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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../introduction.html">Introduction</a></li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Newly Proposed</li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Proposed</li><li class="chapter-item expanded "><a href="../proposed/0000-pre-elves_soft.html">RFC-0000: Pre-ELVES soft concensus</a></li><li class="chapter-item expanded "><a href="../proposed/0000-rewards.html">RFC-0000: Validator Rewards</a></li><li class="chapter-item expanded "><a href="../proposed/0004-remove-unnecessary-allocator-usage.html">RFC-0004: Remove the host-side runtime memory allocator</a></li><li class="chapter-item expanded "><a href="../proposed/0006-dynamic-pricing-for-bulk-coretime-sales.html">RFC-0006: Dynamic Pricing for Bulk Coretime Sales</a></li><li class="chapter-item expanded "><a href="../proposed/0034-xcm-absolute-location-account-derivation.html">RFC-34: XCM Absolute Location Account Derivation</a></li><li class="chapter-item expanded "><a href="../proposed/0035-conviction-voting-delegation-modifications.html"> RFC-0035: Conviction Voting Delegation Modifications</a></li><li class="chapter-item expanded "><a href="../proposed/0044-rent-based-registration.html">RFC-0044: Rent based registration model</a></li><li class="chapter-item expanded "><a href="../proposed/0054-remove-heap-pages.html">RFC-0054: Remove the concept of "heap pages" from the client</a></li><li class="chapter-item expanded "><a href="../proposed/0070-x-track-kusamanetwork.html">RFC-0070: X Track for @kusamanetwork</a></li><li class="chapter-item expanded "><a href="../proposed/0073-referedum-deposit-track.html">RFC-0073: Decision Deposit Referendum Track</a></li><li class="chapter-item expanded "><a href="../proposed/0074-stateful-multisig-pallet.html">RFC-0074: Stateful Multisig Pallet</a></li><li class="chapter-item expanded "><a href="../proposed/0077-increase-max-length-of-identity-pgp-fingerprint-value.html">RFC-0077: Increase maximum length of identity PGP fingerprint values from 20 bytes</a></li><li class="chapter-item expanded "><a href="../proposed/0088-broker-pallet-slashable-deposit-purchaser-reputation-reserved-cores.html">RFC-0088: Add slashable locked deposit, purchaser reputation, and reserved cores for on-chain identities to broker pallet</a></li><li class="chapter-item expanded "><a href="../proposed/00xx-secondary-marketplace-for-regions.html">RFC-0001: Secondary Market for Regions</a></li><li class="chapter-item expanded "><a href="../proposed/00xx-smart-contracts-coretime-chain.html">RFC-0002: Smart Contracts on the Coretime Chain</a></li><li class="chapter-item expanded "><a href="../proposed/0102-offchain-parachain-runtime-upgrades.html">RFC-0000: Feature Name Here</a></li><li class="chapter-item expanded "><a href="../proposed/0106-xcm-remove-fees-mode.html">RFC-0106: Remove XCM fees mode</a></li><li class="chapter-item expanded "><a href="../proposed/0111-pure-proxy-replication.html">RFC-0111: Pure Proxy Replication</a></li><li class="chapter-item expanded "><a href="../proposed/0112-compress-state-response-message-in-state-sync.html">RFC-0112: Compress the State Response Message in State Sync</a></li><li class="chapter-item expanded "><a href="../proposed/0114-secp256r1-hostfunction.html">RFC-0114: Introduce secp256r1_ecdsa_verify_prehashed Host Function to verify NIST-P256 elliptic curve signatures</a></li><li class="chapter-item expanded "><a href="../proposed/0117-unbrick-collective.html">RFC-0117: The Unbrick Collective</a></li><li class="chapter-item expanded "><a href="../proposed/0120-referenda-confirmation-by-candle-mechanism.html">RFC-0120: Referenda Confirmation by Candle Mechanism</a></li><li class="chapter-item expanded "><a href="../proposed/0124-extrinsic-version-5.html">RFC-0124: Extrinsic version 5</a></li><li class="chapter-item expanded "><a href="../proposed/0138-invulnerable-collator-election.html">RFC-0138: Election mechanism for invulnerable collators on system chains</a></li><li class="chapter-item expanded "><a href="../proposed/0145-remove-unnecessary-allocator-usage.html">RFC-0145: Remove the host-side runtime memory allocator</a></li><li class="chapter-item expanded "><a href="../proposed/0150-voting-while-delegating.html">RFC-150: Allow Voting While Delegating</a></li><li class="chapter-item expanded "><a href="../proposed/0152-decentralized-convex-preference-coretime-market-for-polkadot.html">RFC-0152: Decentralized Convex-Preference Coretime Market for Polkadot</a></li><li class="chapter-item expanded "><a href="../proposed/0154-multi-slot-aura.html">RFC-0154: AURA Multi-Slot Collation </a></li><li class="chapter-item expanded "><a href="../proposed/0155-pUSD.html">RFC-0155: pUSD (Polkadot USD over-collateralised debt token)</a></li><li class="chapter-item expanded "><a href="../proposed/RFC-114 Adjust Tipper Track Confirmation Periods.html">RFC-114: Adjust Tipper Track Confirmation Periods</a></li><li class="chapter-item expanded "><a href="../proposed/TODO-stale-nomination-reward-curve.html">RFC-TODO: Stale Nomination Reward Curve</a></li><li class="chapter-item expanded "><a href="../proposed/xxxx-improve-the-security-of-proof-of-possession.html">RFC-XXXX: Adding customized mandatory context to proof of possession statement</a></li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Approved</li><li class="chapter-item expanded "><a href="../approved/0001-agile-coretime.html">RFC-1: Agile Coretime</a></li><li class="chapter-item expanded "><a href="../approved/0005-coretime-interface.html">RFC-5: Coretime Interface</a></li><li class="chapter-item expanded "><a href="../approved/0007-system-collator-selection.html">RFC-0007: System Collator Selection</a></li><li class="chapter-item expanded "><a href="../approved/0008-parachain-bootnodes-dht.html">RFC-0008: Store parachain bootnodes in relay chain DHT</a></li><li class="chapter-item expanded "><a href="../approved/0009-improved-net-light-client-requests.html">RFC-0009: Improved light client requests networking protocol</a></li><li class="chapter-item expanded "><a href="../approved/0010-burn-coretime-revenue.html">RFC-0010: Burn Coretime Revenue</a></li><li class="chapter-item expanded "><a href="../approved/0012-process-for-adding-new-collectives.html">RFC-0012: Process for Adding New System Collectives</a></li><li class="chapter-item expanded "><a href="../approved/0013-prepare-blockbuilder-and-core-runtime-apis-for-mbms.html">RFC-0013: Prepare Core runtime API for MBMs</a></li><li class="chapter-item expanded "><a href="../approved/0014-improve-locking-mechanism-for-parachains.html">RFC-0014: Improve locking mechanism for parachains</a></li><li class="chapter-item expanded "><a href="../approved/0017-coretime-market-redesign.html">RFC-0017: Coretime Market Redesign</a></li><li class="chapter-item expanded "><a href="../approved/0022-adopt-encointer-runtime.html">RFC-0022: Adopt Encointer Runtime</a></li><li class="chapter-item expanded "><a href="../approved/0026-sassafras-consensus.html">RFC-0026: Sassafras Consensus Protocol</a></li><li class="chapter-item expanded "><a href="../approved/0032-minimal-relay.html">RFC-0032: Minimal Relay</a></li><li class="chapter-item expanded "><a href="../approved/0042-extrinsics-state-version.html">RFC-0042: Add System version that replaces StateVersion on RuntimeVersion</a></li><li class="chapter-item expanded "><a href="../approved/0043-storage-proof-size-hostfunction.html">RFC-0043: Introduce storage_proof_size Host Function for Improved Parachain Block Utilization</a></li><li class="chapter-item expanded "><a href="../approved/0045-nft-deposits-asset-hub.html">RFC-0045: Lowering NFT Deposits on Asset Hub</a></li><li class="chapter-item expanded "><a href="../approved/0047-assignment-of-availability-chunks.html">RFC-0047: Assignment of availability chunks to validators</a></li><li class="chapter-item expanded "><a href="../approved/0048-session-keys-runtime-api.html">RFC-0048: Generate ownership proof for SessionKeys</a></li><li class="chapter-item expanded "><a href="../approved/0050-fellowship-salaries.html">RFC-0050: Fellowship Salaries</a></li><li class="chapter-item expanded "><a href="../approved/0056-one-transaction-per-notification.html">RFC-0056: Enforce only one transaction per notification</a></li><li class="chapter-item expanded "><a href="../approved/0059-nodes-capabilities-discovery.html">RFC-0059: Add a discovery mechanism for nodes based on their capabilities</a></li><li class="chapter-item expanded "><a href="../approved/0078-merkleized-metadata.html">RFC-0078: Merkleized Metadata</a></li><li class="chapter-item expanded "><a href="../approved/0084-general-transaction-extrinsic-format.html">RFC-0084: General transactions in extrinsic format</a></li><li class="chapter-item expanded "><a href="../approved/0091-dht-record-creation-time.html">RFC-0091: DHT Authority discovery record creation time</a></li><li class="chapter-item expanded "><a href="../approved/0097-unbonding_queue.html">RFC-0097: Unbonding Queue</a></li><li class="chapter-item expanded "><a href="../approved/0099-transaction-extension-version.html">RFC-0099: Introduce a transaction extension version</a></li><li class="chapter-item expanded "><a href="../approved/0100-xcm-multi-type-asset-transfer.html">RFC-0100: New XCM instruction: InitiateAssetsTransfer</a></li><li class="chapter-item expanded "><a href="../approved/0101-xcm-transact-remove-max-weight-param.html">RFC-0101: XCM Transact remove require_weight_at_most parameter</a></li><li class="chapter-item expanded "><a href="../approved/0103-introduce-core-index-commitment.html">RFC-0103: Introduce a CoreIndex commitment and a SessionIndex field in candidate receipts</a></li><li class="chapter-item expanded "><a href="../approved/0105-xcm-improved-fee-mechanism.html" class="active">RFC-0105: XCM improved fee mechanism</a></li><li class="chapter-item expanded "><a href="../approved/0107-xcm-execution-hints.html">RFC-0107: XCM Execution hints</a></li><li class="chapter-item expanded "><a href="../approved/0108-xcm-remove-testnet-ids.html">RFC-0108: Remove XCM testnet NetworkIds</a></li><li class="chapter-item expanded "><a href="../approved/0122-alias-origin-on-asset-transfers.html">RFC-0122: Asset transfers can alias XCM origin on destination to original origin</a></li><li class="chapter-item expanded "><a href="../approved/0123-pending-code-as-storage-location-for-runtime-upgrades.html">RFC-0123: Introduce :pending_code as intermediate storage key for the runtime code</a></li><li class="chapter-item expanded "><a href="../approved/0125-xcm-asset-metadata.html">RFC-0125: XCM Asset Metadata</a></li><li class="chapter-item expanded "><a href="../approved/0126-introduce-pvq.html">RFC-0126: Introduce PVQ (PolkaVM Query)</a></li><li class="chapter-item expanded "><a href="../approved/0135-compressed-blob-prefixes.html">RFC-0135: Compressed Blob Prefixes</a></li><li class="chapter-item expanded "><a href="../approved/0139-faster-erasure-coding.html">RFC-0139: Faster Erasure Coding</a></li><li class="chapter-item expanded "><a href="../approved/0146-deflationary-fee-proposal.html">RFC-0146: Deflationary Transaction Fee Model for the Relay Chain and its System Parachains</a></li><li class="chapter-item expanded "><a href="../approved/0149-rfc-1-renewal-adjustment.html">RFC-0149: Renewal Adjustment</a></li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Stale</li></ol>
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<p><a href="https://github.com/polkadot-fellows/RFCs/blob/main/text/0105-xcm-improved-fee-mechanism.md">(source)</a></p>
<p><strong>Table of Contents</strong></p>
<ul>
<li><a href="#rfc-0105-xcm-improved-fee-mechanism">RFC-0105: XCM improved fee mechanism</a>
<ul>
<li><a href="#summary">Summary</a></li>
<li><a href="#motivation">Motivation</a></li>
<li><a href="#stakeholders">Stakeholders</a></li>
<li><a href="#explanation">Explanation</a>
<ul>
<li><a href="#examples">Examples</a></li>
</ul>
</li>
<li><a href="#drawbacks">Drawbacks</a></li>
<li><a href="#testing-security-and-privacy">Testing, Security, and Privacy</a></li>
<li><a href="#performance-ergonomics-and-compatibility">Performance, Ergonomics, and Compatibility</a>
<ul>
<li><a href="#performance">Performance</a></li>
<li><a href="#ergonomics">Ergonomics</a></li>
<li><a href="#compatibility">Compatibility</a></li>
</ul>
</li>
<li><a href="#prior-art-and-references">Prior Art and References</a></li>
<li><a href="#unresolved-questions">Unresolved Questions</a></li>
<li><a href="#future-directions-and-related-material">Future Directions and Related Material</a></li>
</ul>
</li>
</ul>
<h1 id="rfc-0105-xcm-improved-fee-mechanism"><a class="header" href="#rfc-0105-xcm-improved-fee-mechanism">RFC-0105: XCM improved fee mechanism</a></h1>
<div class="table-wrapper"><table><thead><tr><th></th><th></th></tr></thead><tbody>
<tr><td><strong>Start Date</strong></td><td>23 July 2024</td></tr>
<tr><td><strong>Description</strong></td><td>Allow multiple types of fees to be paid</td></tr>
<tr><td><strong>Authors</strong></td><td>Francisco Aguirre</td></tr>
</tbody></table>
</div>
<h2 id="summary"><a class="header" href="#summary">Summary</a></h2>
<p>XCM already handles execution fees in an effective and efficient manner using the <code>BuyExecution</code> instruction.
However, other types of fees are not handled as effectively -- for example, delivery fees.
Fees exist that can't be measured using <code>Weight</code> -- as execution fees can -- so a new method should be thought up for those cases.
This RFC proposes making the fee handling system simpler and more general, by doing two things:</p>
<ul>
<li>Adding a <code>fees</code> register</li>
<li>Deprecating <code>BuyExecution</code> and adding a new instruction <code>PayFees</code> with new semantics to ultimately replace it.</li>
</ul>
<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
<p>Execution fees are handled correctly by XCM right now.
However, the addition of extra fees, like for message delivery, result in awkward ways of integrating them into the XCVM implementation.
This is because these types of fees are not included in the language.
The standard should have a way to correctly deal with these implementation specific fees, that might not exist in every system that uses XCM.
The new instruction moves the specified amount of fees from the holding register to a dedicated fees register that the XCVM can use in flexible ways depending on its implementation.
The XCVM implementation is free to use these fees to pay for execution fees, transport fees, or any other type of fee that might be necessary.
This moves the specifics of fees further away from the XCM standard, and more into the actual underlying XCVM implementation, which is a good thing.</p>
<h2 id="stakeholders"><a class="header" href="#stakeholders">Stakeholders</a></h2>
<ul>
<li>Runtime Users</li>
<li>Runtime Devs</li>
<li>Wallets</li>
<li>dApps</li>
</ul>
<h2 id="explanation"><a class="header" href="#explanation">Explanation</a></h2>
<p>The new instruction that will replace <code>BuyExecution</code> is a much simpler and general version: <code>PayFees</code>.
This instruction takes one <code>Asset</code>, takes it from the holding register, and puts it into a new <code>fees</code> register.
The XCVM implementation can now use this <code>Asset</code> to make sure every necessary fee is paid for, this includes execution fees, delivery fees, and any other type of fee
necessary for the program to execute successfully.</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>PayFees { asset: Asset }
<span class="boring">}</span></code></pre></pre>
<p>This new instruction will reserve <strong>the entirety</strong> of the <code>asset</code> operand for fee payment.
There is not concept of returning the leftover fees to the holding register, to allow for the implementation to charge fees at different points during execution.
Because of this, the <code>asset</code> passed in can't be used for anything else during the entirety of the program.
This is different from the current semantics of <code>BuyExecution</code>.</p>
<p>If not all <code>Asset</code> in the <code>fees</code> register is used when the execution ends, then we trap them alongside any possible leftover assets from the holding register.
<code>RefundSurplus</code> can be used to move all leftover fees from the <code>fees</code> register to the <code>holding</code> register.
Care must be taken that this is used only after all possible instructions which might charge fees, else execution will fail.</p>
<h3 id="examples"><a class="header" href="#examples">Examples</a></h3>
<p>Most XCM programs that pay for execution are written like so:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// Instruction that loads the holding register
BuyExecution { asset, weight_limit }
// ...rest
<span class="boring">}</span></code></pre></pre>
<p>With this RFC, the structure would be the same, but using the new instruction, that has different semantics:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// Instruction that loads the holding register
PayFees { asset }
// ...rest
<span class="boring">}</span></code></pre></pre>
<h2 id="drawbacks"><a class="header" href="#drawbacks">Drawbacks</a></h2>
<p>There needs to be an explicit change from <code>BuyExecution</code> to <code>PayFees</code>, most often accompanied by a reduction in the assets passed in.</p>
<h2 id="testing-security-and-privacy"><a class="header" href="#testing-security-and-privacy">Testing, Security, and Privacy</a></h2>
<p>It might become a security concern if leftover fees are trapped, since a lot of them are expected.</p>
<h2 id="performance-ergonomics-and-compatibility"><a class="header" href="#performance-ergonomics-and-compatibility">Performance, Ergonomics, and Compatibility</a></h2>
<h3 id="performance"><a class="header" href="#performance">Performance</a></h3>
<p>There should be no performance downsides to this approach.
The <code>fees</code> register is a simplification that may actually result in better performance, in the case an implementation is doing a workaround to achieve what this RFC proposes.</p>
<h3 id="ergonomics"><a class="header" href="#ergonomics">Ergonomics</a></h3>
<p>The interface is going to be very similar to the already existing one.
Even simpler since <code>PayFees</code> will only receive one asset.
That asset will allow users to limit the amount of fees they are willing to pay.</p>
<h3 id="compatibility"><a class="header" href="#compatibility">Compatibility</a></h3>
<p>This RFC can't just change the semantics of the <code>BuyExecution</code> instruction since that instruction accepts any funds, uses what it needs and returns the rest immediately.
The new proposed instruction, <code>PayFees</code>, doesn't return the leftover immediately, it keeps it in the <code>fees</code> register.
In practice, the deprecated <code>BuyExecution</code> needs to be slowly rolled out in favour of <code>PayFees</code>.</p>
<h2 id="prior-art-and-references"><a class="header" href="#prior-art-and-references">Prior Art and References</a></h2>
<p>The closed RFC PR on the xcm-format repository, before XCM RFCs got moved to fellowship RFCs: https://github.com/polkadot-fellows/xcm-format/pull/53.</p>
<h2 id="unresolved-questions"><a class="header" href="#unresolved-questions">Unresolved Questions</a></h2>
<p>None</p>
<h2 id="future-directions-and-related-material"><a class="header" href="#future-directions-and-related-material">Future Directions and Related Material</a></h2>
<p>This proposal would greatly benefit from an improved asset trapping system.</p>
<p><a href="https://github.com/polkadot-fellows/xcm-format/blob/master/proposals/0037-custom-asset-claimer.md">CustomAssetClaimer</a> is also related, as it directly improves the ergonomics of this proposal.</p>
<p><a href="https://github.com/polkadot-fellows/RFCs/pull/107">LeftoverAssetsDestination</a> execution hint would also similarly improve the ergonomics.</p>
<p><a href="https://github.com/polkadot-fellows/RFCs/pull/106/files">Removal of JIT fees</a> is also related, they are useless with this proposal.</p>
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