Serve gzipped Content
Most modern browsers understand gzipped (compressed) content, so a well-performing page should aim to serve all of its content compressed. Since most images, swf files and other media files are already compressed, you don't need to worry about compressing them.
You do, however, need to take care of serving compressed HTML, CSS, client-side scripts, and any other type of text content. If you make XMLHttpRequests to services that return XML (or JSON, or plain text), make sure your server gzips this content as well.
If you open the Net panel in Firebug (or use LiveHTTPHeaders or some other packet sniffer), you can verify that the content is compressed by looking for a Content-Encoding header in the response, as shown in the following example:
GET /2.2.2/build/utilities/utilities.js HTTP/1.1
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:22.214.171.124) Gecko/20070713 Firefox/126.96.36.199
HTTP/1.x 200 OK
Last-Modified: Wed, 18 Apr 2007 17:36:33 GMT
Expires: Sun, 16 Apr 2017 00:01:52 GMT
Date: Mon, 30 Jul 2007 21:18:16 GMT
In this request, the browser informed the server that it understands gzip and deflate encodings (Accept-Encoding: gzip,deflate) and the server responded with gzip-encoded content (Content-Encoding: gzip).
There's one gotcha when it comes to serving gzipped content: you must make sure that proxies do not get in your way. If an ISP's proxy caches your gzipped content and serves it to all of its customers, chances are that someone with a browser that doesn't support compression will receive your compressed content.
To avoid this you can use the Vary: Accept-Encoding response header to tell the proxy to cache this response only for clients that send the same Accept-Encoding request header. In the example above, the browser said it supports gzip and deflate, and the server responded with some extra information for any proxy between the server and client, saying that gzip-encoded content is okay for any client that sends the same Accept-Encoding content.
There is one additional problem here: some browsers (IE 5.5, IE 6 SP 1, for instance) claim they support gzip, but can actually experience problems reading it (as described on the Microsoft downloads site, and the support site). If you care about people using these browsers (they usually account for less than 1% of a site's visitors) you can use a different header -- Cache-Control: Private -- which eliminates proxy caching completely. Another way to prevent proxy caching is to use the header Vary: *.
To gzip or to Deflate?
If you're confused by the two Accept-Encoding values that browsers send, think of deflate as being just another method for encoding content that's less popular among browsers. It's also less efficient, so gzip is preferred.
Make Sure you Send gzipped Content
Okay, now let's see what you can do to start serving gzipped content in accordance with what your host allows.
Option 1: mod_gzip for Apache Versions Earlier than 2
If you're using Apache 1.2 and 1.3, the mod_gzip module is available. To verify the Apache version, you can check Firebug's Net panel and look for the Server response header of any request. If you can't see it, check you provider's documentation or create a simple PHP script to echo this information to the browser, like so:
<?php echo apache_get_version(); ?>
In the Server header signature, you might also be able to see the mod_gzip version, if it's installed. It might look like something like this:
Server: Apache/1.3.37 (Unix) mod_gzip/188.8.131.52a.....
mod_gzip_item_include mime ^application/json$
mod_gzip_item_include mime ^text/.*$
mod_gzip_item_include file \.html$
mod_gzip_item_include file \.php$
mod_gzip_item_include file \.js$
mod_gzip_item_include file \.css$
mod_gzip_item_include file \.txt$
mod_gzip_item_include file \.xml$
mod_gzip_item_include file \.json$
Header append Vary Accept-Encoding
The first line enables mod_gzip. The next three lines set compression based on MIME-type. The next section does the same thing, but on the basis of file extension. The last line sets the Vary header to include the Accept-Encoding value.
If you want to send the Vary: * header, use:
Header set Vary *
Note that some hosting providers will not allow you to use the Header directive. If this is the case, hopefully you should be able to substitute the last line with this one:
This will also set the Vary header to Accept-Encoding.
Be aware that there might be a minimum size condition on gzip, so if your files are too small (less than 1kb, for example), they might not be gzipped even though you've configured everything correctly. If this problem occurs, your host has decided that the gzipping process overhead is unnecessary for very small files.