craft TCP acknowledgment messages to add another layer of protection.
Since the sender of spoofing packets is often unable to see any replies, a recipient
host can send acknowledgments that should change the TCP window size
or cause packet retransmission, and then observe whether or not the supposed
source responds correctly. If the supposed source does not change the window
size or does not retransmit the packet, the recipient host considers the packet’s
source to be spoofed.