about add deflate support to http_gzip_filter_module

matt at nepheliad.net matt at nepheliad.net
Tue Dec 30 09:53:12 MSK 2008

My readings of the applicable RFCs (2616, 1950, 1951, and 1952) indicate 
that you can't just "strip" the header off a proper RFC-compliant gzip 
stream and call it "deflate". The "gzip" compression is a RFC 1952 gzip 
stream which may contain one or more members. The "deflate" compression is 
an RFC 1950 zlib  + RFC 1951 deflate stream, which differs from a "raw" RFC 
1951 DEFLATE stream.

I realize that many popular web servers simply send a RFC 1951 stream, but 
in my opinion, if it's added to nginx, it ought to be done right. :) I've 
not looked at the relevant code yet, but getting a zlib stream instead of a 
gzip stream should be easy enough, in the end.

Also, due to the fact that a zlib stream is "simpler" than a gzip stream, 
many implementations of zlib are actually faster (though potentially a tiny 
bit larger in terms of total data). So, instead of a separate 'gzip_deflate' 
directive, I'd like to see it implemented as "always on" (in case a User 
Agent only requests deflate for some reason). Then the gzip directive could 
be modified to have 3 options: Off; On (same as 'prefer_gzip'); and 
'prefer_deflate' (for those sites which prefer the efficiency over the 
potential < 10% bandwidth savings). Potentially, there could also be a 
'never_deflate' option to the gzip directive. Honestly, I can't see a huge 
reason for this, myself.

By changing the gzip directive, any upgrading sites would "automagically" 
get the new feature while allowing sites to fine-tune their outgoing 
content. And all without yet-another-directive. And it could be documented 
akin to gzip_proxied:


syntax: gzip off|prefer_gzip|prefer_deflate|on

default: gzip off

context: http, server, location, if (x) location

Enables or disables gzip compression.

   * off - disables compression for all requests
   * prefer_gzip - sends gzip content to user agents who can accept both 
gzip and deflate content
   * prefer_deflate - sends deflate content to user agents who can accept 
both gzip and deflate content
   * on - synonym for "prefer_gzip". For backwards-compatibility with older 

But count this as a message in support of the feature (else I would not have 
bothered typing so much). I'd just like to see it implemented in a way that 
is as useful as possible, and without mimicing "other" web servers' buggy 



From: "Paul Gao" <paulgao at yeah.net>
Date: Monday, December 29, 2008 6:13 PM
To: <nginx at sysoev.ru>
Subject:  about add deflate support to http_gzip_filter_module

> I have a suggestion on improving http_gzip_filter_module and want to 
> discuss
> with you:
> Currently most browsers support both the gzip and deflate compression 
> methods,
> and the algorithms of gzip and deflate are almost the same,
> except that the deflate compression removes an 18 bytes header.
> In order to save bandwidth, I hope to add a 'gzip_deflate' option which 
> could
> support deflate compression when enabled. Default value is Off.
> Any ideas?

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