How Clients and Servers Work Together

1. How Clients and Servers Work Together

2. Objectives • Web Server Protocols • Examine how e-mail server and client software work • Use FTP to transfer files • Initiate and use a Telnet session • Subscribe to and use newsgroups • Learn about gophers and gopher space

3. Web Server Protocols • HTTP and TCP/IP are the two main protocols used with Web servers. • HTTP methods used for browser requests are GET, POST, HEAD, PUT, and DELETE. • The most frequently used method is GET, which requests files from the Web server. • A dialog is a series of commands from the sender to the receiver and replies from the receiver to the sender.

4. Examining E-Mail Client and Server Software • The sender’s computer and e-mail server both use SMTP (Simple Mail Transfer Protocol) to send an e-mail message to its destination. • After the message arrives at the destination e-mail server, it remains there until the recipient requests delivery.

5. Examining E-Mail Client and Server Software (Continued) • The recipient’s e-mail server uses one of two protocols to deliver the message: POP (Post Office Protocol) or IMAP4 (Internet Message Access Protocol, version 4). • Internet protocols are described and defined in an RFC (Request for Comments) sent to the Internet development community. • When e-mail experts speak of error messages created during e-mail transactions, they sometimes call these messages 822 messages.

6. Managing Your E-Mail Clients • E-mail client software communicates with an e-mail server when it sends and receives e-mail. • Some of the most common e-mail clients are Eudora, Outlook Express, Outlook, and Pegasus Mail. • When you configure your e-mail client software for the first time, you need to enter the addresses of your e-mail servers.

7. How E-Mail Clients and Servers Interact • The three e-mail protocols are SMTP, POP, and IMAP. • The goal is to prepare you to respond to e-mail errors by understanding the source of the error and what you must do to resolve it.

8. SMTP • SMTP is the protocol used to send e-mail over the Internet. • SMTP is typical of many client/server protocols in the TCP/IP protocol suite in that character-based commands are issued from the client and the server replies with numeric codes. • An SMTP transaction begins when an e-mail client program sends an e-mail message to a recipient.

9. SMTP (Continued) • SMTP is considered a stateful protocol because it can recognize and interpret the nature of the material being sent, such as commands or data. • In contrast, TCP is considered a stateless protocol because it is not concerned with what is being sent. • TCP establishes the session but does not interpret the transmissions that occur during the session.

10. POP • POP is used when a client downloads its e-mail messages from a server. • First, the client sends the user ID and password to the server. • The server verifies that the user has an e-mail account with the server. • Then a session is established between the client and the server.

11. POP (Continued) • Next, transactions occur as the client requests the mail, and then the session is closed. • This process contains three states: • Authentication • Transaction • Update

12. IMAP • IMAP is expected to replace POP because it offers these additional functions: • Messages can be archived in folders on the server. • Mailboxes can be shared, so multiple users can access the same mail. • Users can easily access multiple mail servers. • Users can choose to read only the header information about an attached file without opening the file. • Attached files need not be downloaded with every message.

13. E-Mail Server Software • An ISP or large business using the Internet or having an intranet is responsible for providing an e-mail server for its subscribers or employees. • E-mail servers most likely are installed on UNIX, Linux, Windows 2000 Server, or Windows Server 2003.

14. Microsoft Exchange Server • Protocols supported by Exchange Server include HTTP, MAPI, POP3, IMAP4, and NNTP protocols. • NNTP (Network News Transfer Protocol) is the protocol used by newsgroups. • MAPI (Messaging Application Programming Interface) is a specification that allows an application to interact with an e-mail client to send and receive e-mail. • Microsoft Exchange Server supports hot backups and dynamic rerouting.

15. IBM Lotus Domino • Lotus Domino by IBM is designed for large companies and ISPs. • It can be run on a variety of server platforms, such as Windows NT Server, Windows 2000 Server, Windows Server 2003, Sun Solaris, IBM OS/400, and several flavors of Linux. • Domino provides a useful feature that allows administrators to remotely manage the e-mail server from the Web browser on any computer in their network.

16. Novell GroupWise • This product is designed for medium to large companies running NetWare or Windows Server platforms. • To run GroupWise, you must set up Novell Directory Services.

17. E-Mail Client Support for HTML • For most of the time e-mail has existed, e-mail messages have consisted of text only. • Recently, HTML e-mail has become very popular. • Eudora, Outlook Express, and Outlook clients now support HTML in the body of e-mail messages.

18. Using FTP to Transfer Files • Web servers (using HTTP) and e-mail software (using SMTP) must encode data so it appears as text when it travels over the Internet. • FTP (File Transfer Protocol) offers an alternative. • FTP can transfer binary files over the Internet without the encoding and decoding overhead, making it a popular protocol for moving files over the Internet. • An FTP site is a computer running an FTP server application.

19. Using FTP to Transfer Files (Continued) • Large organizations might maintain several FTP sites in different parts of the world to speed up download time across the globe. • These are called mirror sites. • A mirror site is a server that contains the same set of files as a heavily used server to off-load some of the burden of providing the files to the community using them. • Mirror sites also serve as a backup for the main server in case the main server fails.

20. How FTP Works • An FTP server identifies users on an FTP site by their user IDs. • FTP client and server software create a session after you are logged on. • The FTP client has access to the file system on the server. • The local computer (the client) issues character-like commands, and the remote computer (the server) replies with numbers that are interpreted by the local computer.

21. FTP Via a Web Browser • Have you ever attempted to download software from a Web site and clicked a hyperlink that says “Click here to download now” or a similar message? • If you carefully note the URL after you click to download the software, you will see that the protocol changes from http:// to ftp:// in the Web browser’s Address box.

22. FTP from a Command Prompt • Most operating systems, including Windows 9x, Windows NT, Windows 2000, and Windows XP, offer FTP client software that runs from a command prompt. • A batch file is a file with a .bat file extension that contains a list of DOS-like commands that can be executed as a group.

23. FTP from a Command Prompt (Continued) • Another protocol similar to FTP is TFTP (Trivial FTP). • TFTP has fewer commands than FTP and can be used only to send and receive files. • It can be used for multicasting in which a file is sent to more than one client at the same time using the UDP (User Datagram Protocol).

24. Initiating and Using Telnet Sessions • A Telnet window on a computer is a command window to a remote computer in which any command can be executed just as though the user were sitting at the computer console. • Telnet is a protocol used to pass commands and replies between the client the the UNIX computer. • All UNIX systems support some form of Telnet.

25. Subscribing to and Using Newsgroups • A newsgroup is a service on the Internet or private network where a group of people can post articles and responses to those articles so information can be shared among the members of the group. • A newsgroup uses NNTP. • This protocol works much like SMTP, whereby commands are issued from the client or requesting server as character-based words followed by arguments, and replies come from the news server in the form of numeric codes followed by descriptive text.

26. Understanding Gophers and Gopher Space • A gopher is a distribution service for text files on the Internet that runs on a UNIX computer using the Gopher protocol. • A gopher service runs on a UNIX computer, tracking the documents available on the server in the form of a hierarchical site menu called gopher space. • When you access the service, you can browse the gopher space by searching these top-down lists.

27. Summary • A server can be software that provides a service to other software or the computer on which the server software is running. • TCP creates sessions that application protocols, such as HTTP and FTP, can use. • Apache Web Server by the Apache Software Foundation is a common Web server that runs on a UNIX or Windows platform. • Protocol dialogs can be used to troubleshoot problems with servers.