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Here Antoine said the following:

This introduced absolute timelock capability through the nLockTime transaction field, which is disabled if all the transactions inputs have an nSequence that is 0xffffffff.

I have a couple of questions:

  1. Is this still a consensus rule?
  2. Does this mean that If I want absolute locktime in my transaction I must set the nSequence of some input to value != 0xffffffff?
  3. By setting some input nSequence value to != 0xffffffff we are enabling relative locktime (32th bit is NOT set to 1) or RBF (32th bit is set to 1). Does it mean that my transaction, for example, must be RBF in order to use absolute locktime?
  4. If all written above is true, does this mean that by setting all nSequence to == 0xffffffff and by that disabling nLocktime and absolute timelock, we can also disable CLTV?
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Yes it's still a consensus rule, as it's been since 2009.

You indeed need to set at least one input's nSequence to something else than 0xffffffff for the absolute timelock to be taken into account. See for instance how OP_CHECKLOCKTIMEVERIFY enforces this.

Your transaction may enable nLockTime without enabling RBF by setting the nSequence to 0xfffffffe. RBF only gets activated by using 0xfffffffd or lower.

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  • Thanks, and what about the last (4th) question? If we set all nSequences to == 0xffffffff, we can disable nLocktime and absolute timelock. And what happened if we have CLTV in the output we consume? Do we also disable CLTV since we disabled nLocktime and we do not take into account the value of the nLocktime field?
    – LeaBit
    Oct 29, 2023 at 16:59
  • When an OP_CLTV executes, it requires that the spending transactions' locktime is active. Oct 29, 2023 at 18:27
  • @PieterWuille Thanks, I asked a question related to it. Can you help? bitcoin.stackexchange.com/questions/120259/…
    – LeaBit
    Oct 29, 2023 at 18:39

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