IEC 62439 PRP: Bumpless recovery for highly available, hard real-time industrial networks

2015 IEEE World Conference on Factory Communication Systems (WFCS)

Tomozei

et al. 2015

Abstract-Reliable packet delivery within stringent delay constraints is of primal importance to industrial processes with hard real-time constraints, such as electrical grid monitoring. Because retransmission and coding techniques counteract the delay requirements, reliability is achieved through replication over multiple fail-independent paths. Existing solutions such as parallel redundancy protocol (PRP) replicate all packets at the MAC layer over parallel paths. PRP works best in local area networks, e.g., sub-station networks. However, it is not viable for IP layer wide area networks which are a part of emerging smart grids. Such a limitation on scalability, coupled with lack of security, and diagnostic inability, renders it unsuitable for reliable data delivery in smart grids. To address this issue, we present a transport-layer design: IP parallel redundancy protocol (iPRP). Designing iPRP poses non-trivial challenges in the form of selective packet replication, soft-state and multicast support. Besides unicast, iPRP supports multicast, which is widely using in smart grid networks. It duplicates only time-critical UDP traffic. iPRP only requires a simple software installation on the end-devices. No other modification to the existing monitoring application, end-device operating system or intermediate network devices is needed. iPRP has a set of diagnostic tools for network debugging. With our implementation of iPRP in Linux, we show that iPRP supports multiple flows with minimal processing and delay overhead. It is being installed in our campus smart grid network and is publicly available. I. INTRODUCTIONSpecific time-critical applications (found for example in electrical networks) have such strict communication-delay constraints that retransmissions following packet loss can be both detrimental and superfluous. In smart grids, critical control applications require reliable information about the network state in quasi-real time, within hard delay constraints of the order of approximately 10 ms. Measurements are streamed periodically (every 20 ms for 50 Hz systems) by phasor measurement units (PMUs) to phasor data concentrators (PDCs). In such settings, retransmissions can introduce delays for successive, more recent data that in any case supersede older ones. Also, IP multicast is typically used for delivering the measurements to several PDCs. Hence, UDP is preferred over TCP, despite its best-effort delivery approach. Increasing the reliability of such unidirectional (multicast) UDP flows is a major challenge.The parallel redundancy protocol (PRP, IEC standard [1]) was proposed as a solution for deployments inside a local area network (LAN) where there are no routers. Communicating devices need to be connected to two cloned (disjoint) bridged networks. The sender tags MAC frames with a sequence number and replicates it over its two interfaces. The receiver discards redundant frames based on sequence numbers.PRP works well in controlled environments, like a substation LAN, where network setup is entirely up ...

mentioning

confidence: 99%

iPRP: Parallel redundancy protocol for IP networks

2015 IEEE World Conference on Factory Communication Systems (WFCS)

Tomozei

et al. 2015

“…In the literature, the desired QoS target is achieved by replication of packets over two or more fail-independent paths using PRP at the MAC-layer [6], [7] or iPRP at the IPlayer [8], [9]. Needless to say, these protocols rely on the existence of fail-independent paths to reap maximum benefits from the replication.…”

Section: Introductionmentioning

confidence: 99%

Experimental validation of the usability of Wi-Fi over redundant paths for streaming phasor data

2016 IEEE International Conference on Smart Grid Communications (SmartGridComm)

Giannakopoulos

et al. 2016

Abstract-Applications performing streaming of phasormeasurement data require low latency and losses from the communication network. Traditionally, such requirements are realized through wired infrastructure. Recently, wireless infrastructure has gained attention due to its low-cost and ease of deployment, but its poor quality-of-service is a strong deterrent for use in mission-critical applications. Recent studies have used measurements to explore the use of packet replication over redundant Wi-Fi paths, for obtaining the desired loss performance without hampering the end-to-end latency. However, these studies are done in a controlled, laboratory environment and do not reflect the real, in-field performance. In this paper, we perform extensive measurements using two co-located directional Wi-Fi links in a real-life setting, to experimentally validate the use of packet replication over Wi-Fi for streaming phasor data. In the setting that we evaluated, we find that the two channels are not fail-independent but the performance achieved with replication is very close to what it would be if they were to be independent. From the loss and latency statistics after replication, we conclude that replicating the phasor data over redundant Wi-Fi paths is a viable option for achieving the desired quality-of-service.

“…As mentioned in Section I, iPRP overcomes the limitations of PRP [9]. The authors of [11] are aware of the fact that PRP is limited to LANs and suggest a direction for developing PRP in an IP environment.…”

Section: Related Workmentioning

confidence: 99%

“…The parallel redundancy protocol (PRP) IEC standard [9] was proposed as a solution to packet replication over multiple fail-independent paths for local area networks (LANs) where there are no routers. Communicating devices need to be connected to two cloned (disjoint) bridged networks.…”

Section: A From Mac-to Ip-layer Parallel Redundancy Protocolmentioning

confidence: 99%

iPRP—The Parallel Redundancy Protocol for IP Networks: Protocol Design and Operation

Tomozei

et al. 2016

IEEE Trans. Ind. Inf.

Abstract-Reliable packet delivery within stringent delayconstraints is of paramount importance to mission-critical computer applications with hard real-time constraints. Because retransmission and coding techniques counteract the delay requirements, reliability may be achieved through replication over multiple fail-independent paths. Existing solutions, such as the parallel redundancy protocol (PRP), replicate all packets at the MAC layer over parallel paths. PRP works best in local area networks. However, it is not viable for IP networks that are a key element of emerging mission-critical systems. This limitation, coupled with diagnostic inability and lack of security, renders PRP unsuitable for reliable data-delivery in these IP networks. To address this issue, we present a transport-layer solution: the IP parallel redundancy protocol (iPRP). Designing iPRP poses non-trivial challenges in the form of selective packetreplication, soft-state and multicast support. iPRP replicates only time-critical unicast or multicast UDP traffic. iPRP requires no modifications to the existing monitoring application, end-device operating system or to the intermediate network devices. It only requires a simple software installation on the end-devices. iPRP has a set of diagnostic tools for network debugging. With our implementation of iPRP in Linux, we show that iPRP supports multiple flows with minimal processing-and-delay overhead. It is being installed in our campus smart-grid network and is publicly available. I. INTRODUCTIONSpecific mission-critical computer applications have hard delay-constraints. Failure to satisfy these constraints can result in economic losses or, even worse, human lives can be endangered in cases when these failures affect safety mechanisms. Notable examples of such applications (often built on top of cyber-physical systems) are process-control and emergencymanagement applications in the oil and gas industry Reliable and timely packet delivery, even in the order of 10 ms, is of utmost importance in satisfying the hard-delay constraints. The classic approaches to reliable communication through coding and retransmission are not compatible with the hard delay-constraints. An alternative is to achieve reliability through replication over multiple fail-independent paths, which is the focus of this paper. More precisely, we present a solution for packet-replication over multiple paths in IP networks. Indeed, as we discuss next, existing solutions apply to MAClayer networks and cannot be transposed to IP networks that are a requirement for many of the aforementioned applications.