*[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](#immediate-finality) 
* [Required validator availability](#required-validator-availability) 
* [Type of Byzantine Fault Tolerance](#type-of-byzantine-fault-tolerance)
* [Speed](#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. 

### Required Validator Availability

For the network to be able to create blocks and progress, the amount of validators (rounded up to the next integer)
required to be online is at least:

* 2/3 for IBFT 2.0.
* 1/2 for Clique.

### Type of Byzantine Fault Tolerance

#### IBFT 2.0 

IBFT 2.0 features a classical BFT consensus protocol that ensures safety (no fork is possible) provided that no more than
(n-1)/3 of the validators (truncated to the integer value) are Byzantine.
For example in an IBFT 2.0 network of:

* 3, no Byzantine nodes are tolerated
* 4-6, 1 Byzantine node is tolerated
* 7-9, 2 Byzantine nodes are tolerated

If more than (n-1)/3 of the validators (truncated to the integer value) are Byzantine, forks and reorganizations may occur 
from any height after which sufficient nodes were compromised. 

#### Clique

Clique features a probabilistic consensus protocol (like Nakamoto) where minor forks always occur. The deeper a block is
in the chain, the more probable it is that the block is final (that is, the block will not be part of a reorganization event).
The higher the network latency is, the more depth is required for a block to be considered stable.

If more than 1/2 of the validators (rounded up to the next integer) are Byzantine, the network is compromised 
and the history of the chain can be re-written.

### Speed 

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

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