Chap 15 Application Layer Andres, Wen-Yuan Liao Department of Computer Science and Engineering De Lin Institute of Technology

Overview Client-Server Redirectors Domain Name System /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, , 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 The client side On the local computer The requestor of the services The server side On a remote computer Provides 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

Redirector A 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 network Expand the capabilities of non-network software

Making/breaking a connection Web The connection was maintained just long enough to download the current Web page Printer The 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 message DNS function

Internet applications The WWW uses the HTTP protocol Remote access programs : Telnet protocol programs : POP3 application layer protocol for electronic mail File utility programs : FTP protocol

message The first is to send the to the user ’ s post office The second is to deliver the from that post office to the user ’ s client

Application Layer Examples Telnet File transfer protocol Hypertext transfer protocol

Telnet All processing and storage take place on the remote computer Works 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 application Requires 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