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Re: Double compression over HTTPS

From: Lieven Govaerts <lgo_at_apache.org>
Date: Tue, 13 Nov 2012 08:41:53 +0100

On Fri, Oct 5, 2012 at 11:51 AM, Lieven Govaerts <lgo_at_apache.org> wrote:
> Hi,
> when OpenSSL is built with zlib, it will automatically compress all data
> sent over an SSL connection. You can see this in the initial handshake
> "Client Hello" and "Server Hello" where client and server agree on the
> compression mechanism to be used.
> If the data being sent or received is already compressed, OpenSSL will
> compress it a second time. This can happen when already compressed binaries
> like .gif or .zip are sent, or when the server uses gzip encoding for a http
> response. This can have impact on performance and memory usage. See Paul
> Querna's blog post about this topic in [1]. This also has been mentioned
> before on svn-dev by Justin in [2].
> Since OpenSSL 1.0 this automatic compression can be disabled at runtime.
> Compression by OpenSSL has some advantages and disadvantages:
> + OpenSSL will compress the full data stream, so for https that includes all
> headers + all small requests and responses which mod_deflate skips.
> + OpenSSL compression is stateful, it will not reset its dictionary between
> every response like gzip/deflate-encoding does, so it will reach better
> compression ratio when content of multiple consecutive requests or responses
> are similar within a 32KB window. Side note: I have done tests with using a
> preset dictionary for zlib for http(s) responses and found the difference
> can be up to 50% extra compression.
> - Where content is already compressed by the application layer (e.g. gzip
> encoding or transferring binary files), OpenSSL will compress these again.
> I have been doing some small-scale testing to see what difference this all
> makes. My test case was using svn to checkout a copy of the subversion trunk
> branch in the asf repository.
> I have tested 4 different scenario's:
> 1. As-is setup, OpenSSL compression enabled + gzip encoding enabled. (double
> compression)
> 2. OpenSSL compression disabled + gzip encoding enabled. (compression
> handled by the application)
> 3. OpenSSL compression disabled + gzip encoding disabled. (no compression at
> all)
> 4. OpenSSL compression enabled + gzip encoding disabled (compression handled
> by OpenSSL)
> I found this particular scenario too small to see a measurable difference in
> memory or cpu usage, although this is interesting to test further.
> Difference in total times are more interesting:
> | bytes read | bytes written | total time
> 1: | 17.50MB | 233-284KB | 59s
> 2: | 18.67MB | 2.13-2.43MB | 1m9s-1m18s
> 3: | 50.35MB | 2.34MB | 103s-108s
> 4: | 15.27MB | 235-260KB | 50s-56s
> You can see from the above reasoning and my test results that it would be
> beneficial to disable gzip encoding when using https if OpenSSL was built
> with zlib. However, in the scenario where large compressed binary files are
> stored in a svn repository, I suppose disabling both OpenSSL compression and
> gzip encoding will provide the best results.
> Given the above I propose the following:
> - Add an option in serf to disable OpenSSL compression
> - Add a function in serf to check if compression is enabled in OpenSSL.
> - In Subversion, don't ask for gzip encoding when working over https with
> compression.
> - In Subversion, if the config option "http-compression" is set to "no",
> disable both OpenSSL compression and gzip encoding.
> Which makes scenario 4 the default, and the user can select for scenario 3
> with the "http-compression" option.
> Patch to disable OpenSSL compression in serf is attached.
> Suggestions? Objections?

This topic seems to be coming up in issue #3980 - serf increases
server load, so I'd like to clear that none of the proposed changes
have been implemented.
The reason is a confirmed security issue with openssl compression:

The proposed approach here is to disable SSL compression completely,
so in terms of the above that leaves us with scenario 2.

Serf doesn't have an option currently to disable SSL compression from
the client side. I plan to add it in the next version.


> [1]: http://journal.paul.querna.org/articles/2011/04/05/openssl-memory-use/
> [2]: http://svn.haxx.se/dev/archive-2011-05/0362.shtml
Received on 2012-11-13 08:42:47 CET

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