Post on 04-Jan-2016
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Chap 15 Application Layer
Andres, Wen-Yuan Liao
Department of Computer Science and Engineering
De Lin Institute of Technology
andres@dlit.edu.tw
http://www.cse.dlit.edu.tw/~andres
OverviewClient-Server
Redirectors
Domain Name System
E-mail/Telnet/FTP/HTTP
Basics of the Application Layer
Application processes
Direct network applications
Indirect network support Making and breaking a connection
Application processes
Supports the communicating component of an application
The OSI layer closest to the end system
Does not provide services to any other OSI layer
Direct interface Browser, e-mail, FTP, Telnet
Indirect interface Word processors, spreadsheets, presentation managers with a network redirector
Application processes
Direct network applications
Client-server applications (most)
FTP, web browsers, and e-mail
The client side On the local computer The requestor of the services
The server side On a remote computerProvides services
WWW
Browsers (NN and IE) are probably the most commonly used network applications
An easy way to understand a Web browser is to compare it to a television remote control
Indirect network support
If a client wants to save a file from a word processor to a network server, the redirector enables the word processing application to become a network client
RedirectorA protocol that works with computer operating systems and network clients instead of specific application programs
Redirectors
Apple File Protocol NetBIOS Extended User Interface (NetBEUI) Novell IPX/SPX protocols Network File System (NFS) of the TCP/IP protocol suite
Redirector
Allows a network administrator to assign remote resources to logical names on the local client (F:, G:, … )
Advantages
The applications on the client never have to recognize the networkExpand the capabilities of non-network software
Making/breaking a connection
Web The connection was maintained just
long enough to download the current Web page
PrinterThe connection was maintained just
long enough to send the document to the print server
Domain Name System
Problems with using IP addresses
The domain name server
Problems with using IP addresses
A domain is a group of computers that are associated by their geographical location or their business type
A domain name is a string of characters and/or numbers
More than 200 top-level domains
Generic names
.edu - educational sites
.com - commercial sites
.gov - government sites
.org - non-profit sites
.net - network service
The domain name server
Domain name -> IP address
The DNS system is set up in a hierarchy that creates different levels of DNS servers
Network Applications
Internet applications
E-mail message
DNS function
Internet applications
The WWW uses the HTTP protocol
Remote access programs : Telnet protocol
E-mail programs : POP3 application layer protocol for electronic mail
File utility programs : FTP protocol
E-mail message
The first is to send the e-mail to the user’s post office
The second is to deliver the e-mail from that post office to the user’s e-mail client
Application Layer Examples
Telnet
File transfer protocol
Hypertext transfer protocol
TelnetAll processing and storage take place on the remote computerWorks mainly at the top three layers of the OSI model Application, presentation , session layers
FTP
Be designed to download files (e.g. receive from the Internet) or upload files (e.g. send to the Internet)
Hypertext transfer protocol
The fastest growing and most used part of the Internet
A Web browser is a client-server applicationRequires both a client and a
server component in order to function
Hyperlink
A hyperlink is an object (word, phrase, or picture) on a Web page that, when clicked, transfers you to a new Web page
Summary
Direct and indirect network applications
The domain name system
Telnet, FTP and HTTP