Redundancy as a Measure of Fault-Tolerance for the Internet of Things: A Review

In this paper we review and analyze redundancy-based fault-tolerant techniques for the IoT as a paradigm to support two of the main goals of computer security: availability and integrity. We organized the presentation in terms of the three main tasks performed by the nodes of an IoT network: sensing, routing, and control. We first discuss how the implementation of fault-tolerance in the three areas is primary for the correct operation of an entire system. We provide an overview of the different approaches that have been used to address failures in sensing and routing. Control devices typically implement state machines that take decisions based on the measurement of sensors and may also ask actuators to execute actions. Traditionally state-machine replication for fault-tolerance is realized through consensus protocols. Most protocols were developed in the 80’s and 90’s. We will review the properties of such protocols in detail and discuss their limitations for the IoT. Since 2008, consensus algorithms took a new direction with the introduction of the concept of blockchain. Standard blockchain based protocols cannot be applied without modifications to support fault-tolerance in the IoT. We will review some recent results in this new class of algorithms, and show how they can provide the flexibility required to support fault-tolerance in control devices, and thus overcome some of the limitations of the traditional consensus protocols