How Clients and Servers Work Together

Objectives Web Server Protocols Examine how 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

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.

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

Examining Client and Server Software (Continued) The recipient’s 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 experts speak of error messages created during transactions, they sometimes call these messages 822 messages.

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

How Clients and Servers Interact The three 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.

SMTP SMTP is the protocol used to send 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 message to a recipient.

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.

POP POP is used when a client downloads its 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.

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

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.

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

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 client to send and receive . Microsoft Exchange Server supports hot backups and dynamic rerouting.

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 server from the Web browser on any computer in their network.

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.

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

Using FTP to Transfer Files Web servers (using HTTP) and 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.

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.

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.

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 to ftp:// in the Web browser’s Address box.

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.

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).

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.

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.

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.

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.