[Babel-users] Loop-freedom [was: emacs...]

[Dave Taht]

On Mon, Nov 12, 2018 at 8:18 AM Juliusz Chroboczek <jch at irif.fr> wrote:
>
> >> do loop-free routing.
>
> > Hah. :)
>
> > I note that babel doesn't actually do that, any more.
>
> Pick a pair (p, id), where p is a prefix and id is a router id.  Consider
> the graph defined by all route table entries indexed by (p, id).  Then
> Babel guarantees that at any time this graph is acyclic.
>
> Since the union of multiple acyclic graphs is not in general acyclic,
> Babel does not guarantee that the forwarding graph for a prefix p is
> acyclic in the case where p is originated at different routers, although
> it does guarantee that any loops disappear in linear time (with a constant
> that depends on the amount of packet loss).  And of course Babel does not
> guarantee loop-freedom in the presence of redistribution between protocols
> (as in your case with DHCP).

I DO get it. :)

The practical thing, is that you won't see unreachables (while a
gateway reboots, for example, or
the routing table is optimized) while duking it out with other metrics
from other protocols. If we really really care about loop freedom for
those short intervals, keeping the babel metric the same when going
unreachable is the way to go.

'course if that route is GC'd we end up with the same problem a few
minutes later.

It would be really good to get all the distro designers and all the
routing protocol daemon writers and all the address assignment daemon
developers in one room, to hammer out one, consistent way to do this
stuff right.

But I'm pretty sure we'd run out of pizza and the ranks decimated by
the ensuing cannibalism long before that universal solution was
achieved, and we'd be back to using sneakernet universally.

(see also the addrlabel discussion for ipv6)

>
> -- Juliusz
>
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-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740