A test for ndp -c was sometimes failed because between the deletion
and the check NS/NA messages were exchanged and a NDP cache was
recreated unexpectedly. To provent this situation, we do ifconfig
shmif0 down of the peer before the test, so the test won't be
interfered by the messages.
- Take a diff between packet dumps and use it for packet checking
- it's resistant against packet reorder
- Seep 2 sec to make sure a NS message is sent
- Disable tentative state check for now
- it's too ephemeral to check robustly
