SSHv2 doesn't actually have compression levels

SSHv2 doesn't actually have compression levels

[Tobias Geerinckx-Rice]

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Guix,

While reading (guix scripts offload) in a (failed) attempt to get 
offloading working, the following comment caught my eye:

  ;; We rely on protocol-level compression from libssh to optimize 
  large data
  ;; transfers.  Warn if it's missing.

which means that we're not currently doing our own compression 
inside the tunnel.

This is significant, because it means that the COMPRESS-LEVEL 
field of BUILD-MACHINE is completely bogus[0], and probably always 
was - unless it once supported SSHv1, which I think we can agree 
is not something we want to support today.  Nobody's offloading 
libreoffice to that old router in the attic.

I'd like to undocument the option, and add a comment noting its 
lack of any useful effect to the code, by next week.

Kind regards,

T G-R

[0]: Take a look at OpenSSH's readconf.c, packet.c, and all calls 
to start_compression_out().

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Re: SSHv2 doesn't actually have compression levels

[Ludovic Courtès]

Hello,

Tobias Geerinckx-Rice <me@tobias.gr> skribis:

> While reading (guix scripts offload) in a (failed) attempt to get
> offloading working, the following comment caught my eye:
>
>  ;; We rely on protocol-level compression from libssh to optimize
> large data
>  ;; transfers.  Warn if it's missing.
>
> which means that we're not currently doing our own compression inside
> the tunnel.
>
> This is significant, because it means that the COMPRESS-LEVEL field of
> BUILD-MACHINE is completely bogus[0], and probably always was - unless

[...]

> [0]: Take a look at OpenSSH's readconf.c, packet.c, and all calls to
> start_compression_out().

I don’t get it: the ‘compression-level’ field defaults to 3, and OpenSSH
does:

  static int
  start_compression_out(struct ssh *ssh, int level)
  {
          if (level < 1 || level > 9)
                  return SSH_ERR_INVALID_ARGUMENT;
          debug("Enabling compression at level %d.", level);

  [...]

  }

What’s wrong?

Ludo’.

Re: SSHv2 doesn't actually have compression levels

[Tobias Geerinckx-Rice]

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Ludo',

Welcome back :-)

Ludovic Courtès 写道：
> I don’t get it: the ‘compression-level’ field defaults to 3, 
> and OpenSSH
> does:
>
>   static int
>   start_compression_out(struct ssh *ssh, int level)
>   {
>           if (level < 1 || level > 9)
>                   return SSH_ERR_INVALID_ARGUMENT;
>           debug("Enabling compression at level %d.", level);
>
>   [...]
>
>   }
>
> What’s wrong?

Exactly!

  ~/openssh-8.0p1 λ grep -r start_compression_out
  packet.c:start_compression_out(struct ssh *ssh, int level)
  packet.c:	if ((r = start_compression_out(ssh, 6)) != 0)
  packet.c:	if ((r = start_compression_out(ssh, 6)) != 0)

Kind regards,

T G-R

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Re: SSHv2 doesn't actually have compression levels

[Ludovic Courtès]

Tobias Geerinckx-Rice <me@tobias.gr> skribis:

> Ludo',
>
> Welcome back :-)
>
> Ludovic Courtès 写道：
>> I don’t get it: the ‘compression-level’ field defaults to 3, and
>> OpenSSH
>> does:
>>
>>   static int
>>   start_compression_out(struct ssh *ssh, int level)
>>   {
>>           if (level < 1 || level > 9)
>>                   return SSH_ERR_INVALID_ARGUMENT;
>>           debug("Enabling compression at level %d.", level);
>>
>>   [...]
>>
>>   }
>>
>> What’s wrong?
>
> Exactly!

Exactly what?  :-)

>  ~/openssh-8.0p1 λ grep -r start_compression_out
>  packet.c:start_compression_out(struct ssh *ssh, int level)
>  packet.c:	if ((r = start_compression_out(ssh, 6)) != 0)

libssh (which is what ‘guix offload’ uses) appears to honor the
user-provided compression level:

--8<---------------cut here---------------start------------->8---
int compress_buffer(ssh_session session, ssh_buffer buf) {
  ssh_buffer dest = NULL;

  dest = gzip_compress(session, buf, session->opts.compressionlevel);
--8<---------------cut here---------------end--------------->8---

Apologies if I’m missing the obvious!

Thanks,
Ludo’.