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149 lines
5.5 KiB
Plaintext
<pre>
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BIP: XX
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Title: Median-Past-TimeLock
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Author: Thomas Kerin <me@thomaskerin.io>
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Mark Friedenbach <mark@friedenbach.org>
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Status: Draft
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Type: Standards Track
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Created: 2015-08-10
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</pre>
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==Abstract==
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This BIP is a proposal to redefine the semantics used to determine a time-locked
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transactions eligibilty for inclusion in a block. The proposal is to use a
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blocks MedianTimePast instead of the included timestamp, ensuring that it
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increases monotonically with each block.
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==Motivation==
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At present, transactions are excluded from the next block if the present time
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or block height is less than that specified in the locktime. Since there is no
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network rule ensuring that block timestamps come in chronological order,
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directly using this can lead to transactions being incorrectly excluded, when
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they ought to be valid.
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This BIP proposes comparing the locktime against the MedianTimePast over the
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last 11 blocks, rather than the time included in the block. The benefit is
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this figure is derived via consensus, and guaranteed to monotonically advance.
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This proposal seeks to ensure reliable behaviour in locktime calculations as
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required by BIP65, BIPXX (OP_CHECKSEQUENCEVERIFY), and BIP68.
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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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blocks time.
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enum { nMedianTimeSpan=11 };
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int64_t GetMedianTimePast() const
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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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const CBlockIndex* pindex = this;
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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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BIP68 proposes to replace IsFinalTx() and CheckFinalTx() with Locktime(), and CheckLocktime(),
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allowing a TxIn’s sequence number to specify a relative locktime.
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Adopting the use of MedianTimePast in comparisons with a locktime, involves modifying
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CheckLocktime() to use the next blocks MedianTimePast, should the LOCKTIME_MEDIAN_TIME_PAST
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flag be set.
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The following function introduces this behaviour:
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int64_t CheckLockTime(const CTransaction &tx, int flags)
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{
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AssertLockHeld(cs_main);
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// By convention a negative value for flags indicates that the
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// current network-enforced consensus rules should be used. In
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// a future soft-fork scenario that would mean an
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// IsSuperMajority check against chainActive.Tip().
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if (flags < 0)
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flags = LOCKTIME_MEDIAN_TIME_PAST;
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// pcoinsTip contains the UTXO set for chainActive.Tip()
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const CCoinsView *pCoinsView = pcoinsTip;
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// CheckLockTime() uses chainActive.Height()+1 to evaluate
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// nLockTime because when LockTime() is called within
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// CBlock::AcceptBlock(), the height of the block *being*
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// evaluated is what is used. Thus if we want to know if a
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// transaction can be part of the *next* block, we need to call
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// LockTime() with one more than chainActive.Height().
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const int nBlockHeight = chainActive.Height() + 1;
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// Timestamps on the other hand don't get any special treatment,
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// because we can't know what timestamp the next block will have,
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// and there aren't timestamp applications where it matters.
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int64_t nBlockTime = GetAdjustedTime();
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if (flags & LOCKTIME_MEDIAN_TIME_PAST)
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nBlockTime -= ((CBlockIndex::nMedianTimeSpan + 1) >> 1) * Params().GetConsensus().nPowTargetSpacing;
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return LockTime(tx, flags, pCoinsView, nBlockHeight, nBlockTime);
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}
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Where a value for LocktimeCutoff is used, the switchover logic is implemented as such:
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int64_t nLockTimeCutoff = (nLockTimeFlags & LOCKTIME_MEDIAN_TIME_PAST)
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? nMedianTimePast
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: pblock->GetBlockTime();
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==Upgrade and Testing Plan==
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TBD
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==Acknowledgements==
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Mark Friedenbach for designing and authoring the actual implementation for Median-
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Past time-lock.
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==Implementations==
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A reference implementation is provided in the following git repository:
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https://github.com/maaku/bitcoin/tree/medianpasttimelock
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==Deployment==
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We reuse the double-threshold switchover mechanism from BIPs 34 and 66, with the
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same thresholds, but for block.nVersion = 4. The new rules are in effect for
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every block (at height H) with nVersion = 4 and at least 750 out of 1000 blocks
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preceding it (with heights H-1000...H-1) also have nVersion = 4. Furthermore,
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when 950 out of the 1000 blocks preceding a block do have nVersion = 4,
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nVersion = 3 blocks become invalid, and all further blocks enforce the new rules.
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It is recommended that this soft-fork deployment trigger include other related
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proposals for improving Bitcoin's lock-time capabilities, such as BIP 65, BIP68
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and OP_CHECKSEQUENCEVERIFY.
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==Compatibility==
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This BIP is not known to introduce any compatibility concerns.
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==References==
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[https://github.com/bitcoin/bips/blob/master/bip-00.mediawiki BIP65: OP_CHECKLOCKTIMEVERIFY]
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[https://github.com/bitcoin/bips/blob/master/bip-00.mediawiki BIP68: Consensus-enforced transaction replacement signaled via sequence numbers]
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[https://github.com/bitcoin/bips/blob/master/bip-00.mediawiki BIP65: OP_CHECKSEQUENCEVERIFY]
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==Copyright==
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This document is placed in the public domain.
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