My preference is height activation + one step per block (i.e. also height).  Height seems KISS.

AFAICT most of the attacks would occur around the already-heavily-watched flag day activation event, in a height based environment, a useful attribute.

However I would like to hear from others about possible attacks with the various approaches, before diverging from the default community approach of switch-based-on-time.

On Fri, Dec 18, 2015 at 3:10 PM, Jorge Timón <jtimon@jtimon.cc> wrote:

Well, if it's not going to be height, I think median time of the previous block is better than the time of the current one, and would also solve Chun Wang's concerns.
But as said I prefer to use heights that correspond to diff recalculation (because that's the window that bip9 will use for the later 95% confirmation anyway).

On Dec 18, 2015 9:02 PM, "Jeff Garzik" <jgarzik@gmail.com> wrote:

From a code standpoint, based off height is easy.

My first internal version triggered on block 406,800 (~May 5), and each block increased by 20 bytes thereafter.

It was changed to time, because time was the standard used in years past for other changes; MTP flag day is more stable than block height.

It is preferred to have a single flag trigger (height or time), rather than the more complex trigger-on-time, increment-on-height, but any combination of those will work.

Easy to change code back to height-based...

On Fri, Dec 18, 2015 at 2:52 PM, Jorge Timón <bitcoin-dev@lists.linuxfoundation.org> wrote:

I agree that nHeight is the simplest option and is my preference.
Another option is to use the median time from the previous block (thus you know whether or not the next block should start the miner confirmation or not). In fact, if we're going to use bip9  for 95% miner upgrade confirmation, it would be nice to always pick a difficulty retarget block (ie block.nHeight % DifficultyAdjustmentInterval == 0).
Actually I would always have an initial height in bip9, for softforks too.
I would also use the sign bit as the "hardfork bit" that gets activated for the next diff interval after 95% is reached and a hardfork becomes active (that way even SPV nodes will notice when a softfork  or hardfork happens and also be able to tell which one is it).
I should update bip99 with all this. And if the 2 mb bump is uncontroversial, maybe I can add that to the timewarp fix and th recovery of the other 2 bits in block.nVersion (given that bip102 doesn't seem to follow bip99's recommendations and doesn't want to give 6 full months as the pre activation grace period).

On Dec 18, 2015 8:17 PM, "Chun Wang via bitcoin-dev" <bitcoin-dev@lists.linuxfoundation.org> wrote:

In many BIPs we have seen, include the latest BIP202, it is the block
time that determine the max block size. From from pool's point of
view, it cannot issue a job with a fixed ntime due to the existence of
ntime roll. It is hard to issue a job with the max block size unknown.
For developers, it is also easier to implement if max block size is a
function of block height instead of time. Block height is also much
more simple and elegant than time.
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