2020/11/2378Retransmission

Timer功能：處理重傳超時發(fā)送方可以重傳報(bào)文段→某些報(bào)文段在傳輸過程中可能丟失或者被丟棄（

lost

or

discarded

）The

waiting

time for

an

ack

of

asegmentUsage（如何使用重傳超時定時器）When

TCPsends

a

segment,

it

creates

aretransmission

timer

for

that

segmentIf

anackis

received

for

that segment

beforetime-out,

the

timer

is

destroyedOtherwise

the

segment

is

retransmitted

and

thetimer

is

reset關(guān)鍵問題：重傳超時（RTO）時間如何確定？?TCP/IP

Protocol

Suite2020/11/2379重傳超時時間RTT

(Round-Trip

Time)：報(bào)文段傳輸?shù)耐禃r延從發(fā)送一個報(bào)文到收到這個報(bào)文的時間重傳超時時間取值一般是基于RTT的例如（一種簡單算法）：Retransmission

timer=2

x

RTTInternet的上的RTT不同TCP連接具有各自不同的路徑長度，時延差異大同一個TCP連接，發(fā)送方從發(fā)送報(bào)文段到收到確認(rèn)的時間隨網(wǎng)絡(luò)的擁塞狀況而改變分組經(jīng)過路由器，路由器產(chǎn)生的時延取決于通信量結(jié)論：分組的傳輸時延無法預(yù)先確定→重傳超時時間無法事先 ，是動態(tài)變化的?TCP/IP

Protocol

Suite2020/11/2380Internet上的100個Packet的往返傳輸時間圖u2020/11/2381RTT的測量In

TCP,

there

can

be

only

be

one

RTTmeasurement

inprogress

at

any

time——RTTM(M：measure)TCP報(bào)文段和它的確認(rèn)并非1－1對應(yīng)某個TCP報(bào)文段的RTT是該報(bào)文段到達(dá)終點(diǎn)并被確認(rèn)所需的時間，雖然這個確認(rèn)可能包含對其它報(bào)文段的確認(rèn)平滑RTTS——Internet中測量的RTTM起伏大，需要平滑——

平均初始：沒有數(shù)值第一次測量后：RTTS=RTTM其它任何測量后：RTTS

=

(

1-

α)

x

RTTS

+

αx

RTTM,

α

=1/8?TCP/IP

Protocol

Suite2020/11/2382RTT偏差（RTTD）初始：沒有數(shù)值第一次測量后：

RTTD

=

RTTM

/

2其它任何測量后：RTTD

=

(

1-β)

x

RTTD

+β|RTTS-RTTM|Β一般取值為1/4?TCP/IP

Protocol

Suite2020/11/2383重傳超時RTO的計(jì)算初值：初始值其它任何次測量后：RTO =

RTTS

+

4×RTTDRTT偏差（RTTD）?TCP/IP

Protocol

Suite2020/11/2384Karn’s

algorithm解決的問題：發(fā)送方無法準(zhǔn)確判斷在啟動重傳后收到ACK所確認(rèn)的報(bào)文段（原始？重

傳？）不對重傳的報(bào)文段更新RTT但需要采用時延補(bǔ)償技術(shù)（針對重傳）——指數(shù)退避Internetworking

with

TCP/IP

Volume

1:Principles,

Protocols,

and

Architectures,Chapter

13?TCP/IP

Protocol

Suite2020/11/23852020/11/C

/I o

oco Sui

e86指數(shù)退避原則：每產(chǎn)生一次重傳，RTO數(shù)值就加倍2020/11/23TCP/IP

Protocol

Suite87Persistence

Timer（持久定時器）功能：To

deal

with

the

loss

of

non-zero

window

size非零窗口值advertisement當(dāng)發(fā)送方窗口關(guān)閉后，直到收方發(fā)送一個確認(rèn)來如果該確認(rèn)丟失，雙方將陷入死鎖UsageWhen

the

sending

TCP

receives

anack

with

a

window

size

ofzero,

it

starts

a

persistence

timerIf

an

ack

with

anon-zerowindow

size

is

not

received

from

thereceiver

before

time-out,

then

senda

probe

segment

（發(fā)送探測報(bào)文段）探測報(bào)文段有序號，但不需要確認(rèn)The

value

of

the

timer

is

set

to

the

value

of

the

retransmissiontimerThe

sender

continues

sending

the

probe

segment

and

doublingand

resetting

the

value

of

the

timer

until

the

value

reaches

athreshold

(usually

60

s)TCP中，不對ACK確認(rèn)Keepalive

Timer功能：To

prevent

a

long

idle

connectionbetween

two

TCPUsageEach

time

the

server

hears

from

a

client,

itresets

this

timerThe

time-out

is

usually

2

hoursIf the

server

does

not

hear

from

the

clientafter

2

hours,

i ds

a

probesegmentIf

there

is

no

response

after

10

probe,

each

ofwhich

is

75s

apart,

then the

server

assumesthat

the

client is

down

and

terminates

theconnection?TCP/IP

Protocol

Suite2020/11/2388Time-waited

TimerUsageWhen

TCP

closes

a

connection,itdoesnot

consider

the

connection

really

closed.The

connection

is

held

in

limbo

（過渡期）for

a

time-waitedperiodThe

value

for

this

timer

is

usually

2times

the

expected

lifetime

of

a

segment( um

Segment

Lifetime,MSL)2

minutes:

an

engineering

choice?TCP/IP

Protocol

Suite2020/11/2389Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/2390Segment（0

~

40

bytes）控制字段Segment差錯控制以字節(jié)為單位流控同UDP標(biāo)識應(yīng)用進(jìn)程Initial

sequence

number?TCP/IP

Protocol

Suite2020/11/2391初始序號（ISN）在建立連接時，通信雙方的TCP各自隨機(jī)產(chǎn)生初始序號例：假設(shè)站點(diǎn)A的TCP向站點(diǎn)B發(fā)起建立連接，其ISN＝2367，意味著：A的第1個TCP報(bào)文段的序號為2367第1個TCP報(bào)文段用于建立連接，數(shù)據(jù)長度＝0A的第2個TCP報(bào)文段的序號為2368第2個TCP報(bào)文段用于建立連接，數(shù)據(jù)長度＝0A的第3個TCP報(bào)文段的序號為2368，第3個TCP報(bào)文段攜帶數(shù)據(jù)，長度≠0問題：假設(shè)第3個TCP報(bào)文段數(shù)據(jù)長度＝1000，那么A的第4個TCP報(bào)文段的序號是多少？2368＋1000＝3368TCP中，建立連接階段的控制報(bào)文段有序號，但數(shù)據(jù)部分為0?TCP/IP

Protocol

Suite2020/11/2392Control

Field控制域（6bit）用于：FlowcontrolConnection

establishment

and

terminationThe

mode

of

data

transfer

in

TCPURG

ACK

PSH

RST

SYN

FINURG:

Urgent

pointer

is

validRST:

Reset

the

connectionACK:

Acknowledgment

is

validSYN:

Synchronize

sequence

numbersPSH:

Request

for

pushFIN:

Terminate

the

connection?TCP/IP

Protocol

Suite2020/11/23932020/11/2394校驗(yàn)和2020/11/23TCP/IP

ProtocolSuite95緊急指針與緊急（帶外）數(shù)據(jù)緊急數(shù)據(jù)：不需要接收進(jìn)程按序緊急指針：只有當(dāng)控制域中的URG置位時，才有效指向帶外數(shù)據(jù)的最后一個字節(jié)帶外數(shù)據(jù)（out-of-band

data，urgent

data）發(fā)送端TCP立即創(chuàng)建報(bào)文段帶外數(shù)據(jù)位于數(shù)據(jù)域的首部接收端TCP：不在數(shù)據(jù)流中排隊(duì)，直接遞交上層提供快速傳遞數(shù)據(jù)的功能UR

GTCPHeaderTCP

Data帶外(緊急)數(shù)據(jù)2020/11/23TCP/IP

ProtocolSuite96Option（

）選項(xiàng)Option單字節(jié)Single-byte多字節(jié)Multiple-byte無操作No

operation最大報(bào)文段長度Max

segmentsize選項(xiàng)結(jié)束End

of

option窗口比例因子Window

scale

factor時間戳TimestampRFC

1323Chapter

10

TCPOverviewTCP

servicesWindow

based

control

ProtocolFlow

controlErrorcontrolTCPtimersCongestion

controlSegmentConnectionTCPoperationTCPpackage?TCP/IP

Protocol

Suite2020/11/2397ConnectionConnection連接由1對端點(diǎn)標(biāo)識:

{(IP1，port1),

(IP2，port2)}連接數(shù)量受機(jī)器內(nèi)存容量限制一個機(jī)器上某個TCP端 可被多個連接共享面向連接的傳輸包括以下幾個操作：連接建立連接拆除連接復(fù)位TCPTCPServerClient

2App3 App2TCPApp1Client

1App2App1 App

3?TCP/IP

Protocol

Suite2020/11/23982020/11/23TCP/IP

Protocol

Suite99連接建立（Connection

Establishment）Active

open（主動打開）發(fā)起建立連接，ClientPassive

open（ 打開）接受連接，Server

打開方一般先于主動打開方啟動，但不發(fā)起連接建立過程，而是在某個端口上等待主動方的連接請求——服務(wù)器TCP連接的任意一方都可以主動打開連接3-way

HandshakingClient

ServerWaits

for

apassive

openRequests

forAn

active

open包含初始序號(同步序號)，不包括確認(rèn)號，也沒定義窗口對第一次握手進(jìn)行確認(rèn)，同時定義

窗口確認(rèn)第二次握手，定義

窗口；可以在第三個報(bào)文段中發(fā)送數(shù)據(jù)D an

be

sent

wi