Re: [bitcoin-dev] Consensus based block size retargeting algorithm (draft)

From: jl2012@xbt•hk
To: Btc Drak <btcdrak@gmail•com>
Cc: Bitcoin Dev <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Consensus based block size retargeting algorithm (draft)
Date: Sat, 29 Aug 2015 15:03:57 -0400	[thread overview]
Message-ID: <252a127e52be46262b09482c39ad286c@xbt.hk> (raw)
In-Reply-To: <CADJgMzvJDSk6oZMZPc4C8ENHhGvspdeF7WURfnY6OPKWs33a7w@mail.gmail.com>

I am quite skeptical about any pay-to-increase proposal because it is 
difficult to predict the game dynamics and determine the right amount of 
penalty. But anyway, here is my response to your revised proposal:

1. I agree with you that there should be a cap in the rate of change, 
and also the maximum possible size. This is already part of BIP100

2. Requiring a higher difficulty is bad for everyone:
  a) it increases the variance of the miner;
  b) average confirmation time for all tx are increased. It may even 
cause a feedback: many tx in mempool -> increase block size -> wait 
longer for confirmation -> more tx in mempool;
  c) difficulty of the next round will be decreased, leading to a greater 
fluctuation in confirmation time.

Instead, you should require miners to burn their coinbase reward. This 
is effectively same as higher difficulty but is good for everyone: a) 
mining variance and confirmation time unchanged; b) all bitcoin holders 
become relatively richer

If you don't want to burn any bitcoin, you may require the miner the 
send to penalty to <840000 + current height> OP_CHECKLOCKTIMEVERIFY, 
which will subsidize the mining when the block reward drops below 1 BTC

3. It is a better idea to allow mining of a bigger block immediately, 
which reduces (but not eliminates) the problem of tragedy of the 
commons. However, you can't use the blocksize as the vote. Mining an 
empty block doesn't mean the miner wants to decrease the block size to 
200 bytes. That will just encourage some miners to fill up a block with 
garbage which does no good for anyone. Therefore, you need to look at 
both the actual block size and the coinbase vote, and always take the 
bigger value to determine the penalty and the max block size of next 
round. If a miner includes nothing in the coinbase, it should be 
consider as a vote for the current max block size.

Btc Drak via bitcoin-dev 於 2015-08-29 06:15 寫到:
> On Sat, Aug 29, 2015 at 1:29 AM, Mark Friedenbach via bitcoin-dev
> <bitcoin-dev@lists•linuxfoundation.org> wrote:
> 
> Mark and Jorge,
> 
> I am very glad you have brought up this particular objection because
> it's something I thought about but was unclear if it was an opinion
> that would be shared by others. I chose to omit it from the proposal
> to see if it would come up during peer review.
> 
> I feel that giving miners a blank cheque to increase blocksize, by any
> means, goes against a key design of bitcoin's security model. Full
> nodes keep miners honest by ensuring by validating their blocks. Under
> any voting-only scheme there is no way for full nodes to keep miners
> in cheque because miner have free reign to increase the blocksize.
> 
> This problem can be solved by introducing a hard cap on blocksize. By
> introducing an upper limit miners now have the freedom to increase
> blocksize but only within defined parameters.  Remember my proposal
> allows blocksize to increase and decrease in such a way that miners
> must collectively agree if they want the size to increase.
> 
> I believe the idea of a hard upper limit has become rather politicised
> but is essential to the security model of bitcoin.
> 
> With respect to the flexicap idea where miners can create a larger
> block by paying extra difficulty, I believe that proposal has a
> critical flaw because, as Gavin pointed out, it makes it very
> expensive (and risky) to include a few extra transactions. I believe
> it suffers from tragedy of the commons because there is no incentive
> for the mining community to reach consensus. Each and every block is
> going to be a gamble, "should we include a few extra transactions at
> the risk of losing the block?". Under my proposal miners can
> collectively agree to change the blocksize. Let's say they want a 10%
> increase, they can collude together to make that increase and once
> reached, it remains until they want to change it again. Yet, the upper
> hard limit keeps the ultimate control of the maximum block size
> squarely in the hands of full nodes.
> 
> Whilst the exact number may be up for discussion, I would propose an
> initial upper limit of 8MB, so under my proposal the blocksize would
> be flexible between 1MB and 8MB.
> 
> An alternative methodology to voting in the coinbase would be to
> change the vote to be the blocksize itself
> 
> 1. miners pay extra difficulty to create a larger block.
> 2. every 2016 blocks the average or median of the last 2016 blocks is
> calculated and becomes the new maximum blocksize limit.
> 
> This would retain incentive to collude to increase blocksize, as well
> as the property of costing to increase while being free to propose
> decrease.
> 
> It would still require an upper blocksize limit in order for full
> nodes to retain control. Without an upper limit, any proposal is going
> to break the security model as full nodes give up some oversight
> control over miners.
> 
> Another way of looking at these ideas is we're raising blocksize hard
> limit (to 8MB or whatever is decided), but making a soft of "softer"
> or inner limit part of consensus. Such a concept is not really
> departing from the current idea of a soft limit except to make it
> consensus enforced. Obviously it's not identical, but I think you can
> see the similarities.
> 
> Does that make sense?
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