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DCP: 0004
Title: Fix LN Sequence Locks View
Author: Dave Collins <davec@decred.org>
Status: Voting
Created: 2019-01-28
License: CC0-1.0
License-Code: ISC
Requires: DCP0003

Table of Contents

Abstract

This proposes required corrections to the view that is used to enforce relative time locks in order to provide full support for the Lightning Network and a wide variety of other use cases.

Motivation

The current view used for determining consensus-enforced relative lock-time semantics in version 2 transactions is more restrictive than called for by the original specification in DCP0003. As a result, it is currently not possible for applications to make full use of their functionality.

This proposal modifies the consensus rules to ensure their full functionality is available.

As a reminder, a small sampling of some of the possible applications that relative locks enable include:

  • Lightning Network
  • Hash Time-Locked Contracts with Relative Timeouts
  • Bidirectional Payment Channels
  • Off-chain Atomic Swaps
  • Escrow with Timeout
See DCP0003 for more information about these applications.

Specification

The view which is used to provide inputs necessary to determine sequence lock calculations for transactions with sequence locks enabled must provide access to the referenced inputs so long as those inputs exist from the point of view of the block from which the sequence locks are being evaluated.

Recall that sequence locks are only enabled for inputs on transactions that are not coinbase or stakebase transactions, have a version greater than or equal to 2, and do not have the disable flag set in the sequence number field.

For reference, the disable flag is bit 31 of the of the sequence number field as shown in the following diagram:

In particular, transaction inputs with sequence locks enabled that are currently rejected by the current consensus rules must be permitted when appearing in transactions that exhibit the following properties:

  • The transaction appears in the regular tree and spends outputs from transactions which appear earlier in the same block
  • The transaction spends outputs from blocks before the block prior to the current block
  • The transaction spends outputs from a transaction shared between the current block and the block prior to it when the block has been disapproved

Rationale

The specific approach of implementing the necessary updates such that they apply to existing version 2 transactions was chosen for three primary reasons:

  • It significantly reduces the number of changes other software needs to make as compared to other approaches such as introducing a new transaction version
  • It provides a cleaner future upgrade path as it will allow the legacy view handling code to be completely removed once the changes specified herein are activated and the acivation height is buried sufficiently deep in the chain history
  • It supports more efficient view implementations which act solely on unspent outputs versus pruned transactions
More generally, there is a strong preference for changes which do not require permanently carrying around legacy baggage that ultimately can't be fixed. The proposed implementation satisfies that goal.

Deployment

This agenda will be deployed to mainnet using the standard Decred on-chain voting infrastructure as follows:

Name Setting
Deployment Version 6
Agenda ID fixlnseqlocks
Agenda Description Modify sequence lock handling as defined in DCP0004
Start Time 1548633600 (Jan 28th, 2019 00:00:00 +0000 UTC)
Expire Time 1580169600 (Jan 28th, 2020 00:00:00 +0000 UTC)
Mask 0x06 (Bits 1 and 2)
Choices
Choice English Description Bits
abstain abstain voting for change 0x00
no keep the existing consensus rules 0x02 (Bit 1)
yes change to the new consensus rules 0x04 (Bit 2)

Compatibility

This is a hard-forking change to the Decred consensus. This means that once the agenda is voted in and becomes locked in, anybody running code that fully validates blocks must upgrade before the activation time or they will risk rejecting a chain containing a transaction that is invalid under the old rules.

Other software that performs full validation will need to modify their consensus enforcement rules accordingly. Since the changes introduced herein are less restrictive than the previous sequence lock rules, and therefore there will not be any transactions in the chain which do not conform to the newly activated rules, new implementations will not require any additional code to handle the previous more restrictive rules.

Reference Implementation

// Only create and use a view which preserves the expected consensus
// semantics for relative lock times via sequence numbers until DCP0004
// becomes active.
legacySeqLockView := expectedView
fixSeqLocksActive, err := b.isFixSeqLocksAgendaActive(node.parent)
if err != nil {
	return err
}
if lnFeaturesActive && !fixSeqLocksActive {
	var err error
	legacySeqLockView, err = b.createLegacySeqLockView(block, parent, view)
	if err != nil {
		return err
	}
}

Pull Requests

New Sequence Lock View Enforcement

A reference implementation of enforcing the new sequence lock view semantics in accordance with the results of the agenda vote is implemented by pull request #1579.

Deployment

A reference implementation of the required agenda definition is implemented by pull request #1578.

Acknowledgements

Collaborators

Thanks to the following individuals who provided valuable feedback during the review process of this proposal (alphabetical order):

Additional References

Copyright

This document is licensed under the CC0-1.0: Creative Commons CC0 1.0 Universal license.

The code is licensed under the ISC License.