[Bitcoin-development] Proposed alternatives to the 20MB step function

Fri May 8 22:43:14 UTC 2015

This is a clever way to tie block size to fees.

I would just like to point out though that it still fundamentally is using
hard block size limits to enforce scarcity. Transactions with below market
fees will hang in limbo for days and fail, instead of failing immediately
by not propagating, or seeing degraded, long confirmation times followed by
eventual success.

Aaron Voisine
co-founder and CEO
breadwallet.com

On Fri, May 8, 2015 at 1:33 PM, Mark Friedenbach <mark at friedenbach.org>
wrote:

> It is my professional opinion that raising the block size by merely
> adjusting a constant without any sort of feedback mechanism would be a
> dangerous and foolhardy thing to do. We are custodians of a multi-billion
> dollar asset, and it falls upon us to weigh the consequences of our own
> actions against the combined value of the entire bitcoin ecosystem. Ideally
> we would take no action for which we are not absolutely certain of the
> ramifications, with the information that can be made available to us. But
> of course that is not always possible: there are unknown-unknowns, time
> pressures, and known-unknowns where information has too high a marginal
> cost. So where certainty is unobtainable, we must instead hedge against
> unwanted outcomes.
>
> The proposal to raise the block size now by redefining a constant carries
> with it risk associated with infrastructure scaling, centralization
> pressures, and delaying the necessary development of a constraint-based fee
> economy. It also simply kicks the can down the road in settling these
> issues because a larger but realistic hard limit must still exist, meaning
> a future hard fork may still be required.
>
> But whatever new hard limit is chosen, there is also a real possibility
> that it may be too high. The standard response is that it is a soft-fork
> change to impose a lower block size limit, which miners could do with a
> minimal amount of coordination. This is however undermined by the
> unfortunate reality that so many mining operations are absentee-run
> businesses, or run by individuals without a strong background in bitcoin
> protocol policy, or with interests which are not well aligned with other
> users or holders of bitcoin. We cannot rely on miners being vigilant about
> issues that develop, as they develop, or able to respond in the appropriate
> fashion that someone with full domain knowledge and an objective
> perspective would.
>
> The alternative then is to have some sort of dynamic block size limit
> controller, and ideally one which applies a cost to raising the block size
> in some way the preserves the decentralization and/or long-term stability
> features that we care about. I will now describe one such proposal:
>
>   * For each block, the miner is allowed to select a different difficulty
> (nBits) within a certain range, e.g. +/- 25% of the expected difficulty,
> and this miner-selected difficulty is used for the proof of work check. In
> addition to adjusting the hashcash target, selecting a different difficulty
> also raises or lowers the maximum block size for that block by a function
> of the difference in difficulty. So increasing the difficulty of the block
> by an additional 25% raises the block limit for that block from 100% of the
> current limit to 125%, and lowering the difficulty by 10% would also lower
> the maximum block size for that block from 100% to 90% of the current
> limit. For simplicity I will assume a linear identity transform as the
> function, but a quadratic or other function with compounding marginal cost
> may be preferred.
>
>   * The default maximum block size limit is then adjusted at regular
> intervals. For simplicity I will assume an adjustment at the end of each
> 2016 block interval, at the same time that difficulty is adjusted, but
> there is no reason these have to be aligned. The adjustment algorithm
> itself is either the selection of the median, or perhaps some sort of
> weighted average that respects the "middle majority." There would of course
> be limits on how quickly the block size limit can adjusted in any one
> period, just as there are min/max limits on the difficulty adjustment.
>
>   * To prevent perverse mining incentives, the original difficulty without
> adjustment is used in the aggregate work calculations for selecting the
> most-work chain, and the allowable miner-selected adjustment to difficulty
> would have to be tightly constrained.
>
> These rules create an incentive environment where raising the block size
> has a real cost associated with it: a more difficult hashcash target for
> the same subsidy reward. For rational miners that cost must be
> counter-balanced by additional fees provided in the larger block. This
> allows block size to increase, but only within the confines of a
> self-supporting fee economy.
>
> When the subsidy goes away or is reduced to an insignificant fraction of
> the block reward, this incentive structure goes away. Hopefully at that
> time we would have sufficient information to soft-fork set a hard block
> size maximum. But in the mean time, the block size limit controller
> constrains the maximum allowed block size to be within a range supported by
> fees on the network, providing an emergency relief valve that we can be
> assured will only be used at significant cost.
>
> Mark Friedenbach
>
> * There has over time been various discussions on the bitcointalk forums
> about dynamically adjusting block size limits. The true origin of the idea
> is unclear at this time (citations would be appreciated!) but a form of it
> was implemented in Bytecoin / Monero using subsidy burning to increase the
> block size. That approach has various limitations. These were corrected in
> Greg Maxwell's suggestion to adjust the difficulty/nBits field directly,
> which also has the added benefit of providing incentive for bidirectional
> movement during the subsidy period. The description in this email and any
> errors are my own.
>
> On Fri, May 8, 2015 at 12:20 AM, Matt Whitlock <bip at mattwhitlock.name>
> wrote:
>
>> Between all the flames on this list, several ideas were raised that did
>> not get much attention. I hereby resubmit these ideas for consideration and
>> discussion.
>>
>> - Perhaps the hard block size limit should be a function of the actual
>> block sizes over some trailing sampling period. For example, take the
>> median block size among the most recent 2016 blocks and multiply it by 1.5.
>> This allows Bitcoin to scale up gradually and organically, rather than
>> having human beings guessing at what is an appropriate limit.
>>
>> - Perhaps the hard block size limit should be determined by a vote of the
>> miners. Each miner could embed a desired block size limit in the coinbase
>> transactions of the blocks it publishes. The effective hard block size
>> limit would be that size having the greatest number of votes within a
>> sliding window of most recent blocks.
>>
>> - Perhaps the hard block size limit should be a function of block-chain
>> length, so that it can scale up smoothly rather than jumping immediately to
>> 20 MB. This function could be linear (anticipating a breakdown of Moore's
>> Law) or quadratic.
>>
>> I would be in support of any of the above, but I do not support Mike
>> Hearn's proposed jump to 20 MB. Hearn's proposal kicks the can down the
>> road without actually solving the problem, and it does so in a
>> controversial (step function) way.
>>
>>
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>
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