Whenever packets are sent from one device to another, they are tagged with a sequence number. Sometimes when packets leaves the sender device, they may not always arrive at the destination in the same sequential order, therefore the sequence number can assist the recipient device in the reassembly process.

Anti-replay monitors the packets with their sequence numbers as they flow across the IPSec tunnel, if the anti-replay feature detects a packet with the same sequence number that has already entered the tunnel, the packet will not be allowed to enter. Only packets with unique sequence numbers are accepted.