TOS byte on port forwarding-only connections

[Catalin Patulea]

Going back to February 2013:
https://secure.ucc.asn.au/hg/dropbear/rev/80af450dae76
https://secure.ucc.asn.au/hg/dropbear/rev/aa689d140928

Matt, at the time you had called out a potential issue with
connections doing only port forwarding staying on IPTOS_LOWDELAY. Now
I'm actually running into that issue.

'ssh -Lx:x:x cat' is a workaround, albeit ugly. Ideally I would like
'ssh -N -Lx:x:x' to also trigger IPTOS_BULK.

I think for that I could start the connection at LOWDELAY, then reduce
to BULK until the first pty session, then set LOWDELAY again. If the
client deletes the pty session but keeps the connection, it will stay
at LOWDELAY - we probably want it to reduce to BULK in that case.

How about a cleaner approach, where we keep a "refcount on lowdelay",
updated when pty channels are created/removed. When the refcount
transitions from 0->1, set LOWDELAY, 1->0, set BULK. I don't think
it's all that much extra code and it will really do the right thing in
many situations.

How would you feel about a patch for that?

Catalin