net.inet.ipsec.test_replay - When set to 1, IPsec will send packets with
the same sequence number. This allows to verify if the other side
has proper replay attacks detection.
net.inet.ipsec.test_integrity - When set 1, IPsec will send packets with
corrupted HMAC. This allows to verify if the other side properly
detects modified packets.
I used the first one to discover that we don't have proper replay attacks
detection in ESP (in fast_ipsec(4)).