feemarket
Abstract
This document specifies the feemarket module which allows to define a global transaction fee for the network.
This module has been designed to support EIP1559 in cosmos-sdk.
The MempoolFeeDecorator
in x/auth
module needs to be overwritten to check the baseFee
along with the minimal-gas-prices
allowing to implement a global fee mechanism which vary depending on the network activity.
For more reference to EIP-1559:
https://github.com/ethereum/EIPs/blob/master/EIPS/eip-1559.md
Concepts
EIP-1559: Fee Market
EIP-1559 describes a pricing mechanism that was proposed on Ethereum to improve to calculation of transaction fees. It includes a fixed-per-block network fee that is burned and dynamically expands/contracts block sizes to deal with peaks of network congestion.
Before EIP-1559 the transaction fee is calculated with
where gasPrice
is the price per gas and gasLimit
describes the amount of gas required to perform the transaction. The more complex operations a transaction requires, the higher the gas limit (see Executing EVM bytecode). To submit a transaction, the signer needs to specify the gasPrice
.
With EIP-1559 enabled, the transaction fee is calculated with
where baseFee
is the fixed-per-block network fee per gas and priorityTip
is an additional fee per gas that can be set optionally. Note, that both the base fee and the priority tip are gas prices. To submit a transaction with EIP-1559, the signer needs to specify the gasFeeCap
, which is the maximum fee per gas they are willing to pay in total. Optionally, the priorityTip
can be specified, which covers both the priority fee and the block's network fee per gas (aka: base fee).
The Cosmos SDK uses a different terminology for gas
than Ethereum. What is called gasLimit
on Ethereum is called gasWanted
on Cosmos. You might encounter both terminologies on Slinky since it builds Ethereum on top of the SDK, e.g. when using different wallets like Keplr for Cosmos and Metamask for Ethereum.
Base Fee
The base fee per gas (aka base fee) is a global gas price defined at the consensus level. It is stored as a module parameter and is adjusted at the end of each block based on the total gas used in the previous block and gas target (block gas limit / elasticity multiplier
):
it increases when blocks are above the gas target,
it decreases when blocks are below the gas target.
Instead of burning the base fee (as implemented on Ethereum), the feemarket
module allocates the base fee for regular Cosmos SDK fee distribution.
Priority Tip
In EIP-1559, the max_priority_fee_per_gas
, often referred to as tip
, is an additional gas price that can be added to the baseFee
in order to incentivize transaction prioritization. The higher the tip, the more likely the transaction is included in the block.
Until the Cosmos SDK version v0.46, however, there is no notion of transaction prioritization. Thus, the tip for an EIP-1559 transaction on Slinky should be zero (MaxPriorityFeePerGas
JSON-RPC endpoint returns 0
).
Effective Gas price
For EIP-1559 transactions (dynamic fee transactions) the effective gas price describes the maximum gas price that a transaction is willing to provide. It is derived from the transaction arguments and the base fee parameter. Depending on which one is smaller, the effective gas price is either the baseFee + tip
or the gasFeeCap
Local vs. Global Minimum Gas Prices
Minimum gas prices are used to discard spam transactions in the network, by raising the cost of transactions to the point that it is not economically viable for the spammer. This is achieved by defining a minimum gas price for accepting txs in the mempool for both Cosmos and EVM transactions. A transaction is discarded from the mempool if it doesn't provide at least one of the two types of min gas prices:
Minimum gas prices are used to discard spam transactions in the network, by raising the cost of transactions to the point that it is not economically viable for the spammer. This is achieved by defining a minimum gas price for accepting txs in the mempool for both Cosmos and EVM transactions. A transaction is discarded from the mempool if it doesn't provide at least one of the two types of min gas prices:
the local min gas prices that validators can set on their node config and
the global min gas price, which is set as a parameter in the
feemarket
module, which can be changed through governance.
The lower bound for a transaction gas price is determined by comparing of gas price bounds according to three cases:
If the effective gas price (
effective gas price = base fee + priority tip
) or the local minimum gas price is lower than the globalMinGasPrice
(min-gas-price (local) < MinGasPrice (global) OR EffectiveGasPrice < MinGasPrice
), thenMinGasPrice
is used as a lower bound.If transactions are rejected due to having a gas price lower than
MinGasPrice
, users need to resend the transactions with a gas price higher or equal toMinGasPrice
.If the effective gas price or the local
minimum-gas-price
is higher than the globalMinGasPrice
, then the larger value of the two is used as a lower bound. In the case of EIP-1559, users must increase the priority fee for their transactions to be valid.
The comparison of transaction gas price and the lower bound is implemented through AnteHandler decorators. For EVM transactions, this is done in the EthMempoolFeeDecorator
and EthMinGasPriceDecorator
AnteHandler
and for Cosmos transactions in NewMempoolFeeDecorator
and MinGasPriceDecorator
AnteHandler
.
If the base fee decreases to a value below the global MinGasPrice
, it is set to the MinGasPrice
. This is implemented, so that the base fee can't drop to gas prices that wouldn't allow transactions to be accepted in the mempool, because of a higher MinGasPrice
.
State
The x/feemarket
module keeps in the state variable needed to the fee calculation:
Only BlockGasUsed in previous block needs to be tracked in state for the next base fee calculation.
Begin Block
The base fee is calculated at the beginning of each block.
Base Fee
Disabling base fee
We introduce two parameters : NoBaseFee
and EnableHeight
NoBaseFee
controls the feemarket base fee value. If set to true, no calculation is done and the base fee returned by the keeper is zero.
EnableHeight
controls the height we start the calculation.
If
NoBaseFee = false
andheight < EnableHeight
, the base fee value will be equal tobase_fee
defined in the genesis and theBeginBlock
will return without further computation.If
NoBaseFee = false
andheight >= EnableHeight
, the base fee is dynamically calculated upon each block atBeginBlock
.
Those parameters allow us to introduce a static base fee or activate the base fee at a later stage.
Enabling base fee
To enable EIP1559 with the EVM, the following parameters should be set :
NoBaseFee should be false
EnableHeight should be set to a positive integer >= upgrade height. It defines at which height the chain starts the base fee adjustment
LondonBlock evm's param should be set to a positive integer >= upgrade height. It defines at which height the chain start to accept EIP1559 transactions
Calculation
The base fee is initialized at EnableHeight
to the InitialBaseFee
value defined in the genesis file.
The base fee is after adjusted according to the total gas used in the previous block.
End Block
The block_gas_used
value is updated at the end of each block.
Block Gas Used
The total gas used by current block is stored in the KVStore at EndBlock
.
It is initialized to block_gas
defined in the genesis.
Keeper
The feemarket module provides this exported keeper that can be passed to other modules, which require access to the base fee value
Events
The x/feemarket
module emits the following events:
BeginBlocker
EndBlocker
Parameters
The x/feemarket
module contains the following parameters:
Client
CLI
A user can query and interact with the feemarket
module using the CLI.
Queries
The query
commands allow users to query feemarket
state.
Base Fee
The base-fee
command allows users to query the block base fee by height.
Example:
Example Output:
Block Gas
The block-gas
command allows users to query the block gas by height.
Example:
Example Output:
Params
The params
command allows users to query the module params.
Example:
Example Output:
gRPC
Queries
AnteHandlers
The x/feemarket
module provides AnteDecorator
s that are recursively chained together into a single Antehandler
. These decorators perform basic validity checks on an Ethereum or Cosmos SDK transaction, such that it could be thrown out of the transaction Mempool.
Note that the AnteHandler
is run for every transaction and called on both CheckTx
and DeliverTx
.
Decorators
MinGasPriceDecorator
Rejects Cosmos SDK transactions with transaction fees lower than MinGasPrice * GasLimit
.
EthMinGasPriceDecorator
Rejects EVM transactions with transactions fees lower than MinGasPrice * GasLimit
.
For
LegacyTx
andAccessListTx
, theGasPrice * GasLimit
is used.For EIP-1559 (aka.
DynamicFeeTx
), theEffectivePrice * GasLimit
is used.
Note: For dynamic transactions, if the feemarket
formula results in a BaseFee
that lowers EffectivePrice < MinGasPrices
, the users must increase the GasTipCap
(priority fee) until EffectivePrice > MinGasPrices
. Transactions with MinGasPrices * GasLimit < transaction fee < EffectiveFee
are rejected by the feemarket
AnteHandle
.
EthGasConsumeDecorator
Calculates the effective fees to deduct and the tx priority according to EIP-1559 spec, then deducts the fees and sets the tx priority in the response.
When there are multiple messages in the transaction, choose the lowest priority in them.
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