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

Abstract

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.

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“…As mentioned in Section III and illustrated in Figure 2c, we use the iPRP redundancy protocol in a communication system with two independent networks. When a packet loss or communication delay takes place in one path, this is repaired by the iPRP via the other path with zero-delay [28]. Note that, as the control loop is executed at a fast pace (in our setup every 100ms), the sporadic loss of packets will not largely affect the system behaviour unless there is a high power jump (this is accounted for in the belief function).…”

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