On Fri, Jan 22, 2016 at 04:36:58PM +0000, Andrew C via bitcoin-dev wrote:
> Spending a time locked output requires setting nSequence to less than
> MAX_INT but opting into RBF also requires setting nSequence to less than
> MAX_INT.

Hi Andrew,

Opt-in RBF requires setting nSequence to less than MAX-1 (not merely
less than MAX), so an nSequence of exactly MAX-1 (which appears in
hex-encoded serialized transactions as feffffff) enables locktime
enforcement but doesn't opt in to RBF.

For more information, please see BIP125:

    https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki

-Dave