Crosschain communications allows information to be communicated between blockchains. Consensus in the context of crosschain communications relates to how participants on one blockchain are convinced of the state of a remote blockchain. It describes how parties associated with a source blockchain come to agreement and communicate with a destination blockchain such that information from the source blockchain can be trusted. This paper surveys crosschain communications protocols, presenting them based on the top-level usage scenarios they are trying to meet: value swapping, crosschain messaging, and blockchain pinning. It analyses how each protocol achieves crosschain consensus, what trust assumptions are made, their ability to operate successfully in Permissionless and Permissioned blockchains contexts, and whether the protocol delivers atomic updates across blockchains.
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