[bitcoin-dev] Fill-or-kill transaction

[jl2012@xbt.hk]

Fill-or-kill tx is not a new idea and is discussed in the Scaling 
Bitcoin workshop. In Satoshi's implementation of nLockTime, a huge range 
of timestamp (from 1970 to 2009) is wasted. By exploiting this unused 
range and with compromise in the time resolution, a fill-or-kill system 
could be built with a softfork.

-----------
Two new parameters, nLockTime2 and nKillTime are defined:

nLockTime2 (Range: 0-1,853,010)
0: Tx could be confirmed at or after block 420,000
1: Tx could be confirmed at or after block 420,004
.
.
719,999: Tx could be confirmed at or after block 3,299,996 (about 55 
years from now)
720,000: Tx could be confirmed if the median time-past >= 1,474,562,048 
(2016-09-22)
720,001: Tx could be confirmed if the median time-past >= 1,474,564,096 
(2016-09-22)
.
.
1,853,010 (max): Tx could be confirmed if the median time-past >= 
3,794,966,528 (2090-04-04)

nKillTime (Range: 0-2047)
if nLockTime2 < 720,000, the tx could be confirmed at or before block 
(nLockTime2 + nKillTime * 4)
if nLockTime2 >= 720,000, the tx could be confirmed if the median 
time-past <= (nLockTime2 - 720,001 + nKillTime) * 2048

Finally, nLockTime = 500,000,000 + nKillTime + nLockTime2 * 2048

Setting a bit flag in tx nVersion will activate the new rules.

The resolution is 4 blocks or 2048s (34m)
The maximum confirmation window is 8188 blocks (56.9 days) or 
16,769,024s (48.5 days)

For example:
With nLockTime2 = 20 and nKillTime = 100, a tx could be confirmed only 
between block 420,080 and 420,480
With nLockTime2 = 730,000 and nKillTime = 1000, a tx could be confirmed 
only between median time-past of 1,495,042,048 and 1,497,090,048

----------------
Why is this a softfork?

Remember this formula: nLockTime = 500,000,000 + nKillTime + nLockTime2 
* 2048

For height based nLockTime2 (<= 719,999)

For nLockTime2 = 0 and nKillTime = 0, nLockTime = 500,000,000, which 
means the tx could be confirmed after 1970-01-01 with the original lock 
time rule. As the new rule does not allow confirmation until block 
420,000, it's clearly a softfork.

It is not difficult to see that the growth of nLockTime will never catch 
up nLockTime2.

At nLockTime2 = 719,999 and nKillTime = 2047, nLockTime = 1,974,559,999, 
which means 2016-09-22. However, the new rule will not allow 
confirmation until block 3,299,996 which is decades to go



For time based nLockTime2 (> 720,000)

For nLockTime2 = 720,000 and nKillTime = 0, nLockTime = 1,974,560,000, 
which means the tx could be confirmed after median time-past 
1,474,560,000 (assuming BIP113). However, the new rule will not allow 
confirmation until 1,474,562,048, therefore a soft fork.

For nLockTime2 = 720,000 and nKillTime = 2047, nLockTime = 
1,974,562,047, which could be confirmed at 1,474,562,047. Again, the new 
rule will not allow confirmation until 1,474,562,048. The 1 second 
difference makes it a soft fork.

Actually, for every nLockTime2 value >= 720,000, the lock time with the 
new rule must be 1-2048 seconds later than the original rule.

For nLockTime2 = 1,853,010 and nKillTime = 2047, nLockTime = 
4,294,966,527, which is the highest possible value with the 32-bit 
nLockTime

----------------
User's perspective:

A user wants his tx either filled or killed in about 3 hours. He will 
set a time-based nLockTime2 according to the current median time-past, 
and set nKillTime = 5

A user wants his tx get confirmed in the block 630000, the first block 
with reward below 10BTC. He is willing to pay high fee but don't want it 
gets into another block. He will set nLockTime2 = 210,000 and nKillTime 
= 0

----------------
OP_CLTV

Time-based OP_CLTV could be upgraded to support time-based nLockTime2. 
However, height-based OP_CLTV is not compatible with nLockTime2. To 
spend a height-based OP_CLTV output, user must use the original 
nLockTime.

We may need a new OP_CLTV2 which could verify both nLockTime and 
nLockTime2

----------------
55 years after?

The height-based nLockTime2 will overflow in 55 years. It is very likely 
a hard fork will happen to implement a better fill-or-kill system. If 
not, we could reboot everything with another tx nVersion for another 55 
years.