[ovs-discuss] Thoughts on gratuitous ARP

[Ihar Hrachyshka]

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
>>
>>
>