RFC-0304/VNCommittees

Validator Node Committee Selection

status: draft

Maintainer(s): Philip Robinson

Licence

The 3-Clause BSD Licence.

Copyright 2019 The Tari Development Community

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Language

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY" and "OPTIONAL" in this document are to be interpreted as described in BCP 14 (covering RFC2119 and RFC8174) when, and only when, they appear in all capitals, as shown here.

Disclaimer

This document and its content are intended for information purposes only and may be subject to change or update without notice.

This document may include preliminary concepts that may or may not be in the process of being developed by the Tari community. The release of this document is intended solely for review and discussion by the community of the technological merits of the potential system outlined herein.

Goals

The aim of this Request for Comment (RFC) is to describe the process an Asset Issuer (AI) will go through in order to select the committee of Validator Nodes (VNs) that will serve a given Digital Asset (DA).

Related Requests for Comment

Description

Abstract

Digital Assets (DAs) are managed by committees of nodes called Validator Nodes (VNs), as described in RFC-0300 and RFC-0302. During the asset creation process, described in RFC-0341, the Asset Issuer (AI) MUST select a committee of VNs to manage their asset. This process consists of the following steps:

  1. Candidate VNs MUST be nominated to be considered for selection by the AI.
  2. The AI MUST employ a CommitteeSelectionStrategy to select VNs from the set of nominated candidates.
  3. The AI MUST make an offer of committee membership to the selected VNs.
  4. Selected VNs MAY accept the offer to become part of the committee by posting the required AssetCollateral.

Nomination

The first step in assembling a committee is to nominate candidate VNs. As described in RFC-0311, an asset can be created with two possible committee_modes - CREATOR_NOMINATION or PUBLIC_NOMINATION:

  • In CREATOR_NOMINATION mode, the AI nominates candidate committee members directly. The AI will have a list of permissioned Trusted Nodes that they want to act as the committee. The AI will contact the candidate VNs directly to inform them of their nomination.
  • In PUBLIC_NOMINATION mode, the AI does not have a list of Trusted Nodes and wants to source unknown VNs from the network. In this case, the AI broadcasts a public call for nomination to the Tari network using the peer-to-peer messaging protocol described in RFC-0172. This call for nomination contains all the details of the asset. VNs that want to participate will then nominate themselves by contacting the AI.

Selection

Once the AI has received a list of nominated VNs, it must make a selection, assuming enough VNs were nominated to populate the committee. The AI will employ some CommitteeSelectionStrategy in order to select the committee from the candidate VNs that have been nominated. This strategy might aim for a perfectly random selection, or perhaps it will consider some metrics about the candidate VNs, such as the length of their VN registrations. These metrics might indicate that they are reliable and have not been blacklisted for poor or malicious performance.

A consideration when selecting a committee in PUBLIC_NOMINATION mode will be the size of the pool of nominated VNs. The size of this pool relative to the size of the committee to be selected will be linked to a risk profile. If the pool contains very few candidates, then it will be much easier for an attacker to have nominated their own nodes in order to obtain a majority membership of the committee. For example, if the AI is selecting a committee of 10 members using a uniformly random selection strategy and only 12 public nominations are received, an attacker only requires control of six VNs to achieve a majority position in the committee. In contrast, if 100 nominations are received and the AI performs a uniformly random selection, an attacker would need to control more than 50 of the nominated nodes in order to achieve a majority position in the committee.

Offer Acceptance

Once the selection has been made by the AI, the selected VNs will be informed and they will be made an offer of membership. If the VNs are still inclined to join the committee, they will accept the offer by posting the AssetCollateral required by the asset to the base layer during the initial Checkpoint transaction built to commence the operation of the asset.