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Create zip-proportional-output-fee-mechanism-pofm
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ZIP: XXX
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Title: Proportional Output Fee Mechanism (POFM)
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Owners: Aditya Bharadwaj <nighthawk24@gmail.com>
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Credits: Madars Vizra, Kris Nuttycombe
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Status: Draft
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Community Status: Request for comments :
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Category: Wallet
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Created: 2022-08-15
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License: MIT
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Terminology
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===========
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The key words "MUST", "SHOULD", "SHOULD NOT", "MAY", "RECOMMENDED",
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"OPTIONAL", and "REQUIRED" in this document are to be interpreted as
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described in RFC 2119. [#RFC2119]_
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"Z" refers to shielded address.
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"T" refers to transparent address.
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"POFM" refers to the proposed change in conventional fee as described in
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this ZIP.
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The term "default transaction fee" in this document is in reference
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to Z to Z, T to Z & Z to T transaction fee paid to miners on the Zcash network
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for the work on including the shielded transaction in a block.
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Abstract
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========
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The goal of this ZIP is to change the conventional transaction fees for
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Shielded transactions and get buy-in from wallet developers, miners & Zcash users.
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With an updated transaction fee formula, miners will be compensated fairly for
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including transactions with a high number of outputs, while still allowing low fees
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for regular shielded transaction use cases.
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Out of Scope for this Proposal
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============
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Discussion around transaction fees for T to T.
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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 zats transaction fees set in ZIP-313.
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The conventional transaction fee presently is 0.00001 ZEC or 1,000 zats per
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ZIP-313, that allowed exploration of novel use cases of the Zcash blockchain.
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The Zcash network has operated for almost 2 years of 1,000 zats fee per shielded
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transaction without consideration for the total number of inputs and outputs in the transaction.
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This has resulted in high output transactions with 1,100 outputs costing the same as
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transactions with 2 outputs.
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Requirements for consensus
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-------------------------
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Wallet developers must update the fees to the proposed formula by Madars and
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Kris Nuttycomb [#madars-1]_
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min_fee = 0.00001 + 0.00001 * max(0, #nullifiers + #shielded outputs - 4)
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The change to the conventional transaction fees must be undertaken soon
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as the Zcash network has been under heavy load with high-output transactions while
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regular shielded transactions with 2 outputs are affected when relying on current
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light wallet infrastructure.
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The following parties need to be part of the consensus:
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* The technical aspects of a changing conventional fee based on outputs
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need to be evaluated.
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* A guarantee from mining groups is required to include the updated POFM
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transactions in the next block.
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* Wallet developers need to update the software to use the new fee.
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* Zcash documentation and community outreach must be undertaken to
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make the change known.
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Requirements for adoption
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-------------------------
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The change to the conventional transaction fees should be undertaken soon
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as it gets difficult to gain consensus with the growth in the network
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of wallets, exchanges, miners, and third parties involved.
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The following parties need to be part of the consensus:
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* Support from mining groups is required to include the updated conventional
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fee transactions in the next block.
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* Wallet developers need to provide a commitment to update the software to use
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the new fee.
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* Zcash documentation and community outreach must be undertaken to make the
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change known.
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Security and privacy considerations
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-----------------------------------
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Unique transaction fees may reveal specific users or wallets or wallet versions,
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which would reduce privacy for those specific users and the rest of the network.
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Hence this change should be accepted by a majority of shielded transaction
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software providers before deploying the change.
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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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Specification
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=============
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Wallets implementing this specification will use a conventional fee in the form of
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min_fee = 0.00001 + 0.00001 * max(0, #nullifiers + #shielded outputs - 4)
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starting from block 180,000.
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Transaction relaying
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--------------------
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zcashd, and potentially other node implementations, implement fee-based
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restrictions on relaying of mempool transactions. Nodes that normally relay
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transactions are expected to do so for transactions that pay at least the
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conventional fee, unless there are other reasons not to do so for robustness
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or denial-of-service mitigation.
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Mempool size limiting
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---------------------
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zcashd limits the size of the mempool as described in [#zip-0401]_. This
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specifies a *low\_fee\_penalty* that is added to the "eviction weight" if the
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transaction pays a fee less than the proposed increase with POFM.
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Support
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=======
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The developers of the following wallets intend to implement the updated fee mechanism:
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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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* ECC Mobile SDKs for iOS & Android
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* zcashd built-in wallet
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Acknowledgements
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================
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Thanks to Madars Vizra for suggesting the fee mechanism. And Kris Nuttycombe to
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suggest optimization.
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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?u=aiyadt>`_
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