Chapter 2 - Application layer. his chapter explores the different application programs, or services, available at the topmost layer, layer five, of the Internet model. In this chapter, we introduce the application layer and its relationships to the other layers. We discuss the client-server paradigm, the only one used today in the Internet. We also discuss addressing, a feature inseparable from the application layer. We then explore the most common application programs used in the Internet including electronic mail, file transfer, remote logging, World Wide Web, list servers, videoconferencing, and chatting. | Chapter 2 Application Layer The client-server paradigm. Electronic mail. File transfer. Remote login. After reading this chapter, the reader should be able to understand: OBJECTIVES The World Wide Web. Other applications such as videoconferencing and chatting. INTRODUCTION Figure 2-1 Position of the application layer in the Internet model CLIENT-SERVER PARADIGM Application-layer programs in the Internet model communicate using the client-server paradigm. Note: Figure 2-2 Client-server model Figure 2-3 Client-server relationship In 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: ADDRESSING Technical Focus: Domain Name System (DNS) The following are traditional and new domain names: Traditional: | Chapter 2 Application Layer The client-server paradigm. Electronic mail. File transfer. Remote login. After reading this chapter, the reader should be able to understand: OBJECTIVES The World Wide Web. Other applications such as videoconferencing and chatting. INTRODUCTION Figure 2-1 Position of the application layer in the Internet model CLIENT-SERVER PARADIGM Application-layer programs in the Internet model communicate using the client-server paradigm. Note: Figure 2-2 Client-server model Figure 2-3 Client-server relationship In 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: ADDRESSING 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 ELECTRONIC MAIL Email 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 2-4 Email delivery Figure 2-5 POP Figure 2-6 Email address Technical Focus: Inside an Email Message An email is made of an envelope and a letter: Figure 2-7 MIME 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 FILE TRANSFER Figure 2-8 FTP GENERAL-PURPOSE APPLICATION: TELNET TELNET is a general-purpose client-server application .
