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Merge pull request #237 from edposnak/ch07-nSequence-fix

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nSequence discussion
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Will Binns 2017-02-05 10:52:13 -06:00 committed by GitHub
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@ -327,15 +327,13 @@ Relative timelocks can be set on each input of a transaction, by setting the nSe
The nSequence field was originally intended (but never properly implemented) to allow modification of transactions in the mempool. In that use, a transaction containing inputs with nSequence value below 2^32^ (0xFFFFFFFF) indicated a transaction that was not yet "finalized". Such a transaction would be held in the mempool until it was replaced by another transaction spending the same inputs with a higher nSequence value. Once a transaction was received whose inputs had an nSequence value of 2^32^ it would be considered "finalized" and mined.
The original meaning of nSequence was never properly implemented and the value of nSequence is customarily set to 2^32^ in transactions that do not utilize timelocks. For transactions with nLocktime or CHECKLOCKTIMEVEIRFY, the nSequence value must be set to less than 2^32^. Customarily, it is set to 2^32^ - 1 (0xFFFFFFFE).
Programmatically, that means that if the most significant bit (1<<31) is not set, it is a flag that means "relative locktime". Otherwise (bit 1<<31 set), the nSequnce value is reserved for other uses such as enabling CHECKLOCKTIMEVERIFY, nLocktime, Opt-In-Replace-By-Fee and other future developments.
The original meaning of nSequence was never properly implemented and the value of nSequence is customarily set to 2^32^ in transactions that do not utilize timelocks. For transactions with nLocktime or CHECKLOCKTIMEVEIRFY, the nSequence value must be set to less than 2^32^ for the timelock guards to have effect. Customarily, it is set to 2^32^ - 1 (0xFFFFFFFE).
===== nSequence as consensus-enforced relative timelock
Since the activation of BIP-68, the new consensus rules apply for any transaction containing an input whose nSequence value is less than 2^31^ (bit 1<<31 is not set).
Since the activation of BIP-68, new consensus rules apply for any transaction containing an input whose nSequence value is less than 2^31^ (bit 1<<31 is not set). Programmatically, that means that if the most significant bit (1<<31) is not set, it is a flag that means "relative locktime". Otherwise (bit 1<<31 set), the nSequnce value is reserved for other uses such as enabling CHECKLOCKTIMEVERIFY, nLocktime, Opt-In-Replace-By-Fee and other future developments.
Transactions can contain inputs with nSequence values less than 2^31^. Any such input is interpreted as having a relative timelock. Meaning that the transaction which includes it is only valid once the input has aged by the relative timelock amount. For example, a transaction with one input with nSequence relative timelock of 30 blocks is only valid when at least 30 blocks have elapsed from the time the UTXO referenced in the input was mined. Since nSequence is a per input field, a transaction may contain any number of timelocked inputs, all of which must have sufficiently aged for the transaction to be valid. A valid transaction can include both timelocked inputs (nSequence < 2^31^) and inputs without a relative timelock (nSequence >= 2^31^).
Transaction inputs with nSequence values less than 2^31^ are interpreted as having a relative timelock. Meaning that the transaction that includes it is only valid once the input has aged by the relative timelock amount. For example, a transaction with one input with nSequence relative timelock of 30 blocks is only valid when at least 30 blocks have elapsed from the time the UTXO referenced in the input was mined. Since nSequence is a per input field, a transaction may contain any number of timelocked inputs, all of which must have sufficiently aged for the transaction to be valid. A valid transaction can include both timelocked inputs (nSequence < 2^31^) and inputs without a relative timelock (nSequence >= 2^31^).
The nSequence value is specified in either blocks or seconds, but in a slightly different format than we saw used in nLocktime. A type-flag is used to differentiate between values counting blocks and values counting time in seconds. The type flag is set in the 23rd least significant bit (ie. value 1<<22). If the type-flag is set, then the nSequence value is interpreted as a multiple of 512 seconds. If the type-flag is not set, the nSequence value is interpreted as a number of blocks.