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Gzip Components


Reducing the amount of time it takes to transfer an HTTP request and response across a network improves the user experience. There are a number of ways to do this. In this three-part article series, you'll learn how to do it by reducing the size of the HTTP response with gzip encoding. This article is excerpted from chapter four of the book High Performance Web Sites: Essential Knowledge for Frontend Engineers, written by Steve Sounders (O'Reilly; ISBN: 0596529309). Copyright © 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
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January 03, 2011
TABLE OF CONTENTS:
  1. · Gzip Components
  2. · What to Compress

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Gzip Components
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The time it takes to transfer an HTTP request and response across the network can be significantly reduced by decisions made by frontend engineers. It’s true that the end user’s bandwidth speed, Internet service provider, proximity to peering exchange points, and other factors are beyond the control of the development team. However, there are more variables that affect response times. Rules 1 and 3 address response times by eliminating unnecessary HTTP requests. If there is no HTTP request then there is no network activity—the ideal situation. Rule 2 improves response times by bringing the HTTP response closer to the user.

Rule 4, examined in this chapter, reduces response times by reducing the size of the HTTP response. If an HTTP request results in a smaller response, the transfer time decreases because fewer packets must travel from the server to the client. This effect is even greater for slower bandwidth speeds. This chapter shows how to use gzip encoding to compress HTTP responses, and thus reduce network response times. This is the easiest technique for reducing page weight and it also has the biggest impact. There are other ways you can reduce the HTML document’s page weight (strip comments and shorten URLs, for example), but they are typically less effective and require more work.

How Compression Works

The same file compression that has been used for decades to reduce file sizes in email messages and on FTP sites is also used to deliver compressed web pages to browsers. Starting with HTTP/1.1, web clients indicate support for compression with the
Accept-Encoding header in the HTTP request.

 Accept-Encoding: gdef, deflate

If the web server sees this header in the request, it may compress the response using one of the methods listed by the client. The web server notifies the web client of this via theContent-Encodingheader in the response.

 Content-Encoding: gzip

Gzip is currently the most popular and effective compression method. It is a free format (i.e., unencumbered by patents or other restrictions) developed by the GNU project and standardized by RFC 1952. The only other compression format you’re likely to see is deflate, but it’s slightly less effective and much less popular. In fact, I have seen only one site that uses deflate: msn.com. Browsers that support deflate also support gzip, but several browsers that support gzip do not support deflate, so gzip is the preferred method of compression.


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