High-availability Seamless Redundancy (HSR) is similar to Parallel Redundancy Protocol (PRP) but is designed to work in a ring topology. Instead of two parallel independent networks of any technology (LAN-A and LAN-B), HSR defines a ring with traffic in opposite directions.
In HSR, to allow the determining and discarding duplicate frames, additional protocol specific information is sent with the data frame. In HSR, the frames are identical except for the path field in their 6 octet HSR header (tag), both directions around a loop. The idea is that one copy of the message will reach the destination node, even if the loop is broken.
Periodically, so called supervision frames, which allow supervision of the status of the redundant network, e.g. broken links, are sent.
Network devices which do not have the ability to communicate by HSR, can be connected to an HSR ring via a RedBox, i.e. redundancy box. The intended recipient of the redundant copies of the HSR frame passes the first copy of the message up the network stack and discards the second one.