Alex,

I am sorry for not communicating more clearly. Mark and I discussed your
concerns from the last meeting and he made the change. The BIP text still
needs to be updated, but the discussed change was added to the PR, albeit
squashed making it more non-obvious. BIP68 now explicitly uses 16 bits with
a bitmask. Please see the use of SEQUENCE_LOCKTIME_MASK
and SEQUENCE_LOCKTIME_GRANULARITY in the PR
https://github.com/bitcoin/bitcoin/pull/6312.

    /* If CTxIn::nSequence encodes a relative lock-time, this mask is
     * applied to extract that lock-time from the sequence field. */
    static const uint32_t SEQUENCE_LOCKTIME_MASK = 0x0000ffff;

    /* In order to use the same number of bits to encode roughly the
     * same wall-clock duration, and because blocks are naturally
     * limited to occur every 600s on average, the minimum granularity
     * for time-based relative lock-time is fixed at 512 seconds.
     * Converting from CTxIn::nSequence to seconds is performed by
     * multiplying by 512 = 2^9, or equivalently shifting up by
     * 9 bits. */
    static const int SEQUENCE_LOCKTIME_GRANULARITY = 9;

I am also much happier with this last tightening up of the specification
because it removes ambiguity. 512s granularity makes sense within the
context of the 10 minute block target.

Thank you for spending so much time carefully considering this BIP and
reference implementation and please let me know if there there are any
remaining nits so we can get those addressed.





On Thu, Oct 15, 2015 at 2:47 PM, Alex Morcos via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Mark,
>
> You seemed interested in changing BIP 68 to use 16 bits for sequence
> number in both the block and time versions, making time based sequence
> numbers have a resolution of 512 seconds.
>
> I'm in favor of this approach because it leaves aside 14 bits for further
> soft forks within the semantics of BIP 68.
>
> It would be nice to know if you're planning this change, and perhaps
> people can hold off on review until things are finalized.
>
> I'd cast my "vote" against BIP 68 without this change, but am also open to
> being convinced otherwise.
>
> What are other peoples opinions on this?
>
> On Thu, Oct 8, 2015 at 9:38 PM, Rusty Russell via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Peter Todd <p...@petertodd.org> writes:
>> > On Tue, Oct 06, 2015 at 12:28:49PM +1030, Rusty Russell wrote:
>> >> Peter Todd via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
>> >> writes:
>> >> > However I don't think we've done a good job showing why we need to
>> >> > implement this feature via nSequence.
>> >>
>> >> It could be implemented in other ways, but nSequence is the neatest and
>> >> most straightforward I've seen.
>> >>
>> >> - I'm not aware of any other (even vague) proposal for its use?
>> Enlighten?
>> >
>> > There's three that immediately come to mind:
>> >
>> > Gregory Maxwell has proposed it as a way of discouraging miners from
>> > reorging chains, by including some of the low-order bits of a previous
>> > block header in nSequence.
>> >
>> > A few people have proposed implementing proof-of-stake blocksize voting
>> > with nSequence.
>>
>> Excellent, thanks!  It's good to have such ideas as a compass.  PoS
>> voting seems like it won't be a problem in 5 bits.
>>
>> The "prevbits" idea would want more bits; naively 64 would be good, but
>> I think there are some tricks we can use to make 32 work OK.  We would
>> have to then split between nLocktime (if available) and multiple
>> nSequence fields, and it would weaken it for some txs.
>>
>> There is one easy solution: change the BIP wording from:
>>
>> -For transactions with an nVersion of 2 or greater,
>> +For transactions with an nVersion of 2,
>>
>> And on every tx bump, we decide whether to keep this scheme (mempool
>> would enforce it always).
>>
>> Cheers,
>> Rusty.
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>>
>
>
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