Application Layer

Chapter 2ApplicationLayerThe client-server paradigm.Electronic mail.File transfer.Remote login.After reading this chapter, the reader should be able to understand:OBJECTIVESThe World Wide Web.Other applications such as videoconferencing and chatting. INTRODUCTION2.1Figure 2-1Position of the application layer in the Internet modelCLIENT-SERVERPARADIGM2.2Application-layer programs in the Internet model communicate using the client-server paradigm.Note:Figure 2-2Client-server modelFigure 2-3Client-server relationshipIn the client-server paradigm, many clients can be served by one server, either one at a time or concurrently.Note:In the client-server paradigm, a client runs for a specific period of time; the server runs continuously.Note:In the client-server paradigm, there are specific servers and normally one general-purpose server. Note:ADDRESSING2.3Technical Focus: Domain Name System (DNS)The following are traditional and new domain names:Traditional: com edu gov int net orgNew: aero biz coop info museum name proELECTRONICMAIL2.4Email is a store-and-forward application on the Internet.Note: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:The task of sending and receiving email is done by a mail transfer agent (MTA).Note:The formal protocol for electronic mail in the Internet is Simple Mail Transfer Protocol or SMTP. Note:Figure 2-4Email deliveryFigure 2-5POPFigure 2-6Email addressTechnical Focus: Inside an Email MessageAn email is made of an envelope and a letter:Figure 2-7MIMETechnical Focus: How MIME worksMIME has five header types that can be added to the original SMTP header:MIME-VersionContent-Transfer-EncodingContent-TypeContent-IDContent-DescriptionFILETRANSFER2.5Figure 2-8FTPGENERAL-PURPOSEAPPLICATION:TELNET2.6TELNET is a general-purpose client-server application program.Note:Figure 2-9Local loginFigure 2-10Remote loginWORLD WIDE WEB(WWW)2.7Figure 2-11Distributed servicesFigure 2-12HypertextFigure 2-13Browser architectureFigure 2-14HTTP transactionFigure 2-15Message categoriesFigure 2-16Request and response messagesFigure 2-17URLBusiness Focus: History of WWWThe 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. Technical Focus: HTMLHypertext 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.VIDEOCONFERENCING2.8Figure 2-18VideoconferencingGROUP DISCUSSION:LISTSERV2.9Figure 2-19ListservTo subscribe, the user sends the request to the subscriber server, not the mailer server.Note: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:CHATTING2.10