[bitcoin-dev] BIP Idea: Marginal Pricing

[William Morriss <wjmelements@gmail.com>]

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Comrades,

Long term, tx fees must support hash power by themselves. The following is
an economic approach to maximize total fee collection, and therefore
hashpower.

*Goals*
Maximize total transaction fees
Reduce pending transaction time
Reduce individual transaction fees

*Challenges*
Validators must agree on the maximum block size, else miners can cheat and
include extra transactions.
Allowing too many transactions per block will increase the cost of the
mining without collecting much income for the network.


*Problem*
In the transaction market, users are the demand curve, because they will
transact less when fees are higher, and prefer altcoins. The block size is
the supply curve, because it represents miners' willingness to accept
transactions.
Currently, the supply curve is inelastic:

​Increasing the block size will not affect the inelasticity for any fixed
block size. The downsides of a fixed block size limit are well-known:
- Unpredictable transaction settlement time
- Variable transaction fees depending on network congestion
- Frequent overpay

*Proposal*
1. Miners implicitly choose the market sat/byte rate with the cheapest-fee
transaction included in their block. Excess transaction fees are refunded
to the inputs.
2. Remove the block size limit, which is no longer necessary.

*Benefits*
- Dynamic block size limit regulated by profit motive
- Transaction fees maximized for every block
- No overpay; all fees are fair

​Miners individually will make decisions to maximize their block-reward
profit.
Miners are incentivized to ignore low-fee transactions because they would
shave the profits of their other transactions and increase their hash time.
Users and services are free to bid higher transaction fees in order to
reach the next block, since their excess bid will be refunded.

The block size limit was added as a spam-prevention measure, but in order
for an attacker to spam the network with low-fee transactions, they would
have to offset the marginal cost of reducing the price with their own
transaction fees. Anti-spam is thus built into the marginal system without
the need for an explicit limit.

Rarely, sections of the backlog would become large enough to be profitable.
This means every so many blocks, lower-fee transactions would be included
en masse after having been ignored long enough. Low-fee transactions thus
gain a liveness property not previously enjoyed: low-fee transactions will
eventually confirm. Miners targeting these transactions would be at a
noteworthy disadvantage because they would be hashing a larger block. I
predict that this scheme would result in two markets: a backlog market and
a real-time market. Users targeting the backlog market would match the
price of the largest backlog section in order to be included in the next
backlog block.

*Examples*

Scenario 1
Sat/byte Bytes Reward
400 500000 200000000
300 700000 210000000
200 1000000 200000000
100 1500000 150000000
50 5000000 250000000
20 10000000 200000000
A miner would create a 5MB block and receive 0.25 BTC

Scenario 2
Sat/byte Bytes Reward
400 600000 240000000
300 700000 210000000
200 1000000 200000000
100 1800000 180000000
50 4000000 200000000
20 10000000 200000000
A miner would create a 600KB block and receive 0.24 BTC

Thanks,
William Morriss

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