[X2Go-Dev] Traffic shaping nightmare - missing SSH TOS bits

John A. Sullivan III jsullivan at opensourcedevel.com
Tue Feb 21 07:05:15 CET 2012


Hello, all.  I've been working on a project to implement traffic shaping
to conform to 95th percentile billing including our X2Go virtual
desktops.  One of the challenges is prioritizing the interactive X2Go
traffic (screen, keyboard, mouse) differently from bulk traffic like
local share transfers, printing, and possibly sound.  I thought this
would be trivial because of the SSH transport since SSH sets the
minimize-delay bit for interactive traffic and the maximize-throughput
bit for bulk transfers.  Thus we could distinguish the packets by
looking at the TOS bits.  This is how the default Linux pfifo_fast qdisc
does it anyway.

I took a packet trace just to confirm it and, to my horror, I see that
we are not setting any of the TOS bits and thus not prioritizing
interactive traffic even with the default traffic shaping.

I'm guessing this is a casualty of moving to libssh.  Is there any way
to set the TOS bits for interactive to 0x10, bulk to 0x08 as it is in
OpenSSH?.  It would be really nice if sound could be configured either
way according the the administrator's preference.  Unfortunately, I did
not see anything in the libssh documentation to show how to do this
although I do not claim to understand it.

This is a critical issue for any WAN based deployments like ours.
Thanks - John




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