tutorial 1-schema

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AnIntroductionToInternetProgrammingWithSocketWrench.NET


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AnIntroductionToInternetProgramming Page2

IntroductionTheTransmissionControlProtocol/InternetProtocol(TCP/IP)isthecommonlanguagespokenbyevery

deviceandapplicationthatcommunicatesovertheInternet.Asadeveloperwhoisinterestedin

integratingInternetfunctionalityintoyourownsoftware,itisimportanttounderstandthe

fundamentalsofhowTCP/IPworks.Thistutorialwillexplainthebasicconceptsbehindnetwork

programming,andhelpgetyoustarteddevelopingyourfirstInternetapplication.

TheexamplesincludedinthistutorialaregearedprimarilytowardsVisualBasicdevelopers;however,

thegeneralconceptscanbeappliedtoany.NETprogramminglanguage.Toworkwiththeseexamples,

youshouldhaveVisualStudio2005andSocketWrench.NETinstalled.SocketWrenchisthegeneral

purposeTCP/IPnetworkingcomponentthatweusethroughoutourexamples.

SocketWrenchisalsopartofalargercollectionofInternetcomponentsinourSocketToolsfamilyof

products.WithSocketTools,youcaneasilytakeadvantageoffeaturessuchasuploadingand

downloadingfiles,sendingemailmessages,managingyourInboxonyourmailserver,submittingdata

toaweb

server,

executing

remote

commands

and

much

more.

While

SocketWrench

makes

it

easy

to

createyourowncustomclientandserverapplications,SocketToolsfurthersimplifiesapplication

developmentbyenablingyoutoincorporatehighlevelInternetfunctionalityinyoursoftwarewithjust

afewlinesofcode.MoreinformationandafreeevaluationcopyofbothSocketWrenchandSocketTools

isavailableontheCatalystDevelopmentwebsiteatwww.catalyst.com

WindowsSocketsAPITheWindowsSocketsspecificationwascreatedbyagroupofcompanies,includingMicrosoft,inan

efforttostandardizetheTCP/IPsuiteofprotocolsunderWindows.PriortoWindowsSockets,each

vendor

developed

their

own

proprietary

libraries,

and

although

they

all

had

similar

functionality,

the

differencesweresignificantenoughtocauseproblemsforthesoftwaredeveloperswhousedthem.

Afterchoosingtouseaspecificvendor'slibrary,thedeveloperwaslockedintothatparticular

implementationbecauseaprogramwrittenagainstonevendor'sproductwouldnotworkwith

another's.WindowsSocketswasofferedasasolution,leavingdevelopersandtheirendusersfreeto

chooseanyvendor'sTCP/IPimplementationwiththeassurancethattheproductwouldcontinueto

work.Today,thereareafewthirdpartyTCP/IPproductsavailablefortheWindowsplatform;however,

mostsystemsusetheMicrosoftTCP/IPlibrariesthatareincludedaspartofthebaseoperatingsystem

andithasbecomethestandardnetworkinginterfaceontheWindowsplatform.

SocketWrenchisacomponentthatusestheWindowsSocketsAPItosimplifythedevelopmentof

Internetapplications.Itoffersahigherlevelinterface,enablingthedevelopertosetproperties,invoke

methodsandcreateeventhandlerstorespondtonetworkevents.InprogramminglanguageslikeVisual

Basic,itprovidesamorenaturalprogramminginterfacethatavoidsmuchoftheerrorpronedrudgery

commonlyassociatedwithsocketsprogramming.BysimplyreferencingSocketWrenchinyourproject,

settingsomepropertiesandrespondingtoevents,youcanquicklyandeasilywriteanInternetenabled

application.However,beforewegetstartedwithSocketWrench,we'llcoverthebasicterminologyand

conceptsbehindsocketsprogrammingingeneral.

Copyright2007CatalystDevelopmentCorporation.Allrightsreserved.
http://www.catalyst.com/http://www.catalyst.com/http://www.catalyst.com/


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ProtocolStandardsInthecontextofInternetprogramming,aprotocolestablishesthe"rulesoftheroad"thateach

computermustfollowsothatallofthesystemsinthenetworkcanunderstandthedatabeing

exchangedbetweenthem.Therearetwogenerallevelsofprotocolsthataretypicallydiscussed.The

firstconsistsofthenetworkingprotocolsthatgovernhowtwoormorecomputersystemscommunicate

withoneanother.TwoexamplesoftheselowerlevelnetworkingprotocolswouldbeTCPandUDP,

whicharediscussedlaterinthistutorial.

Thesecondtypeofprotocolisonewhichdetermineshowapplicationsexchangeinformationand

performcertaintasks.AnexampleofthesehigherlevelprotocolswouldbeHTTPandFTP,whichare

usedtocommunicatewithwebandfileservers.Thesehigherlevelapplicationprotocolsthenusethe

lowerlevelnetworkingprotocolssuchasTCPtocommunicateovertheInternet.

MostInternetprotocols,includingtheprotocolsthatwediscussinthistutorial,aredescribedin

technicaldocumentscalledRequestforComments(RFCs).Thesedocumentsarepublishedthroughthe

InternetSociety,

an

international

organization

of

computer

scientists

and

engineers

that

promote

Internetstandards.EventuallysomeoftheseproposedInternetprotocolsareadoptedbytheInternet

EngineeringTaskForce(IETF),whichformalizesthemasstandardprotocols.

Anumberisusedtoreferencethestandardsdocumentforeachprotocol.Forexample,thestandards

documentfortheFileTransferProtocolisnumber959,andiscommonlyreferredtoasRFC959.Alistof

theavailableRFCdocumentsisavailableattheIETFwebsite:

http://www.ietf.org/

ItsimportanttokeepinmindthatRFCsaretechnicaldocumentsthatdescribetheimplementation

detailsof

aparticular

protocol,

and

are

not

meant

to

provide

an

overview

of

the

protocol

in

laymans

termsorexplainhowtouseaparticularprotocol.Forexample,RFC959describeshowFTPis

implemented,butitwillnotexplainhowtouseanapplicationtodownloadafile.Thelanguageusedin

RFCsalsotendstobeplatformneutral.Inotherwords,youwonttypicallyfindspecificimplementation

informationfortheWindowsoperatingsystem.Thosekindsofplatformspecificdetailsarelefttothe

programmer,andnoassumptionsaremadeabouttheunderlyinghardwareoroperatingsystem.

TheTransmissionControlProtocolWhentwocomputerswishtoexchangeinformationoveranetwork,thereareseveralcomponentsthat

must

be

in

place

before

the

data

can

actually

be

sent

and

received.

Of

course,

the

physical

hardware

mustexist,whichistypicallyeitheranetworkinterfacecard(NIC)oraserialcommunicationsportfor

dialupnetworkingconnections.Inadditiontothephysicalconnection,computersalsoneedtousea

protocolthatdefinestheparametersofthecommunicationbetweenthem.Oneofthemostpopular

protocolsinusetodayisTCP/IP,whichstandsforTransmissionControlProtocol/InternetProtocol.

Byconvention,TCP/IPisusedtorefertoasuiteofprotocols,allbasedontheInternetProtocol(IP).

Unlikeasinglelocalnetwork,whereeverysystemisdirectlyconnectedtoeachother,aninternetisa

http://www.ietf.org/http://www.ietf.org/http://www.ietf.org/


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collectionofnetworks,combinedintoasingle,virtualnetwork.TheInternetProtocolprovidesthe

meansbywhichanysystemonanynetworkcancommunicatewithanotheraseasilyasiftheywereon

thesamephysicalnetwork.Eachsystem,commonlyreferredtoasahost,isassignedauniquenumber

thatcanbeusedtoidentifyitoverthenetwork.ThemostcommonversionoftheInternetProtocolused

todayisIPv4(version4)whichusesa32bitvaluefortheaddress.Typically,thisaddressisbrokeninto

four8bit

numbers

separated

by

periods.

This

is

called

dot

notation,

and

looks

something

like

"192.43.19.64".Somepartsoftheaddressareusedtoidentifythenetworkthatthesystemisconnected

to,andtheremainderidentifiesthesystemitself.TherearethreeclassesofInternetaddresses,

generallyreferredtoasclassA,BandC.TheruleofthumbisthatclassAaddressesareassigned

toverylargenetworks,classBaddressesareassignedtomediumsizednetworks,andclassC

addressesareassignedtosmallernetworkswithlessthanapproximately250systems.Itshouldbe

notedthatthelatestversionoftheInternetProtocolisIPv6(version6),whichsupports128bit

addresses.However,atthispointIPv4isstillthemostwidelyusedversionoftheprotocolandtheuseof

IPv6remainslargelylimitedtoresearchandgovernmentalnetworks.Itisgenerallypredictedthatthe

widespreadadoptionofIPv6willbephasedinoverthenextfivetotenyears.

WhenasystemsendsdataoverthenetworkusingtheInternetProtocol,itissentindiscreteunitscalled

datagrams,alsocommonlyreferredtoaspackets.Adatagramconsistsofaheaderfollowedby

applicationdefineddata.Theheadercontainstheaddressinginformationthatisusedtodeliverthe

datagramtoitsdestination,muchlikeanenvelopeisusedtoaddressandcontainpostalmail.Andlike

postalmail,thereisnoguaranteethatadatagramwillactuallyarriveatitsdestination.Infact,

datagramsmaybelost,duplicatedordeliveredoutoforderduringtheirtravelsoverthenetwork.

Needlesstosay,thiskindofunreliabilitycancausealotofproblemsforsoftwaredevelopers.Whats

reallyneededisareliable,straightforwardwaytoexchangedatawithouthavingtoworryaboutlost

packets

or

jumbled

data.

Tofillthisneed,theTransmissionControlProtocol(TCP)wasdeveloped.BuiltontopofIP,TCPoffersa

reliable,fullduplexbytestreamthatmaybereadandwrittentoinafashionsimilartoreadingand

writingafile.Theadvantagestothisareobvious:theapplicationprogrammerdoesntneedtowrite

codetohandledroppedoroutoforderdatagrams,andinsteadcanfocusontheapplicationitself.And

becausethedataispresentedasastreamofbytes,existingcodecanbeeasilyadoptedandmodifiedto

useTCP.

BeforetwoprogramscanbegintoexchangedatausingTCP,theymustestablisha"connection"with

eachother.Thisisdonewithathreewayhandshakeinwhichbothsidesexchangepackets,establishing

theinitial

packet

sequence

numbers.

The

client

is

responsible

for

initiating

the

connection,

while

the

serversresponsibilityistowait,listenandrespondtoincomingconnections.



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Thefirststepisfortheclienttosendasynchronization(SYN)packettotheserverthatcontainsits

sequencenumber.Next,theserverrespondswithanacknowledgementandsynchronization(SYNACK)

packetthat

contains

its

sequence

number

in

response.

Finally,

the

client

sends

an

acknowledgement

(ACK)backtotheserver.Oncetheconnectionhasbeenestablished,bothsidesmaysendandreceive

datauntiltheconnectionisclosed.Thisseriesofstepsisimportantbecauseitensuresthatthebytesof

dataexchangedbetweentheclientandserverwillbereceivedinthesameordertheyweresent.

TCPisknownasastreamorientedprotocolbecausedataisexchangedbetweentheclientandserveras

astreamofbytes.WhileTCPwillguaranteethatthedatawillarriveintact,withthebytesreceivedinthe

sameorderthattheywerewritten,thereisnoguaranteethattheamountofdatareceivedinasingle

readoperationonthesocketwillmatchtheamountofdatawrittenbytheremotehost.

Forexample,consideraserverthatsendsdatatoaclientinfourseparateoperations,eachcontaining

1024bytes

of

data.

While

it

is

convenient

to

think

of

these

as

discrete

blocks

of

data,

TCP

considers

it

a

streamof4096bytes.Theclientmayreceivethatdatainasinglereadonthesocket,returningall4096

bytes.Alternatively,itmayreadthesocket,andonlyreceivethefirst1460bytes;subsequentreadsmay

returnanother1460bytes,followedbytheremaining1176bytes.Applicationswhichmakeassumptions

abouttheamountofdatatheycanreadorwriteinasingleoperationmayworkinsomeenvironments,

suchasonalocalnetwork,butfailonslowerconnections.

AgeneralruletousewhendesigninganapplicationusingTCPistoconsiderhowtheprogramwould

handlethesituationwherethereadoperationonlyreturnsasinglebyte.Iftheapplicationcancorrectly

handlethiskindofextremecase,thenitshouldfunctioncorrectlyevenunderadversenetwork

conditions.

Insomesituationsitmaybedesirabletodesigntheapplicationtoexchangeinformationasdiscrete

messagesorblocksofdata.Whilethisisn'tdirectlysupportedbyTCP,itcanbeimplementedontopof

thedatastream.Thereareseveralmethodsthatcanbeusedtoaccomplishthis,dependingonthe

requirementsoftheapplication:



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1. Exchangethedataasfixedlengthstructures.Thisisthesimplestapproach,andhasverylittleornooverhead.Theclientandservercaneitherusepredefinedvalues,ornegotiatethesizeofthe

datastructureswhentheconnectionisestablished.

2. Prefixvariablelengthdatastructureswiththenumberofbytesbeingsent.Thelengthvaluecould

be

expressed

either

as

anative

integer

value,

or

as

afixed

length

string

that

is

converted

toanumericvaluebytheapplication.Thisallowsthereceivertoreadthisfixedlengthvalue,

andthenusethatvaluetodeterminehowmanyadditionalbytesmustbereadtoobtainthe

completemessageordatastructure.

3. Prefixthedatawithauniquebyteorbytesequencethatwouldnormallynotbefoundinthedatastream.Thiswouldbefollowedbythedataitself,withacompletemessagereceivedwhen

anotheruniquebytesequenceisencountered.Alternatively,auniquebytesequencecouldbe

usedtoterminateamessage.ThisistheapproachthatmanyInternetapplicationprotocolsuse,

suchasFTP,SMTPandPOP3.Commandsaresentasoneormoreprintablecharacters,

terminatedwithacarriagereturn(CR)andlinefeed(LF)bytesequencethattellstheremote

hostthatacompletecommandhasbeenreceived.

4. Acombinationoftheabovemethods,usinguniquebytesequences.ThemessagelengthandevenadditionalinformationsuchasaCRC32checksumorMD5hashcanbeusedtovalidate

theintegrityofthedata.Thiswouldeffectivelycreatean"envelope"aroundthedatabeing

exchanged,andadditionalcheckscouldbemadetoensurethatthedatahasbeenreceivedand

processedcorrectly.

Regardlessofthemethodused,forbestperformanceitisrecommendedthattheapplicationbufferthe

datareceivedandthenprocessthedataoutofthatbuffer.Whenusingasynchronous(nonblocking)

sockets,the

application

should

read

all

of

the

data

available

on

the

socket,

typically

in

aloop

which

adds

thedatatothebufferandexitingtheloopwhenthereisnomoredataavailableatthattime.

TheUserDatagramProtocolUnlikeTCP,theUserDatagramProtocol(UDP)doesnotpresentdataasastreamofbytes,nordoesit

requirethatyouestablishaconnectionwithanotherprograminordertoexchangeinformation.Datais

exchangedindiscreteunitscalleddatagrams,whicharesimilartoIPdatagrams.Infact,theonly

featuresthatUDPoffersoverrawIPdatagramsareportnumbersandanoptionalchecksum.

UDPissometimesreferredtoasanunreliableprotocolbecausewhenaprogramsendsaUDPdatagram

overthenetwork,thereisnowayforittoknowthatitactuallyarrivedatitsdestination.Thismeans

thatthesenderandreceivermusttypicallyimplementtheirownacknowledgementprotocolontopof

UDP.MuchoftheworkthatTCPdoestransparently(suchasgeneratingchecksums,acknowledgingthe

receiptofpackets,retransmittinglostpacketsandsoon)mustbeperformedbytheapplicationitself.

WiththelimitationsofUDP,youmightwonderwhyitsusedatall.UDPhastheadvantageoverTCPin

twocriticalareas:speedandpacketoverhead.BecauseTCPisareliableprotocol,itgoesthroughgreat



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lengthstoinsuredataarrivesatitsdestinationintact,andasaresult,itexchangesafairlyhighnumber

ofpacketsoverthenetwork.UDPdoesnthavethisoverheadandisconsiderablyfasterthanTCP.In

situationswherespeedisparamount,orthenumberofpacketssentoverthenetworkmustbekepttoa

minimum,UDPisoftenthepreferredsolution.AcommonuseofUDPiswithvideoandaudiostreaming,

wherethedownsideofpotentialpacketlossisoutweighedbythebenefitsofafastertransferrateand

reducedbandwidth.

HostnamesBeforeanapplicationcanexchangedatawithanothercomputersystem,itmusthaveseveralessential

piecesofinformation.ThefirstistheInternetProtocoladdressoftheremotehost.Thisaddressis

typicallyexpressedeitherasitsnumericalvalueindotnotation,orbyalogicalnamecalledahostname.

Likeanaddressindotnotation,hostnamesaredividedintoseveralpiecesseparatedbyperiods,called

domains.Domainshaveahierarchicalstructure,wherethetopleveldomainsdefinethetypeof

organizationtowhichthatnetworkbelongs,andsubdomainsfurtheridentifythespecificnetwork.

Inthis

figure,

the

top

level

domains

are

"gov"

(government

agencies),

"com"

(commercial

organizations),"org"(organizations)and"net"(serviceproviders).Thefullyqualifieddomainnameis

specifiedbynamingthehostandeachparentsubdomainaboveit,separatingthemwithperiods.For

example,thefullyqualifieddomainnameforthe"jupiter"hostwouldbe"jupiter.catalyst.com".The

system"jupiter"ispartofthe"catalyst"domain(acompanyslocalnetwork),whichinturnispartofthe

"com"topleveldomain(adomainusedbyallcommercialenterprises).



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Inordertouseahostnameinsteadofadotaddresstoidentifyaspecificsystemornetwork,theremust

besomecorrelationbetweenthetwo.Thisisaccomplishedbyoneoftwomeans:alocalhosttableora

domainnameserver.AhosttableisatextfilethatliststheIPaddressofahost,followedbythenames

thatitsknownby.Typicallythisfileisnamedhostsandisfoundinthe

C:\Windows\System32\Drivers\Etcfolder.

Adomainnameserver,ontheotherhand,isasystemthatcanbepresentedwithahostnameandwill

returnthathostsIPaddress.Thisapproachisadvantageousbecausethehostinformationfortheentire

networkismaintainedinonecentralizedlocation,ratherthanbeingscatteredaboutoneveryhoston

thenetwork.

ServicePortsInadditiontotheIPaddressoftheremotesystem,anapplicationalsoneedstoknowhowtoaddress

thespecificprogramwithwhichitwishestocommunicate.Thisisaccomplishedbyspecifyingaservice

port,

a

16

bit

number

that

uniquely

identifies

an

application

running

on

the

system.

Instead

of

numbers,

however,servicenamesareusuallyusedinstead.Likehostnames,servicenamesareusuallymatchedto

portnumbersthroughalocalfile,commonlycalledservices.Thisfileliststhelogicalservicename,

followedbytheportnumberandprotocolusedbytheserver.Likethehostsfile,thisfileisalsofoundin

theC:\Windows\System32\Drivers\Etcfolder.

AnumberofstandardservicenamesareusedbyInternetbasedapplicationsandthesearereferredto

aswellknownservices.Theseservicesaredefinedbyastandardsdocumentandincludecommon

applicationprotocolssuchasFTP,POP3,SMTPandHTTP.

Rememberthataservicenameorportnumberisawaytoaddressanapplicationrunningonaremote

host.Because

aparticular

service

name

is

used,

it

doesnt

guarantee

that

the

service

is

available,

just

as

dialingatelephonenumberdoesntguaranteethatthereissomeoneathometoanswerthecall.

SocketsTheprevioussectionsdescribedwhatinformationaprogramneedstocommunicateoveraTCP/IP

network.Thenextstepisfortheprogramtocreatewhatiscalledasocket,acommunicationsendpoint

thatcanbelikenedtoatelephone.However,creatingasocketbyitselfdoesntletyouexchange

information,justlikehavingatelephoneinyourhousedoesntmeanthatyoucantalktosomeoneby

simplytakingitoffthehook.Youneedtoestablishaconnectionwiththeotherprogram,justasyou

needto

dial

atelephone

number,

and

to

do

this

you

need

the

address

of

the

application

to

which

you

wanttoconnect.Thisaddressconsistsofthreekeyparts:theprotocolfamily,InternetProtocol(IP)

addressandtheserviceportnumber.

WevealreadytalkedabouttheIPaddressandserviceport,butwhatstheprotocolfamily?Itsa

numberwhichisusedtologicallydesignatethegrouptowhichagivenprotocolbelongs.Sincethe

socketinterfaceisgeneralenoughtobeusedwithseveraldifferentprotocols,theprotocolfamilytells



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theunderlyingnetworksoftwarewhichprotocolisbeingusedbythesocket.Inourcase,theInternet

Protocolfamilywillalwaysbeusedwhencreatingsockets.Withtheprotocolfamily,IPaddressofthe

systemandtheserviceportnumber,yourereadytoestablishaconnection.

Client-Server

Applications

ProgramswrittentouseTCParedevelopedusingtheclientservermodel.Asmentionedpreviously,

whentwoprogramswishtouseTCPtoexchangedata,oneoftheprogramsmustassumetheroleofthe

client,whiletheothermustassumetheroleoftheserver.Theclientapplicationinitiateswhatiscalled

anactiveopen.Itcreatesasocketandactivelyattemptstoconnecttoaserver.Theserverapplication

createsasocketandpassivelylistensforincomingconnectionsfromclients,performingwhatiscalleda

passiveopen.

Whentheclientinitiatesaconnection,theserverisnotifiedthatsomeprocessisattemptingtoconnect

withit.Byacceptingtheconnection,theservercompleteswhatiscalledavirtualcircuit,alogical

communications

pathway

between

the

two

programs.

Its

important

to

note

that

the

act

of

accepting

a

connectioncreatesanewsocket;theoriginalsocketremainsunchangedsothatitcancontinuetobe

usedtolistenforadditionalconnections.Whentheservernolongerwishestolistenforconnections,it

closestheoriginalpassivesocket.

Toreview,therearefivesignificantstepsthataprogramwhichusesTCPmusttaketoestablishand

completeaconnection.Theserversidewouldfollowthesesteps:

1. Createasocket.2. Listenforincomingconnectionsfromclients.3. Accepttheclientconnection,creatinganewsocket.4. Sendandreceiveinformation.5. Closethesocketwhenfinished,terminatingtheconversation.

Inthecaseoftheclient,thesestepsarefollowed:

1. Createasocket.2. Specifytheaddressandserviceportoftheserverprogram.3. Establishtheconnectionwiththeserver.4. Sendandreceiveinformation.5. Closethesocketwhenfinished,terminatingtheconversation.

Onlystepstwoandthreearedifferent,dependingonifitsaclientorserverapplication.Whencreating

aserverapplication,itisgenerallydesirabletouseamultithreadeddesign.Afteracceptingthe



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connection,athreadiscreatedtohandlethesocketI/Oforthatparticularsession.Whenthe

connectionisclosed,thethreadcaneitherbeterminatedorputtosleepuntilanewclientconnection

hasbeenaccepted.Thiskindofimplementationhasanumberofadvantages,particularlyonsystems

withmultipleprocessors.Amultithreadedserverisabletohandlealargernumberofclientconnections

moreefficiently,andgenerallyitiseasiertoimplementanddebugbecauseeachclientsessionoperates

independentlyfrom

the

others.

AsynchronousSocketsOneofthefirstissuesyoullencounterwhendevelopingyourInternetapplicationisthedifference

betweensynchronousandasynchronoussockets.Wheneveryouperformsomeoperationonasocket,it

maynotbeabletocompleteimmediatelyandreturncontrolbacktoyourprogram.Forexample,

readingdatafromasocketcannotcompleteuntilsomedatahasbeensentbytheremotehost.Ifthere

isnodatawaitingtoberead,oneoftwothingscanhappen:themethodcanwaituntilsomedatahas

beenwrittenonthesocket,oritcanreturnimmediatelywithanerrorthatindicatesthatthereisno

datato

be

read.

Thefirsttypeofsocketiscalledasynchronousorblockingsocket.Inotherwords,theprogramis

"blocked"untiltherequestfordatahasbeensatisfied.Whentheremotesystemdoeswritesomedata

onthesocket,thereadoperationwillcompleteandexecutionoftheprogramwillresume.Thesecond

typeofsocketiscalledanasynchronousornonblockingsocket,andrequiresthattheapplication

recognizetheerrorconditionandhandlesthesituationappropriately.

Programsthatuseasynchronoussocketstypicallyuseoneoftwomethodswhensendingandreceiving

data.Onemethodistohavetheprogramperiodicallyattempttoreadorwritedatafromthesocket,

typicallyusingatimer.However,thiscanresultinhigherCPUutilizationandnegativelyimpactthe

overallperformanceofthesystem.Thepreferredmethodistousewhatiscalledasynchronous

notification.Thismeansthattheprogramisnotifiedwheneverasocketeventtakesplace,andinturn

canrespondtothatevent.Forexample,iftheremoteprogramwritessomedatatothesocket,anevent

isgeneratedsothatprogramknowsitcanreadthedatafromthesocketatthatpoint.

Bydefault,socketI/Oissynchronousandcontrolisnotreturnedtotheprogramuntiltheblocking

socketoperationhascompleted.However,thiscanintroducesomeproblemsontheWindowsplatform.

Whenthesocketblocks,itwilleithercausethecurrentthreadtosleep,oritwillprocessWindows

messagessenttotheapplication.Thiscaneitherresultintheprogramappearingtobecomenon

responsiveoritmaybereenteredatadifferentpointwiththeblockedoperationparkedonthe

program'sstack.

For

example,

consider

aprogram

that

attempts

to

read

some

data

from

the

socket

whenabuttonispressed.Becausenodatahasbeenwrittenyet,itblocksandtheprogramgoesintoa

messageloop.Theuserthenpressesadifferentbutton,whichcausescodetobeexecuted,whichin

turnattemptstoreaddatafromthesocket,andsoon.

Toresolvethisissuewithblockingsockets,theWindowsSocketsstandardstatesthattheremayonlybe

oneoutstandingblockedcallperthreadofexecution.Thismeansthatapplicationsthatarereentered



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(asintheexampleabove)willencountererrorswhenevertheytrytotakesomeactionwhileablocking

functionisalreadyinprogress.Thecreationofbackgroundworkerthreadstoperformblockingsocket

operationsisacommonapproachtoaddressthisissue.Thisallowsthemainuserinterfacethreadto

continuetoprocessWindowsmessagesandremainresponsivetotheuser.

Itshould

be

noted

that

there

are

advantages

to

using

blocking

sockets.

In

most

cases,

the

application

designandimplementationissimpler,andoverallthroughput(therateatwhichdataissentand

received)isgenerallyhigherwithblockingsocketsbecauseitdoesnothavetogothroughanevent

mechanismtonotifytheapplicationofachangeinstatus.

TheSocketWrenchcomponentfacilitatestheuseofnonblockingsocketsbyfiringeventswhen

appropriate.Forexample,anOnReadeventisgeneratedwhenevertheremotehostwritesonthe

socket,whichtellsyourapplicationthatthereisdatawaitingtoberead.

Insummary,therearethreegeneralapproachesthatcanbetakenwhenbuildinganapplication:

Useasynchronous(blocking)socket.Inthismode,theprogramwillnotresumeexecutionuntilthesocketoperationhascompleted.Ifmultiplesimultaneousconnectionsmustbeestablished

bytheapplication,itisrecommendedthatyouuseamultithreadeddesign,whereeachsession

ismanagedbyitsownworkerthread.

Useanasynchronous(nonblocking)socket.Thismodeallowsyourapplicationtorespondtoevents.Forexample,whentheremotesystemwritesdatatothesocket,anOnReadeventis

generatedforthecontrol.Yourapplicationcanrespondbyreadingthedatafromthesocket,

andperhapssendsomedataback,dependingonthecontextofthedatareceived.

Useacombinationofblockingandnonblockingsocketoperations.Theabilitytoswitchbetweenblockingandnonblockingmodes"onthefly"providesapowerfulandconvenientway

toperformsocketoperations.

Ifyoudecidetouseasynchronoussocketsinyourapplication,itsimportanttokeepinmindthatyou

mustcheckthereturnvaluefromeveryreadandwriteoperation,sinceitispossibleyoumaynotbe

abletosendorreceiveallthespecifieddata.

Developersfrequentlyencounterproblemswhentheywriteaprogramthatassumesagivennumberof

bytescanalwaysbewrittento,orreadfrom,thesocket.Inmanycases,theprogramworksasexpected

whendevelopedandtestedonalocalareanetwork,butfailsunpredictablywhentheprogramis

releasedto

auser

that

has

aslower

network

connection

(such

as

aserial

dial

up

connection

to

the

Internet).Byalwayscheckingthereturnvaluesoftheseoperations,youinsurethatyourprogramwill

workcorrectly,regardlessofthespeedorconfigurationofthenetwork.

SecureCommunicationsSecurityandprivacyisaconcernforeveryonewhousestheInternet,andtheabilitytoprovidesecure

transactionsovertheInternethasbecomeoneofthekeyrequirementsformanybusinessapplications.



12/29


SocketWrenchhastheabilitytoestablishsecureconnectionswithremoteservers,aswellasfunctionas

asecureserveritself.Althoughmostofthetechnicalissuessuchasdataencryptionarehandled

internallybythecontrolandlibrary,ageneralunderstandingofthestandardsecurityprotocolsisuseful

whendesigningyourownapplications.

Whenyou

establish

aconnection

to

aserver

over

the

Internet

(for

example,

aweb

server),

the

data

that

youexchangeistypicallyroutedoverdozensofcomputersystemsuntilitreachesitsdestination.Any

oneofthesesystemsmaymonitorandlogthedatathatitforwards,andthereisnowayforeitherthe

senderorreceiverofthatdatatoknowifthishasbeendone.ExchanginginformationovertheInternet

couldbelikenedtotalkingwithsomeoneinapublicrestaurant.Anyonecanchoosetolistentowhat

you'resaying,andunlesstheyintroducethemselves,youhavenoideawhotheyareorifthey'veeven

heardwhatyousaid.

ToensurethatprivateinformationcanbesecurelyexchangedovertheInternet,twobasicrequirements

mustbemet:theremustbeawaytosendthatinformationsothatonlythesenderandthereceivercan

understandwhatisbeingexchanged,andtheremustbeawayforthemtodeterminethattheyeachare

infactwhotheyclaimtobe.Thesolutiontothefirstproblemistouseencryption,whereakeyisused

toencryptanddecryptthedatausingamathematicalformula.Thesecondproblemisaddressedby

usingdigitalcertificates.ThesecertificatesareissuedbyaCertificationAuthority(CA),whichisatrusted

thirdpartyorganizationwhoverifiestheidentityoftheindividualorcompanywhoisissueda

certificate.Thesetwoconcepts,encryptionanddigitalcertificates,arecombinedtoprovidethemeans

tosendandreceiveinformationsecurelyovertheInternet.

TheSecureSocketsLayer(SSL)protocolwasoriginallydevelopedbyNetscape,andisstillthemost

commonprotocolinusetoday.ThelatestimprovementstoSSLhaveresultedintheTransportLayer

Security(TLS)protocol,anditisbeginningtoreplaceSSLasthestandardforsecurecommunications

overthe

Internet.

Microsoft

also

developed

aprotocol

similar

to

SSL

called

the

Private

Communication

Technology(PCT)protocol;however,itisnotwidelyused.Eachoftheseprotocolsweredesignedto

provideessentiallythesamething:aprivateexchangeofencrypteddatabetweenthesenderand

receiver,makingitunreadablebyanintermediatesystem.Usingtherestaurantanalogy,itwouldbeasif

twopeoplewerespeakinginalanguagethatonlytheycouldunderstand.Althoughsomeonesittingat

thenexttablecouldlisteninontheconversation,theywouldn'thaveanyideawhatwasactuallybeing

said.

Asecureconnection,forexamplebetweenawebbrowserandaserver,beginswithwhatiscalledthe

handshakephasewheretheclientandserveridentifythemselves.Whentheclientfirstconnectswith

theserver

it

sends

amessage

to

the

server

and

the

server

responds

with

its

digital

certificate,

along

with

itspublickeyandinformationaboutwhattypeofencryptionitwouldliketouse.Next,theclient

generatesamasterkeyandsendsthiskeytotheserver,whichauthenticatesit.

Oncetheclientandserverhavecompletedthisexchange,keysaregeneratedwhichareusedtoencrypt

anddecryptthedatathatisexchanged.Withthehandshakecompleted,asecureconnectionbetween

theclientandserverisestablished.SocketWrenchhandlesthehandshakephaseofthesecure



13/29


connectiontransparentlyanddoesnotrequireanyadditionalprogramming.Ifasecureconnection

cannotbeestablished,anerrorisreturnedandthenetworkconnectionisclosed.

Afterthehandshakephasehascompleted,theclientmaychoosetoexaminethedigitalcertificate

returnedbytheserver.Theinformationcontainedinthecertificateincludesthedateitwasissued,the

dateit

expires,

information

about

the

organization

who

issued

the

certificate

(called

the

issuer)

and

to

whomthecertificatewasissued(calledthesubjectofthecertificate).Theclientmayalsovalidatethe

statusofthecertificate,determiningifitwasissuedbyatrustedCertificationAuthorityandwas

returnedbythesamecompanyorindividualitwasissuedto.

Theremaybecertaincaseswheretheclientdeterminesthere'saproblemwiththecertificate(for

example,ifthecertificate'scommonnamedoesnotmatchthedomainnameoftheserver),butchooses

tocontinuecommunicatingwiththeserver.Notetheconnectionwiththeserverwillstillbesecurein

thiscase.Inothercases,forexampleifthecertificatehasexpired,theclientmaychoosetoterminate

theconnectionandwarntheuser.

DigitalCertificatesWithsecureInternetconnections,digitalcertificatesareusedtoexchangepublickeysfordata

encryptionandtoprovideidentificationinformation.Thisinformationtypicallyincludestheorganization

thatwasissuedthecertificate,itsphysicallocationandsoon.Thecertificateitselfisusedtovalidatethe

publickeyactuallybelongstotheentityitwasissuedto.Thecertificatealsoincludesinformationabout

theCertificationAuthority(CA)whoissuedthecertificate.

TheCertificationAuthorityisresponsibleforvalidatingtheinformationprovidedbythatorganization,

andthendigitallysigningthecertificate,establishingarelationshipbetweenthetwoentities.When

othersvalidate

the

certificate,

they

know

it

has

been

issued

by

atrusted

third

party.

Forexample,let'ssaythatacompanywantstoimplementasecuresitesothatpeoplecanorder

productsonline.Theywouldprovideinformationabouttheircompany(organizationalcontacts,

financialinformationandsoon)toatrustedthirdpartyorganizationsuchasVerisign.Verisignwould

thenverifythattheinformationtheyprovidedwascompleteandcorrect,andthenwouldissueasigned

certificatetothem,whichtheyinstallontheirserver.

Whenauser(clientsystem)connectstotheirserverandchecksthecertificate,theyseethatitwas

issuedbyVerisign,atrustedCertificationAuthority.Inessence,theuserissayingthatbecausetheytrust

Verisign,andVerisigntruststhecompanythecertificatewasissuedto,theywilltrustthecompanyas

well.

ToestablishthisrelationshipbetweentheCertificationAuthorityandtheorganizationthecertificateis

issuedto,thereneedstobearootcertificatewhichhasbeensignedbythesametrustedorganization.

Thisservesasthebeginningofthecertificationpaththatisusedtovalidatesignedcertificates.Using

theaboveexample,ontheuser'ssystemthereisarootcertificateforVerisign,signedbyVerisign.



14/29


Rootcertificatesaremaintainedinthelocalsystem'scertificatestorewhichisessentiallyadatabaseof

digitalcertificates.Thisdatabaseisstructuredsothatdifferenttypesofcertificatescanbeorganizedin

onecentrallocationonthesystem,andastandardinterfaceisprovidedtoenumerateandvalidate

thesecertificates.Certificatesareassociatedwithastorename,allowingthemtobeeasilycategorized.

Forexample,rootcertificatesarestoredunderthename"Root",whileauser'spersonalcertificates

(alongwith

their

private

keys)

are

stored

under

the

name

"My".

WhentheWindowsoperatingsystemisinstalled,thereisacertificatestorethatcontainstheroot

certificatesforthemajorCertificationAuthorities.However,therearesituationswhereadditional

certificatesmayneedtobeaddedtothesystem.Tofacilitatethis,thesystemmanagementconsolecan

beusedtoinstallcertificates,aswellasexportorremovecertificatesfromthecertificatestore.When

managingyoursystem'scertificatestore,youshouldexercisethesamecautionaswhenyoumake

changestothesystemregistry.Inadvertentlyremovingacertificatecouldresultinerrorswhen

attemptingtoaccesssecuresystems.

Ingeneral,

the

one

situation

where

certificate

management

becomes

important

is

when

you

want

to

developyourownsecureserver.Thisisbecauseyourserverneedstohaveasignedcertificatetosendto

theclientinordertoestablishthesecureconnection.Forgeneralpurposecommercialapplications,this

generallymeansyouwouldneedtoobtainacertificatesignedbyaCertificationAuthoritysuchas

Verisign.Thiscertificatewouldthenbeinstalledinthecertificatestoreontheserver.



15/29


Fordevelopmentandtestingpurposesitmaybeinconvenienttopurchaseacertificate.Therealsomay

besituationsinwhichanorganizationwishestofunctionasitsownCertificationAuthorityandissue

certificatesthemselves.Thisallowstheorganizationtocontrolhowcertificatesaremanagedandcanbe

idealforsecureapplicationsthataredesignedforthecorporateintranet.SocketWrenchincludes

CreatCert,autilitythatenablesyoutocreateyourownselfsignedrootcertificatesandserver

certificates.For

more

information,

refer

to

the

documentation

included

with

SocketWrench.

WebClientExampleTogetstartedworkingwithSocketWrench,wellcreateasimpleVisualBasicapplicationthatconnects

toawebserverandretrievesthecontentsofanHTMLpageonthatserver.Forthisexample,youllneed

MicrosoftVisualStudio2005,SocketWrench.NETandaworkingInternetconnection.

ThefirststepafterstartingVisualStudioistocreateanewVisualBasicproject.SelectFile|New|

Projectfromthemenu,thenselectVisualBasicastheprojecttypeandWindowsApplicationasthe

project

template.

A

new

project

will

be

created

with

a

single,

empty

form.

Aftertheprojecthasbeencreated,thenextstepistoaddSocketWrench.NETtoyournewproject.

SelectProject|AddReferencefromthemenuandadialogwillbedisplayedwhichliststheavailablecomponents.ScrolldowntowhereSocketTools.SocketWrenchisdisplayed,selectitandthenclickon

theOKbutton.

TheUserInterface



16/29


AftertheSocketWrenchcomponenthasbeenreferencedinyourproject,thenextstepistocreatethe

Inthisexample,thelargeTextBoxcontrolinthecenteroftheformhastheMultilin

userinterface.WelladdseveralLabelandTextBoxcontrols,andaButtoncontrol.

epropertysetto

True,theWordWrappropertyissettoFalse,andtheScrollBarspropertyissettoBoth.Thiswillenable

ts.

t

number,atimeoutperiodinseconds,andthedocumentthatyouwishtoview.ClickingtheOKbutton

examplecomplete,thenextstepistowritethecodethatwill

retrievethedocumentfromtheserver.Todothis,selectView|Codefromthemenu,andaneditorwill

ch

ThefirststepiscreatinganinstanceoftheSocketWrenchcomponentandtheninitializingthatinstance.

ch,wellcreateaprivatemembervariableintheformsclassthat

I mpor t s Socket Tool s. Socket Wr ench

ustoviewthecompletedocumentthatsretrievedfromthewebserverandscrollthroughtheconten

Wehave

also

initialized

the

TextBox

controls

with

default

values

that

well

use

with

this

example.

Whentheapplicationiscompleted,youwillbeabletospecifythenameofthewebserver,thepor

willestablishaconnectionwiththeserver,requestthedocumentandthenretrievethecontentsofthe

documentthattheserverreturns.

Withtheuserinterfaceportionofthe

displaytheemptyformclass.

InitializingSocketWren

TocreateaninstanceofSocketWren

referencesthecomponent,andnameitasSocket:



17/29


Publ i c Cl ass For m1ol s. Socket Wr ench

d Cl ass

toinitializethatinstanceofthecomponentwhentheformiscreated.Thisisdoneby

theInitialize

method,

and

it

should

be

the

first

method

that

you

call.

SocketWrench

uses

explicit

x. Show( "Unabl e t o i ni t i al i ze Socket Wr ench component " )End

lizemethodfails,itwillreturnavalueofFalse.Inthatcase,wellsimplydisplayanerror

andterminatetheprogram.Failurecanindicatethattherewasaproblemloadingthe

solves

tanceofSocketWrenchisnolongerbeingused. Becausesocketsarealimitedsystem

Me. For mCl osed

d Sub

lizemethodistypicallyinvokedimmediatelybeforetheapplicationterminates.Itwillclose

activenetworkconnectioncreatedbythatinstanceofSocketWrench,andwillreleaseallofthe

Pr i vat e Socket As New Socket ToEn

Next,weneed

callinginitializationlikethisbecauseitdynamicallyloadsthenetworkinglibrariesthatitrequires.Inturn,this

allowsyourapplicationtohandleerrorconditionsgracefullyiftheinitializationprocessfailsforsome

reason.AgoodplacetocalltheInitializemethodisintheformsLoadevent,sowellcreateanevent

handlerthatdoesthis:

Pr i vat e Sub For m1_Load( ByVal sender As Obj ect , ByVal e As Syst em. Event Ar gs)Handl es Me. Load

I f Not Socket . I ni t i al i ze( ) ThenMessageBo

End I f

End Sub

IftheInitia

message

networkingsubsystem,oraninternalproblemwiththeSocketWrenchcomponentorhowitwas

installed.Ifyoucontinuetoencounterthiserror,tryreinstallingSocketWrenchandseeifthatre

theproblem.

Foreach

call

to

the

Initialize

method,

there

should

be

acorresponding

call

to

the

Uninitialize

method

whenthatins

resource,itsagoodideatoexplicitlyreleasethemwhentheyrenolongerneeded.Althoughthe.NET

garbagecollectorwilleventuallydothisforyou,itsrecommendedthatyoudothisexplicitly.Agood

placetocalltheUninitializemethodisintheFormClosedevent:

Pr i vat e Sub For m1_For mCl osed( ByVal sender As Obj ect , ByVal e AsSyst em. Wi ndows. For ms. For mCl osedEvent Ar gs) Handl es

Socket . Uni ni t i al i ze( )

En

TheUninitia

any

systemresourcesthatwereallocatedforit.Oncethisisdone,thatinstanceofthecomponentcanno

longerbeused.



18/29


EstablishingaConnection

AfteraninstanceoftheSocketWrenchcomponenthasbeencreatedandinitialized,thenextstepisto

server.WelldothisintheClickeventhandlerforourButton

.

thod

,soitsrecommendedthatyoureviewthetechnicalreferencefor

moreinformation.Inthisexample,wellusetheservername,portnumberandtimeoutvaluetheuser

t t on1. Cl i ck

er

di spl ay.

alizeafewpropertiestothevaluesprovidedbytheuser.TheHostNamepropertyspecifies

meorIPaddressoftheserverthatwewanttoconnectto.TheRemotePortproperty

specifiestheportnumberthatwillbeusedtoestablishtheconnection.TheTimeoutpropertyspecifies

rorhasoccurred.Inthatcase,we

displayamessageboxwithadescriptionofthelasterrorusingtheLastErrorStringproperty.Thereis

rthat

meaningfulifthepreviousmethodindicatesanerrorhasactuallyoccurred,eitherby

returningavaluethatindicatesfailureorthrowinganexception.Thismeansyoushouldnevercheckthe

establishaconnectionwiththeweb

controlthatwasplacedontheform

Toconnect

to

the

server,

well

use

the

Connect

method.

Its

important

to

note

that

the

Connect

me

canbecalledinseveraldifferentways

provides.TheTextBox1controlspecifiestheservernameorIPaddress,theTextBox2controlspecifies

theportnumber,andtheTextBox3controlspecifiesthetimeoutperiodinseconds.Hereswhatour

Clickeventhandlerlookslikesofar:

Pr i vat e Sub But t on1_Cl i ck( ByVal sender As Obj ect , ByVal e AsSyst em. Event Ar gs) Handl es Bu

' I ni t i al i ze t he Socket Wr ench pr oper t i es, set t i ng t he ser v ' host name, r emot e por t number and t i meout per i od.

Socket . Host Name = Text Box1. TextSocket . Remot ePor t = I nt eger . Par se(TextBox2. Text )Socket . Ti meout = I nt eger . Par se(TextBox3. Text )

' Est abl i sh a connect i on wi t h t he web server , andai l s ' an er r or message i f t he connect i on at t empt f

I f Not Socket . Connect ( ) Then

MessageBox. Show( Socket . Last Er r orSt r i ng)Exi t Sub

End I f

End Sub

First,weiniti

ehostnath

thenumberofsecondstowaitfortheconnectiontobeestablished.

Next,wecalltheConnectmethodandcheckthereturnvalue.IfthemethodreturnsTrue,the

connectionhasbeenestablishedsuccessfully.IfitreturnsFalse,aner

alsoaproperty

named

LastError

which

returns

anumeric

error

code

that

identifies

the

last

erro

hasoccurred.

OneimportantthingtokeepinmindusingtheLastErrorandLastErrorStringpropertiesisthattheir

valuesareonly



19/29


valueoftheLastErrorpropertytodetermineifthepreviousmethodhasfailed;instead,youshould

alwayscheckthemethodsreturnvalue.

RequestingtheDocument

lished,thenextstepistorequestthedocumentfromtheweb

we

erto

Pr i vat e Sub But t on1_Cl i ck( ByVal sender As Obj ect , ByVal e As

i es, set t i ng t he ser ver

( TextBox2. Text )

nd di spl ay

t Er r or St r i ng)

he GET command to t he ser ver usi ng t he Wr i t eLi ne

i ed.

TheTextBox4controlspecifiesthenameofthedocumentwewanttoretrievefromtheserver. Ifwe

be

Oncetheconnectionhasbeenestab

server.To

do

this,

well

send

the

GET

command

to

the

server,

along

with

the

name

of

the

document

wouldliketoview.Tosendthiscommand,wellusetheWriteLinemethodwhichsendsalineoftextto

theremotehost,terminatingitwithacarriagereturnandlinefeedcharactersequence.Thisisvery

similartohowalineoftextiswrittentoatextfile.HeresourupdatedcodeintheClickeventhandl

sendtherequesttotheserver:

Syst em. Event Ar gs) Handl es But t on1. Cl i ck

' I ni t i al i ze the Socket Wr ench pr oper t' host name, r emot e por t number and t i meout per i od.

Socket . Host Name = Text Box1. TextSocket . Remot ePor t = I nt eger . Par seSocket . Ti meout = I nt eger . Par se(TextBox3. Text )

' Est abl i sh a connect i on wi t h t he web ser ver , a' an er r or message i f t he connect i on at t empt f ai l s.

I f Not Socket . Connect ( ) ThenMessageBox. Show( Socket . Las

Exi t SubEnd I f

' Send t

' method, r equest i ng the document t hat t he user has speci fI f Not Socket . Wr i t eLi ne( "GET " + TextBox4. Text ) Then

MessageBox. Show( Socket . Last Err orSt r i ng)Socket . Di sconnect ( )Exi t Sub

End I f

d SubEn

use/asthedocumentname,ittellstheserverwewantthedefaultindexpage.TheGETcommandis

combinedwith

the

name

of

the

document

and

sent

to

the

server

using

the

WriteLine

method.

It

is

importanttonotethisisthesimplestformoftheGETcommand,andisusedfordemonstration

purposesherebecauseofitssimplicity.ForanexampleofhowamorecomplexGETrequestcan

performed,refertotheViewPageexampleincludedwithSocketWrench.NET.



20/29


IftheWriteLinemethodreturnsTrue,thenthatindicatesthecommandhasbeensentsuccessfullyto

theserver.IfitreturnsFalse,thentheoperationhasfailedandamessageboxisdisplayedtotheuser.If

thatoccurs,wealsoterminatetheconnectiontotheserverbycallingtheDisconnectmethod.

ItsimportanttonotethatWriteLineisnottheonlywaythatyoucansenddatatoaserver.For

example,SocketWrench

also

provides

amore

general

purpose

Write

method

which

can

be

used

to

send

datastoredinabytearray,andcanbeusedwhenbinary(nontextual)datamustbeexchangedwiththe

server.TheresalsothehigherlevelWriteStreammethodwhichcanbeusedtosendverylargeamounts

ofdatainasinglemethodcall.Formoreinformation,refertotheTechnicalReferencethatisincluded

withSocketWrench.

ReadingtheDocument

Afterthecommandrequestingthedocumenthasbeensent,thenextstepistoreadthecontentsofthe

documentreturnedbytheserver.ThisisdoneusingtheReadLinemethod,whichreadsalineoftext

fromtheserverandreturnsitinastring.WecalltheReadLinemethodinaloop,readingeachlineof

text

returned

by

the

server,

until

there

is

no

more

data

available

to

be

read.

Here

is

the

complete

code

fortheClickeventhandler:

Pr i vat e Sub But t on1_Cl i ck( ByVal sender As Obj ect , ByVal e AsSyst em. Event Ar gs) Handl es But t on1. Cl i ck

' I ni t i al i ze t he Socket Wr ench pr oper t i es, set t i ng t he ser ver' host name, r emot e por t number and t i meout per i od.


' Est abl i sh a connect i on wi t h the web ser ver , and di spl ay' an er r or message i f t he connect i on at t empt f ai l s.

I f Not Socket . Connect ( ) ThenMessageBox. Show( Socket . Last Er r orSt r i ng)Exi t Sub

End I f

' Send t he GET command t o t he ser ver usi ng t he Wr i t eLi ne' method, r equest i ng t he document t hat t he user has speci f i ed.

I f Not Socket . Wr i t eLi ne( "GET " + TextBox4. Text ) ThenMessageBox. Show( Socket . Last Er r orSt r i ng)Socket . Di sconnect ( )

Exi t SubEnd I f

' Decl ar e t he st r i ng buf f er t hat wi l l cont ai n t he t ext' r et ur ned by the server , and a Bool ean var i abl e.

Di m str Buf f er As St r i ng = St r i ng. Empt yDi m bCont i nue As Bool ean =Tr ue



21/29


' Cal l t he ReadLi ne met hod i n a l oop unt i l i t r et ur ns a' val ue of Fal se. For each l i ne of t ext t hat i s r ead,' append i t t o t he mul t i l i ne TextBox cont r ol .

DobCont i nue = Socket . ReadLi ne( st r Buf f er )I f bCont i nue Then

Text Box5. AppendText ( st r Buf f er + vbCr Lf )End I f

Loop Unt i l bCont i nue = Fal se

' Di sconnect f r om t he ser ver , r el easei ng t he socket handl e' t hat was al l ocat ed f or t hi s sessi on.

Socket . Di sconnect ( )

End Sub

Whenthereisnomoredataavailabletoberead,theReadLinemethodwillreturnFalse,andtheloop

will

be

exited.

At

that

point,

the

only

remaining

step

is

to

terminate

the

connection

by

calling

the

Disconnectmethod.Thisgracefullyclosestheconnectionandreleasesthesockethandlethatwas

allocatedfortheconnection.

Keepinmindthatthereisalsomorethanonewayyoucanreaddatafromtheserver.Whilethe

ReadLinemethodisdesignedtoreadlinesoftext,thereisalsoageneralpurposeReadmethodthatcan

beusedtoreceivebinary(nontextual)dataandstoreitinabytearray.Theresalsothehigherlevel

ReadStreammethodwhichcanreceiveanarbitrarilylargeamountofdatainasinglemethodcalland

returnitinabufferthatyouprovide.Formoreinformationaboutthesemethods,pleasereviewthe

TechnicalReferenceincludedwithSocketWrench.

Nowthat

we

have

completed

the

code

for

the

Button

controls

Click

event,

the

final

step

is

to

run

the

example.PressingF5willexecutetheprogram,andclickingtheOKbuttonwillretrievetheindexpage

fromtheCatalystDevelopmentwebserver.Althoughthiswasaverysimpleexample,itdemonstrated

theimportantaspectsofcreatinganInternetapplicationusingSocketWrench:

1. UsingtheInitializemethodaftercreatinganinstanceofthecomponent.Thisshouldbedoneshortlyafterthenewinstanceiscreated,andbeforeanyothermethodsare

invoked.

2. UsingtheConnectmethodtoestablishaconnectionusingaspecifichostnameandportnumber.ThereturnvaluefromtheConnectmethodwilltellyouiftheconnectionwas

successful,or

ifan

error

has

occurred.

3. SendingdatatotheserverusingtheWriteLinemethod,andreceivingdatafromtheserverusingtheReadLinemethod.Thereturnvaluewillindicateifthemethodwas

successful,iftherewasanerrororthereisnomoredataavailabletoread.

4. TerminatingtheconnectionbycallingtheDisconnectmethod.Thismethodshouldbeinvokedwhenthesessionhascompleted,releasingthesockethandlethatwasallocated

bythepreviouscalltotheConnectmethod.



22/29


ThesestepsarecommontovirtuallyeveryapplicationyoullcreatewithSocketWrench,andaresimilar

tothestepsdiscussedintheprevioussectionaboutclientserverapplications.

CreatingaSecureConnection

Tocompleteourexample,wellmakeafewmodificationstoalsosupportsecureconnectionstotheweb

serverusing

the

standard

SSL

and

TLS

protocols.

One

of

the

benefits

of

SocketWrench

is

that

this

is

a

simplechange,typicallyrequiringthatyousetandcheckthevalueofafewproperties.Theinitialclient

serverhandshake,certificatevalidationandthedataencryptionanddecryptionareallhandledforyou

automatically.

AccordingtotheHypertextTransferProtocol(HTTP),thestandardprotocolforwebservers,thedefault

portnumberforstandard(nonsecure)connectionsisport80.ForsecureconnectionsusingSSL/TLS,the

defaultportnumberis443.Toaccommodatethis,wellmodifytheexampletocheckiftheuserhas

specifiedport443,andiftheyhave,establishasecureconnectiontotheserver.Aftersettingthe

HostName,RemotePortandTimeoutproperties,addthiscode:

' I f t he user has speci f i ed por t 443, t hen t hi s shoul d be a' secur e connect i on t o t he web server

I f Socket . Remot ePor t = 443 ThenSocket . Secur e =Tr ue

End I f

TheSecurepropertytellsSocketWrenchthatyouwishtoestablishasecureconnectionwiththeremote

host.Inmostcases,thisisallyouwillhavetodotoenablethesecurityfeaturesinyourapplication.

SocketWrenchwillautomaticallyselecttheappropriatesecurityprotocolforyou,checkthecertificate

returnedbytheremotehostandnegotiateforthestrongestencryptionalgorithmsupportedbyboth

theclientandserver.

Onceasecureconnectionhasbeenestablished,anumberofothersecurityrelatedpropertiesbecome

availabletothecontrol.Thesepropertiesfallintotwogeneralgroups,returninginformationeither

aboutthesecureconnectionitself,orabouttheserver'sdigitalcertificate.Thepropertieswhichprovide

informationabouttheconnectionare:

CipherStrength Thispropertyreturnsinformationabouttherelativestrengthoftheencryption

thatisbeingusedtosecurethedata.Thevaluereturnedisactuallythelength

ofthekey(inbits)usedbytheencryptionalgorithm,andwilltypicallybe40,56

or128.

A

key

length

of

40

bits

is

considered

weak,

while

akey

length

of

56

bits

isconsideredtobemoderateand128bitorhigherkeysareconsideredvery

secure.

HashStrength Thispropertyreturnsinformationaboutthestrengthofthemessagedigest

(hash)thatwasselected.Commonvaluesreturnedbythispropertyare128and

160.



23/29


SecureCipher Thispropertyidentifiestheencryptionalgorithmthatwasselected.The

algorithmssupportedareRC2,RC4,DES,andTripleDES.Themostcommonly

usedalgorithmsareAESandRC4.

SecureHash Thispropertyidentifiesthemessagedigest(hash)algorithmthatwasselected.

Thealgorithms

supported

are

SHA

and

MD5.

The

most

commonly

used

messagedigestisMD5.Thisalgorithmisusedduringthehandshakephase

betweentheclientandserver,andismadeavailabletotheclientfor

informationalpurposes.

SecureKeyExchange Thispropertyidentifiesthekeyexchangealgorithmthatwasselected.ThealgorithmssupportedareRSA,KEAandDiffieHellman.Themostcommonly

usedkeyexchangealgorithmisRSA.

SecureProtocol Thispropertyidentifiestheprotocolusedtoestablishthesecureconnection.

TheprotocolssupportedareSSL2.0,SSL3.0,PCT1.0andTLS1.0.

Inadditiontoinformationaboutthesecureconnection,thereareseveralpropertieswhichreturn

informationabouttheremoteserver'sdigitalcertificate.Thesepropertiesare:

CertificateExpires Thispropertyreturnsthedatetheserver'scertificateexpires.Ifthisvalueis

earlierthanthecurrentdate,thecertificatehasexpired.Inthatcase,itis

recommendedthatyoualerttheuserthatthecertificateisnolongervalid.

CertificateIssued Thispropertyreturnsthedatetheserver'scertificatewasissuedbythe

certificateauthority.Ifthisdateislaterthanthecurrentdate,orlaterthanthe

datethecertificatewasissued,thecertificateisinvalid.

CertificateIssuer Thispropertyreturnsinformationabouttheorganizationthatissuedthe

certificate.Thedataisreturnedasastringwhichcontainsoneormoretagged

nameandvaluepairs.

CertificateStatus Thispropertyreturnsinformationaboutthestatusofthecertificate.Theclient

isresponsibleforcheckingthisvalue,andbasedonthevaluereturned,decide

iftheconnectionshouldbeterminatedornot.

CertificateSubject Thispropertyreturnsinformationabouttheorganizationtowhichthecertificatewasissued.LiketheCertificateIssuerproperty,thisproperty

returnsastring

which

contains

one

or

more

tagged

name

and

value

pairs.

ItisrecommendedyourapplicationimmediatelycheckthevalueoftheCertificateStatuspropertyafter

thesecureconnectionhasbeenestablished.Thisallowsyourapplicationtomakethedecisionasto

whetherornotitissafetocommunicatewiththeserverbasedonthestatusofthedigitalcertificateit

returns.Forexample,usingtheabovecodetheCertificateStatuspropertywouldreturnavalueof



24/29


SecurityCertificate.certificateValid,whichindicatesthecertificateisvalid. AfterthecalltotheConnect

method,addthiscode:

' I f t hi s i s a secur e connect i on, t hen check t he st at us' of t he ser ver ' s cert i f i cat e. I f t he cer t i f i cat e i s not' val i d, war n t he user and t er mi nat e the connect i on.

I f Socket . Secur e And _Socket . Cer t i f i cat eSt at us Secur i t yCer t i f i cat e. cer t i f i cat eVal i d ThenMessageBox. Show( "Ther e i s a pr obl em wi t h t he si t e cer t i f i cat e")Socket . Di sconnect ( )Exi t Sub

End I f

Althoughthisexampleterminatestheconnectionifthecertificatecannotbevalidated,keepinmindthis

isnotrequired.Forexample,youcouldmodifytheprogramtodisplayamessageboxindicatingthereis

aproblemwiththecertificateandaskingiftheywanttocontinue.Evenifthecertificatecannotbe

validated,the

connection

is

still

encrypted.

However,

an

invalid

certificate

can

indicate

the

site

has

been

compromised,soitisalwaysrecommendedyoustronglyadviseyourusersofthepotentialrisks.

Ifyouwantedtodisplayspecificinformationaboutacertificate,forexample,thenameofthe

organizationthatissuedthecertificateorthenameofthecompanythatitwasissuedto,youwould

needtousetheCertificateIssuerandCertificateSubjectproperties.Thesearestringpropertiesthat

returnoneormorecommaseparatedvalues.Eachvalueprovidesinformationaboutthecertificate.For

example,iftheCertificateSubjectpropertyreturnsthisstring:

C=US, O="RSA Dat a Secur i t y, I nc. "

Thatwould

specify

two

pieces

of

information

that

are

available

for

this

certificate:

1. C=US2. O="RSADataSecurity,Inc."

EachofthesevaluesconsistsofanidentifiercalledanRDN(RelativeDistinguishedName)anditsdata.

Sincethesecondvaluecontainsacomma,itisenclosedinquotes,andthisneedstobeaccountedfor

whenparsingthestring.ThereareapredefinedsetofRDNsdefinedbytheX.500standardwhichare

usedincertificates.ThemostcommonlyusedRDNsinX.509certificatesare:

RDN DescriptionC

TheISO

standard

two

character

country

code.

S Thenameofthestateorprovince.

L Thenameofthecityorlocality.

O Thenameofthecompanyororganization.

OU Thenameofthedepartmentororganizationalunit.

CN Thecommonname;withX.509certificates,thisisthedomainnameofthe

sitethecertificatewasissuedto.



25/29


So,forexample,ifyouwantedtodeterminethedomainnamethatacertificatewasissuedto,you

wouldneedtoreadthevalueoftheCertificateSubjectpropertyandparsetheresultingstringforthe

"CN"(CommonName)RDN.

Withthechangeswevemadetotheexampletosupportsecureconnections,hereswhatourcomplete

ButtonClickeventhandlerlookslike:

Pr i vat e Sub But t on1_Cl i ck( ByVal sender As Obj ect , ByVal e AsSyst em. Event Ar gs) Handl es But t on1. Cl i ck

' I ni t i al i ze t he Socket Wr ench pr oper t i es, set t i ng t he ser ver' host name, r emot e por t number and t i meout per i od.


' I f t he user has speci f i ed por t 443, t hen t hi s shoul d be a' secur e connect i on t o t he web ser ver

I f Socket . Remot ePor t = 443 ThenSocket . Secur e =Tr ue

End I f

' Est abl i sh a connect i on wi t h the web ser ver , and di spl ay' an er r or message i f t he connect i on at t empt f ai l s.

I f Not Socket . Connect ( ) ThenMessageBox. Show( Socket . Last Er r orSt r i ng)Exi t Sub

End I f

' I f t hi s i s a secur e connect i on, t hen check t he st at us' of t he ser ver ' s cert i f i cat e. I f t he cer t i f i cat e i s not' val i d, war n t he user and t er mi nat e the connect i on.

I f Socket . Secur e And _Socket . Cer t i f i cat eSt at us Secur i t yCer t i f i cat e. cer t i f i cat eVal i d ThenMessageBox. Show( "Ther e i s a pr obl em wi t h t he si t e cer t i f i cat e")Socket . Di sconnect ( )Exi t Sub

End I f

' Send t he GET command t o t he ser ver usi ng t he Wr i t eLi ne' method, r equest i ng t he document t hat t he user has speci f i ed.

I f Not Socket . Wr i t eLi ne( "GET " + TextBox4. Text ) ThenMessageBox. Show( Socket . Last Er r orSt r i ng)Socket . Di sconnect ( )Exi t Sub

End I f

' Decl ar e t he st r i ng buf f er t hat wi l l cont ai n t he t ext



26/29


' r et ur ned by the server , and a Bool ean var i abl e.

Di m st r Buf f er As St r i ng = Str i ng. Empt yDi m bCont i nue As Bool ean =Tr ue

' Cal l t he ReadLi ne met hod i n a l oop unt i l i t r et ur ns a' val ue of Fal se. For each l i ne of t ext t hat i s r ead,' append i t t o t he mul t i l i ne TextBox cont r ol .

DobCont i nue = Socket . ReadLi ne( st r Buf f er )I f bCont i nue Then

Text Box5. AppendText ( st r Buf f er + vbCr Lf )End I f

Loop Unt i l bCont i nue = Fal se

' Di sconnect f r om t he ser ver , r el easei ng t he socket handl e' t hat was al l ocat ed f or t hi s sessi on.

Socket . Di sconnect ( )

End Sub

Insummary,toestablishasecureconnectionyouneedtosettheSecurepropertytoavalueofTrue,

andthenyoushouldcheckthevalueoftheCertificateStatuspropertyaftertheConnectmethodhas

returned.Ifthecertificateisvalid,youcanproceednormally.Ifthereisaproblemwiththecertificate,

youshouldeitherterminatetheconnectionoralerttheuserthereisaproblemandletthemmakethat

decision.

DebuggingApplications

Oneoftheissuesthateverydeveloperhastocontendwithisproblemsthatariseinanapplicationafter

it'sbeendistributedtoendusers.AnderrorsrelatedtoWindowsSocketsprogrammingcanbeeven

moredifficulttotrackdownbecausetherearesomanyvariablesinvolved(suchastheplatform,

operatingsystemversion,systemconfiguration,andsoon).Toaddressthesedifficultproblems,the

SocketWrenchcontrolhasthebuiltinabilitytologtheWindowsSocketsAPIfunctioncallsthatare

made.Therearethreepropertiesrelatedtocreatingalogfile:Trace,TraceFileandTraceFlags.Setting

thesepropertiesenablesyourapplicationtodynamicallymanagefunctiontracingfeaturesavailableto

thecontrol.TheTracepropertyisaBooleanflagwhichsimplyenablesordisablesthefunctiontracing

feature.

TheTraceFilepropertyspecifiesthenameofatracelogfileinwhicheachfunctionanditsparameters

willbewritten.Ifthispropertyisnotexplicitlyset,thenafilenamedSocketTools.logwillbecreatedin

thesystem'stemporarydirectory(thedirectoryspecifiedbytheTEMPenvironmentvariable).The

TraceFlagspropertyspecifieswhattypeofloggingwillbeperformedbythecontrol,andmaybesetto

oneoffourvalues:0(TRACE_ALL)inwhichallfunctionswillbelogged,1(TRACE_ERROR)inwhichonly

errorswillbelogged,2(TRACE_WARNING)inwhichcasebothwarningsanderrorswillbewrittentothe



27/29


logfile,and4(TRACE_HEXDUMPinwhichallfunctionswillbelogged,togetherwithASCIIand

hexadecimaldisplaysofalldatathatissentorreceivedonsockets.Bydefault,allfunctioncallsare

loggedbythecontrol(TRACE_ALL).

Thetracefilehasthefollowingformat:

EXAMPLE1 I NF: WSAAsyncSel ect ( 46, 0xcc4, 0x7e9, 0x27) r et urned 0

EXAMPLE1 WRN: connect ( 46, 192. 0. 0. 1: 1234, 16) r eturned - 1 [ 10035]

EXAMPLE1 ERR: accept( 46, NULL, 0x0) r et urned - 1 [ 10038]

Thefirstcolumncontainsthenameoftheprocessbeingtraced.Thesecondcolumnidentifiesifthe

recordisreportinginformation,awarning,oranerror.WhatfollowsisthenameoftheWindows

Socketsfunctionbeingcalled,theargumentspassedtothefunctionandthefunction'sreturnvalue.Ifa

warningorerrorisreported,theerrorcodeisappendedtotherecordwithinbrackets.

Whenreadingatracelog,therearetwocommonthingsthatyouwillsee:

1. Theerrorcode10035,whichcorrespondstotheWindowsSocketserrorWSAEWOULDBLOCKisanormaloccurrenceonconnectcalls,andshouldnotbetakenasacauseforconcernbyitself.

2. Thenormalreturnvaluefortheselectfunctionisgreaterthanzero,typicallyavalueofone.Aselectcallthatreturnszerousuallyindicatesatimeout.Areturnvalueof1indicatesanerror

condition.

Ifparametersarepassedasintegervalues,theyarerecordedindecimal.Iftheparameterorreturn

valueisamemoryaddress,itisrecordedasahexadecimalvalueprefixedwith"0x".Aspecialtypeof

pointer,calledanullpointer,isrecordedasNULL.

Thosefunctionswhichexpectsocketaddressesaredisplayedintheformataa.bb.cc.dd:nnnn.Thefirst

fournumbersseparatedbyperiodsrepresenttheIPaddress,andthenumberfollowingthecolon

representstheportnumberinhostbyteorder.Notethatinthesecondlineoftheaboveexample,the

applicationisattemptingtoconnecttoasystemwiththeIPaddress192.0.0.1onport1234.

Toenablelogginginyourapplication,youalsoneedtoredistributeanextrafilecalled

SocketTools.TraceLog.dllandinstallitinthesamedirectoryasyourapplication.Ifyouhavesetthe

TracepropertytoTrue,butthislibrarycannotbeloaded,thenSocketWrenchwillsilentlyresettheTrace

propertytoFalse.NotethatthislibrarywillonlyprovideloggingcapabilitytotheSocketWrench

component;itisnotageneralpurposelibraryforloggingWindowsSocketsfunctionsandwillnotlogthe

functioncallsmadebyanyotherapplicationorcomponent.

Thereareseveralwaysyoucouldincorporatetracelogginginyoursoftware.Thesimplestwouldbea

menuitemoracommandlineswitch(like/Debug)inwhichtheTracepropertywouldbesettoTrue.A

morecomplexapproachwouldbetoincludeadialogorpropertysheetwhichallowstheusertospecify

thelogfilenameandoptions.Whenanendusercallsfortechnicalsupportandisencounteringa

problemyouthinkmaybenetworkrelated,youcaninstructthemtoenabletheloggingfeatureand

emailorfaxyouacopyofthelogfile.Inturn,ifitisaproblemyoudon'tunderstand,youcansendthe



28/29


logfiletoaCatalystDevelopmentsupporttechnicianwhocananalyzethelogandprovideyouwith

additionalinformationabouttheproblem.

Rememberthatifyoudonotusetheloggingfeaturesatanytimeduringtheexecutionofyourprogram,

thereisnoadditionalperformancepenalty.Ifyoudoenableloggingatsomepoint,thelibrarywillbe

loadedand

memory

will

be

allocated

by

the

logging

functions.

These

functions

open,

append

to

the

tracelogfile,flushandthenclosethefileforeachWindowsSocketsfunctioncallthatismade.This

insuresthatthelastfunctioncalledisloggedincaseofageneralprotectionfaultorotherabnormal

terminationoftheprogram.However,becauseofthefileI/Ooverhead,it'srecommendedthatyour

programrenameorremovethelogfilebeforebeginninganewtrace.

AdvancedDevelopmentUsingSocketToolsSocketWrenchispartofapackagedevelopedbyCatalystcalledSocketTools.Inadditiontothe

comprehensive,butfairlylowlevel,accessthatSocketWrenchprovides,SocketToolsincludes

components

and

libraries

for

many

of

the

popular

Internet

application

protocols.

There

are

several

differenteditionsofSocketToolsavailable,andalleditionsprovideroyaltyfreeredistributionlicensing

andathirtydaymoneybackguarantee.EvaluationcopiesofalloftheSocketToolsEditionsareavailable

fordownloadingfromourwebsite.

SocketTools.NET Edition

TheSocketTools.NETEditionisacollectionofmanagedcodeclasses,designedtosimplifythe

integrationofInternetfunctionalityintoapplicationsbuiltusingtheVisualStudiodevelopment

platform.SocketTools.NETisidealfortheVisualBasicorC#developerwhorequirestheeaseofuseand

rapiddevelopmentfeaturesofacomponent,withoutthecomplexitiesofthenativesocketclassesand

withoutrequiringindepthknowledgeofhowthevariousInternetprotocolsareimplemented.TheSocketToolsSecure.NETEditionhastheadditionalfeatureofintegratedsupportforsecure,

encryptedconnectionsusingtheindustrystandardSecureSocketsLayer(SSL)andTransportLayer

Security(TLS)protocols.Yourdataisprotectedbyindustrialstrength128bitencryption,and

SocketToolsincludessupportforclientcertificatesandotheradvancedfeatures.Youcanalsocreate

yourown,customsecureclientandserverapplicationsandthere'snoneedforyoutounderstandthe

detailsofcertificatemanagement,dataencryptionorhowthesecurityprotocolswork.Allittakesisa

fewlinesofcodetoenablethesecurityfeatureswithveryminimalchangestoanyexistingcode.

SocketToolsLibraryEdition

TheSocketTools

Library

Edition

includes

standard

Windows

dynamic

link

libraries

(DLLs)

which

can

be

usedinawidevarietyofprogramminglanguagessuchasVisualStudio.NET,VisualC++,VisualBasicand

Delphi.TheLibraryEditionisidealforthedeveloperwhorequiresthehighperformance,minimum

resourceutilizationandflexibilityofalowerlevelinterface,withouttheinherentoverheadofActiveX

controlsorCOMlibraries.



29/29


TheSocketToolsLibraryEditionAPIhasover800functionswhichcanbeusedtodevelopapplications

thatmeetawiderangeofneeds.SocketToolscoversitall,includinguploadinganddownloadingfiles,

sendingandretrievingemail,remotecommandexecution,terminalemulation,andmuchmore.

Inadditiontotheabove,theSocketToolsSecureLibraryEditionhasintegratedsupportforsecure,

encryptedconnections

using

the

industry

standard

Secure

Sockets

Layer

(SSL)

and

Transport

Layer

Security(TLS)protocols.ItprovidesaninterfaceforallofthemajorInternetprotocolsincludingHTTPS,

FTPS,SMTPS,POP3SandIMAPS.Creatingasecureconnectiononlyrequiresafewadditionallinesof

code,anddoesntrequirethatyouunderstandthecomplexdetailsofdataencryptionorcertificate

validation.TheSecureLibraryEditionhandlesallofthedetails,enablingyoutoaddsecurityfeaturesto

yourapplicationquicklyandeasily.

SocketToolsScriptingEdition

TheSocketToolsScriptingEditionincludesCOMcomponentswhichcanbeusedinawidevarietyof

scriptinglanguagessuchasVBScript,JScriptandPHP.Inaddition,theScriptingEditioncomponentscan

also

be

used

with

Visual

Studio.NET,

Visual

C++

and

Visual

Basic.

The

Scripting

Edition

is

ideal

for

the

developerwhorequirestheflexibility,easeofuseandrapiddevelopmentfeaturesofacomponent

designedspecificallyforclientandserversidescripting.

Inadditiontotheabove,theSocketToolsSecureScriptingEditionhasintegratedsupportforsecure,

encryptedconnectionsusingtheindustrystandardSecureSocketsLayer(SSL)andTransportLayer

Security(TLS)protocols.TheSecureEditionsimplementthemajorsecureprotocolssuchasHTTPS,FTPS,

SMTPS,POP3S,IMAPSandmore.

SocketToolsVisualEdition

TheSocketToolsVisualEditionincludesActiveXcontrols(OCXs)whichcanbeusedinawidevarietyof

programminglanguages

such

as

Visual

Studio.NET,

Visual

C++

and

Visual

Basic.

The

Visual

Edition

is

ideal

forthedeveloperwhorequirestheflexibility,easeofuseandrapiddevelopmentfeaturesofa

componentwithoutthecomplexitiesofworkingwiththeWindowsSocketsAPIorindepthknowledge

ofhowthevariousInternetprotocolsareimplemented.

TheSocketToolsVisualEditionconsistsofeighteenActiveXcontrolswhichcanbeusedtodevelop

applicationsthatmeetawiderangeofneeds.SocketToolscoversitall,includinguploadingand

downloadingfiles,sendingandretrievingemail,remotecommandexecutionandterminalemulation.

Ifyou'redevelopingcommercialapplicationswhichrequiretheabilitytoestablishsecure,encrypted

connections

using

the

Secure

Sockets

Layer

(SSL)

and

Transport

Layer

Security

(TLS)

protocols,

the

SocketToolsSecureVisualEditionincludessupportforallofthemajorsecureprotocolsincludingHTTPS,

FTP,SMTPS,POP3SandIMAPS. Yourdataisprotectedbyindustrialstrength128bitencryption,and

SocketToolsincludessupportforclientcertificatesandotheradvancedfeatures.Allittakesissettinga

fewpropertiestoenablethesecurityfeatures,withveryminimalchangestoanyexistingcode.

FormoreinformationaboutSocketTools,pleasevisittheCatalystDevelopmentwebsiteat

http://www.catalyst.com/products/sockettools/

tutorial 1-schema

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