Application Layer protocols- OSI Model Layers

Application layer design, protocols

1. APPLICATIONLAYER
ShashikantV. Athawale
Assistant Professor
Department of Computer Engineering,
AISSMS COE ,Pune

2. AGENDA
OSI Model Block Diagram
OSI Encapsulation Process
Application Protocol Design
Application Layer Protocols
1. HTTP(Hypertext Transfer Protocol)
2. DNS(Domain Name System)
3. FTP(File Transfer Protocol)
4. TELNET
5. DHCP(Dynamic Host Configuration Protocol)
6. SMTP(Simple Mail Transfer Protocol)
Summary
References
2

3. OSI MODELBLOCKDIAGRAM
3
Application-Layer Protocols

4. AGENDA
OSI Model Block Diagram
OSI Encapsulation Process
Application Protocol Design
Application Layer Protocols
1. HTTP(Hypertext Transfer Protocol)
2. DNS(Domain Name System)
3. FTP(File Transfer Protocol)
4. TELNET
5. DHCP(Dynamic Host Configuration Protocol)
6. SMTP(Simple Mail Transfer Protocol)
Summary
References
4

5. OSI ENCAPSULATION PROCESS
5
Application-Layer Protocols

6. AGENDA
OSI Model Block Diagram
OSI Encapsulation Process
Application Protocol Design
Application Layer Protocols
1. HTTP(Hypertext Transfer Protocol)
2. DNS(Domain Name System)
3. FTP(File Transfer Protocol)
4. TELNET
5. DHCP(Dynamic Host Configuration Protocol)
6. SMTP(Simple Mail Transfer Protocol)
Summary
References
6

7. APPLICATIONPROTOCOLDESIGN
Thinkofdifferentpeople/teams,workingontheclientandserver
programs.
–Differentprogramminglanguages.
–Diversehardware,operatingsystems.
Beunambiguous,precise.
–Considerpotentialerrorconditions.
Allowforfutureextensions.
–Leaveroomforadditionaldata,meta-data.
Donotreplicateservicesprovidedbylowerlayerprotocols
–e.g.,checksum 7
Application-Layer Protocols

8. AGENDA
OSI Model Block Diagram
OSI Encapsulation Process
Application Protocol Design
Application Layer Protocols
1. HTTP(Hypertext Transfer Protocol)
2. DNS(Domain Name System)
3. FTP(File Transfer Protocol)
4. TELNET
5. DHCP(Dynamic Host Configuration Protocol)
6. SMTP(Simple Mail Transfer Protocol)
Summary
References
8

9. APPLICATIONLAYERPROTOCOLS
Anapplicationlayerprotocoldefineshowanapplicationprocesses
(clientsandservers),runningondifferentendsystems,pass
messagestoeachother.
Inparticular,anapplicationlayerprotocoldefines:
–Thetypesofmessages,e.g.,requestmessagesandresponse
messages.
–Thesyntaxofthevariousmessagetypes,i.e.,thefieldsinthe
messageand
howthefieldsaredelineated.
–Thesemanticsofthefields,i.e.,themeaningoftheinformationthat
the
fieldissupposedtocontain;
–Rulesfordeterminingwhenandhowaprocesssendsmessages
andrespondstomessages.
9
Application-Layer Protocols

10. CONTINUE..
ManyInternetapplication-layerprotocolsarefullyspecifiedin
Request.
ForCommentsdocuments(RFCs)andarethereforeinthepublic
domain.
–Forexample,theHTTP1.1specificationisincludedinRFC2068,
whichwas
finalizedandmadepublicJanuary1997.
–Ifabrowser(HTTPclient)developerfollowstherulesoftheHTTP
1.1RFC,
thebrowserwillbeabletoretrieveWebpagesfromanyWebserver
thathasalsohasfollowedtherulesoftheHTTP1.1RFC.
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Application-Layer Protocols

11. CONTINUE..
Application layer protocols as follows:
1.HTTP(Hypertext Transfer Protocol)
2.DNS(Domain Name System)
3.FTP(File Transfer Protocol)
4.TELNET
5.DHCP(Dynamic Host Configuration Protocol)
6.SMTP(Simple Mail Transfer Protocol)
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Application-Layer Protocols

12. 1. HTTP
HypertextTransferProtocolisusedtotransferfilesthatmakeup
thewebpagesoftheWorldWideWeb.
HTTP:TCPport80.
WasoriginallydevelopedtopublishandretrieveHTMLpages.
usedfordistributed,collaborativeinformationsystems.
HTTPisusedacrosstheworldwidewebfordatatransferandisone
ofthemostusedapplicationprotocols.
HTTPspecifiesarequest/responseprotocol.
Whenaclient,typicallyawebbrowser,sendsarequestmessageto
aserver,theHTTPprotocoldefinesthemessagetypestheclient
usestorequestthewebpageandthemessagetypestheserver
usestorespond.
Thethreecommonmessagetypesare:
oGET
oPOST
oPUT
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Application-Layer Protocols

13. CONTINUE..
GET is a client request for data. A web browser sends the GET
message to request pages from a web server.
POST and PUT are used to send messages that upload data to the
web server.
HTTP protocol using GET.
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Application-Layer Protocols

14. 2. DNS
HOSTNAMES:
IPAddressesaregreatforcomputers
–IPaddressincludesinformationusedforrouting.
IPaddressesaretoughforhumanstoremember.
IPaddressesareimpossibletoguess.
–everguessedatthenameofaWWWsite?
TheDomainNameSystem:
Thedomainnamesystemisusuallyusedtotranslateahost
nameintoanIPaddress.
Domainnamescompriseahierarchysothatnamesareunique,
yeteasytoremember.
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Application-Layer Protocols

15. CONTINUE..
DNS Hierarchy
educom org jp
rpialbany
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Application-Layer Protocols

16. CONTINUE..
Host name structure:
•Eachhostnameismadeupofasequenceoflabelsseparatedby
periods.
–Eachlabelcanbeupto63characters
–Thetotalnamecanbeatmost255characters.
•Examples:
–whitehouse.gov
–barney.the.purple.dinosaur.com
–monica.cs.rpi.edu
DomainName
•Thedomainnameforahostisthesequenceoflabelsthatleadfrom
thehost(leafnodeinthenamingtree)tothetopoftheworldwide
namingtree.
•Adomainisasubtreeoftheworldwidenamingtree.
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Application-Layer Protocols

17. CONTINUE..
Top level domains:
•edu,gov,com,net,org,mil,…
•Countrieseachhaveatopleveldomain(2letterdomainname).
•Newtopleveldomainsinclude:
.aero.biz.coop.info.name.pro
DNS Organization
•Distributed Database
–Theorganizationthatownsadomainnameisresponsiblefor
runningaDNSserverthatcanprovidethemappingbetween
hostnameswithinthedomaintoIPaddresses.
–So-somemachinerunbyRPIisresponsibleforeverythingwithin
therpi.edudomain.
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Application-Layer Protocols

18. CONTINUE..
DNS Clients:
•ADNSclientiscalledaresolver.
•AcalltogetByName(host)ishandledbyaresolver(typicallypartof
theclient).
•MostUnixworkstationshavethefile/etc/resolv.confthatcontains
thelocaldomainandtheaddressesofDNSserversforthatdomain.
Nslookup:
•nslookupisaninteractiveresolverthatallowstheuserto
communicatedirectlywithaDNSserver.
•nslookupisusuallyavailableonUnixworkstations.
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Application-Layer Protocols

19. CONTINUE..
DNS Servers:
•Servers handle requests for their domain directly.
•Servers handle requests for other domains by contacting remote
DNS server(s).
•Servers cache external mappings.
oDNS Message Format:
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Application-Layer Protocols

20. CONTINUE..
DNS Message Header:
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Application-Layer Protocols

21. 3. FTP
promotesharingoffiles.
encourageindirectuseofremotecomputers.
shielduserfromvariationsinfilestorage.
transferdatareliablyandefficiently.
“FTP,althoughusabledirectlybyauserataterminal,isdesigned
mainlyforusebyprograms”.
Tosuccessfullytransferfiles,FTPrequirestwoconnectionsbetween
theclientandtheserver:oneforcommandsandreplies,andthe
otherfortheactualfiletransfer.
TheclientestablishesthefirstconnectiontotheserveronTCPport
21.Thisconnectionisusedforcontroltraffic,consistingofclient
commandsandserverreplies.
TheclientestablishesthesecondconnectiontotheserveroverTCP
port20.Thisconnectionisfortheactualfiletransferandiscreated
everytimeafileistransferred.
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Application-Layer Protocols

22. CONTINUE..
Theclientcandownload(pull)afilefromtheserverorupload(push)
afiletotheserver.
FTPProcess
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Application-Layer Protocols

23. 4. TELNET
TELNETisaprotocolthatprovides“ageneral,bi-directional,eight-bit
byteorientedcommunicationsfacility”.
telnetisaprogramthatsupportstheTELNETprotocoloverTCP.
ManyapplicationprotocolsarebuiltupontheTELNETprotocol.
TELNETservice:
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Application-Layer Protocols

24. CONTINUE..
The TELNET Protocol
oTCP connection
odata and control over the same connection.
oNetwork Virtual Terminal
onegotiated options
24
Application-Layer Protocols

25. CONTINUE..
Network Virtual Terminal
ointermediaterepresentationofagenericterminal.
oprovidesastandardlanguageforcommunicationofterminalcontrol
functions.
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Application-Layer Protocols

26. CONTINUE..
Negotiated Options:
oAllNVTssupportaminimalsetofcapabilities.
oSometerminalshavemorecapabilitiesthantheminimalset.
oThe2endpointsnegotiateasetofmutuallyacceptableoptions
(characterset,echomode,etc).
oTheprotocolforrequestingoptionalfeaturesiswelldefinedand
includesrulesforeliminatingpossiblenegotiation“loops”.
oThesetofoptionsisnotpartoftheTELNETprotocol,sothatnew
terminalfeaturescanbeincorporatedwithoutchangingtheTELNET
protocol.
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Application-Layer Protocols

27. CONTINUE..
Control Functions:
oTELNETincludessupportforaseriesofcontrolfunctionscommonly
supportedbyservers.
oThisprovidesauniformmechanismforcommunicationof(the
supported)controlfunctions.
oInterrupt Process (IP)
–suspend/abort process.
oAbort Output (AO)
–process can complete, but send no more output to user’s terminal.
oAre You There (AYT)
–check to see if system is still running.
oErase Character (EC)
–delete last character sent
–typically used to edit keyboard input.
oErase Line (EL)
–delete all input in current line. 27
Application-Layer Protocols

28. 5. DHCP
DHCP enables clients on a network to obtain IP addresses and other
information from a DHCP server.
DHCP allows a host to obtain an IP address dynamically when it
connects to the network.
The DHCP server is contacted by sending a request, and an IP
address is requested.
The DHCP server chooses an address from a configured range of
addresses called a pool and assigns it to the host client for a set
period.
different ways of having DHCP servers arranged in next diagram
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Application-Layer Protocols

29. CONTINUE..
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Application-Layer Protocols

30. 7. SMTP
Protocoloriginatedin1982(RFC821,JonPostel)
Standardmessageformat(RFC822,2822,D.Crocker)
Goal:Totransfermailreliablyandefficiently
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Application-Layer Protocols

31. CONTINUE..
SMTPclientsandservershavetwo
maincomponents:
UserAgents–Preparesthe
message,enclosesitinanenvelope.
(ex.Thunderbird,Eudora)
MailTransferAgent–Transfersthe
mailacrosstheinternet(ex.
Sendmail,Exim)
Analogoustothepostalsystemin
manyways
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Application-Layer Protocols

32. CONTINUE..
SMTP also allows the use of
Relays allowing other MTAs
to relay the mail.
Mail Gateways are used to
relay mail prepared by a
protocol other than SMTP and
convert it to SMTP.
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Application-Layer Protocols

33. CONTINUE..
Connection establishment:
33
Application-Layer Protocols

34. CONTINUE..
Message Progress:
34
Application-Layer Protocols

35. CONTINUE..
Connection Termination:
35
TCP Connection Termination
Application-Layer Protocols

36. CONTINUE..
Limitations in SMTP:
OnlyusesNVT7bitASCIIformat
Howtorepresentotherdatatypes?
Noauthenticationmechanisms
Messagesaresentun-encrypted
Susceptibletomisuse(Spamming,
fakingsenderaddress)
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Application-Layer Protocols

37. SUMMARY
OSI model has 7 layers.
Inparticular,anapplicationlayerprotocoldefines:
–Thetypesofmessages,e.g.,requestmessagesandresponse
messages.
Applicationlayerhas6protocols:
1. HTTP(Hypertext Transfer Protocol)
2. DNS(Domain Name System)
3. FTP(File Transfer Protocol)
4. TELNET
5. DHCP(Dynamic Host Configuration Protocol)
6. SMTP(Simple Mail Transfer Protocol)
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38. REFERENCES
Web Sites:
http://www.faqs.org/rfcs/rfc821.html (RFC 821).
http://www.faqs.org/rfcs/rfc2821.html (RFC 2821)
http://www.tcpipguide.com/free/t_SMTPSpecialFeaturesCapa
bilitiesandExtensions.html
http://cns.utoronto.ca/usg/technotes/smtp-intro.html
http://computer.howstuffworks.com/email5.htm
Books:
Programmer's Guide to Internet Mail by John Rhoton
TCP/IP Illustrated Volume 1 by Richard Stevens
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THANK YOU…!!!