[ovs-discuss] Thoughts on gratuitous ARP

[Miguel Angel Ajo Pelayo]

I have a patch ready for ovn to send REPLIES additionally to the requests
so, if we believe it makes sense, I can send it to the dev list.

On Thu, Nov 16, 2017 at 4:18 PM, Ihar Hrachyshka <ihrachys at redhat.com>
wrote:

> In neutron reference implementation, initially we were sending ARP
> REPLYs only that didn't work with existing kernels in specific
> scenarios. We added ARP REQUESTs (while continuing sending REPLYs). I
> think it makes sense to send both types because some nodes may e.g.
> know about REPLYs and not REQUESTs. If linux kernel had the mistake of
> ignoring gratuitous REQUESTs till very recently, there may be other,
> more niche networking stacks also exposing inconsistent behavior.
>
> Ihar
>
> On Thu, Nov 16, 2017 at 6:22 AM, Miguel Angel Ajo Pelayo
> <majopela at redhat.com> wrote:
> > Sorry, it's the other way around.
> >
> > REQUEST is what neutron reference solution started using (ANSWER was the
> > previous type of ARP packet which was leading to issues with the buggy
> > kernels).
> >
> > Since ovn-controller emits gratuitous ARPs as broadcast ARP requests,
> that
> > should
> > work.
> >
> > @Ihar, if you can confirm that our understanding is correct, that'd be
> > great, I also
> > see that your upstream kernel patch is really modifying the behaviour to
> > also
> > catch ANSWER packets with sha == tha, which should be equivalent as per
> RFC.
> >
> > On Wed, Nov 15, 2017 at 8:10 PM, Miguel Angel Ajo Pelayo
> > <majopela at redhat.com> wrote:
> >>
> >> See inline email, I wasn't subscribed to ovs-discuss, sorry :)
> >>
> >>
> >> On Nov 15, 2017 2:32 PM, "Miguel Angel Ajo Pelayo" <majopela at redhat.com
> >
> >> wrote:
> >>
> >>
> >> We're finding that sometimes reused floating IP addresses
> >> won't be reachable, for some reason. And I remembered that,
> >> we found the same issue once for the reference
> >> solution once, it was fixed here: [1]
> >>
> >> Basically because the linux kernel, under some conditions will
> >> ignore the gARP requests, and reset a timeout value that would
> >> keep ignoring those requests. But if it received a REPLY
> >> packet instead, it worked.
> >>
> >> I believe that we may want to do the same in ovn controller.
> >>
> >> [1]
> >> https://github.com/openstack/neutron/commit/
> 82831b9d8d4bd61f90610df9eca8c7f6e447f8d8
> >>
> >>
> >
>
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