How does XRP Ledger consensus work.

The XRP Ledger (XRPL) uses a unique consensus protocol (often called the XRP Ledger Consensus Protocol or originally the Ripple Protocol Consensus Algorithm / RPCA). It is not Proof-of-Work (like Bitcoin) or Proof-of-Stake (like Ethereum). Instead, it relies on a federated Byzantine agreement system among trusted validators.

Core Idea

Validators (specialized servers) repeatedly propose and vote on sets of transactions until a supermajority agrees. Once agreement is reached, those transactions are applied to create a new validated ledger version. This happens every 3–5 seconds.

Key Components

1. Unique Node List (UNL) Each server maintains its own list of validators it trusts not to collude against it.

• This is called the Unique Node List (UNL).
• Most operators use a recommended default UNL (dUNL) published by the XRP Ledger Foundation (and previously others).
• The dUNL typically includes ~35 diverse, independent validators (universities, exchanges, businesses, individuals).
• Ripple itself runs only a small minority of these.
• High overlap between different servers’ UNLs is important to prevent network forks.

2. Consensus Rounds (the process)

• Transactions enter the network and are shared among servers.
• Validators form an initial proposal (a candidate set of valid transactions they have seen).
• They exchange proposals with the validators on their UNL.
• In successive rounds, they adjust their proposals to match what a growing percentage of their trusted validators support.
• The threshold rises until a final 80% supermajority of the UNL agrees on the same set of transactions.
• Those transactions are applied (in a deterministic order) to the previous ledger, producing a new validated ledger.
• Any transactions that did not reach the threshold are held over and can be proposed in the next round.

3. Fault Tolerance

• The network can continue progressing as long as fewer than ~20% of trusted validators are faulty or malicious.
• Confirming an invalid transaction would require more than 80% of a UNL to collude.
• If the percentage of faulty validators is between ~20% and 80%, the network typically stalls (stops making progress) rather than accepting bad data.
• This design prioritizes safety (no double-spends or invalid ledgers) over continuous liveness in extreme cases.

Advantages of This Design

• Very fast finality (3–5 seconds).
• Extremely low energy use (no mining or staking competitions).
• Low transaction costs.
• No single central operator controls the ledger.
• Anyone can run a validator, but influence depends on being included in others’ UNLs.

Summary Flow

1. Transactions submitted → shared across the network.
2. Validators propose candidate transaction sets.
3. Iterative voting among trusted peers (UNL) until ≥80% agreement.
4. Agreed transactions applied → new ledger version validated.
5. Process repeats continuously.

This consensus model is what enables the XRP Ledger’s combination of speed, low cost and relative energy efficiency compared with traditional blockchains.