[Babel-users] failing over faster?

[Juliusz Chroboczek]

> 8+ years ago, with ahcp and babel, and a network configured to use
> that with a single static ip address on both the ethernet and wifi, I
> could do that. My own networks were setup that way, anyway... I did it
> all the time. It was wonderful. I never had to think about it.

Dave, the plan is to do exactly that with shncpd and babeld -- think of
shncpd as ahcpdv2.  Please try running babeld and shncpd (-M) on the host,
and if it doesn't work as well as ahcpd, we'll fix it.

> It was massively disconcerting to attempt to move back into the
> "regular" world where wifi and ethernet were treated as distinct,
> where taking an interface offline lost its address,

Right.  One difference between ahcpd and shncpd -M is that the former uses
a single address, while the latter uses one address per interface.  The
workaround is to keep the interface up, even if it is unconnected.
Since -M is out of spec anyway, I can be convinced to change that.

> where taking a new /64 was considered mandatory,

That's what -M is for.

> and no host changes allowed,

We're not Homenet, Dave, we're independent researchers.  Just because
Homenet rejects something doesn't mean we shouldn't do the right thing.
My personal opinion is that having reasonable support for unchanged hosts
is a goodness, but we shouldn't shy from designing better hosts.

> I've harped on a need for atomic updates, but I still think that
> a userspace routing daemon simply can't react fast enough to a change in
> an ethernet routing table to prevent no-route messages being sent to one
> or more flows on a busy link when it goes down.

Higher-layer protocols should be able to survive ICMP unreachable by
retrying after a few jiffies.  TCP certainly does, and if your protocol
doesn't, it's a bug in the protocol.

> A newer problem that I haven't thunk much about before was that babel
> aims for a stable route, so if I have 3 routes - one stable, but
> lousy, and both the better routes flap twice in under 60 seconds or
> so, we end up choosing the stablest route, sometimes for a very long
> time.

Yes, over the years babeld has been tuned to prefer stable routes.  Have
you tried playing with -M?  I'm quite open to changing its default value.

-- Juliusz