[Bitcoin-development] [BIP draft] Consensus-enforced transaction replacement signalled via sequence numbers

Stephen Morse stephencalebmorse at gmail.com
Tue Jun 2 04:34:34 UTC 2015

I see, so OP_SEQUENCEVERIFY will have a value pushed on the stack right
before, and then check that the input spending the prevout has nSequence
corresponds to at least the sequence specified by the stack value. Good
idea! Keeps the script code from depending on external chain specific data,
which is nice.

Hopefully we can repurpose one of the OP_NOPs for CHECKLOCKTIMEVERIFY and
one for OP_CHECKSEQUENCEVERIFY. Very complementary.


On Tue, Jun 2, 2015 at 12:16 AM, Mark Friedenbach <mark at friedenbach.org>

> You are correct! I am maintaining a 'checksequenceverify' branch in my git
> repository as well, an OP_RCLTV using sequence numbers:
> https://github.com/maaku/bitcoin/tree/checksequenceverify
> Most of the interesting use cases for relative lock-time require an RCLTV
> opcode. What is interesting about this architecture is that it possible to
> cleanly separate the relative lock-time (sequence numbers) from the RCLTV
> opcode (OP_CHECKSEQUENCEVERIFY) both in concept and in implementation. Like
> CLTV, the CSV opcode only checks transaction data and requires no
> contextual knowledge about block headers, a weakness of the other RCLTV
> proposals that violate the clean separation between libscript and
> libconsensus. In a similar way, this BIP proposal only touches the
> transaction validation logic without any impact to script.
> I would like to propose an additional BIP covering the CHECKSEQUENCEVERIFY
> opcode and its enabling applications. But, well, one thing at a time.
> On Mon, Jun 1, 2015 at 8:45 PM, Stephen Morse <stephencalebmorse at gmail.com
> > wrote:
>> Hi Mark,
>> Overall, I like this idea in every way except for one: unless I am
>> missing something, we may still need an OP_RCLTV even with this being
>> implemented.
>> In use cases such as micropayment channels where the funds are locked up
>> by multiple parties, the enforcement of the relative locktime can be done
>> by the first-signing party. So, while your solution would probably work in
>> cases like this, where multiple signing parties are involved, there may be
>> other, seen or unforeseen, use cases that require putting the relative
>> locktime right into the spending contract (the scriptPubKey itself).
>> When there is only one signer, there's nothing that enforces using an
>> nSequence and nVersion=2 that would prevent spending the output until a
>> certain time.
>> I hope this is received as constructive criticism, I do think this is an
>> innovative idea. In my view, though, it seems to be less fully-featured
>> than just repurposing an OP_NOP to create OP_RCLTV. The benefits are
>> obviously that it saves transaction space by repurposing unused space, and
>> would likely work for most cases where an OP_RCLTV would be needed.
>> Best,
>> Stephen
>> On Mon, Jun 1, 2015 at 9:49 PM, Mark Friedenbach <mark at friedenbach.org>
>> wrote:
>>> I have written a reference implementation and BIP draft for a soft-fork
>>> change to the consensus-enforced behaviour of sequence numbers for the
>>> purpose of supporting transaction replacement via per-input relative
>>> lock-times. This proposal was previously discussed on the mailing list in
>>> the following thread:
>>> http://sourceforge.net/p/bitcoin/mailman/message/34146752/
>>> In short summary, this proposal seeks to enable safe transaction
>>> replacement by re-purposing the nSequence field of a transaction input to
>>> be a consensus-enforced relative lock-time.
>>> The advantages of this approach is that it makes use of the full range
>>> of the 32-bit sequence number which until now has rarely been used for
>>> anything other than a boolean control over absolute nLockTime, and it does
>>> so in a way that is semantically compatible with the originally envisioned
>>> use of sequence numbers for fast mempool transaction replacement.
>>> The disadvantages are that external constraints often prevent the full
>>> range of sequence numbers from being used when interpreted as a relative
>>> lock-time, and re-purposing nSequence as a relative lock-time precludes its
>>> use in other contexts. The latter point has been partially addressed by
>>> having the relative lock-time semantics be enforced only if the
>>> most-significant bit of nSequence is set. This preserves 31 bits for
>>> alternative use when relative lock-times are not required.
>>> The BIP draft can be found at the following gist:
>>> https://gist.github.com/maaku/be15629fe64618b14f5a
>>> The reference implementation is available at the following git
>>> repository:
>>> https://github.com/maaku/bitcoin/tree/sequencenumbers
>>> I request that the BIP editor please assign a BIP number for this work.
>>> Sincerely,
>>> Mark Friedenbach
>>> ------------------------------------------------------------------------------
>>> _______________________________________________
>>> Bitcoin-development mailing list
>>> Bitcoin-development at lists.sourceforge.net
>>> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
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