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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">RFC-0105: XCM improved fee mechanism</a></li><li class="chapter-item expanded "><a href="../approved/0107-xcm-execution-hints.html" class="active">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/0107-xcm-execution-hints.md">(source)</a></p>
<p><strong>Table of Contents</strong></p>
<ul>
<li><a href="#rfc-0107-xcm-execution-hints">RFC-0107: XCM Execution hints</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></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-0107-xcm-execution-hints"><a class="header" href="#rfc-0107-xcm-execution-hints">RFC-0107: XCM Execution hints</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>Add a mechanism for configuring particular XCM executions</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>A previous XCM RFC (https://github.com/polkadot-fellows/xcm-format/pull/37) introduced a <code>SetAssetClaimer</code> instruction.
This idea of instructing the XCVM to change some implementation-specific behavior is useful.
In order to generalize this mechanism, this RFC introduces a new instruction <code>SetHints</code>
and makes the <code>SetAssetClaimer</code> be just one of many possible execution hints.</p>
<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
<p>There is a need for specifying how certain implementation-specific things should behave.
Things like who can claim the assets or what can be done instead of trapping assets.
Another idea for a hint:</p>
<ul>
<li><code>AssetForFees</code>: to signify to the executor what asset the user prefers to use for fees.</li>
<li><code>LeftoverAssetsDestination</code>: for depositing leftover assets to a destination instead of trapping them</li>
</ul>
<h2 id="stakeholders"><a class="header" href="#stakeholders">Stakeholders</a></h2>
<ul>
<li>Runtime devs</li>
<li>Wallets</li>
<li>dApps</li>
</ul>
<h2 id="explanation"><a class="header" href="#explanation">Explanation</a></h2>
<p>A new instruction, <code>SetHints</code>, will be added.
This instruction will take a single parameter of type <code>Hint</code>, an enumeration.
The first variant for this enum is <code>AssetClaimer</code>, which allows to specify a location that should be able to claim trapped assets.
This means the instruction <code>SetAssetClaimer</code> would also be removed, in favor of this.</p>
<p>In Rust, the new definitions would look as follows:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>enum Instruction {
// ...snip...
SetHints(BoundedVec&lt;Hint, NumVariants&gt;),
// ...snip...
}
enum Hint {
AssetClaimer(Location),
// more can be added
}
type NumVariants = /* Number of variants of the `Hint` enum */;
<span class="boring">}</span></code></pre></pre>
<h2 id="drawbacks"><a class="header" href="#drawbacks">Drawbacks</a></h2>
<p>The <code>SetHints</code> instruction might be hard to benchmark, since we should look into the actual hints being set to know how much weight to attribute to it.</p>
<h2 id="testing-security-and-privacy"><a class="header" href="#testing-security-and-privacy">Testing, Security, and Privacy</a></h2>
<p><code>Hint</code>s are specified on a per-message basis, so they have to be specified at the beginning of a message.
If they were to be specified at the end, hints like <code>AssetClaimer</code> would be useless if an error occurs beforehand and assets get trapped before ever reaching the hint.</p>
<p>The instruction takes a bounded vector of hints so as to not force barriers to allow an arbitrary number of <code>SetHint</code> instructions.</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>None.</p>
<h3 id="ergonomics"><a class="header" href="#ergonomics">Ergonomics</a></h3>
<p>The <code>SetHints</code> instruction provides a better integration with barriers.
If we had to add one barrier for <code>SetAssetClaimer</code> and another for each new hint that's added, barriers would need to be changed all the time.
Also, this instruction would make it simpler to write XCM programs.
You only need to specify the hints you want in one single instruction at the top of your program.</p>
<h3 id="compatibility"><a class="header" href="#compatibility">Compatibility</a></h3>
<p>None.</p>
<h2 id="prior-art-and-references"><a class="header" href="#prior-art-and-references">Prior Art and References</a></h2>
<p>The previous RFC PR in the xcm-format repository before XCM RFCs moved to fellowship RFCs: https://github.com/polkadot-fellows/xcm-format/pull/59.</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>None.</p>
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