1. 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

2. Overview Client-Server Redirectors Domain Name System E-mail/Telnet/FTP/HTTP

4. Basics of the Application Layer Application processes Direct network applications Indirect network support Making and breaking a connection

5. 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

6. Direct interface Browser, e-mail, FTP, Telnet Indirect interface Word processors, spreadsheets, presentation managers with a network redirector Application processes

7. 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 computer Provides services

9. 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

10. 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

11. Redirector A protocol that works with computer operating systems and network clients instead of specific application programs

13. Redirectors Apple File Protocol NetBIOS Extended User Interface (NetBEUI) Novell IPX/SPX protocols Network File System (NFS) of the TCP/IP protocol suite

14. Redirector Allows a network administrator to assign remote resources to logical names on the local client (F:, G:, … )

16. Advantages The applications on the client never have to recognize the network Expand the capabilities of non-network software

17. 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

18. Domain Name System Problems with using IP addresses The domain name server

19. 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

20. Generic names.edu - educational sites.com - commercial sites.gov - government sites.org - non-profit sites.net - network service

21. The domain name server Domain name -> IP address The DNS system is set up in a hierarchy that creates different levels of DNS servers

22. Network Applications Internet applications E-mail message DNS function

23. 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

24. 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

27. Application Layer Examples Telnet File transfer protocol Hypertext transfer protocol

28. 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

29. FTP Be designed to download files (e.g. receive from the Internet) or upload files (e.g. send to the Internet)

30. 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

31. Hyperlink A hyperlink is an object (word, phrase, or picture) on a Web page that, when clicked, transfers you to a new Web page

32. Summary Direct and indirect network applications The domain name system Telnet, FTP and HTTP