Ethernet has become ubiquitous - even in niche domains such as automotive and industrial networks.
In this blog we will look at two industrial-ethernet protocols: PRP (Parallel Redundancy Protocol) and HSR (High-availability Seamless Redundancy).
Specified in IEC 62439-3, PRP and HSR are designed to provide zero-recovery-time redundancy to prevent downtime in mission-critical networks.
While PRP achieves redundancy by duplicating networks, HSR creates a ring topology - both ensuring that a single network failure doesn’t disrupt communication. These protocols are vital in environments like power substations, industrial automation, and railway systems where even milliseconds of network interruption can have severe consequences.
Parallel Redundancy Protocol (PRP)
PRP ensures network redundancy by sending duplicate packets over two independent networks. The end devices (nodes) are equipped with two network interfaces connected to two separate LANs (LAN A and LAN B).
- A sender transmits identical frames over both LANs.
- A receiver accepts the first frame to arrive and discards the duplicate from the second LAN.
- If one network fails, the receiver still gets the frames via the other network, ensuring seamless communication.
This design has the following advantages
- Zero recovery time: No network reconfiguration is needed after a failure.
- Redundancy without special switches: Works with standard Ethernet switches.
- Independent LANs: Each LAN can operate independently.
PRP is used in critical systems in power substations, industrial automation, and transportation.
High-availability Seamless Redundancy (HSR)
HSR provides redundancy using a single LAN configured as a ring topology. Devices are interconnected in a ring, and every device has two network ports.
- A frame is sent in both directions around the ring.
- The receiver processes the first frame it receives and discards duplicates.
- If a link in the ring fails, the frame can still reach its destination via the other path.
This leads to the following features
- Zero recovery time: Communication continues without interruption during a link failure.
- Ring topology: Requires HSR-compatible devices; no standalone switches are needed.
- Frame duplication: Ensures redundancy at the device level.
HSR is used in critical systems in railway systems, industrial plants, and utility grids.
Frame format
Conceptually the frame format is similar for PRP and HSR with differences in their placement.
PRP is added as a trailer to an Ethernet frame.
+-----------------+---------+-------------+-----+
| Ethernet Header | Payload | PRP Trailer | FCS |
+-----------------+---------+-------------+-----+
HSR is added as a header to an Ethernet frame similar to VLAN tags.
+-----------------+---------+---------+-----+
| Ethernet Header | HSR Tag | Payload | FCS |
+-----------------+---------+---------+-----+
Both the PRP/HSR tags are 6 bytes long and contain the following fields.
- PRP/HSR EtherType: 16 bits
- Path Id: 4 bits
- Frame Size: 12 bits
- Seq Number: 16 bits
In a future blog post we will look at how to generate traffic for these protocols using Ostinato.
Would you like to use Ostinato to test other industrial-ethernet protocols like IEC 61850 or SV or GOOSE? Let us know!