This is in contrast to the companion standard HSR (IEC 62439-3 Clause 5), with which PRP shares the operating principle.
PRP and HSR are independent of the application-protocol and can be used by most Industrial Ethernet protocols in the IEC 61784 suite.
Cost impact is low since it makes little difference if the spares lie on the shelf or are actually working in the plant.
The two frames travel through their respective LANs until they reach a destination node (DANP) with a certain time skew.
PRP provides zero-time recovery and allows to check the redundancy continuously to detect lurking failures.
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.
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.
This gave a good error detection coverage but made difficult the transition from PRP to the High-availability Seamless Redundancy (HSR) protocol, which uses a ring topology instead of parallel networks.
This allowed building transparent PRP-HSR connection bridges and nodes that can operate both as PRP (DANP) and HSR (DANH).