The success of blockchain as the underlying technology for
cryptocurrencies has opened up possibilities for its use in other
application domains as well. The main advantages of blockchain for its
potential use in other areas are its inherent security mechanisms and
immunity to data manipulation attacks. A blockchain relies on a
consensus method for agreeing on any new data. Most of the consensus
methods which are currently used for the blockchain of different
cryptocurrencies require high computational power and thus are not
suitable for resource-constrained systems.

In this article, we survey the various blockchain-based consensus
methods that are applicable to resource-constrained IoT devices and
networks. In a typical IoT network, there exist several devices with
limited computational and communication capabilities. Most often, these
devices cannot perform intensive computations and are starved for
bandwidth. Therefore, we discuss the possible measures that can be taken
to reduce the computational power and convergence time for the
underlying consensus methods. We also talk about some of the
alternatives to the public blockchain, such as private blockchain and
tangle, along with their potential adoption for IoT networks.
Furthermore, we review the existing consensus methods that have been
implemented and explore the possibility of utilizing them to realize a
blockchainbased IoT network. Some of the open research challenges
including AI-enabled blockchains are also put forward.