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126 lines
4.7 KiB
126 lines
4.7 KiB
<pre>
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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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</pre>
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==Abstract==
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This BIP is a proposal to redefine the semantics used in determining a
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time-locked transaction's eligibility for inclusion in a block. The
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median of the last 11 blocks is used instead of the block's timestamp,
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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
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present time or block height is less than or equal to that specified
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in the locktime. Since the consensus rules do not mandate strict
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ordering of block timestamps, this has the unfortunate outcome of
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creating a perverse incentive for miners to lie about the time of
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their blocks in order to collect more fees by including transactions
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that by wall clock determination have not yet matured.
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This BIP proposes comparing the locktime against the median of the
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past 11 block's timestamps, rather than the timestamp of the block
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including the transaction. Existing consensus rules guarantee this
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value to monotonically advance, thereby removing the capability for
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miners to claim more transaction fees by lying about the timestamps of
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their block.
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This proposal seeks to ensure reliable behaviour in locktime calculations
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as required by BIP65 (CHECKLOCKTIMEVERIFY) and matching the behavior of
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BIP68 (sequence numbers) and BIP112 (CHECKSEQUENCEVERIFY).
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==Specification==
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The values for transaction locktime remain unchanged. The difference is only in
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the calculation determining whether a transaction can be included. Instead of
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an unreliable timestamp, the following function is used to determine the current
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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. This BIP proposes
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that after activation calls to IsFinalTx() within consensus code use
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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
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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
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implementation of this BIP.
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Thanks go to Gregory Maxwell who came up with the original idea,
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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
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approximately an hour longer to confirm than would be expected under
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the old rules. This is not known to introduce any compatibility
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concerns with existing protocols.
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==References==
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[https://github.com/bitcoin/bips/blob/master/bip-0009.mediawiki BIP9: Versionbits]
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[https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki BIP65: OP_CHECKLOCKTIMEVERIFY]
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[https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki BIP68: Consensus-enforced transaction replacement signaled via sequence numbers]
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[https://github.com/bitcoin/bips/blob/master/bip-0112.mediawiki BIP112: CHECKSEQUENCEVERIFY]
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[http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-August/010396.html Softfork deployment considerations]
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[https://gist.github.com/sipa/bf69659f43e763540550 Version bits]
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==Copyright==
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This document is placed in the public domain.
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