12 are required to implement time based replay protection nonce based replay protection is optional the basic principle of time stamp replay protection is that the node generating a message inserts the current time of day into the message and the node receiving the message checks that this time stamp is sufficiently close to its own time of day the two nodes must have adequately synchronized time of day clocks when sending a registration request the pcf 12 places a time stamp in the high order 32 bits of the identification element the identification element is among the elements protected by the authentication code and therefore cannot be changed by the malicious agent without knowledge of the shared secret the time stamp is the current time specified in universal time coordinated utc time the low order 32 bits could comprise either the fractional seconds associated with the current time or a pseudo random number the value placed in the identification element of a registration request must increase monotonically that is the value placed in the identification element of the registration request must be greater than that used in any previous registration request since the pdsn 14 uses the value in the identification element as a sequence number.