I recently enabled compression in IIS6 but it seems to be working for certain users and not for others. I've verified that it's working for me with Fiddler and Firebug.

For example, I have a particular static file with "sc-bytes" of 2,223, 7,272, and 7,296 in my log file. The largest file size is the uncompressed size.

Here are some sample User Agents and the file sizes for Status 200 requests:

bytes: 2,223

  • Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+2.0.50727)

bytes: 7,296

  • Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+2.0.50727)
  • Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.1;+SV1; +.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+.NET+CLR+3.0.04506.30)

What can I do to start troubleshooting / solve this? Is this something I can control or not?

Web Gardens are enabled, if that makes a difference.


Some older browsers did not handle compressed content correctly, so default server setups try to detect these (from the agent strings) and refuse to send compressed content if such a client is detected.

Also, a browser (or other HTTP client) needs to identify itself as accepting compressed content in the request that it sends - if the relevant accepts-* headers are not present in the request then the response will not be compressed. This is not something you can control server-side. The users may not be able to control it either if they are running through a proxy that does not pass on the relevant headers.

I'm not sure why you would see some requests of 7272 bytes though, if the actual file size is the 7296. How many times has this occurred? If it is only once then it is most likely just a random error that you'll never get to the bottom of. If it is happening regularly then it would be worthy of more detailed investigation.

  • Some sort of corporate proxy sounds plausible. i checked and the 7272 only happened once, so it's probably just a fluke. – Greg Dec 16 '09 at 1:10
  • One extra thing to remember when analysing user-agent names in log files - some browsers and utilities give incorrect user-agent information (or none at all) and some proxies may alter or remove that header too. – David Spillett Dec 16 '09 at 9:55

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