The anti-replay window protocol is used to secure IP against an adversary that can insert (possibly replayed) messages in the message stream from a source computer to a destination computer in the Internet. In this paper, we discuss this important protocol and point out a potential problem faced by the protocol, in which severe reorder of messages can cause the protocol to discard a lot of good messages. We then introduce a controlled shift mechanism that can reduce the number of discarded good

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... messages by sacrificing a relatively small number of messages. We use simulation to show that the modified protocol is more effective than the original protocol when a severe reorder of messages occurs. In particular, we show that the modified protocol reduces the number of discarded good messages by up to 70%.