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.


On Mon, Jun 1, 2015 at 9:49 PM, Mark Friedenbach <m...@friedenbach.org>

> 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
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