P663

TransportProtocols

2

其它的網絡系統結構

?其它網絡體系結構

-ARPANET

—SNA(SystemNetworkArchitecture)

-DNA(DECNET)

-TCP/IP

其它的網絡系統結構

ISO/OSITCP/1PARPASNADNA

應用層用戶層端用戶端用戶

表示層功能管理服務網絡應用

應用文件傳輸協議

會話層遠程通信協議數據流控制會話控制

傳輸層TCP/UDPHOST-HOST傳輸控制網絡服務

源IMP-目的IMP

路由控制

網絡層IP通路傳輸控制

控制傳輸組

IMP-IMP

數據鏈路層數據鏈路數據鏈路控制

數據鏈路控制

物理層物理層物理層物理層物理層

P602

IPv6

?Expandedaddressspace

?Improvedoptionmechanism

?Addressautoconfiguration

?Increasedaddressingflexibility

?Supportforresourceallocation

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?Hop-by-HopOptionheader

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?Authenticationheader

?EncapsulatingSecurityPayloadheader

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P604

IPv6Header

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TCP/IP協議族圖示

H/WDeviceDriversandMediaAccessProtocols

TCP/IPOSI

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Space

Session

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8

Figure16.1InternetworkingProtocolsContext9

InternetworkingProtocolsinContext

10

TCP

AbbreviationofTransmissionControlProtocol,andpronouncedasseparate

letters.TCPisoneofthemainprotocolsinTCP/IPnetworks.WhereastheIP

protocoldealsonlywithpackets,TCPenablestwohoststoestablisha

connectionandexchangestreamsofdata.TCPguaranteesdeliveryofdataand

alsoguaranteesthatpacketswillbedeliveredinthesameorderinwhichthey

weresent.

TCPstandsforTransmissionControlProtocol.ItisdescribedinSTD-7/RFC-793.

TCPisaconnection-orientedprotocolthatisresponsibleforreliable

communicationbetweentwoendprocesses.Theunitofdatatransferrediscalled

astream,whichissimplyasequenceofbytes.

Beingconnection-orientedmeansthatbeforeactuallytransmittingdata,youmust

opentheconnectionbetweenthetwoendpoints.Thedatacanbetransferredin

fullduplex(sendandreceiveonasingleconnection).Whenthetransferisdone,

youhavetoclosetheconnectiontofreesystemresources.Bothendsknowwhen

thesessionisopened(begin)andisclosed(end).Thedatatransfercannottake

placebeforebothendshaveagreedupontheconnection.Theconnectioncanbe

closedbyeitherside;theotherisnotified.Provisionismadetoclosegracefullyor

justaborttheconnection.

Beingstreamorientedmeansthatthedataisananonymoussequenceofbytes.

Thereisnothingtomakedataboundariesapparent.Thereceiverhasnomeansof

knowinghowthedatawasactuallytransmitted.Thesendercansendmanysmall

datachunksandthereceiverreceiveonlyonebigchunk,orthesendercansenda

bigchunk,thereceiverreceivingitinanumberofsmallerchunks.Theonlything

thatisguaranteedisthatalldatasentwillbereceivedwithoutanyerrorandin

thecorrectorder.Shouldanyerroroccur,itwillautomaticallybecorrected

(retransmittedasneeded)ortheerrorwillbenotifiedifitcan'tbecorrected.

Attheprogramlevel,theTCPstreamlooklikeaflatfile.Whenyouwrite

datatoaflatfile,andreaditbacklater,youareabsolutelyunableto

knowifthedatahasbeenwritteninonlyonechunkorinseveralchunks.

Unlessyouwritesomethingspecialtoidentifyrecordboundaries,thereis

nothingyoucandotolearnitafterward.Youcan,forexample,useCRor

CRLFtodelimityourrecordsjustlikeaflattextfile.

Attheprogramminglevel,TWSocketisfairlysimpletouse.Tosenddata,

youjustneedtocalltheSendmethod(oranyvariationsuchasSendStr)

togivethedatatobetransmitted.TWSocketwillputitinabufferuntilit

canbeactuallytransmitted.Eventuallythedatawillbesentinthe

background(theSendmethodreturnsimmediatelywithoutwaitingforthe

datatobetransmitted)andtheOnDataSenteventwillbegeneratedonce

thebufferisemptied.

Toreceivedata,aprogrammustwaituntilitreceivestheOnDataAvailable

event.Thiseventistriggeredeachtimeadatapacketcomesfromthe

lowerlevel.TheapplicationmustcalltheReceivemethodtoactuallyget

thedatafromthelow-levelbuffers.YouhavetoReceiveallthedata

availableoryourprogramwillgoinanendlessloopbecauseTWSocket

willtriggertheOnDataAvailableagainifyoudidn'tReceiveallthedata.

Asthedataisastreamofbytes,yourapplicationmustbepreparedto

receivedataassentfromthesender,fragmentedinseveralchunksor

mergedinbiggerchunks.Forexample,ifthesendersent"Hello"and

then"World!",itispossibletogetonlyoneOnDataAvailableeventand

receive"HelloWorld!"inonechunk,ortogettwoevents,onefor"Hello"

andtheotherfor"World!".Youcanevenreceivemoresmallerchunkslike

"Hei","Iowo"and"rid!".Whathappensdependsontrafficload,router

algorithms,randomerrorsandmanyotherparametersyoucan'tcontrol.

Onthesubjectofclient/serverapplications,mostapplicationsneedto

knowcommandboundariesbeforebeingabletoprocessdata.Asdata

boundariesarenotalwayspreserved,youcannotsupposeyourserverwill

receiveasinglecompletecommandinoneOnDataAvailableevent.You

canreceiveonlypartofarequestormaybetwoormorerequestmerged

inonechunk.Toovercomethisdifficulty,youmustusedelimiters.

MostTCP/IPprotocols,likeSMTP,POP3,FTPandothers,useCR/LFpair

ascommanddelimiter.EachclientrequestissentasiswithaCR/LFpair

appended.Theserverreceivesthedataasitarrives,assemblesitina

receivebuffer,scansforCR/LFpairstoextractcommandsfromthe

receivedstream,andremovesthemfromthereceivebuffer.

UDP

ShortforUserDatagramProtocol,aconnectionlessprotocolthat,like

TCP,runsontopofIPnetworks.UnlikeTCP/IP,UDP/IPprovidesvery

fewerrorrecoveryservices,offeringinsteadadirectwaytosendand

receivedatagramsoveranIPnetwork.It'susedprimarilyfor

broadcastingmessagesoveranetwork.

UDPstandsforUserDatagramProtocol.ItisdescribedinSTD-6/RFC-

768andprovidesaconnectionlesshost-to-hostcommunicationpath.

UDPhasminimaloverhead:;eachpacketonthenetworkiscomposedof

asmallheaderanduserdata.ItiscalledaUDPdatagram.

UDPpreservesdatagramboundariesbetweenthesenderandthe

receiver.Itmeansthatthereceiversocketwillreceivean

OnDataAvailableeventforeachdatagramsentandtheReceivemethod

willreturnacompletedatagramforeachcall.Ifthebufferistoosmall,

thedatagramwillbetruncated.Ifthebufferistoolarge,onlyone

datagramisreturned,theremainingbufferspaceisnottouched.

UDPisconnectionless.Itmeansthatadatagramcanbesentatany

momentwithoutprioradvertising,negotiationorpreparation.Justsend

thedatagramandhopethereceiverisabletohandleit.

UDPisanunreliableprotocol.Thereisabsolutelynoguaranteethatthe

datagramwillbedeliveredtothedestinationhost.Buttobehonest,the

failurerateisverylowontheInternetandnearlynullonaLANunlessthe

bandwidthisfull.

Notonlythedatagramcanbeundelivered,butitcanbedeliveredinan

incorrectorder.Itmeansyoucanreceiveapacketbeforeanotherone,

evenifthesecondhasbeensentbeforethefirstyoujustreceived.Youcan

alsoreceivethesamepackettwice.

Yourapplicationmustbepreparedtohandleallthosesituations:missing

datagram,duplicatedatagramordatagramintheincorrectorder.Youmust

programerrordetectionandcorrection.Forexample,ifyouneedto

transfersomefile,you'dbettersetupakindofzmodemprotocol.

ThemainadvantagesforUDParethatdatagramboundariesarerespected,

youcanbroadcast,anditisfast.

Themaindisadvantageisunreliabilityandthereforecomplicatedto

programattheapplicationlevel.

ADDRESSING

TCPandUDPusethesameaddressingscheme.AnIPaddress(32bits

number,alwayswrittenasfour8-bitnumberexpressedasunsigned3-digit

decimalnumbersseparatedbydotssuchas6)andaport

number(a16-bitnumberexpressedasaunsigneddecimalnumber).

TheIPaddressisusedbythelow-levelprotocol(IP)toroutethedatagram

tothecorrecthostonthespecifiednetwork.Thentheportnumberisused

toroutethedatagramtothecorrecthostprocess(aprogramonthehost).

Foragivenprotocol(TCPorUDP),asinglehostprocesscanexistatatime

toreceivedatasenttothegivenport.Usuallyoneportisdedicatedtoone

process.

tcpvs.udpforfiletransfer

ithinktcpisanoverusedprotocol;ithinkudpisanunderusedprotocol,

thisisanargumenti'vebeenhavingquiteabitwithpeoplelately,soi've

decidedi'lllayoutmyreasoningheresoidon'thavetotypeorreciteitat

peopleoverandover.

sinceifirstwrotethis(jan1998orso),tcpextensionsthatfixmanyofmy

complaints(selectiveacknowledgment,largewindows,tcpfortransactions)

haveseenmorewidespreadimplementation,whiletheyareastepinthe

rightdirection(well,t/tcpwasaterriblesecurityblunder...),tcpwillalways

beastreamprotocol,andthuswillneverbeanoptimaltransportforsome

advantagesoftcp

?theoperatingsystemdoesallthework,youjustsitbackandwatchthe

show.noneedtohavethesamebugsinyourcodethateveryoneelsedid

ontheirfirsttry;it'sallbeenfiguredoutforyou.

?sinceit'sintheos,handlingincomingpacketshasfewercontext

switchesfromkerneltouserspaceandback;allthereassembly,acking,

flowcontrol,etcisdonebythekernel.

?tcpguaranteesthreethings:thatyourdatagetsthere,thatitgetsthere

inorder,andthatitgetstherewithoutduplication.(thetruth,thewhole

truth,andnothingbutthetruth...)

?routersmaynoticetcppacketsandtreatthemspecially.theycanbuffer

andretransmitthem,andinlimitedcasespreackthem.

?tcphasgoodrelativethroughputonamodemoraIan.

disadvantagesoftcp

?theoperatingsystemmaybebuggy,andyoucan'tescapeit.itmaybe

inefficient,andyouhavetoputupwithit.itmaybeoptimizedfor

conditionsotherthantheonesyouarefacing,andyoumaynotbeableto

retuneit.

?tcpmakesitverydifficulttotryharder;youcansetafewsocket

options,butbeyondthatyouhavetotoleratethebuiltinflowcontrol.

?tcpmayhavelotsoffeaturesyoudon'tneed,itmaywastebandwidth,

time,oreffortonensuringthingsthatareirrelevanttothetaskathand.

?tcphasnoblockboundaries;youmustcreateyourown.

?routersontheinternettodayareoutofmemory,theycan'tpaymuch

attentiontotcpflyingby,andtrytohelpit.designassumptionsoftcp

breakdowninthisenvironment.

?tcphasrelativelypoorthroughputonalossy,highbandwidth,high

latencylink,suchasasatelliteconnectionoranoverfullt1.

?tcpcannotbeusedforbroadcastormulticasttransmission.

?tcpcannotconcludeatransmissionwithoutalldatainmotionbeing

explicitlyacked.

disadvantagesofudp

?therearenoguaranteeswithudp.apacketmaynotbedelivered,or

deliveredtwice,ordeliveredoutoforder;yougetnoindicationofthis

unlessthelisteningprogramattheotherenddecidestosaysomething,

tcpisreallyworkinginthesameenvironment;yougetroughlythesame

servicesfromipandudp.however,tcpmakesupforitfairlywell,andina

standardizedmanner.

?udphasnoflowcontrol.implementationisthedutyofuserprograms.

?routersarequitecarelesswithudp.theyneverretransmititifitcollides,

anditseemstobethefirstthingdroppedwhenarouterisshorton

memory,udpsuffersfromworsepacketlossthantcp.

advantagesofudp

?itdoesn'trestrictyoutoaconnectionbasedcommunicationmodel,so

startuplatencyindistributedapplicationsismuchlower,asisoperating

systemoverhead.

?allflowcontrol,acking,transactionlogging,etcisuptouserprograms;a

brokenosimplementationisnotgoingtogetinyourway.additionally,

youonlyneedtoimplementandusethefeaturesyouneed.

?therecipientofudppacketsgetsthemunmangled,includingblock

boundaries.

?broadcastandmulticasttransmissionareavailablewithudp.19

disadvantagesoftcpforfiletransfer

?startuplatencyissignificant,ittakesatleasttwicertttostartgetting

databack.

?tcpallowsawindowofatmost64k,andtheackingmechanismmeans

thatpacketlossismisdetected,tcpstallseasilyunderpacketloss,tcpis

morethrottledbyrttthanbandwidth.

?tcptransferservershavetomaintainaseparatesocket(andoften

separatethread)foreachclient.

?loadbalancingiscrudeandapproximate,especiallyonlocalnetworks

thatallowcollisions,twosimultaneoustcptransfershaveatendencyto

fightwitheachother,evenifthesenderisthesame.

advantagesofudpforfiletransfer

?latencycanbeaslowasrttiftheprotocolissuitablydesigned.

?flowcontrolisuptouserspace;windowscanbeinfinite,artificialstalls

nonexistant,latencywelltolerated,andmaximumspeedsenforcedonly

byrealnetworkbandwidth,yetactualspeedschosenbyagreementof

senderandreceiver.

?receivinganimagesimultaneouslyfrommultiplehostsismucheasier

withudp,asissendingonetomultiplehosts,especiallyiftheyhappento

bepartofthesamebroadcastormulticastgroup.

asinglesendinghostwithmultipletransfersproceedingcanbalance20

themwithexcellentprecision.

TheInternetrunsonahierarchicalprotocolstack.Asimplifiedversionof

thisisshowninfigure1.ThelayercommontoallInternetapplicationsis

theIP(InternetProtocol)layer.Thislayerprovidesaconnectionless,

unreliablepacketbaseddeliveryservice.Itcanbedescribedas

connectionlessbecausepacketsaretreatedindependentlyofallothers.

Theserviceisunreliablebecausethereisnoguaranteeofdelivery.

Packetsmaybesilentlydropped,duplicatedordelayedandmayarrive

outoforder.Theserviceisalsocalledabesteffortservice,allattemptsto

deliverapacketwillbemade,withunreliabilityonlycausedbyhardware

faultsorexhaustedresources.

AsthereisnosenseofaconnectionattheIPleveltherearenosimple

methodstoprovideaqualityofservice(QoS).QoSisarequestfroman

applicationtothenetworktoprovideaguaranteeonthequalityofa

connection.Thisallowsanapplicationtorequestafixedamountof

bandwidthfromthenetwork,andassumeitwillbeprovided,oncethe

QoSrequesthasbeenaccepted.Alsoafixeddelay,i.e.nojitterandin

orderdeliverycanbeassumed.AnetworkthatsupportsQoSwillbe

protectedfromcongestionproblems,asthenetworkwillrefuse

connectionsthatrequestlargerresourcesthancanbesupplied.An

exampleofanetworkthatsupportsQoSisthecurrenttelephonenetwork,

whereeverycallisguaranteedthebandwidthforthecall.Mostusersat

somepointhaveheardtheoverloadedsignalwherethenetworkcannot21

providetherequestedresourcerequiredtomakeacall.

Theapplicationdecideswhichtransportprotocolisused.Thetwo

protocolsshownhere,TCPandUDParethemostcommonlyusedones.

TCPprovidesareliableconnectionandisusedbythemajorityofcurrent

Internetapplications.TCP,besidesbeingresponsibleforerrorchecking

andcorrecting,isalsoresponsibleforcontrollingthespeedatwhichthis

dataissent.TCPiscapableofdetectingcongestioninthenetworkand

willbackofftransmissionspeedwhencongestionoccurs.Thesefeatures

protectthenetworkfromcongestioncollapse.

Asdiscussedintheintroduction,VolPisareal-timeservice.Forreal-time

propertiestobeguaranteedtobemet,anetworkwithQoSmustbeused

toprovidefixeddelayandbandwidth.IthasalreadybeensaidthatIP

cannotprovidethis.Thisthenpresentsachoice.IfIPisarequirement,

whichtransportlayershouldbeusedtoprovideasystemthatismost

likelytomeetreal-timeconstraints.

AsTCPprovidesfeaturessuchascongestioncontrol,itwouldbethe

preferredprotocoltouse.UnfortunatelyduetothefactthatTCPisa

reliableservice,delayswillbeintroducedwheneverabiterrororpacket

lossoccurs.Thisdelayiscausedbyretransmissionofthebrokenpacket,

alongwithanysuccessivepacketsthatmayhavealreadybeensent.This

canbealargesourceofjitter.22

TCPusesacombinationoffouralgorithmstoprovidecongestioncontrol,

slowstart,congestionavoidance,fastretransmitandfastrecovery.These

algorithmsallusepacketlossasanindicationofcongestion,andallalter

thenumberofpacketsTCPwillsendbeforewaitingforacknowledgments

ofthosepackets.Thesealterationsaffectthebandwidthavailableandalso

changedelaysseenonalink,providinganothersourceofjitter.

ApplicationWWWFTPE-mailNFSVoIPDNS

TuaiuportTCPUDP

NetuaockIP

Plwskal

4TEthernetAAL-5HDLC

SimplifiedIPprotocolstack

23

Combined,TCPraisesjittertoanunacceptablelevelrenderingTCP

unusableforreal-timeservices.Voicecommunicationhastheadvantage

ofnotrequiringacompletelyreliabletransportlevel.Thelossofapacket

orbiterrorwilloftenonlyintroduceaclickoraminorbreakintothe

output.

ForthesereasonsmostVoIPapplicationsuseUDPforthevoicedata

transmission.UDPisathinlayerontopofIPthatprovidesawayto

distinguishamongmultipleprogramsrunningonasinglemachine.UDP

alsoinheritsallofthepropertiesofIPthatTCPattemptstohide.UDPis

thereforealsoapacketbased,connectionless,best-effortservice.Itisup

totheapplicationtosplitdataintopackets,andprovideanynecessary

errorcheckingthatisrequired.

Becauseofthis,UDPallowsthefastestandmostsimplewayof

transmittingdatatothereceiver.Thereisnointerferenceinthestream

ofdatathatcanbepossiblyavoided.Thisprovidesthewayforan

applicationtogetasclosetomeetingreal-timeconstraintsaspossible.

UDPhoweverprovidesnocongestioncontrolsystems.Acongestedlink

thatisonlyrunningTCPwillbeapproximatelyfairtoallusers.WhenUDP

dataisintroducedintothislink,thereisnorequirementfortheUDPdata

ratestobackoff,forcingtheremainingTCPconnectionstobackoffeven

further.ThiscanbethoughofasUDPdatanotbeinga“goodcitizen”.

Theaimofthisprojectistocharacterisethequantityofthisdropoffin24

TCPperformance.

TCPUDP

?Connection-Oriented?Connectionless

?Reliabilityindeliveryof?Noattempttofragment

messagesmessages

?Splittingmessagesinto?Noreassemblyand

datagramsynchronization

?Keeptrackoforder(or?Incaseoferror,messageis

sequence)retransmitted

?Noacknowledgment

?Usechecksumsfordetecting

errors

?Reliabilityisamust?Serverandclientmessagesfit

completelywithinapacket

?MessagesexceedUDPpacket?Theserverhandlesmultiple

sizeclients(UDPisstateless)

P664

ApplicationprogramApplicationprogram

(Process)(Process)

DomainofTCPprotocol

26

P664

Socket:(hostIP,port)

TELNETTELNET

(Client)(Server)

27

P665

ConnectionOrientedTransportProtocolMechanisms

?ConnectionOrientedMechanisms

-Logicalconnection

—Establishment

—Maintenance

—Termination

-Reliable

-e.g.TCP

?Datalink:X.25,LAPF,LLCofaLAN

-Addressing

-Multiplexing

-Flowcontrol

-Connectionestablishment/termination

?ReliableSequencingNetworkService

使用端口號標識FTP連接和Web連接

0007E942AC280007E963CE53192.168.1,5192.168.1,7130521UserdataCRC

MACDAMACSAIPSAIPDAS-PortD-Port

FTPConnection

FTP

FTPServerPort:21FTPClientPort:1305

S

D

WebServerPort:80￡5WebClientPort:1089

DestinationSource

0007E942AC28WEB0007E963CE53

WEBConnection

0007E942AC280007E963CE53192.168.1,5192.168.1,7108980UserdataCRC

MACDAMACSAIPSAIPDAS-PortD-Port

29

P665

ReliableSequencingNetworkService

?Assumearbitrarylengthmessage

?Assumevirtually100%reliabledeliveryby

networkservice

一e.g.reliablepacketswitchednetworkusingX.25

—e.g.framerelayusingLAPFcontrolprotocol

—e.g.IEEE802.3usingconnectionorientedLLC

service

?Transportserviceisendtoendprotocolbetween

twosystemsonsamenetwork

30

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