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From Ben Noordhuis <>
Subject Re: Vary:User-Agent, best practices, and making the web faster.
Date Sat, 04 Jun 2011 23:58:40 GMT
On Sun, Jun 5, 2011 at 00:34, Joshua Marantz <> wrote:
> It was with some reluctance that I brought this up.  It occurs to me that
> this idea propagates the sort of spec violations that led to this issue
> (inappropriate user of Vary:User-Agent) in the first place.   However, I'm
> trying to figure out how to improve compliance to support legitimate uses of
> Vary:User-Agent without causing mod_pagespeed to become significantly less
> ineffective across a broad range of sites.
> We have found that putting complaints in Apache logs mostly causes disks to
> fill and servers to crash -- although that does get it noticed :).  The
> problem, put another way, is that mod_pagespeed cannot distinguish
> legitimate uses of Vary:User-Agent, so it really has no business complaining
> in logs.  Complaining in docs is fine; but some existing mod_pagespeed users
> that simply type "sudo yum update" will later notice a performance-drop and
> may not consult the docs to figure out why.
> I'm also trying to grok the first response from Eric:
> It's because of the other (dated) canned exceptions that set/unset
> no-gzip/gzip-only-text/html based on the User-Agent, to second-guess
> browsers that send AE:gzip but can't properly deal with it.
> Going backwards:  which browsers send AE:gzip but can't properly deal with
> it?   Does IE6 have that issue or is it only true of IE5?   I know that IE6
> has had issues with compression in the past but they appear to be addressed
> by patches issued by Microsoft four and a half years ago:
>;en-us;Q312496.  Moreover
> IE6 is shrinking in market
> share<>(~
> 10%) and IE5 does not appear in the pie-chart at all.

This was indeed a (since fixed) problem with IE6. I haven't seen the
gzip issue crop up since but that is purely anecdotal.

> And I still don't understand how that relates to Vary:User-Agent.  What's
> really at issue here seems more related to proxies; is that right?  That
> proxies were not respecting Accept-Encoding, but sending gzipped content to
> browsers that did not want it?  Is that still a problem?  Which proxies were
> broken?  Are they still broken?

Some popular OSS packages depend on Vary: User-Agent to make
downstream proxies (reverse or forward) do the right thing.

> And, while I understand the reluctance to help me figure out from our module
> what values were passed to SetEnvIfNoCase and Header, I would like to see
> whether there's agreement that the Apache 2.2 docs for mod_deflate are no
> longer appropriate -- and in fact harmful.

I've been mulling it over for 10 minutes and I can't decide. It's
harmful because it leads to a proliferation of cached objects (bad)
but removing it from the documentation will break things for someone
somewhere (also bad).

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