From: Florian Westphal <fw@strlen.de>
Date: Wed, 29 Oct 2014 13:23:07 +0100

We could do that, if you prefer.

I tried to come up with a scenario though, where sysctl_tcp_ecn=0, and
then we want to enable 'passive' ecn for incoming connections only on
a particular route without announcing ecn to the peer. I haven't been
able to find any -- I think if you deem 'route to x' safe for ecn it
might as well be enabled for both initiator and responder.  The original
patch would be sufficient for that.

IOW, is 'ecn from a to b but not b to a' a sensible requirement?

I think you have to apply the same logic for the sysctl (there's a
reason to only support ECN passively) as you do for the route feature
because you can logically look at the sysctl as applying to the
default route.

Unrelated to this patch, but I'd like to see sysctl_tcp_ecn=1 as a
default at one point (almost no routers set CE bit at this time, perhaps
that would change if ecn usage is more widespread).

Now you're talking.

So, either passive ECN support makes sense or it does not.  To me, no
matter what the argument, it doesn't matter what realm (whole system,
specific routes) you apply that argument to.