travel through their respective LANs until they reach a destination node (DANP) with a certain time skew. The destination node accepts the first frame of a pair and discards the second (if it arrives). Therefore, as long as one LAN is operational, the destination application always receives one frame. PRP provides zero-time recovery and allows to check the redundancy continuously to detect lurking failures. Frame format To simplify the detection of duplicates, the frames are identified by their source address and a sequence number that is incremented for each frame sent according to the PRP protocol. The sequence number, the frame size, the path identifier and an Ethertype are appended just before the Ethernet checksum in a 6-octet PRP trailer. This trailer is ignored (considered as padding) by all nodes that are unaware of the PRP protocol, and therefore these singly attached nodes (SAN) can operate in the same network. NOTE: all legacy devices should accept Ethernet frames up to 1528 octets, this is below the theoretical limit of 1535 octets. Implementation The two Ethernet interfaces of a node use the same MAC address. This is allowed since the two LANs have no connection. Therefore, PRP is a layer 2 redundancy, which allows higher layer network protocols to operate without modification. A PRP node needs only one IP address. Especially, the ARP protocol will correctly relate the MAC to the IP address. Clock synchronization IEC 62439-3 Annex C specifies the Precision Time Protocol Industry Profile that support a clock synchronization over PRP with an accuracy of 1 μs after 15 network elements, as profile of IEEE Std 1588 Precision Time Protocol. Clocks can be doubly attached according to PRP, but since the correction is different according to the path, the duplicate discard method of PRP is not applicable. Also, delay measurement messages (Pdelay_Req & Pdelay_Resp) are not duplicated since they are link-local. About every second, a master clock sends two copies of a Sync message, but not at exactly the same time since the ports are separate, therefore the original Syncs have already different time stamps. A slave receives the two Sync messages at different times and applies the Best Master Clock Algorithm (BMCA), and when the two Sync come from the same grandmaster, the clock quality is used as a tie-breaker. A slave will normally listen to one port and supervise the other, rather than switching back and forth