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Semester 1Semester 1

CHAPTER 15CHAPTER 15

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ContentContent

• Basics of the Application Layer.

• Network Applications.

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BASIC OF BASIC OF

THE APPLICATION LAYERTHE APPLICATION LAYER

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FunctionsFunctions

• The application layer (Layer 7) supports the communicating component of an application.

• The application layer:– Identifies and establishes the availability of

intended communication partners.

– Synchronizes cooperating applications.

– Establishes agreement on procedures for error recovery and control of data integrity.

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CharacteristicsCharacteristics

• The application layer does not provide services to any other OSI layer.

• However, it does provide services to application processes lying outside the scope of the OSI model.

• Example :– Browser, Email, FTP, Telnet ...

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InterfacesInterfaces

• Direct network application: – The application layer provides a direct

interface for the rest of the OSI model by using network applications.

– Client server: WEB, FTP, Mail …

• Indirect network application: – The application layer provides a an indirect

interface for the rest of the OSI model by using standalone applications.

– Redirector: word processor, network driver, network printer …

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RedirectorRedirector

• Redirector works with computer OS instead of specific application programs.

• Redirector allows to assign remote resources to logical names on the local computer.

• Examples of redirectors are:

– NetBEUI.

– Novell IPX/SPX.

– NFS in TCP/IP.

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Client-Server applicationClient-Server application

• The client side is located on the local computer and is the requestor of the services.

• The server side is located on a remote computer and provides services in response to the client’s requests.

• The client side is located on the local computer and is the requestor of the services.

• The server side is located on a remote computer and provides services in response to the client’s requests.

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Network driverNetwork driver

• The applications on the client never have to recognize the network.

• Redirectors expand the capabilities of non-network software.

• The applications on the client never have to recognize the network.

• Redirectors expand the capabilities of non-network software.

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Make and Break a connectionMake and Break a connection

• The connection to the server was maintained only long enough to process the transaction.

– Example : WWW, E-mail ...

• The connection to the server was maintained until the user determines that he/she has finished.

– Example : FTP, Telnet ...

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NETWORK APPLICATIONSNETWORK APPLICATIONS

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Application protocolsApplication protocols

• DNS

• E-mail

• Telnet

• FTP

• WWW

• …

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DNS: DNS: Problems with using IP addressProblems with using IP address

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DNSDNS

• The DNS server is a device on a network that manages domain names and responds to requests from clients to translate a domain name into the associated IP address.

• The DNS system is set up in a hierarchy that creates different levels of DNS servers.

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DNS:DNS: System System

vnnvnn comcom eduedu govgov

comcom eduedu govgov ukukfrfrvnvn

..

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..

DNS:DNS: Database Database

vnvn

comcom

cttctt

www.ctt.com.vn 203.162.50.100

www

203.162.4.10

203.162.50.1

203.162.0.1

63.63.0.1

www – 203.162.50.100mail – 203.162.50.101Lab – 203.160.100.1

www – 203.162.50.100mail – 203.162.50.101Lab – 203.160.100.1

ctt – 203.162.50.1aaa – 203.162.70.201bbb – 203.160.9.7

ctt – 203.162.50.1aaa – 203.162.70.201bbb – 203.160.9.7

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DNS:DNS: Resolve Resolve www.yahoo.comwww.yahoo.com

vnnvnn yahooyahoo

comcomvnvn

..

Address of com server

Address of com serverAddress of yahoo.com

server

Address of yahoo.com

server

Address of www.yahoo.com

Address of www.yahoo.com

Address ofwww.yahoo.com

Address ofwww.yahoo.com

RequestRequestRequestRequest

ReplyReplyReplyReply

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EmailEmail

• E-mail client applications, such as Eudora, Outlook Express, and Netscape Mail, work with the POP3 protocol.

• For security, when message recipients check their e-mail they are often prompted for a password. The password can also be saved in many e-mail programs.

• An e-mail address consists of the recipient’s username and post-office address: ctt@yahoo.com.

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TelnetTelnet

• Telnet software provides the ability to remotely access another computer.

• The Telnet application works mainly at the application, presentation and session layers of the OSI model.

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FTPFTP

• FTP is designed to download files or upload files on the Internet.

• An FTP session is established the same way in which a Telnet session is established. Just like Telnet, the FTP session is maintained until the client terminates it, or there is some sort of communication error.

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WWWWWW

• Web pages are created with a format language called Hypertext Markup Language (HTML).

• Hyperlinks make the World Wide Web easy to navigate. A hyperlink is an object on a Web page that, when clicked, transfers you to a new Web page.

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ReviewReview

• Functions of the application layer.

• Direct and indirect network applications.

• Domain name service.

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