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  WIP: WIP-0015
  Layer: Consensus (hard fork)
  Title: Amendment to WIP-0007
  Authors: Mario Cao 
  Discussions-To: `#dev-general` channel on Witnet Community's Discord server
  Status: Final
  Type: Standards Track
  Created: 2021-06-14
  License: BSD-2-Clause

Abstract

This proposal aims to amend the data request weight specification (described in the WIP-0007) to match the current implementation in witnet-rust.

Motivation and rationale

The implementation of the data request transaction weight formula, included in the pull request witnet-rust#1301, differed from what was previously specified in the WIP-0007.

The transaction weight formulae are considered consensus critical as they are also used to determine if a block is valid by checking if the aggregated transaction weights exceed or not a certain value, known as bucket size. Therefore, to guarantee interoperability with other Witnet nodes, it is paramount that the specification from the WIP-0007 and the witnet-rust reference implementation should be the same.

Current reference implementation vs. specification

The reference implementation in witnet-rust differed from the specification of the WIP-0007 because it is applying the factor alpha to all the components of the formula as:

DR_weight = DR_OUTPUT_size*alpha + W*COMMIT + W*REVEAL*beta + TALLY*beta + W*OUTPUT_SIZE + (N*INPUT_SIZE + M*OUTPUT_SIZE)*alpha

However, the WIP-0007 defines the data request weight as:

DR_weight = DR_size*alpha + W*COMMIT + W*REVEAL*beta + TALLY*beta + W*OUTPUT_SIZE

The main differences between both equations are:

  1. the WIP-0007 is heavily subsidizing "join transactions" by neglecting the size of the transaction input components.
  2. the WIP-0007 neglects OUTPUT_SIZE components as they have fixed sized (i.e. data requests can only have 0 or 1 output components).
  3. the witnet-rust reference implementation was wrongly using the factor alpha, which currently is harmless because alpha=1.
  4. the witnet-rust reference implementation takes into account the sizes of the input and output components.

The heavy subsidy towards join transactions poses, however, an availability threat that may affect the network stability. It also enables Denial-of-Service (DoS) attacks because malicious users may create large data requests with many input components to create large blocks that cannot be propagated fast enough within an epoch (45 seconds).

Specification

This document proposes to update the data request weight definition as:

DR_weight = DR_OUTPUT_size*alpha + W*COMMIT + W*REVEAL*beta + TALLY*beta + (W+M)*OUTPUT_SIZE + N*INPUT_SIZE

The underlying reasons for the update are:

  • It does not affect the current witnet-rust implementation as alpha=1.
  • The updated formula takes into consideration the input and output components, thus reducing the attack surface, especially against DoS.

Amend WIP-0007 to match the reference implementation

(1) Update the data request weight definition in WIP-0007 above specified.

Update witnet-rust reference implementation logic

(2) Update the witnet-rust implementation by removing the factor alpha from the last element of the sum (i.e. (N*INPUT_SIZE + M*OUTPUT_SIZE)).

Backwards compatibility

Consensus

This proposal introduces an update in the data request weight computation, which can be considered a consensus critical change. However, due to the use of a factor equalized to the unit, the values extracted by applying the previous and the updated formula will be the same.

Therefore, there is enough evidence to assert that for the currently used parameters, this amendment can be considered backward compatible with nodes running the witnet-rust reference implementation.

Libraries and Clients

Known libraries and clients are based on the witnet-rust reference implementation and, therefore, this amendment should not affect them.

Reference Implementation

A reference implementation for the proposed protocol improvement can be found as a pull request in the witnet-rust repository.

Adoption Plan

An adoption plan is not needed.

Acknowledgments

This proposal has been cooperatively discussed and devised by many individuals from the Witnet development community.