*[Byzantine fault tolerant]: Ability to function correctly and reach consensus despite nodes failing or propagating incorrect information to peers.

# Comparing Proof of Authority Consensus Protocols  

Pantheon implements the Clique and IBFT 2.0 Proof of Authority consensus protocols. Proof of Authority 
consensus protocols are used when participants are known to each other and there is a level of trust between them. 
For example, in a permissioned consortium network. 

Proof of Authority consensus protocols allow faster block times and have a much greater throughput of transactions 
than the Ethash Proof of Work consensus protocol used on the Ethereum MainNet. 

In Clique and IBFT 2.0, a group of nodes in the network act as signers (Clique) or validators (IBFT 2.0). These nodes propose, validate, 
and add blocks to the blockchain. Nodes are added to or removed from the signer/validator pool by the existing group of nodes voting. 

!!! note
     For the rest of this page, the term validator is used to refer to signers and validators. 

## Properties 
   
Properties to consider when comparing Clique and IBFT 2.0 are: 

* Immediate finality 
* Minimum number of validators 
* Liveness 
* Speed 

### Immediate Finality 

IBFT 2.0 has immediate finality. When using IBFT 2.0 there are no forks and all valid blocks are included in the main chain.

Clique does not have immediate finality. Implementations using Clique must be aware of forks and chain reorganizations occurring. 

### Minimum Number of Validators 

IBFT 2.0 requires 4 validators to be Byzantine fault tolerant. 

Clique can operate with a single validator but operating with a single validator offers no redundancy if
the validator fails. 

### Liveness 

Clique is more fault tolerant than IBFT 2.0. Clique tolerates up to half to the validators failing. IBFT 2.0 networks 
require greater than or equal to 2/3 of validators to be operating to create blocks. For example, in an IBFT 2.0 network of:

* 4-5, 1 unresponsive node is tolerated 
* 6-8, 2 unresponsive nodes are tolerated 

Networks with 3 or less validators are able to produce blocks but do not guarantee finality when operating 
in adversarial environments.

!!! important 
    We recommend not using IBFT 2.0 networks with 3 nodes for production purposes.  

### Speed 

Reaching consensus and adding blocks is faster in Clique networks. For Clique, the probability of a fork 
increases number as the of validators increases. 

For IBFT 2.0, the time to add new blocks increases as the number of validators increases.