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108 lines
4.0 KiB
Plaintext
<pre>
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BIP: 105
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Title: Consensus based block size retargeting algorithm
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Author: BtcDrak <btcdrak@gmail.com>
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Status: Draft
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Type: Standards Track
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Created: 2015-08-21
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</pre>
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==Abstract==
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A method of altering the maximum allowed block size of the Bitcoin protocol
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using a consensus based approach.
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==Motivation==
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There is a belief that Bitcoin cannot easily respond to raising the
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blocksize limit if popularity was to suddenly increase due to a mass adoption
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curve, because co-ordinating a hard fork takes considerable time, and being
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unable to respond in a timely manner would irreparably harm the credibility of
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bitcoin.
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Additionally, predetermined block size increases are problematic because they
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attempt to predict the future, and if too large could have unintended
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consequences like damaging the possibility for a fee market to develop
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as block subsidy decreases substantially over the next 9 years; introducing
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or exacerbating mining attack vectors; or somehow affect the network in unknown
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or unpredicted ways. Since fixed changes are hard to deploy, the damage could be
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extensive.
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Dynamic block size adjustments also suffer from the potential to be gamed by the
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larger hash power.
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Free voting as suggested by BIP100 allows miners to sell their votes out of band
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at no risk, and enable the sponsor the ability to manipulate the blocksize.
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It also provides a cost free method or the larger pools to vote in ways to
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manipulate the blocksize such to disadvantage or attack smaller pools.
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==Rationale==
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By introducing a cost to increase the block size ensures the mining community
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will collude to increase it only when there is a clear necessity, and reduce it
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when it is unnecessary. Larger miners cannot force their wishes so easily
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because not only will they have to pay extra a difficulty target, then can be
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downvoted at no cost by the objecting hash power.
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Using difficulty as a penalty is better than a fixed cost in bitcoins because it
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is less predictable.
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In order to prevent miners having complete control over blocksize, an upper
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limit is required at protocol level. This feature ensures full nodes retain
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control over consensus, remembering full nodes are the mechanism to keep miners
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honest.
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==Specification==
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The initial block size limit shall be 1MB.
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Each time a miner creates a block, they may vote to increase or decrease the
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blocksize by a maximum of 10% of the current block size limit. These votes will
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be used to recalculate the new block size limit every 2016 blocks.
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Votes are cast using the block's coinbase transaction scriptSig.
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As per BIP34, the coinbase transaction scriptSig starts with a push of the block
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height. The next push is a little-endian number representing the preferred block
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size in bytes. For example, 0x4c(OP_PUSHDATA1) 0x03(size of constant) 0x80 0x84 0x1e(2MB)
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or 0x4c(OP_PUSHDATA1) 0x04(size of constant) 0x80 0x96 0x98 0x00(10MB).
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If a miner votes for an increase, the block hash must meet a difficulty target
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which is proportionally larger than the standard difficulty target based on the
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percentage increase they voted for.
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Votes proposing decreasing the block size limit do not need to meet a higher
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difficulty target.
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Miners can vote for no change by voting for the current block size.
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For blocks to be valid the blockhash must meet the required difficulty target
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for the vote otherwise the block is invalid and will be rejected.
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Every 2016 blocks, the block size limit will be recalculated by the median of
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all votes in the last 2016 blocks. This will redefine the block size limit for
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the next 2016 blocks.
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Blocks that are larger than the calculated base block size limit are invalid and
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will be rejected.
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The base block size limit may not reduce below 1MB or increase above 8MB (the exact
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number for the upper limit requires further discussion).
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==Acknowledgements==
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This proposal is based on ideas and concepts derived from the writings of
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Meni Rosenfeld and Gregory Maxwell.
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==References==
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[[bip-0034.mediawiki|BIP34]]
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==Copyright==
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This work is placed in the public domain.
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