High Performance Web Sites

Chapter 2. HTTP Overview

Before diving into the specific rules for making web pages faster, it's important to understand the parts of the HyperText Transfer Protocol (HTTP) that affect performance. HTTP is how browsers and servers communicate with each other over the Internet. The HTTP specification was coordinated by the World Wide Web Consortium (W3C) and Internet Engineering Task Force (IETF), resulting in RFC 2616. HTTP/1.1 is the most common version today, but some browsers and servers still use HTTP/1.0.

HTTP is a client/server protocol made up of requests and responses. A browser sends an HTTP request for a specific URL, and a server hosting that URL sends back an HTTP response. Like many Internet services, the protocol uses a simple, plaintext format. The types of requests are GET, POST, HEAD, PUT, DELETE, OPTIONS, and TRACE. I'm going to focus on the GET request, which is the most common.

A GET request includes a URL followed by headers. The HTTP response contains a status code, headers, and a body. The following example shows the possible HTTP headers when requesting the script yahoo_2.0.0-b2.js.

GET /us.js.yimg.com/lib/common/utils/2/yahoo_2.0.0-b2.js HTTP/1.1
Host: us.js2.yimg.com
User-Agent: Mozilla/5.0 (...) Gecko/20061206 Firefox/1.5.0.9

HTTP/1.1 200 OK
Content-Type: application/x-javascript
Last-Modified: Wed, 22 Feb 2006 04:15:54 GMT
Content-Length: 355

var YAHOO=...

Compression

The size of the response is reduced using compression if both the browser and server support ...