Some comments:

 	* The 75% rule is meaningless here. Since this is a pure relaxation of
rules, there is no such thing as "invalid version 4 blocks"

	* 

The implication threshold is unclear. Is it 95% or 80%?

 	* Softfork requires a very high threshold (95%) to "attack" the
original fork. This makes sure that unupgraded client will only see the
new fork.
 	* In the case of hardfork, however, the new fork is unable to attack
the original fork, and unupgraded client will never see the new fork.
The initiation of a hardfork should be based on its acceptance by the
economic majority, not miner support. 95% is an overkill and may
probably never accomplished. I strongly prefer a 80% threshold rather
than 95%.

 	* As I've pointed out, using 20-percentile rather than median creates
an incentive to 51% attack the uncooperative minority.
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-August/010690.html

Having said that, I don't have a strong feeling about the use of
20-percentile as threshold to increase the block size. That means the
block size is increased only when most miners agree, which sounds ok to
me. 

However, using 20-percentile as threshold to DECREASE the block size
could be very dangerous. Consider that the block size has been stable at
8MB for a few years. Everyone are happy with that. An attacker would
just need to acquire 21% of mining power to break the status quo and
send us all the way to 1MB. The only way to stop such attempt is to 51%
attack the attacker. That'd be really ugly. 

For technical and ethical reasons, I believe the thresholds for increase
and decrease must be symmetrical: increase the block size when the
x-percentile is bigger than the current size, decrease the block size
when the (100-x)-percentile is smaller than the current size. The
overall effect is: the block size remains unchanged unless 80% of miners
agree to. 

 	* Please consider the use of "hardfork bit" to signify the hardfork:

https://www.reddit.com/r/bitcoin_devlist/comments/3ekhg2/bip_draft_hardfork_bit_jl2012_at_xbthk_jul_23_2015/


https://github.com/jl2012/bips/blob/master/hardforkbit.mediawiki 

 	* Or, alternatively, please combine the hardfork with a softfork. I'm
rewriting the specification as follow (changes underlined):

 	* Replace static 1M block size hard limit with a floating limit
("hardLimit").

	* 

hardLimit floats within the range 1-32M, inclusive.

	* 

Initial value of hardLimit is 1M, preserving current system.
 	* Changing hardLimit is accomplished by encoding a proposed value
within a block's coinbase scriptSig.

 	* Votes refer to a byte value, encoded within the pattern "/BVd+/"
Example: /BV8000000/ votes for 8,000,000 byte hardLimit. If there is
more than one match with with pattern, the first match is counted.
 	* Absent/invalid votes and votes below minimum cap (1M) are counted as
1M votes. Votes above the maximum cap (32M) are counted as 32M votes.
 	* A new hardLimit is calculated at each difficult adjustment period
(2016 blocks), and applies to the next 2016 blocks.
 	* Calculate hardLimit by examining the coinbase scriptSig votes of the
previous 12,000 blocks, and taking the 20th percentile and 80th
percentile.
 	* New hardLimit is the median of the followings:

 	* min(current hardLimit * 1.2, 20-percentile)
 	* max(current hardLimit / 1.2, 80-percentile)
 	* current hardLimit

 	* version 4 block: the coinbase of a version 4 block must match this
pattern: "/BVd+/"
 	* 70% rule: If 8,400 of the last 12,000 blocks are version 4 or
greater, reject invalid version 4 blocks. (testnet4: 501 of last 1000)
 	* 80% rule ("Point of no return"): If 9,600 of the last 12,000 blocks
are version 4 or greater, reject all version <= 3 blocks. (testnet4: 750
of last 1000)
 	* Block version number is calculated after masking out high 16 bits
(final bit count TBD by versionBits outcome).

Jeff Garzik via bitcoin-dev 於 2015-09-02 23:33 寫到:
> BIP 100 initial public draft:
> https://github.com/jgarzik/bip100/blob/master/bip-0100.mediawiki [1]
> 
> Emphasis on "initial" This is a starting point for the usual open
> source feedback/iteration cycle, not an endpoint that Must Be This
> Way.
> 
> 
> 
> Links:
> ------
> [1] https://github.com/jgarzik/bip100/blob/master/bip-0100.mediawiki
> 
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