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