Problem
A replay attack is one in which an attacker obtains a copy of an authenticated packet and later transmits it to the intended destination. The receipt of duplicate, authenticated IP packets may disrupt service in some way or may have some other undesired consequence. The Sequence Number field in the IPsec authentication header is designed to thwart such attacks. Because IP is a connectionless, unreliable service, the protocol does not guarantee that packets will be delivered in order and does not guarantee that all packets will be delivered. Therefore, the IPsec authentication document dictates that the receiver should implement a window of size W, with a default of W = 64. The right edge of the window represents the highest sequence number, N, so far received for a valid packet. For any packet with a sequence number in the range from N - W + 1 to N that has been correctly received (i.e., properly authenticated), the corresponding slot in the window is marked Deduce from the figure how processing proceeds when a packet is received and explain how this counters the replay attack.