 Replay is an attack in which data is maliciously retransmitted by an attacker. Assume, for example, that Alice asks Bob to pay 100 euros. The request is sent to Bob, but the attacker is able to intercept that request. The attacker retransmits the message again, possibly as part of a masquerade attack. Bob receives also this second message and believes that now 200 euros should be paid in total. Replay requires the ability to intercept data, for example, by using a wiretap, as well as the ability to masquerade as Alice while resending the message to Bob. Protection against replay attacks is generally possible by including in every message a sequence number. Since the IP protocol may deliver packets out of sequence, replay protection algorithms may include a window mechanism to accommodate that. In some cases, such as the SNP version 3 protocol, replay protection may be based on timers.