  Distributed ledger technology has gained wide popularity and adoption since
the emergence of bitcoin in 2008 which is based on proof of work (PoW). It is a
distributed, transparent and immutable database of records of all the
transactions or events that have been shared and executed among the
participants. All the transactions are verified and maintained by multiple
nodes across a network without a central authority through a distributed
cryptographic mechanism, a consensus protocol. It forms the core of this
technology that not only validates the information appended to the ledger but
also ensures the order in which it is appended across all the nodes. It is the
foundation of its security, accountability and trust. While many researchers
are working on improving the current protocol to be quantum resistant,
fault-tolerant, and energy-efficient. Others are focused on developing
different variants of the protocol, best suited for specific use cases. In this
paper, we shall review different consensus protocols of distributed ledger
technologies and their implementations. We shall also review their properties,
concept and similar-work followed by a brief analysis.
