[ovs-dev] [patch_v2 1/3] ovn: Skip logical switch "router type" port arp responder install.

Darrell Ball dlu998 at gmail.com
Mon Oct 3 21:21:59 UTC 2016

On Mon, Oct 3, 2016 at 10:54 AM, Han Zhou <zhouhan at gmail.com> wrote:

> On Sun, Oct 2, 2016 at 2:14 PM, Darrell Ball <dlu998 at gmail.com> wrote:
> >
> >
> >
> > On Sun, Oct 2, 2016 at 11:27 AM, Han Zhou <zhouhan at gmail.com> wrote:
> >>
> >> On Sat, Oct 1, 2016 at 4:34 PM, Darrell Ball <dlu998 at gmail.com> wrote:
> >> >
> >> > Do not install any potential logical switch "router type"
> >> > port arp responders.  Logical router port arp responders
> >> > should be sufficient in this respect.
> >> > It seems a little wierd for a logical switch not proxying
> >> > for a remote VIF to be responding to arp requests and we
> >> > are not functionally using this capability in ovn.
> >> >
> >> Hi Darrell,
> >>
> >> The arp responder for patch port is useful e.g. when a VM pings the
> default gateway IP. Would removing the flow cause the arp request get
> flooded? And what's the benefit of removing it here?
> >
> >
> > 1) Modelling: I would expect the L3 gateway arp responder to be
> associated with the L3
> > gateway router datapath, at the very least. That way, the modeling is
> correct and we don't have a situation where, for example, a phantom gateway
> router is never even downloaded to a HV,
> > but is "responding" or rather appearing to respond to arp requests.
> >
> Ok, I see your concern. To achieve this expectation, it may be done in a
> way that is similar as the regular LS ports: reply ARP only if
> Logical_Switch_Port.up = true. When gateway router is bound to a chassis we
> can set the LS patch port up to true. And for distributed routers we can
> set patch port up directly. This way we can avoid responding ARP before
> gate router is bound.

I think you missed the main aspect.
There is a layering violation in doing this and also a modeling issue.
The key idea can be summarized as "A logical router should respond to arps
to itself" rather than some logical switch proxying that.
This has implications for cases where an IP address is shared by several
and then the binding is used to designate the gateway used.

> However, I wonder even this change may not be needed. For my understanding
> ARP is just to resolve address. Do you see any real problem of replying
> even if the gateway router is not yet bound? I don't think this is a
> problem of modeling. It might look weird just because it behaves slightly
> different from traditional view. I would prefer keep the simplicity.

Having both logical switch and logical router arp responders for the same
gateway router
is not simpler; it is more complicated
I suggest having a single arp responder built by the associated logical

> > 2) We install an arp responder for the logical routers, including L3
> gateway(s) today (see below).
> > We check for inport in this rule and this inport is only associated with
> the L3 gateway HV.
> > So only the L3 gateway HV should respond. Meaning, if there is a
> response, the L3 gateway
> > datapath is really there.
> But the L2 flooding would still happen, right?

Of course;
Since a L3 gateway resides on a remote HV only, the packets need to
traverse the
network to confirm reachability and binding of that L3 gateway.

> >
> > 3) Usually, there are a limited number of L3 gateways and therefore
> associated bindings.
> > Also, for VMs participating in south<->north traffic, the bindings are
> less likely
> > to timeout since there are multiple uses of the L3 gateway for each VM.
> >
> With a big L2, even a small percent of VM doing ARP will cause annoying
> flooding. Moreover, considering containers come and go frequently this
> would be more common. So I think it is still better to suppress ARP for
> south-north if there is no real problem.

I don't buy it.

Today, we skip using arp responders for packets arriving on localnet and
vtep ports,
meaning the arp requests go to all VMs.
This would be a much more serious issue since external abuse is possible.

This L3 gateway case is more limited and other approaches are possible to
We discussed this internally and we are otherwise thinking to have a user
configuration for arp responders in general.

If we really cannot tolerate a few containers coming and going then we have
a serious
problem that already exists for localnet and vtep cases as well as pure L2

> >
> >>
> >>
> >>
> >> Han
> >
> >

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