1. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Chapter 2 Application Layer

2. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 The client-server paradigm. Electronic mail. File transfer. Remote login. After reading this chapter, the reader should be able to understand: O BJECTIVES The World Wide Web. Other applications such as videoconferencing and chatting.

3. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 INTRODUCTIONINTRODUCTION 2.1

4. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-1 Position of the application layer in the Internet model

5. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 CLIENT-SERVERPARADIGMCLIENT-SERVERPARADIGM 2.2

6. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Application-layer programs in the Internet model communicate using the client-server paradigm. Note:

7. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-2 Client-server model

8. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-3 Client-server relationship

9. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In the client-server paradigm, many clients can be served by one server, either one at a time or concurrently. Note:

10. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In the client-server paradigm, a client runs for a specific period of time; the server runs continuously. Note:

11. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In the client-server paradigm, there are specific servers and normally one general-purpose server. Note:

12. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 ADDRESSINGADDRESSING 2.3

13. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Domain Name System (DNS) The following are traditional and new domain names: Traditional: com edu gov int net org New: aero biz coop info museum name pro

14. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 ELECTRONICMAILELECTRONICMAIL 2.4

15. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Email is a store-and-forward application on the Internet. Note:

16. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A user agent controls the composing, reading, forwarding, replying, and saving of email messages. The user agent is not responsible for sending or receiving email. Note:

17. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 The task of sending and receiving email is done by a mail transfer agent (MTA). Note:

18. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 The formal protocol for electronic mail in the Internet is Simple Mail Transfer Protocol or SMTP. Note:

19. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-4 Email delivery

20. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-5 POP

21. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-6 Email address

22. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Inside an Email Message An email is made of an envelope and a letter:

23. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-7 MIME

24. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: How MIME works MIME has five header types that can be added to the original SMTP header: MIME-Version Content-Transfer-Encoding Content-Type Content-ID Content-Description

25. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 FILETRANSFERFILETRANSFER 2.5

26. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-8 FTP

27. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 GENERAL-PURPOSEAPPLICATION:TELNETGENERAL-PURPOSEAPPLICATION:TELNET 2.6

28. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 TELNET is a general-purpose client-server application program. Note:

29. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-9 Local login

30. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-10 Remote login

31. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 WORLD WIDE WEB (WWW) 2.7

32. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-11 Distributed services

33. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-12 Hypertext

34. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-13 Browser architecture

35. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-14 HTTP transaction

36. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-15 Message categories

37. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-16 Request and response messages

38. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-17 URL

39. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Business Focus: History of WWW The idea of the World Wide Web started in 1989 at the European Particle Physics Laboratory, CERN, in Geneva, Switzerland. Tim Bernes-Lee needed to create a large database for physics research, which he found impossible to do using one single computer. The obvious solution was to let each piece of information be stored on an appropriate computer and let the computers be linked together through hypertext. In 1993, the University of Illinois, under the supervision of Anderson, created the first graphical browser called Mosaic. In 1994, Anderson and some colleagues started Netscape, one of the most popular browsers today. Another widely used browser is Microsoft Explorer.

40. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: HTML Hypertext Markup Language (HTML) is a language for creating web pages. The term markup language comes from the book publishing industry. Before a book is typeset and printed, a copy editor reads the manuscript and corrects it with marks. These marks tell the designer how to format the text. For example, to make part of a text displayed in boldface with HTML, we must include the beginning and ending boldface tags (marks) in the text, as shown below: This is the text to be bold The two tags and are instructions for the browser.

41. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 VIDEOCONFERENCINGVIDEOCONFERENCING 2.8

42. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-18 Videoconferencing

43. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 GROUP DISCUSSION: LISTSERV LISTSERV 2.9

44. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 2-19 Listserv

45. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 To subscribe, the user sends the request to the subscriber server, not the mailer server. Note:

46. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 To send an email to every member of the group, the user sends it to the mailer server, which then automatically relays it to every subscriber. Note:

47. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 CHATTINGCHATTING 2.10