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What does unique node list (UNL) mean in the context of the XRP Ledger?

Unique Node List (UNL) in the XRP Ledger

Definition

In the context of the XRP Ledger (XRPL), a Unique Node List (UNL) refers to a list of trusted validator nodes that each XRPL server (node) maintains for the purpose of reaching consensus on transaction ordering and ledger state1234. Unlike traditional blockchains that might use proof-of-work or proof-of-stake, XRPL uses a federated consensus mechanism (also called Proof-of-Association), where trust relationships between nodes are central.

How the UNL Works

  • Custom Trust: Each node sets its own UNL, choosing which validators it trusts to properly participate in consensus. This allows nodes to independently verify the ledger's correctness, rather than relying solely on default settings156.
  • Default Lists: While nodes can create their own UNL, many use one of several default UNLs curated by organizations such as the XRPL Foundation or Ripple. These default lists name validators that are widely considered trustworthy by the community1.
  • Consensus Mechanism: During consensus, each node communicates only with the validators on its UNL. When 80% or more of a node’s UNL reach agreement on a "candidate ledger," that version is accepted by the node as the new ledger version1.
  • Adjustment & Safety: The system includes mechanisms such as the Negative UNL, which adjusts for validators currently offline to ensure the network functions safely despite node churn3.

Importance

  • Decentralization through Trust: The UNL system ensures power is not concentrated by allowing anyone to run a validator node and by letting each participant independently select their set of trusted validators.
  • Security: Malicious actors cannot arbitrarily join consensus—influence comes only when enough nodes add a validator to their UNL1.

Key Takeaways

  • The UNL underpins XRPL's consensus: it's a personal, flexible, and trust-driven list of validating nodes.
  • It is not fixed—each node picks its own UNL, and changes can be made at any time163.
  • Security is rooted in distributed trust, not financial stake or proof of work.
References:
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