BIP 0113

BIP: 113
 Layer: Consensus (soft fork)
 Title: Median time-past as endpoint for lock-time calculations
 Author: Thomas Kerin <me@thomaskerin.io>
 Mark Friedenbach 
 Comments-Summary: No comments yet.
 Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0113
 Status: Final
 Type: Standards Track
 Created: 2015-08-10
 License: PD
==Abstract==
This BIP is a proposal to redefine the semantics used in determining a
time-locked transaction's eligibility for inclusion in a block. The
median of the last 11 blocks is used instead of the block's timestamp,
ensuring that it increases monotonically with each block.
==Motivation==
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. 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.
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. 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.
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).
==Specification==
The values for transaction locktime remain unchanged. The difference is only in 
the calculation determining whether a transaction can be included. Instead of 
an unreliable timestamp, the following function is used to determine the current 
block time for the purpose of checking lock-time constraints:
 enum { nMedianTimeSpan=11 };
 
 int64_t GetMedianTimePast(const CBlockIndex* pindex)
 {
 int64_t pmedian[nMedianTimeSpan];
 int64_t* pbegin = &pmedian[nMedianTimeSpan];
 int64_t* pend = &pmedian[nMedianTimeSpan];
 for (int i = 0; i < nMedianTimeSpan && pindex; i++, pindex = pindex->pprev)
 *(--pbegin) = pindex->GetBlockTime();
 std::sort(pbegin, pend);
 return pbegin[(pend - pbegin)/2];
 }
Lock-time constraints are checked by the consensus method IsFinalTx().
This method takes the block time as one parameter. This BIP proposes
that after activation calls to IsFinalTx() within consensus code use
the return value of `GetMedianTimePast(pindexPrev)` instead.
The new rule applies to all transactions, including the coinbase transaction.
A reference implementation of this proposal is provided by the
following pull request:
https://github.com/bitcoin/bitcoin/pull/6566
==Deployment==
This BIP is to be deployed by "versionbits" BIP9 using bit 0.
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).
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).
This BIP must be deployed simultaneously with BIP68 and BIP112 using the same deployment mechanism.
==Acknowledgements==
Mark Friedenbach for designing and authoring the reference
implementation of this BIP.
Thanks go to Gregory Maxwell who came up with the original idea,
in #bitcoin-wizards on 2013-07-16.
Thomas Kerin authored this BIP document.
==Compatibility==
Transactions generated using time-based lock-time will take
approximately an hour longer to confirm than would be expected under
the old rules. This is not known to introduce any compatibility
concerns with existing protocols.
==References==
BIP9: Versionbits
BIP65: OP_CHECKLOCKTIMEVERIFY
BIP68: Consensus-enforced transaction replacement signaled via sequence numbers
BIP112: CHECKSEQUENCEVERIFY
Softfork deployment considerations
Version bits
==Copyright==
This document is placed in the public domain.