[ovs-dev] [PATCH RFC 0/3] - send garp on localnet

[Ramu Ramamurthy]

Problem:
When a VM comes up with interface on localnet, external
switches on the broadcast segment have to update their
port->mac mappings and routers have to update their arp caches.
This normally happens when the VM initiates traffic.
on the localnet interface. But in some scenarios, when the
VM remains silent, the VM is unreachable externally. This can
happen when the VM is migrated or when the VM is reusing an earlier used
IP but with a different mac. The fix is to for OVN to initiate gratuitous ARP
which updates external routers/switches on localnet.

This problem has been reported at:
https://bugs.launchpad.net/networking-ovn/+bug/1545897

What to generate:
When ipv4 and mac are defined on the port, we can generate a gratuitous ARP
request or a reply. ARP requests seem to be preferred over ARP replies
as suggested here:
https://tools.ietf.org/html/rfc5227#section-3
In this change, a broadcast garp reply is generated.

How many to generate:
3 to 5 arps spaced over 1-2 minutes seems to be reasonable to allow
for losses. For comparison qemu generates 5 rarps when a VM port comes up.

In this change, the garp packet is input to the port, and goes through
the pipelines. An alternative is to output directly
onto the localnet patch port.