nLocktime for Bitcoin Core

Question

I was reading this while doing some research: https://github.com/achow101/wallet-fingerprinting/blob/main/fingerprints.md and had some questions:

What is the default nLocktime for a transaction in Bitcoin Core?

Was it ever changed because I remember it being zero at some point?

Answer 1

What is the default nLocktime for a transaction in Bitcoin Core?

Wallet source code

In version 23.0 the wallet\spend.c includes this

/**
 * Return a height-based locktime for new transactions (uses the height of the
 * current chain tip unless we are not synced with the current chain
 */
static uint32_t GetLocktimeForNewTransaction(interfaces::Chain& chain, const uint256& block_hash, int block_height)
{
    uint32_t locktime;
    // Discourage fee sniping.
    //
    // For a large miner the value of the transactions in the best block and
    // the mempool can exceed the cost of deliberately attempting to mine two
    // blocks to orphan the current best block. By setting nLockTime such that
    // only the next block can include the transaction, we discourage this
    // practice as the height restricted and limited blocksize gives miners
    // considering fee sniping fewer options for pulling off this attack.
    //
    // A simple way to think about this is from the wallet's point of view we
    // always want the blockchain to move forward. By setting nLockTime this
    // way we're basically making the statement that we only want this
    // transaction to appear in the next block; we don't want to potentially
    // encourage reorgs by allowing transactions to appear at lower heights
    // than the next block in forks of the best chain.
    //
    // Of course, the subsidy is high enough, and transaction volume low
    // enough, that fee sniping isn't a problem yet, but by implementing a fix
    // now we ensure code won't be written that makes assumptions about
    // nLockTime that preclude a fix later.
    if (IsCurrentForAntiFeeSniping(chain, block_hash)) {
        locktime = block_height;

So bitcoin core sets locktime to the current block height to discourage fee sniping.

Release Notes

Release notes for version 0.11 (updated Aug 2015) say

Wallet

#2340 811c71d Discourage fee sniping with nLockTime

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When using RPCs

Locktime is zero when transaction is created manually using RPCs like createpsbt or createrawtransaction. Else a random nearest block height is used in bitcoin core. (Source: OP experimentation)

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Documented meaning of Locktime

It may help to consider the meaning of Locktime

The Protocol Documentation says

The block number or timestamp at which this transaction is unlocked:

Value	Description
0	Not locked
< 500000000	Block number at which this transaction is unlocked
>= 500000000	UNIX timestamp at which this transaction isunlocked

If all TxIn inputs have final (0xffffffff) sequence numbers then lock_time is irrelevant. Otherwise, the transaction may not be added to a block until after lock_time (see NLockTime).

If we look at some transactions in block 654,915 we can see that most transactions have either no lock (0) or a block number ...

C> blockchain -file blk02300.dat -block 100

Transactions in block 00000000000000000002FC97B15D50E38CEB0C6A83DA4344CE836B01FC841EEA

No.	V	Type	Ins	Outs	Lock	Satoshi	Id
1	1	Segwit	1	3	0	743862941	36FD1CE479F30EF1CA1E44F7B2CF63D21C5CA8DBB04C1D5551A26735DB0266B8
2	1	Legacy	1	2	0	99992768	D889E605D3DD5986C89A66F446FEB228A7BCE58278C5F62EED579DB730148001
3	1	Segwit	3	1	0	7685144	268705614DCB68AED6A3750172F445EDCE6179D149BE769AF08D253B7A21F22D
4	2	Legacy	2	1	0	7907240	F3E237ED279F039DA0FAAAF5285362FAD18BADFA9E97455007F39D2C0D275112
5	2	Legacy	6	36	654900	5739473461	8F5B79B4CAC8EEF4525996E7B0B1B02DF83F9199816F27BA7FDD3E57C098D069
6	2	Segwit	1	1	0	1310690	819977BFF99145BCD38024FE0A3BC5B03A82A0A5FAA4A8A8073DB2D1893CC4D0
7	1	Segwit	1	3	0	1376557	378F17EB675E8998D9E1B4283E2AA3E8FB0213827296E6FAB54C8ED78C5E3777
8	1	Segwit	2	5	0	558656062	800C2349DF26295A3AFFBAD35CBB98D82A26AD7F69C8A6BF78ACFE4483414130
9	2	Legacy	1	2	0	9845471	119308B6C68AFB33D621202B704EB3D77867C7CDF7496FBDDB3176EA33481992
10	2	Legacy	1	2	0	9845471	618CD9BBE59448307C2941183A6AB7AD9DB1837B5139BDFC578108D5B710D980
11	1	Segwit	1	1	0	3469300	28AAF790C652C2B07F67FF4FD0E37997CC562B8DB92633DF5E8684399E15DACB
12	1	Legacy	1	1	0	375540	1EE38AFEB454980F694E654713D01188EBCC91BFC4213874CAA5376C0B37CD66
13	1	Legacy	1	1	0	223479	FE02E255EF4200A1D16C5DEBB442933C4A22EA544E55452B905D47EA99ED9675
14	1	Legacy	1	1	0	1127700	CB227B308DC46B4B312BAE6704D6D3B7E2FC8CE3CB7B17E26BB1DB9EE5BAEFB8
15	1	Segwit	1	1	0	877025	EA1871DC8D71A2C8B9224DC3937DF90324AFBD3D266EBBD27AC7A4FE2FB15F89
16	1	Segwit	1	2	0	40824845	2C0607F2B4FA6A15CABBEDABAFDA198CC4A98D18EA134963CAC08E26FF31C3E8
17	2	Legacy	1	2	654914	550200	24EBB2CB89EAE962E89939D48BD35D731791D1ABE8AC911FC8909BCEB299D9F9
18	2	Legacy	1	2	654914	134435250	A7E6E408D619DEF6E43D00B03ACFD247DE7268EB34E751D437BB03961954DFF6
19	2	Segwit	1	2	654862	463598	DAF9BF664D3C4CFEB1D625351922C51867EE1C2A648BBE5281D5505D7DE54ECF
20	1	Segwit	2	2	0	6316861	7FBD3F3079E0AA2404D28800329974F2D9C2D1115E3BE39DAD4898CFC123C8BC

I'd guess most wallets make the default 0 since my quick look at this block seemed to show about 90% with that value.

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Footnote: If we assume general use of block height in locktime is consistent with the approach in Bitcoin core, we can probably use this to see how fee rate affected delays in getting a transaction mined. Transactions 17 and 18 above were mined quickly, transaction 19 probably had to wait 52 blocks? (8-9 hours?)