The issue is that some vdevs, like the arp proxy, want to
send traffic back out the same port that received it in some
cases, but the ingress port has been handled in such a way
that vdevs not at the head of the chain lose the information
about the original physical ingress port.  There are two
ways to fix this - change the controller (primarily in the
Lease class), or change HP4.  Changing HP4 looks a lot easier,
and though composition abstraction flexibility is no longer
as straightforward, it is still there.  Specifically, a vdev
in a virtual network wanting to return traffic back to another
vdev will have to do so via explicit forwarding instead of
simply copying ingress_port to egress_spec.  The compiler can
be modified to accommodate this change without requiring the
user to be aware of it.