Web Database Applications with PHP and MySQL, 2nd Edition

Appendix D. Web Protocols

In this appendix, we introduce some of the protocols and standards of the Web. We cover the following topics:

• Network basics, including IP addresses and ports.

• The structure of URLs (Uniform Resource Locators).

• When to use the GET and POST methods.

• The HTTP request and response model, including response code classes.

Network Basics

This section briefly introduces the function of the TCP and IP protocols, and explains how IP addresses and ports are used for communication on the Internet.

TCP/IP

The Transmission Control Protocol (TCP) and the Internet Protocol (IP) manage the sending and receiving of messages over the Internet.

The Web is a network application that uses the services of TCP and IP. When a web browser requests a page from a web server, the TCP/IP services provide a virtual connection—a virtual circuit—between the two communicating systems. (The connection is virtual because the Internet doesn't operate like an old telephone network. It doesn't create an actual circuit dedicated to a particular call.)

Once a connection is established and acknowledged, the two systems can communicate by sending messages. These messages can be large, such as the binary representation of an image, and TCP may fragment the data into a series of IP datagrams. An IP datagram is like a postage envelope: it holds all or part of a message and it's labeled with a destination address and other fields that manage its transmission from the sender to the receiver.

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