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 reference implementation is available at the following git repository:I request that the BIP editor please assign a BIP number for this work.Sincerely,Mark Friedenbach