Request regarding CHAN_INPUT_MAX

[Nico Williams]

On Fri, Apr 10, 2026 at 03:32:35PM -0400, Chris Rapier wrote:
> By restricting the advertised window to the maximum of the OpenSSH client I
> can at least get a ~16MB receive window. If you can increase the limit to
> 32MB (or whatever) I can improve throughput when OpenSSH clients are sending
> data to HPN-SSH.
> 
> I can spend some more time nailing this down if you are potentially open to
> this change.

The problem with this sort of change is that it effectively creates the
risk of the sorts of pathologies SSHv1 had that motivated SSHv2, that
bulk transfers on low-bandwidth paths will swamp out interactive
channels, causing ssh to feel unresponsive.

The SSHv2 nested flow control model has been called a "hand brake", and
it is a hand brake.  But multiplexing channels without flow control has
its own problems.

"HPN" needs to be an option that can be set / reset.  I don't know
whether it should be the default or not (it should be the default for
scp, but maybe not for sftp, and maybe not for ssh), and ideally it
should be adaptive in some manner (but, how?), but it has to at least be
possible to turn it off.

IMO,

Nico
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