Another interesting relaxation of the time synchronization requirement is that, if we assume that the receiver's clock drift is negligible during a period of time, then the receiver can receive the data stream from the sender before doing a time synchronization and authenticate the data later after a time synchronization. The receiver only needs to store the arrival time of each packet, so that it can evaluate the security condition after it performed the time synchronization. This is highly useful for many applications, for example a router can use TESLA to authenticate itrace messages [3], and the victim can authenticate the routers' IP markings afterwards when it wants to trace an attacker by performing an approximate time synchronization with the router [31].