HTTP: The Definitive Guide

Connections

Now that we’ve sketched what HTTP’s messages look like, let’s talk for a moment about how messages move from place to place, across Transmission Control Protocol (TCP) connections.

TCP/IP

HTTP is an application layer protocol. HTTP doesn’t worry about the nitty-gritty details of network communication; instead, it leaves the details of networking to TCP/IP, the popular reliable Internet transport protocol.

TCP provides:

• Error-free data transportation

• In-order delivery (data will always arrive in the order in which it was sent)

• Unsegmented data stream (can dribble out data in any size at any time)

The Internet itself is based on TCP/IP, a popular layered set of packet-switched network protocols spoken by computers and network devices around the world. TCP/IP hides the peculiarities and foibles of individual networks and hardware, letting computers and networks of any type talk together reliably.

Once a TCP connection is established, messages exchanged between the client and server computers will never be lost, damaged, or received out of order.

In networking terms, the HTTP protocol is layered over TCP. HTTP uses TCP to transport its message data. Likewise, TCP is layered over IP (see Figure 1-9).

Figure 1-9. HTTP network protocol stack

Connections, IP Addresses, and Port Numbers

Before an HTTP client can send a message to a server, it needs to establish a TCP/IP connection between ...