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ZIP 317: work-in-progress editing.
Signed-off-by: Daira Hopwood <daira@jacaranda.org>
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Daira Hopwood
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zip-0317.rst
226
zip-0317.rst
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@ -78,34 +78,106 @@ Requirements
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Specification
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=============
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This specification defines three parameters that are used to calculate the
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Fee calculation
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---------------
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This specification defines two parameters that are used to calculate the
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conventional fee:
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=================== ==============================================
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====================== ==============================================
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Parameter Units
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=================== ==============================================
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`marginal_fee` zatoshis per logical action (as defined below)
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`grace_window_size` logical actions
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=================== ==============================================
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====================== ==============================================
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:math:`marginal\_fee` zatoshis per logical action (as defined below)
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:math:`grace\_actions` logical actions
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====================== ==============================================
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Wallets implementing this specification SHOULD use a conventional fee
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calculated in zatoshis per the following formula::
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calculated in zatoshis per the following formula:
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logical_actions = max(transparent_inputs, transparent_outputs) +
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2*sprout_joinsplits +
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max(sapling_inputs, sapling_outputs) +
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orchard_actions
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.. math::
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conventional_fee = marginal_fee * max(grace_actions, logical_actions)
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\begin{array}{rcl}
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logical\_actions &=& \max(transparent\_inputs, transparent\_outputs) \;+ \\
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& & 2 \cdot sprout\_joinsplits \;+ \\
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& & \max(sapling\_inputs, sapling\_outputs) \;+ \\
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& & orchard\_actions \\
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conventional\_fee &=& marginal\_fee \cdot \max(grace\_actions, logical\_actions)
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\end{array}
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The parameters are set to the following values:
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* `marginal_fee = 5000`;
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* `grace_window_size = 2`.
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* :math:`marginal\_fee = 5000`;
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* :math:`grace\_actions = 2`.
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It is not a consensus requirement that fees follow this formula; however,
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wallets SHOULD create transactions that pay this fee, in order to reduce
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information leakage, unless overridden by the user.
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Rationale for logical actions
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'''''''''''''''''''''''''''''
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The intention is to make the fee paid for a transaction depend on its
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impact on the network, without discriminating between different protocols
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(Orchard, Sapling, or transparent). The impact on the network depends on
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the numbers of inputs and outputs.
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A previous proposal used :math:`inputs + outputs` instead of logical actions.
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This would have disadvantages Orchard transactions, as a result of an
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Orchard Action combining an input and an output. The effect of this
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combining is that Orchard requires padding of either inputs or outputs
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to ensure that the number of inputs and outputs are the same. Usage of
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Sapling and transparent protocols does not require this padding, and
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so this could have effectively discriminated against Orchard.
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Rationale for the chosen parameters
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'''''''''''''''''''''''''''''''''''
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Grace Actions
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~~~~~~~~~~~~~
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**Why not just charge per-action, without a grace window?**
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* This ensures that there is no penalty to padding a 1-action
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transaction to a 2-action transaction. Such padding is desirable
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to reduce information leakage from input and output arity, and
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is the standard approach used by `zcashd` and the mobile SDK
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transaction builder.
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* Without a grace window, an input with value below the marginal
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fee would never be worth including in the resulting transaction.
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With a grace window, an input with value below :math:`marginal\_fee`
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*is* worth including, if a second input is available that covers
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both the primary output amount and the conventional transaction
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fee.
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**Why a grace window of 2?**
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A 1-in, 2-out (or 2-action) transaction is the smallest possible
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transaction that permits both an output to a recipient, and a
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change output. However, as stated above, `zcashd` and the mobile
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SDK transaction builder will pad the number of inputs to at least 2.
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Let :math:`min\_actions` be the minimum number of logical actions
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that can be used to execute economically relevant transactions that
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produce change. Due to the aforementioned padding, :math:`min\_actions = 2`.
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Having a grace window size greater than :math:`min\_actions` would
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increase the cost to create such a minimal transaction. If the
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cost for a minimal transaction is bounded above by :math:`B`, then
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possible choices of :math:`marginal\_fee` are bounded above by
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:math:`B / max(min\_actions, grace\_actions)`. Therefore, the
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optimal choice of :math:`grace\_actions` to maximize the cost of
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denial-of-service attacks that use many logical actions, without
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imposing an undue penalty on minimal transactions, is
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:math:`grace\_actions = min\_actions = 2`.
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Marginal Fee
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~~~~~~~~~~~~
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This returns the conventional fee for a minimal transaction (as
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described above) to the original conventional fee of 10000 zatoshis
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specified in [#zip-0313]_, and imposes a non-trivial cost for
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potential denial-of-service attacks.
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Transaction relaying
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--------------------
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@ -119,7 +191,7 @@ Mempool size limiting
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---------------------
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zcashd and zebrad limit the size of the mempool as described in [#zip-0401]_.
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This specifies a *low\_fee\_penalty* that is added to the "eviction weight"
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This specifies a :math:`low\_fee\_penalty` that is added to the "eviction weight"
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if the transaction pays a fee less than the conventional transaction fee.
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This threshold is modified to use the new conventional fee formula.
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@ -132,10 +204,57 @@ previously used the conventional transaction fee defined in ZIP 313 to
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decide on transaction inclusion, it is expected to instead use the formula
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specified in this ZIP.
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Open Issues
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-----------
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> TODO: Remove this section once a decision is made.
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Security and Privacy considerations
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===================================
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Non-standard transaction fees may reveal specific users or wallets or wallet
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versions, which would reduce privacy for those specific users and the rest
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of the network. However, the advantage of faster deployment argued against
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synchronizing the change in wallet behaviour at a specific block height.
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Long term, the issue of fees needs to be revisited in separate future
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proposals as the blocks start getting consistently full. Wallet developers
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and operators should monitor the Zcash network for rapid growth in
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transaction rates, and consider further changes to fee selection and/or
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other scaling solutions if necessary.
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Denial of Service
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-----------------
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A transaction-rate-based denial of service attack occurs when an attacker
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generates enough transactions over a window of time to prevent legitimate
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transactions from being mined, or to hinder syncing blocks for full nodes
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or miners.
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There are two primary protections to this kind of attack in Zcash: the
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block size limit, and transaction fees. The block size limit ensures that
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full nodes and miners can keep up with the blockchain even if blocks are
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completely full. However, users sending legitimate transactions may not
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have their transactions confirmed in a timely manner.
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This proposal does not alter how fees are paid from transactions to miners.
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Deployment
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==========
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Wallets SHOULD deploy these changes immediately. Nodes SHOULD deploy the
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change to the :math:`low\_fee\_penalty` threshold described in
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`Mempool size limiting`_ immediately.
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Nodes can deploy restrictions to their policies for relaying, mempool
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acceptance, and/or mining once a sufficient proportion of wallets in the
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ecosystem are observed to be paying at least the updated conventional
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transaction fee. Node developers SHOULD coordinate on deployment
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schedule.
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Considered Alternatives
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=======================
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This section describes alternative proposals that have not been adopted.
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Possible alternatives for the parameters:
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@ -151,77 +270,6 @@ Possible alternatives for the parameters:
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of inputs/outputs to be non-proportional above the `grace_window_size`. This
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is no longer expressible with the formula specified above.)
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Rationale for logical actions
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'''''''''''''''''''''''''''''
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The intention is to make the fee paid for a transaction depend on its
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impact on the network, without discriminating between different protocols
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(Orchard, Sapling, or transparent). The impact on the network depends on
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the numbers of inputs and outputs.
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A previous proposal used `inputs + outputs` instead of logical actions.
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This would have disadvantages Orchard transactions, as a result of an
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Orchard Action combining an input and an output. The effect of this
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combining is that Orchard requires padding of either inputs or outputs
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to ensure that the number of inputs and outputs are the same. Usage of
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Sapling and transparent protocols does not require this padding, and
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so this could have effectively discriminated against Orchard.
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Security and Privacy considerations
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===================================
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Non-standard transaction fees may reveal specific users or wallets or wallet
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versions, which would reduce privacy for those specific users and the rest
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of the network. However, the advantage of faster deployment argued against
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synchronizing the change in wallet behaviour at a specific block height.
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Long term, the issue of fees needs to be re-visited in separate future
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proposals as the blocks start getting consistently full. New ZIPs with
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scaling solutions, will need to be evaluated and applied.
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Denial of Service Vulnerability
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-------------------------------
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A transaction-rate-based denial of service attack occurs when an attacker
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generates enough transactions over a window of time to prevent legitimate
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transactions from being mined, or to hinder syncing blocks for full nodes
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or miners.
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There are two primary protections to this kind of attack in Zcash: the
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block size limit, and transaction fees. The block size limit ensures that
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full nodes and miners can keep up with the blockchain even if blocks are
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completely full. However, users sending legitimate transactions may not
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have their transactions confirmed in a timely manner.
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Variable fees could mitigate this kind of denial of service: if there are
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more transactions available than can fit into a single block, then a miner
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will typically choose the transactions that pay the highest fees. If
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legitimate wallets were to increase their fees during this condition, the
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attacker would also increase the fees of their transactions. It is
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sometimes argued that this would impose a cost to the attacker that would
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limit the time window for which they can continue the attack. However, there
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is little evidence that the actual costs involved would be a sufficient
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disincentive.
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This proposal does not alter how fees are paid from transactions to miners.
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Wallet developers and operators should monitor the Zcash network for rapid
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growth in transaction rates.
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Deployment
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==========
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Wallets SHOULD deploy these changes immediately. Nodes SHOULD deploy the
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change to the *low\_fee\_penalty* threshold described in `Mempool size limiting`_
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immediately.
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Nodes can deploy restrictions to their policies for relaying, mempool
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acceptance, and/or mining once a sufficient proportion of wallets in the
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ecosystem are observed to be paying at least the updated conventional
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transaction fee. Node developers SHOULD coordinate on deployment
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schedule.
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Endorsements
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