This chapter covered a little bit about three popular applicationse-mail, FTP, and the World Wide Web. However, many more applications exist, and there's much more depth to the applications than covered here. Transparent file and print services, database services, remote login, and other options for file transfer and e-mail make the mix of applications and application protocols rich. Hopefully, after seeing a little detail about some of the applications and their protocols, you have noticed many similarities, like the following:
In particular, e-mail uses an organizational model like the postal service. It uses SMTP and POP3 protocols to deliver e-mail. SMTP is used to send e-mail to servers, and POP3 is used to retrieve mail from its POP3 server. The e-mail transfer includes a header, defined by the Internet Message Formats RFC, as well as the text of the e-mail. The clients and servers know where to forward the e-mail based on the two-part e-mail address. File transferin particular with FTPuses a model like a warehouse to store goods. Any user who has proper authority can put files on the server. Later, other clients can log in to the FTP server and get copies of the files. Finally, web browsing uses a model like retail stores. First, a website is built, which includes the server hardware and software and web pages. Later, an end user can browse the web pages of that site, much like when you go to a bricks-and-mortar retail store and browse the items in the store. Browsers use HTTP to retrieve files from the server; oftentimes, the first file of a web page, frequently in HTML format, gives instructions to the browser that it should obtain other files to get everything needed to display the web page correctly. |