## 1 Introduction  
High-availability seamless redundancy (HSR) was standardized by the International Electrotechnical Commission (IEC) in IEC 62439-3 Clause 5 [1] and is one of the redundancy protocols selected for substation automation systems (SAS) in the IEC 61850 standard [2]. HSR is a redundancy protocol for Ethernet networks that provides zero switchover time in case of a network component failure. HSR is mainly applied for ring topologies. It is based on the duplication of every frame sent in a ring topology. Each copy of the frame is injected in a different direction of the ring. In the fault-free state of the network, the destination node receives two identical frames, passes the first frame to its upper layers, and discards the duplicate. In the case of failure of one network component such as a link failure or a node failure, only one frame is lost. The application on the destination node operates with the remaining frame undisturbed. Therefore, even in the case of a link or node failure, there is no communication interruption in the network. This feature of the HSR protocol makes it very useful for time-critical and mission-critical applications, such as SAS systems, automation networks, and military applications. The HSR principles and operations are described in detail in [1; 2; 3; 4; 5; 6; 7].  
The major drawback of HSR is that it generates too much unnecessary redundant unicast traffic in HSR networks. This drawback results in the degradation of network performance and may cause congestion and delay. Several techniques have been proposed to reduce the unnecessary traffic in HSR networks. These HSR traffic reduction techniques can be classified into the following two categories: traffic filtering-based techniques and predefined path-based techniques. Traffic filtering-based techniques, such as quick removing (QR) [8; 9; 10; 11], traffic control (TC) [12], pork locking (PL) [11; 13; 14], enhanced port locking (EPL) [15], and filtering HSR traffic (FHT) [16] filter unicast traffic for rings in HSR networks, and preventing the traffic from doubling and circulating in the rings. Predefined path-based techniques, such as optimal dual path (ODP) [17], dual virtual paths (DVP) [11; 18; 19], and ring-based dual paths (RDP) [20] establish two separate paths, called dual paths, for each connection pair of terminal nodes or rings before transmitting traffic in HSR networks. The dual paths are then used to forward unicast frames from a source to a destination, instead of duplicating and forwarding frames to all parts of the networks, as in the standard HSR protocol.  
In this paper, we provide a comparison of the current HSR traffic reduction techniques in the literature. The concepts, operational principles, performance, advantages, and disadvantages of these techniques are investigated, summarized, and compared.  
The rest of this paper is organized as follows: in Section 2, we briefly introduce several existing redundancy protocols. Next, in Section 3, we introduce the HSR protocol and its issues. Section 4 describes traffic filtering-based techniques. Section 5 describes predefined path-based techniques. Finally, we provide our conclusions in Section 6.