计算机网络 运输层

Chapter3：TransportLayer

Ourgoalsgoals：Overview：

runderstandprinciplesrtransportlayerservices

behindtransportlayerrmultiplexing/demultiplexing

services：

rconnectionlesstransport：UDP

HAmultiplexing/demultiplex

rprinciplesofreliabledata

ing

transfer

HAreliabledatatransfer

rconnection-orientedtransport：

侬flowcontrol

TCP

2congestioncontrol

mreliabletransfer

rinstantiationand

mflowcontrol

implementationinthe

Internetmconnectionmanagement

rprinciplesofcongestioncontrol

rTCPcongestioncontrol

3：TransportLayer3-1

Transportservices

Jication

datam

rprovidelogicalcommunicationphysic<

betweenapp'processes

runningondifferenthosts

mRelyonnetworklayer

rProvidemultiplexing/de

multiplexingservices

rReliabilitycontrolication

networl

加Errorcheckingdatalink

physical

HARetansmittingthelostsegments

3：TransportLayer3-2

Transport-layerprotocolsCTCP.UDP)

Internettransportservices：

rreliable,in-orderunicast

delivery(TCP)

mcongestioncontrol

mflowcontrol

mconnectionmanagement

,,

runreliableC'best-effort)/

unorderedunicastor

multicastdelivery：UDP

rBothuseerrorchecking

includedintheirheaders

rservicesnotavailable：

mreal-time

mbandwidthguarantees

mreliablemulticast

3：TransportLayer3-3

Multiplexing/demultiplexing

segment-unitofdata

Demultiplexing：delivering

exchangedbetween

receivedsegmentsto

transportlayerentitiescorrectapplayerprocesses

KATPDU：transport

protocoldataunit

receiver

3：TransportLayer3-4

Multiplexing/demultiplexing

-Multiplexing：------------------------------

gatheringdatafrommultiple<-------------32bits----------------►

appprocesses,enveloping

datawithheadertocreatesourceport#destport#

segmentsandpassingthesegments

toIPlayerotherheaderfields

multiplexing/demultiplexing：

rbasedonsender,receiverportapplication

numbers,IPaddressesdata

msource,destinationport#sin(message)

eachsegment

KArecall：well-knownportnumbers

forspecificapplicationsTCP/UDPsegmentformat

(http：80zf+p21,telnet23)

3：TransportLayer3-5

Multiplexing/demultiplexing：examples

Differentprocessesusedifferentsourceportinaclient

Webclient

hostC

sourceport：x

hostAdest.port：23

SourceIP：CSourceIP：C

.sourceport：23DestIP：BbestIP：B

destport：xsourceport：ysourceport：x

dest.port：80dest.port：80

portuse：simpletelnetapp

5QU"2IP：A

DestIP：BWeb

Webclientsourceport：xserverB

hostA

portuse：Webserver

3：TransportLayer3-6

UDPmu什iplexing/damultiplexing

rPortnumber：1024^65535

rTypically,portnumberofclientapplicationmaybe

assignedautomatically;serverportnumberis

specified

3：TransportLayer3-7

TCPmultiplexing/demultiplexing

rFour-tupleneeded

恒sourceIPaddr,destinationipaddr.

SourcePortnumber,destinationportnumber

rEveryclientapplicationwithportnumberdirectstoasocket

onserver

3：TransportLayer3-8

UDP：UserDatagramProtocol[RFC7681

r''nofrills/1''barebones"Internettransport

protocol

mMultiplexing/demultimplexing

mError-checking

MTheapplicationisalmostdirectlytalkingwithIP

r''besteffort1'service,UbPsegmentsmaybe：

mloSt

^deliveredoutofordertoapp

rconnectionless：

mnohandshakingbetweenUbPsender,receiver

n^eachUbPsegmenthandledindependentlyof

others

3：TransportLayer3-9

UDPApplications

FeaturesofUbPTypicallyapplications

rsimple：noconnectionstateRemotefileserver(NFS)

atsender,receiverStreamingmultimedia

mNobuffers.noccparameters

Internettelephony

恒Nosn,noacknumber

Networkmanagement

rsmallsegmentheader(8

Bytes)Routingprotocol(RIP)

rFast：nocontrols：Nametranslation(bNS)

mUbPcanblastawayasfastasMulticasting

desired,besteffort

Real-timeinvolved

rnoconnectionestablishmentapps(RTP)

TFTP

3：TransportLayer3-10

UDPsegmerrTstructure(RFC768)

r8bytes<-------------32bits----------------►

header

Length,insourceport#destport#

SourceportbytesofUDPj**lengthchecksum

segment,

^Destinationincluding

portheader

mLength

Application

mchecksumdata

rDatafield(message)

UDPsegmentformat

3：TransportLayer3-11

UDPchecksum

Goal：detect''errors”(e.g.,flippedbits)intransmitted

segment

Receiver：

Sender：rcomputechecksumof

receivedsegment

rtreatsegmentcontents

assequenceof16-bitrcheckifcomputedchecksum

integersequalschecksumfieldvalue：

rchecksum：addition(l'smANO-errordetected

complementsum)of恒YES-noerrordetected

segmentcontentsmBut,doesnotcorrectit

rNegatingBit

rsenderputschecksum

valueintoUDPchecksum

field

3：TransportLayer3-12

Exampleforchecksum

rSender：rReceiver：

加316-b汁words：MEallwordsandchecksum：

作0110011001100110

恒+0101010101010101rn0110011001100110

〃0000111100001111加+0101010101010101

rn0000111100001111

馆11001010110010103001101010011010(

恒OG玲1:m1111111111111111

rn0011010100110101

rIfthesum<>alllzmeans

HAputthisvalueintoatleastanerroroccurs

checksumfield

3：TransportLayer3-13

PrinciplesofReliabledatatransfer

rMaybeimplementedinapp.,transport,linklayers

rcharacteristicsofunreliablechannelwilldetermine

complexityofreliabledatatransferprotocol(rdt)

u

o

_

-

o

ounreliabledatatransferprotocol(udt)

=sending]

doreceiver

dprocessIQroce8I

o|data||data|

IrdJsend()

J)relioblechannel)eliverdata()

O行

dreliabledata

s>reliabledata

uotransferprotocoltransferprotocol

o(sendingside)(receivingside)

占

udtsend()fIpacket|

|pack。"Jkrdtrcv()

nrelioblechonnel)」

(a)providedservice(b)serviceimplementation

3：TransportLayer3-14

Buildingreliabledatatransferprotocol

deliverjdata():calledby

rdttodeliverdatatoupper

|datal^deliverdata()

reliabledatareceiving

transferprotocol

[receivingside)side

3：TransportLayer3-15

Buildingreliabledatatransferprotocol

Wewillspecifytherdtfromsimplenessto

complexity

rincrementallydevelopsender,receiversidesof

reliabledatatransferprotocol(rdt)

rconsideronlyunidirectionaldatatransfer

mbutcontrolinfowillflowonbothdirections!

usefinitestatemachines(FSM)tospecify

av-ntcausingstatdtransition

sender,receiveractionstakenonstatetransition

state：wheninthis

"state"nextstate

uniquelydetermined

bynextevent

3：TransportLayer3-16

Rdtl.O：reliabletransferoverareliablechannel

underlyingchannelperfectlyreliable

mnobiterrors

mnolossofpackets

separateFSAAsforsender,receiver：

Msendersendsdataintounderlyingchannel

mreceiverreaddatafromunderlyingchannel

UnidirectionalchanneLnofeedbackisneeded

'VWaitfor^\rdt_send(data)'VWaitforVXrdt_rcv(packet)

(callfrom))_(callfrom)jextract(packet,data)

abovepacket=makepkt(data)belowdeliver_data(data)

udt_send(packe^

senderreceiver

3：TransportLayer3-17

Rdt2.0：channelwithbiterrors

runderlyingchannelmayflipbitsinpacket

出Candetectbutcannotcorrect：checksum

rthequestion：howtorecoverfromerrors：

恒acknowledgements(ACKs)：receiverexplic汁lytellssender

thatpktreceivedOK

negativeacknowledgements(NAKs)：receiverexplic汁ly

tellssenderthatpkthaderrors

加senderretransmitspktonreceiptofNAK

加1b汁longdatamaybesufficientsforNAKand1forACK

rnewmechanismsinrdt2・0:

加errordetection

mreceiverfeedback：controlmsgs(ACK,NAK)rcvr->sender

出Retransmission

ThisistheknownARQ-automaticRepeatrequest!

3：TransportLayer3-18

rdt2,Q:FSAAspecification

rdtsend(data)

sndpkt=make_pkt(data,checksum)receiver

udt_send(sndpkt)

rdt_rcv(rcvpkt)&&

isNAK(rcvpkt)

rdt_rcv(rcvpkt)&&

udt_send(sndpk^corrupt(rcvpkt)

udt_send(NAK)

rdtrcv(rcvpkt)&&isACK(rcvpkt)

A

udt_re-send(sndpk5

sender

rdt_rcv(rcvpkt)&&

notcorrupt(rcvpkt)

extract(revpkt,data)

deliver_data(data)

udt_send(ACK)

3：TransportLayer3-19

rdt2.0：operationwithnoerrors

rdt_rcv(rcvpkt)&&

corrupt(rcvpkt)

udt_send(NAK)

&rdtrcv(rcvpkp&&

notcorrupt(rcvpkt)

extract(revpkt,data)

deliver_data(data)

udtsend(ACK)

3：TransportLayer3-20

rdt2.0：errorscenario

rdtsend(data)

snkpkt=make_pkt(data,checksum)

udtsend(sndpkl)

is

WaitforWaitforrdt_rcv(rcvp峪)&&

cafromACKorudt_send(sndpk^corrupt(rcvpkt)

aboveNAK

udtsend(NAK)

rdt_rcv(rcvpkt)&&isACK(rcvpkt)

Waitfor

Acafrom

beow

rdtrcv(rcvpk,&&

notcorrupt(rcvpkt)

extract(revpkt,data)

deliver_data(data)

udtsend(ACK)

3：TransportLayer3-21

rdt2.0hasafatalflaw!

Whathappensif

ACK/NAKcorrupted?Handlingduplicates：

rsenderdoesn'tknowwhatrsenderaddssequence

happenedatreceiver!numbertoeachpkt

rcan'tjustretransmit：rsenderretransmitscurrent

possibleduplicatepktifACK/NAKgarbled

rreceiverdiscards(doesrit

Whattodo?deliverup)duplicatepkt

rsenderACKs/NAKs

receiversACK/NAK?WhatThisisrd+2.1

ifsenderACK/NAKlost?一stopandwait-------------------

rretransmit,butthismightSendersendsonepacket,

causeretransmissionofthenwa汁sforreceiver

correctlyreceivedpkt!response

rErrorcorrectionforbit

error

3：TransportLayer3-22

rdt2.1:sender,handlesgarbledACK/NAKs

rdt_send(data)

sndpkt=make_pkt(0,data,checksum)

)

udt_send(sndpktrdt_rcv(rcvpkt)&&

(corrupt(rcvpkt)||

WaitforWaitforisNAK(rcvpkt))

ACKor

call0fron

NAKOudt_send(sndpkt)

<above/

rdt_rcv(rcvpkt)

&¬corrupt(rcvpkl)rdt_rcv(rcvpkt)

&&isACK(rcvpkt)&¬corrupt(rcvpkt)

&&isACK(rcvpkt)

A

A

WaitforWaitfor

ACKorca1from

rdt_rcv(rcvpkt)&&NAK1bove

(corrupt(rcvpkt)

isNAK(rcvpkt))rdt_send(data)

udt_send(sndpkt)sndpkt=make_pkt(1,data,checksum)

udt_send(sndpk^

3：TransportLayer3-23

rdt2.1:receiver,handlesgarbledACK/NAKs

rdt_rcv(rcvpk1)&¬corrupt(rcvpkl)

&&hasseqO(scvpkt)

rdt_rcv(rcvpkt)&&extract(rcvpkt,data)

corrupt(rcvpkt)

deliverdata(data)/,..oo

sndpkt=makepktO(ACK,chksum)r-「cv(r：vp)

sndpkt=make_pktO(NAK,chksumgnd(snd血c°:「upt(「cvpkt)

udt_send(sndpk^

sndpkt=make_pkt1(NAK,chksum)

udt_send(sndpk^

rdt_rcv(rcvpkt)&&

notcorrupt(rcvpk^&&rdt_rcv(rcvpkt)&&

hasseql(rcvpkt)notcorrupt(rcvpkt)&&

hasseqO(rcvpkt)

sndpkt=make_pkt1(ACK,chksum)

udt_send(sndpk1)sndpkt=make_pktO(ACK,chksum)

udt_send(sndpk^

rdt_rcv(rcvpk^&¬corrupt(rcvpkt)

&&hasseql(rcvpkt)

Discardduplication!

Discardduplication!extract(rcvpkt,dat^

deliver_data(data)

sndpkt=make_pkt1(ACK,chksum)_

udt_send(sndpkt)3：TransportLayer3-24

rdt2.1:discussion

Sender：Receiver：

rseq#addedtopktrmustcheckifreceived

rtwoseq.#'s(0,1)willpacketisduplicate

suffice.Why?恒stateindicateswhether

0or1isexpectedpkt

rmustcheckifreceivedseq#

ACK/NAKcorrupted

rnote：receivercannot

rtwiceasmanystatesknowifitslast

0

恒statemust''rememberACK/NAKreceivedOK

whether"current"pkt

has0or1seq.#atsender

3：TransportLayer3-25

rdt2.2：aNAK-freeprotocol

rsamefunctionalityasrd+2.1,using

ACKsonly

rinsteadofNAKZreceiversendsACK

forlastpktreceivedOK

•receivermustexplicitlyincludeseq#of

pktbeingACKed

rduplicateACKatsenderresultsinsame

actionasNAK：retransmitcurrentpkt

3：TransportLayer3-26

rd+2.2:sender

rdt_send(data)

sndpkt=make_pkt(0,data,checksum)

udt_send(sndpkt)

rdt_rcv(rcvpkt)&&

(corrupt(rcvpk。||

5Waitfor

WaitforisACKfrcvpktJ))

call0fromACK

above>0udt_send(sndpk1)

senderFSM

rdt_rcv(rcvpkt)rdt_rcv(rcvpkt)

&¬corrupt(rcvpk^&¬corrupt(rcvpk^

&&JsACK(ircypkt/1)&&isACK(rcvpkt,O)

A

WaitforA

Waitfor

ca1from

rdt_rcv(rcvpkt)&&ACK

above

(corrupt(rcvpkt)||

isACK(rcvpkt,O))rdtsend(data)

udt_send(sndpkt)sndpkt=make_pkt(1,data,checksum)

udt_send(sndpkt)

3：TransportLayer3-27

rdt2.2：receiver

rdt_rcv(rcvpkt)&¬corrupt(rcvpk^

&&hasseqO(rcvpkt)

extract(rcvpkt,data)

deliver_data(data)rdt_rcv(rcvpkt)&&

rdt_rcv(rcvpkt)&&sndpkT=make_pkt(ACKO,chksum)corrupt(rcvpkt)

notcorrupt(rcvpkt)&&udt_send(sndpk^udt_send(sndpk^

hasseql(rcvpkt)WaitforO

Waitfor

udt_send(sndpk^receiverFSM1fromrdt_rcv(rcvpkt)&&

0from<below/_)notcorrupt(rcvpkl)&&

beowhas_seqO(rcvpkt)

rdt_rcv(rcvpkt)&¬corrupt(rcvpkt)的

rdt_rcv(rcvpkt)&&&&hasseq1(rcvpkt)udt_send(sndpkt)

corrupt(rcvpkt)

udt_send(sndpk^extract(rcvpkt,data)

deliver_data(data)

sndpkt=make_pkt(ACK1,chksum)

udt_send(sndpk。

3：TransportLayer3-28

rdt3.0：channelswitherrorsandloss

Nawassumption：Approach：senderwaits

underlyingchannelcan“reasonable"amountof

alsolosepackets(datatimeforACK

orACKs)rretransmitsifnoACK

kwchecksum,seq.#,ACKs,receivedinthistime

retransmissionswillberifpkt(orACK)isjustdelayed

ofhelp,butnotenough(notlost)：

Q：howtodealwithloss?HAretransmissionwillbe

duplicate,butuseofseq.

Msenderwa汁suntil

#'salreadyhandlesthis

certaindataorACK

lost,thenretransmits加receivermustspecifyseq

#ofpktbeingACKed

恒Howlongtimeis

needed?rrequirescountdowntimer

3：TransportLayer3-29

rdt3.0sender

rdt_send(data)

rdt_rcv(rcvpkt)&&

Ack(rcvpkt,1)

卜sndpkt=make_pkt(0,data,checksum)(corrupt(rcvpkt)||

\udt_send(sndpkt)isACK(rcvpkt,1))

rdtrcv(rcvpkt)start_timerA

AWait

Waitfortimeout

callOfromfor

udt_send(sndpkt)

<aboveyACKO

start_timer

rdt_rcv(rcvpkt)

&¬corrupt(rcvpkt)rdt_rcv(rcvpkt)

&&isACK(rcvpkt,1)&¬corrupt(rcvpkt)

stop_timer&&isACK(rcvpktQ)

stop_timer

WaitWaitfor

timeoutforca1from

udt_send(sndpkt)CLACK1above

start_timerrdtrcv(rcvpkt)

rdt_send(data)A

rdt_rcv(rcvpkt)&&

(corrupt(rcvpkt)||sndpkt=make_pkt(1,data,checksum)

isACK(rcvpkt,O))udt_send(sndpkt)Ack(rcvpkt,O)

starttimer

A

3：TransportLayer3-30

Rdt3.0receiver

rdt_rcv(rcvpkt)&¬corrupt(rcvpkt)

&&has_seqO(rcvpkt)

extract(rcvpkt,data)

deliver_data(data)

sndpkt=make_pkt(ACKO,chksum)

udt_send(sndpkt)

rdt_rcv(rcvpkt)&&rdt_rcv(rcvpkt)&&

(corrupt(rcvpkt)||(corrupt(rcvpkt)||

WaitforhasseqO(rcvpkt))

waitfo、receiverFSM

0from1from

udt_send(sndpkt)below.belowyudt_send(sndpkt)

rdt_rcv(rcvpkt)&¬corrupt(rcvpkt)

&&hasseq1(rcvpkt)

extract(rcvpkt,dat^

deliver_data(data)

sndpkt=make_pkt(ACK1ychksum)

udt_send(sndpk^3：TransportLayer3-31

rdt3.0inaction

senderreceiver

sendpktO

revpktO

sendACKO

revACKO

sendpktl

(a)operationwithnoloss

(b)lostpacket

3：TransportLayer3-32

rdt3.0inaction

(c)lostACK