SSH1 vs. SSH2 - compression level

Damien Miller djm at
Sat Nov 8 08:26:26 EST 2003

On Fri, 2003-11-07 at 02:36, Tomasz Chmielewski wrote:
> Hello,
> I was searching for this information virtually everywhere, but as I 
> couldn't find it - I'm asking here.
> I was wondering, why setting the Compression Level was removed in SSH2, 
> and if on, is always set to 6.

IIRC protocol 2 doesn't allow negotiation of compression levels, so 
we have to choose something. 6 is a good tradeoff between CPU and size.

> In SSH1 it was possible to set the Compression Level from 1 to 9.
> I have made some tests with Compression Levels using scp: SSH1, 
> compression 9 (highest available for SSH1), vs. SSH2, compression 6 (the 
> only available for SSH2), and, no wonder, SSH1 *always* won, no matter 
> if it was tar'red /etc (lots of txt files), a long pdf file, or even 
> long avi file.

I'm pretty surprised that compressionlevel made any difference with avi
or pdf files, which are usually precompressed. Your speed differences
were probably due to the fact that protocol 1 is more lightweight (crc
instead of MAC, etc - thus also less secure).


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