The sequence number in the ESP header in IPsec is used to prevent replay attacks. Here, the receiver will only accept one packet with each sequence number and reject a packet with a sequence number...

The sequence number in the ESP header in IPsec is used to prevent replay attacks. Here, the receiver will only accept one packet with each sequence number and reject a packet with a sequence number that has already been used. What then prevents an attacker from sending a packet with a higher sequence number to cause the original packet with this sequence number from being accepted?