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Andrew Groot
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4.5 KiB
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99 lines
4.5 KiB
Markdown
# BIP-113
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BIP: 113
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Layer: Consensus (soft fork)
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Title: Median time-past as endpoint for lock-time calculations
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Author: Thomas Kerin <me@thomaskerin.io>
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Mark Friedenbach <mark@friedenbach.org>
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Comments-Summary: No comments yet.
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Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0113
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Status: Final
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Type: Standards Track
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Created: 2015-08-10
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License: PD
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## Abstract
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This BIP is a proposal to redefine the semantics used in determining a time-locked transaction's eligibility for inclusion in a block.
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The median of the last 11 blocks is used instead of the block's timestamp, ensuring that it increases monotonically with each block.
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## Motivation
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At present, transactions are excluded from inclusion in a block if the present time or block height is less than or equal to that specified in the locktime.
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Since the consensus rules do not mandate strict ordering of block timestamps, this has the unfortunate outcome of creating a perverse incentive for miners to lie about the time of their blocks in order to collect more fees by including transactions that by wall clock determination have not yet matured.
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This BIP proposes comparing the locktime against the median of the past 11 block's timestamps, rather than the timestamp of the block including the transaction.
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Existing consensus rules guarantee this value to monotonically advance, thereby removing the capability for miners to claim more transaction fees by lying about the timestamps of their block.
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This proposal seeks to ensure reliable behaviour in locktime calculations as required by BIP65 (`CHECKLOCKTIMEVERIFY`) and matching the behavior of BIP68 (sequence numbers) and BIP112 (`CHECKSEQUENCEVERIFY`).
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## Specification
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The values for transaction locktime remain unchanged.
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The difference is only in the calculation determining whether a transaction can be included.
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Instead of an unreliable timestamp, the following function is used to determine the current block time for the purpose of checking lock-time constraints:
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enum { nMedianTimeSpan=11 };
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int64_t GetMedianTimePast(const CBlockIndex* pindex)
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{
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int64_t pmedian[nMedianTimeSpan];
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int64_t* pbegin = &pmedian[nMedianTimeSpan];
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int64_t* pend = &pmedian[nMedianTimeSpan];
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for (int i = 0; i < nMedianTimeSpan && pindex; i++, pindex = pindex->pprev)
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*(--pbegin) = pindex->GetBlockTime();
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std::sort(pbegin, pend);
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return pbegin[(pend - pbegin)/2];
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}
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Lock-time constraints are checked by the consensus method IsFinalTx().
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This method takes the block time as one parameter.
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This BIP proposes that after activation calls to IsFinalTx() within consensus code use the return value of `GetMedianTimePast(pindexPrev)` instead.
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The new rule applies to all transactions, including the coinbase transaction.
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A reference implementation of this proposal is provided by the following pull request:
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https://github.com/bitcoin/bitcoin/pull/6566
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## Deployment
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This BIP is to be deployed by "versionbits" BIP9 using bit 0.
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For Bitcoin '''mainnet''', the BIP9 '''starttime''' will be midnight 1st May 2016 UTC (Epoch timestamp 1462060800) and BIP9 '''timeout''' will be midnight 1st May 2017 UTC (Epoch timestamp 1493596800).
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For Bitcoin '''testnet''', the BIP9 '''starttime''' will be midnight 1st March 2016 UTC (Epoch timestamp 1456790400) and BIP9 '''timeout''' will be midnight 1st May 2017 UTC (Epoch timestamp 1493596800).
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This BIP must be deployed simultaneously with BIP68 and BIP112 using the same deployment mechanism.
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## Acknowledgements
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Mark Friedenbach for designing and authoring the reference implementation of this BIP.
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Thanks go to Gregory Maxwell who came up with the original idea, in #bitcoin-wizards on 2013-07-16.
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Thomas Kerin authored this BIP document.
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## Compatibility
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Transactions generated using time-based lock-time will take approximately an hour longer to confirm than would be expected under the old rules.
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This is not known to introduce any compatibility concerns with existing protocols.
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## References
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[BIP9: Versionbits](/protocol/forks/bip-0009)
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[BIP65: OP_CHECKLOCKTIMEVERIFY](/protocol/forks/bip-0065)
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[BIP68: Consensus-enforced transaction replacement signaled via sequence numbers](/protocol/forks/bip-0068)
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[BIP112: CHECKSEQUENCEVERIFY](/protocol/forks/bip-0112)
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[Softfork deployment considerations](http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-August/010396.html)
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[Version bits](https://gist.github.com/sipa/bf69659f43e763540550)
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## Copyright
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This document is placed in the public domain.
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