[Ecn-sane] mosh ecn bits washed out

[Dave Taht]

On Fri, Mar 19, 2021 at 1:25 AM Pete Heist <pete at heistp.net> wrote:
>
> Meaning, the negotiation succeeds and ECT(0) is set going up, but
> zeroed on packets by the time they come down?

There has never been any "negotiation" for setting the ect(0) bit in mosh.
It's been set since 2012 on all udp packets.

I reasoned at the time, that since mosh is the tool of desparate sysadmins
everywhere coping with excessive latency and jitter on everything (the local
echo of keystrokes is a godsend), that turning further enabling CE would
help, or, if stuff with that bit set was dropped, be the focus of
outrage by sysadmins
everywhere and ecn support quickly reverted.

Nothing went wrong. :)

mosh has an extremely robust response to marks, reducing the rate to 2 frames
per second from as much as 60 fps on receipt of a single CE. Not going
any further is analogous to the  "subpacket window" problem ecn
enabled tcps have, and attempting to
sustain that low frame rate essential for mosh's operation.

Ironically, since I last bothered to look, mosh gained ipv6 support, but the
parsing of cmsg and the setsockopt for ecn capability were not updated for that.

Also ironically, mosh packets were originally marked AF42 but it stopped working
on one of MITs networks, so the dscp setting was removed, but ecn kept.

https://github.com/mobile-shell/mosh/blob/master/src/network/network.cc#L166

It would be trivial to sce enable mosh.

>
> The negotiation can also be blocked with iptables --ecn-tcp-remove,
> which just zeroes out ECE and CWR, preventing negotiation
> (https://git.netfilter.org/iptables/tree/extensions/libipt_ECN.c), but
> I doubt that's very commonly done.
>
> On Thu, 2021-03-18 at 13:00 -0700, Dave Taht wrote:
> > mosh, which has long had excellent support for ecn, appears to be
> > getting the
> > ecn bit washed out along my path from california to england.
> >
> > ecn survives up that way, but not down.
> >
> > Just a single data point thus far.
> >
>
>


-- 
"For a successful technology, reality must take precedence over public
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dave at taht.net <Dave Täht> CTO, TekLibre, LLC Tel: 1-831-435-0729