One reason for the current explosive growth in network applications is the availability of good tools for creating them. The World Wide Web is possibly the most important of these tools. It runs on a super network known as the Internet. This name reflects the fact that the Internet connects disparate networks around the world. The Internet is an example of a TCP/IP network.

When companies use this technology within their proprietary networks, the result is called an intranet. When companies share an information system based on this technology with suppliers or customers, the result is called an extranet.

Tcl/Tk provides solid support for those of you who wish to write applications based on this technology. This chapter introduces you to client/server technology over TCP/IP networks as it is supported by Tcl. Questions of how to provide security in an environment that passes scripts from computer to computer are discussed in Chapter 21, "Security In a World of Push."

Because servers typically deal with many clients at once, they require a style of programming that can schedule activities that will happen in parallel. Event-driven programming fills this need. It is supported by Tcl and is introduced in this chapter.

• TCP/IP Networks as Seen from Tcl
• The World Wide Web as a Client/Server Application
• Creating a Web Client in Tcl
• The Event Loop
• Timeouts
• Background Error Handling
• Client/Server the Tcl Way
• More about the Event Loop
• The http Package
• A Brief HTML Introduction
• Solutions to Exercises