2022-09-20 02:12:40 -07:00
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ZIP: 317
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Title: Proportional Transfer Fee Mechanism
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Owners: Aditya Bharadwaj <nighthawk24@gmail.com>
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Credits: Madars Virza
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Kris Nuttycombe
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Daira Hopwood
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Jack Grigg
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Francisco Gindre
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2022-10-04 14:17:04 -07:00
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Status: Draft
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2022-09-20 02:12:40 -07:00
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Category: Standards / Wallet
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2022-10-04 14:17:04 -07:00
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Obsoletes: ZIP 313
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2022-09-20 02:12:40 -07:00
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Created: 2022-08-15
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License: MIT
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Discussions-To: <https://forum.zcashcommunity.com/t/zip-proportional-output-fee-mechanism-pofm/42808>
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Pull-Request: <https://github.com/zcash/zips/pull/631>
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2022-10-04 14:17:04 -07:00
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Terminology
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===========
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The key word "SHOULD" in this document is to be interpreted as described in
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RFC 2119. [#RFC2119]_
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The term "conventional transaction fee" in this document is in reference
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to the value of a transaction fee that is conventionally used by wallets,
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and that a user can reasonably expect miners on the Zcash network to accept
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for including a transaction in a block.
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The terms "Mainnet, "Testnet", and "zatoshi" in this document are defined
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as in [#protocol-networks]_.
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Abstract
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========
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The goal of this ZIP is to change the conventional fees for transactions
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and get buy-in from wallet developers, miners and Zcash users.
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Motivation
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==========
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In light of recent network activity, it is time to review and update the
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standard 1,000 zatoshi transaction fee set in ZIP 313 [#zip-0313]_.
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The conventional transaction fee presently is 0.00001 ZEC or 1,000 zatoshis, as
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specified in ZIP 313. This allowed exploration of novel use cases of the Zcash
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blockchain. The Zcash network has operated for almost 2 years at a conventional
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transaction fee of 1,000 zatoshis, without consideration for the total number
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of inputs and outputs in each transaction. Under this conventional fee, some
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usage of the chain has been characterized by high-output transactions with
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1,100 outputs paying the same conventional fee as a transaction with 2 outputs.
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The objective of the new fee policy, once it is enforced, is for fees paid by
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transactions to fairly reflect the processing costs that they impose on various
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participants in the network. This will tend to discourage usage patterns that
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cause either intentional or unintentional denial of service, while still
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allowing low fees for regular transaction use cases.
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Requirements
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============
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* The conventional fee formula should not favour or discriminate against any
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of the Orchard, Sapling, or transparent protocols.
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* The fee for a transaction should scale linearly with the number of inputs
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and/or outputs.
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* Users should not be penalised for sending transactions constructed
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with padding of inputs and outputs to reduce information leakage.
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(The default policy employed by zcashd and the mobile SDKs pads to
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two inputs and outputs for each shielded pool used by the transaction).
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* Users should be able to spend a small number of UTXOs or notes with value
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below the marginal fee per input.
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Specification
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=============
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2022-10-04 15:57:01 -07:00
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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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Parameter Units
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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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.. math::
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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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* :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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2022-10-04 14:17:04 -07:00
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Transaction relaying
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--------------------
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zcashd, zebrad, and potentially other node implementations, implement
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fee-based restrictions on relaying of mempool transactions. Nodes that
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normally relay transactions are expected to do so for transactions that pay
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at least the conventional fee as specified in this ZIP, unless there are
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other reasons not to do so for robustness or denial-of-service mitigation.
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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 :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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Block production
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----------------
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Miners, mining pools, and other block producers, select transactions for
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inclusion in blocks using a variety of criteria. Where the criteria
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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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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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* marginal_fee = 250 in @nuttycom's proposal.
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* marginal_fee = 1000 adapted from @madars' proposal.
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* marginal_fee = 2500 in @daira's proposal.
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* marginal_fee = 1000 for Shielded, Shielding and De-shielding
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transactions, and marginal_fee = 10000 for Transparent transactions
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adapted from @nighthawk24's proposal.
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(In @madars' and @nighthawk24's original proposals, there was an additional
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`base_fee` parameter that caused the relationship between fee and number
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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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Endorsements
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============
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The following entities/groups/individuals expressed their support for the
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updated fee mechanism:
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*Developer Groups or Sole OSS contributors*
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* Zecwallet Suite (Zecwallet Lite for Desktop/iOS/Android & Zecwallet FullNode)
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* Nighthawk Wallet for Android & iOS
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To express and request your support to be added to this ZIP please comment
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below indicating:
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* (group) name/pseudonym
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* affiliation
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* contact
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or, conversely e-mail the same details to the Owner of the ZIP.
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> TODO: Endorsements may depend on specific parameter choices. The ZIP
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> Editors should ensure that the endorsements are accurate before merging
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> this ZIP.
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Acknowledgements
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================
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Thanks to Madars Virza for initially proposing a fee mechanism similar to that
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proposed in this ZIP [#madars-1]_, and to Kris Nuttycombe, Jack Grigg, Daira Hopwood,
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Francisco Gindre, Greg Pfeil, and Teor for suggested improvements.
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References
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==========
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.. [#RFC2119] `RFC 2119: Key words for use in RFCs to Indicate Requirement Levels <https://www.rfc-editor.org/rfc/rfc2119.html>`_
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.. [#protocol-networks] `Zcash Protocol Specification, Version 2022.3.8. Section 3.12: Mainnet and Testnet <protocol/protocol.pdf#networks>`_
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.. [#madars-1] `Madars concrete soft-fork proposal <https://forum.zcashcommunity.com/t/zip-reduce-default-shielded-transaction-fee-to-1000-zats/37566/89>`_
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.. [#zip-0313] `ZIP 313: Reduce Conventional Transaction Fee to 1000 zatoshis <zip-0313.rst>`_
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.. [#zip-0401] `ZIP 401: Addressing Mempool Denial-of-Service <zip-0401.rst>`_
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