Chapte r 8

1. Chapter8 ApplicationLayer Compiled By: Ashish Kr. Jha

2. 2 • HypertextTransferProtocol(HTTP) • Internet(orTheWeb)isadistributedclient/serverinformation • system. • ManyapplicationsarerunningconcurrentlyovertheWeb,suchaswebbrowsing/surfing,e-mail,filetransfer,audio&videostreaming,andsoon.Inorderforpropercommunicationtotakeplacebetweentheclientandtheserver,theseapplicationsmustagreeonaspecificapplication-levelprotocolsuchasHTTP,FTP,SMTP,POP,andetc. Compiled By: Ashish Kr. Jha

3. 3 • HypertextTransferProtocol(HTTP) • HTTP(HypertextTransferProtocol)isperhapsthemost • popularapplicationprotocolusedintheInternet(orTheWEB). • HTTPisanasymmetricrequest-responseclient-serverprotocol.AnHTTPclientsendsarequestmessagetoanHTTPserver.Theserver,inturn,returnsaresponsemessage.Inotherwords,HTTPisapullprotocol,theclientpullsinformationfromtheserver(insteadofserverpushesinformationdowntotheclient). Compiled By: Ashish Kr. Jha

4. 4 • HypertextTransferProtocol(HTTP) • Webserverlistenatportnumber80. • HTTPisimplementedintwoprograms:aclientprogramandserverprogram. • Theclientprogramandserverprograms,executingondifferentendsystems,talktoeachotherbyexchangingHTTPmessages. • HTTPdefinesthestructureofthesemessagesandhowthe • clientandserverexchangethemessages. • HTTPisastatelessprotocol.Inotherwords,thecurrentrequestdoesnotknowwhathasbeendoneinthepreviousrequests. • HTTPpermitsnegotiatingofdatatypeandrepresentation,soastoallowsystemstobebuiltindependentlyofthedatabeingtransferred. • UsesTCPasunderlyingtransportprotocol Compiled By: Ashish Kr. Jha

5. 5 • HTTP • AWebpage(alsocalledadocument)consistsofobjects.Anobjectisasimplyfile--suchasaHTMLfile,aJPEGimage,aGIFimage,aJavaapplet,anaudioclip,etc.--thatisaddressablebyasingleURL.MostWebpagesconsistofabaseHTMLfileandseveralreferencedobjects. • Forexample,ifaWebpagecontainsHTMLtextandfiveJPEGimages,thentheWebpagehassixobjects:thebaseHTMLfileplusthefiveimages.ThebaseHTMLfilereferencestheotherobjectsinthepagewiththeobjects'URLs. • EachURLhastwocomponents:thehostnameoftheserverthathousestheobjectandtheobject'spathname. • Forexample,theURLwww.someSchool.edu/someDepartment/picture.gifhaswww.someSchool.eduforahostnameand • /someDepartment/picture.gifforapathname. Compiled By: Ashish Kr. Jha

6. 6 • HTTP • WheneveryouissueaURLfromyourbrowsertogetawebresourceusingHTTP,e.g.http://www.test101.com/index.html,thebrowserturnstheURLintoarequestmessageandsendsittotheHTTPserver.TheHTTPserverinterpretstherequestmessage,andreturnsyouanappropriateresponsemessage,whichiseithertheresourceyourequestedoranerrormessage.Thisprocessisillustratedbelow: Compiled By: Ashish Kr. Jha

7. 7 • HTTP/1.0non-persistentconnection • SingleHTTPRequestandResponseallowedforeachTCP • Connection • www.someSchool.edu/someDepartment/home.index Compiled By: Ashish Kr. Jha

8. 8 • HTTP/1.0non-persistentconnection • www.someSchool.edu/someDepartment/home.index Compiled By: Ashish Kr. Jha

9. 9 Timingofanon-persistentconnection Compiled By: Ashish Kr. Jha

10. 10 • HTTP/1.1persistentconnection • Assizeofwebpagesincreased,HTTP1.0wasnomore • efficient • WithHTTP1.1,itwaspossibletosendadditionalrequests • andgetadditionalresponsesinasingleTCPconnection. • InanHTTP/1.1persistentconnection,theserverleavestheTCPconnectionopenaftersendingaresponse. • Subsequentrequestsandresponses(pipelinedornot) • betweenthesameclientcanbesentonthesameconnection. Compiled By: Ashish Kr. Jha

11. 11 • HTTPRequestMessage • HTTPrequestmessageconsistsof: • RequestLine • request-method-namerequest-URIHTTP-version • RequestHeaders • request-header-name:request-header-value1,request-header-value2... • RequestMessageBody Compiled By: Ashish Kr. Jha

12. 12 • HTTPRequestMethods • HTTPprotocoldefinesasetofrequestmethods.AclientcanuseoneoftheserequestmethodstosendarequestmessagetoanHTTPserver. • ▫ GET:AclientcanusetheGETrequesttogetawebresourcefromtheserver. • ▫ HEAD:AclientcanusetheHEADrequesttogettheheaderthataGETrequestwouldhaveobtained.Sincetheheadercontainsthelast-modifieddateofthedata,thiscanbeusedtocheckagainstthelocalcachecopy. • ▫ POST:Usedtopostdatauptothewebserver. • ▫ PUT:Asktheservertostorethedata. • ▫ DELETE:Asktheservertodeletethedata. • ▫ TRACE:Asktheservertoreturnadiagnostictraceoftheactionsittakes. • ▫ OPTIONS:Asktheservertoreturnthelistofrequestmethodsit • supports. • ▫ CONNECT:Usedtotellaproxytomakeaconnectiontoanotherhostandsimplyreplythecontent,withoutattemptingtoparseorcacheit.ThisisoftenusedtomakeSSLconnectionthroughtheproxy. Compiled By: Ashish Kr. Jha

13. 13 • HTTPResponseMessage • HTTPResponsemessageconsistsof: • ▫ StatusLine • HTTP-versionstatus-codereason-phrase • ▫ ResponseHeader • response-header-name:response-header-value1,response-header-value2,... ▫ ResponseMessageBody Compiled By: Ashish Kr. Jha

14. 14 • SomeHTTPresponsestatuscodes • • 200OK • ▫ requestsucceeded,requestedobjectinthismessage • 301MovedPermanently • ▫ requestedobjectmoved, • message(Location) • 304NotModified newlocationspecifiedin this • ▫ InresponsetotheIf-Modified-SinceconditionalGETrequest,theservernotifiesthattheresourcerequestedhasnotbeenmodified. • 400BadRequest • ▫ requestmessagenotunderstoodbyserver • 404NotFound • ▫ requesteddocumentnotfoundonthisserver • 408RequestTimeout • 505HTTPVersionNotSupported. Compiled By: Ashish Kr. Jha

15. 15 • HTTPS • UseTCPatTransportlayer. • Listenatportnumber443. • HypertextTransferProtocolSecure(HTTPS)isawidely-usedcommunicationsprotocolforsecurecommunicationoveracomputernetwork,withespeciallywidedeploymentontheInternet. • Technically,itisnotaprotocolinitself;rather,itistheresultofsimplylayeringtheHypertextTransferProtocol(HTTP)ontopof theSSL (SecureSocketLayer)orTLS(TransportLayer protocol,thusaddingthesecuritycapabilitiesof Security) • SSL/TLStostandardHTTPcommunications. • NeedtorequesttheSSLcertificatefromtheCertificateAuthoritytodeployHTTPS. • Encryption/DecryptionMechanismisusedbetweenclientandserverfortransferringdata. • LikefromGoDaddy,VeriSignetc. Compiled By: Ashish Kr. Jha

16. 16 HTTPS Compiled By: Ashish Kr. Jha

17. 17 • FileTransferProtocol(FTP) • FTP,likeHTTP,runsontopofTCP. • However,unlikeHTTP,FTPusestwo parallelTCP • connectionstotransferafile,acontrolconnection(port#21)andadataconnection(port#20).WesayFTPsendsitscontrolinformationout-ofband. • AlsounlikeHTTP,FTPmaintainsstate.Inparticular,FTPremembersthecurrentdirectoryandearlierauthentication. Compiled By: Ashish Kr. Jha

18. 18 • FileTransferProtocol(FTP) • Protocolforexchangingfilesfromonehosttoanotherhost • typicallyformyourcomputertoawebserver. • Thetransferisasynchronous,meaningnotatthesametime, • andthereforefasterthanotherprotocols. • Downloading– • ▫ copyingfilestoyourcomputer • Uploading– • ▫ transmitafilefromyoucomputertoanothercomputeroninternet Compiled By: Ashish Kr. Jha

19. 19 • FileTransferProtocol(FTP) • FTPestablishestwoconnectionsbetween theclientand • server. • Oneisfordatatransferandtheotherisforthecontrol • information. • Thecontrolconnectionusessimplerulesofcommunication.Onlyonelineofcommandoralineofresponseistransferredatatime. • Butthedataconnectionusesmorecomplexrulesdueto • varietyofdatatypesbeingtransferred. • FTPusesport21forcontrolconnectionandport20forthe • dataconnection. • ControlconnectionismaintainedduringtheentireFTPsession. • Thedataconnectionisfirstopened,fileistransferredand • connectionisclosed.Thisisdonefortransferringeachfile. Compiled By: Ashish Kr. Jha

20. 20 FileTransferProtocol(FTP) Compiled By: Ashish Kr. Jha

21. 21 FTPExample Compiled By: Ashish Kr. Jha

22. 22 • WinSCP • WinSCP(WindowsSecureCoPy)isafreeandopensource • SFTP,SCP,andFTPclientforMicrosoftWindows. • Itsmainfunctionissecurefiletransferbetweenalocalanda • remotecomputer. • Beyondthis,WinSCPoffersbasicfilemanagerandfilesynchronizationfunctionality. • Forsecuretransfers,itusesSecureShell(SSH)andsupports • theSCPprotocolinadditiontoSFTP. • WinSCPisbasedontheimplementationoftheSSHprotocolfromPuTTYandFTPprotocolfromFileZilla. Compiled By: Ashish Kr. Jha

23. 23 WinSCP Compiled By: Ashish Kr. Jha

24. 24 • PuTTY • PuTTYisafreeandopensourceterminalemulatorapplicationwhichcanactasaclientfortheSSH,Telnet,rlogin,andrawTCPcomputingprotocolsandasaserialconsoleclient. • Thename"PuTTY"hasnodefinitivemeaning,though"tty"isthenameforaterminalintheUnixtradition,usuallyheldtobeshortforTeletype. Compiled By: Ashish Kr. Jha

25. 25 PuTTY Compiled By: Ashish Kr. Jha

26. 26 • ElectronicMail • MailSendProtocol • ▫ SimpleMailTransferProtocol(SMTP) • PortNumber25/TCP • RelayMailFromOnedomaintoanotherorwithinsamedomain • MailAccessProtocol • ▫ PostOfficeProtocolv3(POP3) • Portnumber110/TCP • AccessmailfromMailserver • ▫ InternetMailAccessProtocol(IMAP) • WebBasedmailaccessprotocol • PortNumber143/TCP Compiled By: Ashish Kr. Jha

27. 27 HighLevelViewofE-mailSystem Compiled By: Ashish Kr. Jha

28. 28 SampleSMTPInteraction ▫ S:220hamburger.edu ▫ C:HELOcrepes.fr ▫ S:250Hellocrepes.fr,pleasedtomeetyou ▫ C:MAILFROM:<alice@crepes.fr> ▫ S:250alice@crepes.fr...Senderok ▫ C:RCPTTO:<bob@hamburger.edu> ▫ S:250bob@hamburger.edu...Recipientok ▫ C:DATA ▫ S:354Entermail,endwith"."onalinebyitself ▫ C:Doyoulikeketchup? ▫ C:Howaboutpickles? ▫ C:. ▫ S:250Messageacceptedfordelivery ▫ C:QUIT ▫ S:221hamburger.educlosingconnection Compiled By: Ashish Kr. Jha

29. 29 E-mailSystem WhenbothsenderandreceiverareconnectedtothemailserverviaaLANoraWAN,weneedtwoUAs,twopairsofMTAs(clientandserver),andapairofMAAs(clientandserver).Thisisthemostcommonsituationtoday. Compiled By: Ashish Kr. Jha

30. 30 • ComponentsofEmailSystem • UserAgent • ▫ Thefirstcomponentofanelectronicmailsystemistheuseragent(UA).Itprovidesservicetotheusertomaketheprocessofsendingandreceivingamessageeasier. • MessageTransferAgent • ▫ Theactualmailtransferisdonethroughmessagetransferagents(MTAs).Tosendmail,asystemmusthavetheclientMTA,andtoreceivemail,asystemmusthaveaserverMTA.TheformalprotocolthatdefinestheMTAclientandserverintheInternetiscalledSimpleMailTransferProtocol(SMTP).TwopairsofMTAclient-serverprogramsareusedinthemostcommonsituation. • MessageAccessAgents • ▫ ThefirstandthesecondstagesofmaildeliveryuseSMTP.However,SMTPisnotinvolvedinthethirdstagebecauseSMTPisapushprotocol;itpushesthemessagefromtheclienttotheserver.Ontheotherhand,thethirdstageneedsapullprotocol;theclientmustpullmessagesfromtheserver.Thethirdstageusesamessageaccessagent. Compiled By: Ashish Kr. Jha

31. 31 • SMTP(SimpleMailTransferProtocol) • SMTPisaprincipleapplication-layerprotocolforInternet • e-mail. • ItusesthereliabletransferserviceofTCP,usesport25 • Directtransfer:sendingservertoreceivingserver • Threephasesoftransfer: • ▫ 1)handshaking(greeting) • 3)closure • Messagesmustbein7-bitASCII • 7-bitASCIIrestrictionisabit 2)transferofmessages ofpain:-requiresbinary multimediadatatobeencodedtoASCIIbeforebeingsent overSMTP Compiled By: Ashish Kr. Jha

32. 32 SendEmail Compiled By: Ashish Kr. Jha

33. 33 Scenario:AlicesendsmessagetoBob SMTPclientsendsAlice’s messageovertheTCPconnection Bob’smailserverplacesthe messagein Bob’smailbox 6)Bobinvokeshisuseragenttoreadmessage 1)AliceusesUAtocomposemessageto:bob@someschool.edu 2)Alice’s UAsendsmessageto hermailserver;messageplacedinmessagequeue 3)ClientsideofSMTPopensTCPconnectionwith Bob’s mailserver 1 useragent mail server mail user agent server 2 6 3 4 5 Compiled By: Ashish Kr. Jha

34. 34 • MailAccessProtocols(PullProtocols) • SMTPwasaMailTransferProtocolorpushProtocolandusedto • pushthemailmessageup tothereceiver’smailserver. • MailAccessProtocol:retrievalfromserver • ▫ HTTPisalsousedtoComposeandretrieveEmails. • ▫ AlsocalledWebbasedemail. • ▫ Eg.Hotmail,YahooMailEtc. • POP:PostOfficeProtocol(POP3) • ▫ authorization(agent<-->server)anddownload(deletedfromserver) • ▫ TCPPortno.110 • IMAP:InternetMailAccessProtocol • ▫ morefeatures(morecomplex). • ▫ TCPPortno.143 • ▫ Remotemanipulationofstoredmessagesonserver Compiled By: Ashish Kr. Jha

35. 35 POP3andIMAP AccessProtocolPOP3orIMAP useragent SMTP SMTP user agent receiver’smail server sender’s mail server Compiled By: Ashish Kr. Jha

36. 36 • POP3(PostOfficeProtocol) • POP3beginswhenuseragentopensaTCPconnectionto • themailserveronport110. • ThenPOP3progressesthrough3phases: • ▫ Authorization:useragentsendsausernameandpassword • toauthenticatetheuser • ▫ Transaction:useragentretrievesmessage,user-agentcanmarkmessagesfordeletion,removedeletionmark • ▫ Update:occursafterclienthasissuedthequitcommand,endingPOP3session,atthistimemailserverdeletethemarkedmessages Compiled By: Ashish Kr. Jha

37. 37 • POP3 • POP3isextremelysimplemailaccessprotocoland • functionalityislimited. • POP3canbeconfiguredtodownloadanddelete(default • configuration)ordownloadandkeep. • Aproblemwithdownloadanddeletemode–usermaywanttoaccesshismailmessagesformmultiplemachines. • DuringPOP3session,POP3servermaintainssomestateinformationaboutwhichusermessageshavebeenmarkedfordeletion. • However,POP3serverdoesnotcarrystateinformation • acrossPOP3session. Compiled By: Ashish Kr. Jha

38. 38 POP3 Compiled By: Ashish Kr. Jha

39. 39 • IMAP(InternetMAP) • Userprefertomaintainafolderhierarchyonremoteserver • thatcanbeaccessedfromanycomputer. • ThisisnotpossiblewithPOP3howeverIMAPprotocolis • definedtosolvetheissue. • IMAPhasmorefeaturealongwithmorecomplexity. • Whenmessagefirstarrives,itisassociated withrecipient’s • INBOXfolder. • Therecipientcanthenmovemessagefromonefolderto • another. • UnlikePOP3,IMAPmaintainsstateinformationacrossIMAPsession,-forexample,namesoffoldersandwhichmessagesareassociatedwithwhichfolders. • IMAPenabletoobtaincomponentsofmessages.(usefulforlowbandwidth). Compiled By: Ashish Kr. Jha

40. 40 • MIME(MultipurposeInternetMailExtensions) • Electronicmailhasasimplestructure.Itssimplicity,however,comeswithaprice.ItcansendmessagesonlyinNVT(NetworkVirtualTerminal)7-bitASCIIformat.Inotherwords,ithassomelimitations. • MultipurposeInternetMailExtensions(MIME)isasupplementaryprotocolthatallowsnon-ASCIIdatatobesentthroughe-mail.MIMEtransformsnon-ASCIIdataatthesendersitetoNVTASCIIdataanddeliversittotheclientMTAtobesentthroughtheInternet.Themessageatthereceivingsiteistransformedbacktotheoriginaldata. Compiled By: Ashish Kr. Jha

41. 41 MIME Compiled By: Ashish Kr. Jha

42. 42 • CommonMIMEtypes • Content-Type:type/subtype;parameters • ▫ Text • plain;html • ▫ Image • jpeg;gif • ▫ Audio • basic;32kadpcm • ▫ Video • quicktime • ▫ Application • msword;octet-stream Compiled By: Ashish Kr. Jha

43. 43 SecuringEmailUsingPrettyGoodPrivacy(PGP) • prettygoodprivacy(PGP)isane-mailencryptionschemethathasbecomeade-factostandard,withthousandsofusersallovertheglobe. • Dependingontheversion,thePGPsoftwareusesMD5orSHAforcalculatingthemessagedigest;CAST,Triple-DESorIDEAforsymmetrickeyencryption;andRSAforthepublickeyencryption. • Inaddition,PGPprovidesdatacompression. • WhenPGPisinstalled,thesoftwarecreatesapublickeypairfortheuser. • Thepublickeycanbepostedontheuser'sWebsiteorplacedinapublickeyserver. • Theprivatekeyisprotectedbytheuseofapassword.Thepasswordhastobeenteredeverytimetheuseraccessestheprivatekey. • PGPgivestheusertheoptionofdigitallysigningthemessage,encryptingthemessage,orbothdigitallysigningandencrypting. Compiled By: Ashish Kr. Jha

44. 44 PGP Compiled By: Ashish Kr. Jha

45. 45 • DNS(DomainNameSystem) • Ahierarchical,distributeddatabasethatcontainsmappingsofDNSdomainnamestovarioustypesofdata,suchasIPaddresses. • DNSenablesthelocationofcomputersandservicesbyuser-friendlynames,anditalsoenablesthediscoveryofotherinformationstoredinthedatabase. • DNSprovidestranslationbetweenhostnameandIPaddress. • DNSnamesareuser-friendly,whichmeansthattheyare • easiertorememberthanIPaddresses. • DNSnamesremainmoreconstantthanIPaddresses.AnIPaddressforaservercanchange,buttheservernameremainsthesame. • DNSallowsuserstoconnecttolocalserversusingthesamenamingconventionastheInternet. • UsesUDPportno.53. Compiled By: Ashish Kr. Jha

46. 46 Hierarchicalstructureofadomainnamespace Compiled By: Ashish Kr. Jha

47. 47 • NameServers • RootDomains • ▫ Therootdomainisatthetopofthehierarchyandisrepresentedasaperiod(.). • ▫ TheInternetrootdomainismanagedbyseveral • organizations,includingNetworkSolutions,Inc. • ▫ 13RootNameServers • Top-LevelDomains • ▫ Top-leveldomainsaretwo-orthree-characternamecodes. • ▫ Top-leveldomainsaregroupedbyorganizationtypeorgeographiclocation. • SecondLevelDomains Compiled By: Ashish Kr. Jha

48. 48 TypesofNameServers rootDNSserver • LocalNameServers • RootNameserver • Authoritativenameserver 2 3 6 7 TLDDNSserver localDNSserver dns.poly.edu 4 5 1 8 authoritativeDNSserver dns.cs.umass.edu requestinghost cis.poly.edu gaia.cs.umass.edu Compiled By: Ashish Kr. Jha

49. 49 • TypesofNameServers • Top-leveldomain(TLD)servers:responsibleforcom,org, • net,edu,etc,andalltop-levelcountrydomainsuk,fr,ca,jp. • ▫ NetworksolutionsmaintainsserversforcomTLD • ▫ EducationalinstitutionsuseforeduTLD • AuthoritativeDNSservers:organization’sDNSservers, providingauthoritativehostnametoIPmappingsfororganization’sservers(e.g.,Webandmail). • ▫ Canbemaintainedbyorganizationorserviceprovider • LocalDNSservers:Doesnotstrictlybelongtohierarchy • ▫ EachISP(residentialISP,company,university)hasone. • ▫ Also called defaultnameserver • ▫WhenahostmakesaDNSquery,queryissenttoitslocalDNSserver • ▫ Actsasaproxy,forwardsqueryintohierarchy. Compiled By: Ashish Kr. Jha

50. 50 • DNSServices • HostnametoIPaddresstranslation • Hostaliasing • ▫ Canonical(Real)andaliasnames • Mailserveraliasing • Loaddistribution • ▫ ReplicatedWebservers:setofIPaddressesforonecanonicalname Compiled By: Ashish Kr. Jha