Vary: Accept-Encoding

Vary: Accept-Encoding

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:

Example request:

GET /2.2.2/build/utilities/utilities.js HTTP/1.1 
Host: yui.yahooapis.com User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.5) Gecko/20070713 Firefox/2.0.0.5 
Accept-Encoding: gzip,deflate

Example response:

HTTP/1.x 200 OK 
Last-Modified: Wed, 18 Apr 2007 17:36:33 GMT 
Vary: Accept-Encoding 
Content-Type: application/x-javascript 
Content-Encoding: gzip 
Cache-Control: max-age=306470616 
Expires: Sun, 16 Apr 2017 00:01:52 GMT Date: Mon, 30 Jul 2007 21:18:16 GMT Content-Length: 22657 
Connection: keep-alive

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: *.