So I've created 2 new repositories with changed rules regarding sequencenumbers:

https://github.com/maaku/bitcoin/tree/sequencenumbers2

This repository inverts (un-inverts?) the sequence number. nSequence=1 means 1 block relative lock-height. nSequence=LOCKTIME_THRESHOLD means 1 second relative lock-height. nSequence>=0x80000000 (most significant bit set) is not interpreted as a relative lock-time.

https://github.com/maaku/bitcoin/tree/sequencenumbers3

This repository not only inverts the sequence number, but also interprets it as a fixed-point number. This allows up to 5 year relative lock times using blocks as units, and saves 12 low-order bits for future use. Or, up to about 2 year relative lock times using seconds as units, and saves 4 bits for future use without second-level granularity. More bits could be recovered from time-based locktimes by choosing a higher granularity (a soft-fork change if done correctly).

On Tue, Aug 25, 2015 at 3:08 PM, Mark Friedenbach <mark@friedenbach.org> wrote:
To follow up on this, let's say that you want to be able to have up to 1 year relative lock-times. This choice is somewhat arbitrary and what I would like some input on, but I'll come back to this point.

 * 1 bit is necessary to enable/disable relative lock-time.

 * 1 bit is necessary to indicate whether seconds vs blocks as the unit of measurement.

 * 1 year of time with 1-second granularity requires 25 bits. However since blocks occur at approximately 10 minute intervals on average, having a relative lock-time significantly less than this interval doesn't make much sense. A granularity of 256 seconds would be greater than the Nyquist frequency and requires only 17 bits.

 * 1 year of blocks with 1-block granularity requires 16 bits.

So time-based relative lock time requires about 19 bits, and block-based relative lock-time requires about 18 bits. That leaves 13 or 14 bits for other uses.

Assuming a maximum of 1-year relative lock-times. But what is an appropriate maximum to choose? The use cases I have considered have only had lock times on the order of a few days to a month or so. However I would feel uncomfortable going less than a year for a hard maximum, and am having trouble thinking of any use case that would require more than a year of lock-time. Can anyone else think of a use case that requires >1yr relative lock-time?

TL;DR

On Sun, Aug 23, 2015 at 7:37 PM, Mark Friedenbach <mark@friedenbach.org> wrote:

A power of 2 would be far more efficient here. The key question is how long of a relative block time do you need? Figure out what the maximum should be ( I don't know what that would be, any ideas?) and then see how many bits you have left over.

On Aug 23, 2015 7:23 PM, "Jorge Timón" <bitcoin-dev@lists.linuxfoundation.org> wrote:
On Mon, Aug 24, 2015 at 3:01 AM, Gregory Maxwell via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> Seperately, to Mark and Btcdrank: Adding an extra wrinkel to the
> discussion has any thought been given to represent one block with more
> than one increment?  This would leave additional space for future
> signaling, or allow, for example, higher resolution numbers for a
> sharechain commitement.

No, I don't think anybody thought about this. I just explained this to
Pieter using "for example, 10 instead of 1".
He suggested 600 increments so that it is more similar to timestamps.
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