(source)

Table of Contents

• RFC-0100: New XCM instruction: InitiateAssetsTransfer
  • Summary
  • Motivation
  • Stakeholders
  • Explanation
    • Example usage: transferring 2 different asset types across 3 chains
  • Drawbacks
  • Testing, Security, and Privacy
  • Performance, Ergonomics, and Compatibility
    • Performance
    • Ergonomics
    • Compatibility
  • Prior Art and References
  • Unresolved Questions
  • Future Directions and Related Material

RFC-0100: New XCM instruction: InitiateAssetsTransfer

Summary

This RFC proposes a new instruction that provides a way to initiate on remote chains, asset transfers which transfer multiple types (teleports, local-reserve, destination-reserve) of assets, using XCM alone.

The currently existing instructions are too opinionated and force each XCM asset transfer to a single transfer type (teleport, local-reserve, destination-reserve). This results in inability to combine different types of transfers in single transfer which results in overall poor UX when trying to move assets across chains.

Motivation

XCM is the de-facto cross-chain messaging protocol within the Polkadot ecosystem, and cross-chain assets transfers is one of its main use-cases. Unfortunately, in its current spec, it does not support initiating on a remote chain, one or more transfers that combine assets with different transfer types.
For example, ParachainA cannot instruct AssetHub to teleport ForeignAssetX to ParachainX alongside USDT (which has to be reserve transferred) using current XCM specification.

There currently exist DepositReserveAsset, InitiateReserveWithdraw and InitiateTeleport instructions that initiate asset transfers on execution, but they are opinionated in the type of transfer to use. Combining them is also not possible, because as a result of their individual execution, a message containing a ClearOrigin instruction is sent to the destination chain, making subsequent transfers impossible after the first instruction is executed.

The new instruction proposed by this RFC allows an XCM program to describe multiple asset transfer types, then execute them in one shot with a single remote_xcm program sent to the target chain to effect the transfer and subsequently clear origin.

Multi-hop asset transfers will benefit from this change by allowing single XCM program to handle multiple types of transfers and reduce complexity.

Bridge asset transfers greatly benefit from this change by allowing building XCM programs to transfer multiple assets across multiple hops in a single pseudo-atomic action.
For example, allows single XCM program execution to transfer multiple assets from ParaK on Kusama, through Kusama Asset Hub, over the bridge through Polkadot Asset Hub with final destination ParaP on Polkadot.

With current XCM, we are limited to doing multiple independent transfers for each individual hop in order to move both "interesting" assets, but also "supporting" assets (used to pay fees).

Stakeholders

• Runtime users
• Runtime devs
• Wallet devs
• dApps devs

Explanation

Detail-heavy explanation of the RFC, suitable for explanation to an implementer of the changeset. This should address corner cases in detail and provide justification behind decisions, and provide rationale for how the design meets the solution requirements.

We can specify the desired transfer type for some asset(s) using:

#![allow(unused)]
fn main() {
/// Specify which type of asset transfer is required for a particular `(asset, dest)` combination.
pub enum AssetTransferFilter {
	/// teleport assets matching `AssetFilter` to `dest`
	Teleport(AssetFilter),
	/// reserve-transfer assets matching `AssetFilter` to `dest`, using the local chain as reserve
	ReserveDeposit(AssetFilter),
	/// reserve-transfer assets matching `AssetFilter` to `dest`, using `dest` as reserve
	ReserveWithdraw(AssetFilter),
}
}

This RFC proposes 1 new XCM instruction:

#![allow(unused)]
fn main() {
/// Cross-chain transfer matching `assets` in the holding register as follows:
///
/// Assets in the holding register are matched using the given list of `AssetTransferFilter`s,
/// they are then transferred based on their specified transfer type:
///
/// - teleport: burn local assets and append a `ReceiveTeleportedAsset` XCM instruction to
///   the XCM program to be sent onward to the `dest` location,
///
/// - reserve deposit: place assets under the ownership of `dest` within this consensus system
///   (i.e. its sovereign account), and append a `ReserveAssetDeposited` XCM instruction
///   to the XCM program to be sent onward to the `dest` location,
///
/// - reserve withdraw: burn local assets and append a `WithdrawAsset` XCM instruction
///   to the XCM program to be sent onward to the `dest` location,
///
/// The onward XCM is then appended a `ClearOrigin` to allow safe execution of any following
/// custom XCM instructions provided in `remote_xcm`.
///
/// The onward XCM also potentially contains a `BuyExecution` instruction based on the presence
/// of the `remote_fees` parameter (see below).
///
/// Parameters:
/// - `dest`: The location of the transfer destination.
/// - `remote_fees`: If set to `Some(asset_xfer_filter)`, the single asset matching
///   `asset_xfer_filter` in the holding register will be transferred first in the remote XCM
///   program, followed by a `BuyExecution(fee)`, then rest of transfers follow.
///   This guarantees `remote_xcm` will successfully pass a `AllowTopLevelPaidExecutionFrom` barrier.
/// - `remote_xcm`: Custom instructions that will be executed on the `dest` chain. Note that
///   these instructions will be executed after a `ClearOrigin` so their origin will be `None`.
///
/// Safety: No concerns.
///
/// Kind: *Command*.
///
/// Errors:
InitiateAssetsTransfer {
	destination: Location,
	assets: Vec<AssetTransferFilter>,
	remote_fees: Option<AssetTransferFilter>,
	remote_xcm: Xcm<()>,
}
}

An InitiateAssetsTransfer { .. } instruction shall transfer to dest, all assets in the holding register that match the provided assets and remote_fees filters. These filters identify the assets to be transferred as well as the transfer type to be used for transferring them. It shall handle the local side of the transfer, then forward an onward XCM to dest for handling the remote side of the transfer.

It should do so using same mechanisms as existing DepositReserveAsset, InitiateReserveWithdraw, InitiateTeleport instructions but practically combining all required XCM instructions to be remotely executed into a single remote XCM program to be sent over to dest.

Furthermore, through remote_fees: Option<AssetTransferFilter>, it shall allow specifying a single asset to be used for fees on dest chain. This single asset shall be remotely handled/received by the first instruction in the onward XCM and shall be followed by a BuyExecution instruction using it. The rest of the assets shall be handled by subsequent instructions, thus also finally allowing single asset buy execution barrier security recommendation.

The onward XCM, following the assets transfers instructions, ClearOrigin or DescendOrigin instructions shall be appended to stop acting on behalf of the source chain, then the caller-provided remote_xcm shall also be appended, allowing the caller to control what to do with the transferred assets.

Example usage: transferring 2 different asset types across 3 chains

• Transferring ROCs as the native asset of RococoAssetHub and PENs as the native asset of Penpal,
• Transfer origin is Penpal (on Rococo) and the destination is WestendAssetHub (across the bridge),
• ROCs are native to RococoAssetHub and are registered as trust-backed assets on Penpal and WestendAssetHub,
• PENs are native to Penpal and are registered as teleportable assets on RococoAssetHub and as foreign assets on WestendAssetHub,
• Fees on RococoAssetHub and WestendAssetHub are paid using ROCs.

We can transfer them from Penpal (Rococo), through RococoAssetHub, over the bridge to WestendAssetHub by executing a single XCM message, even though we'll be mixing and matching all types of transfers along the path:

#![allow(unused)]
fn main() {
Penpal::execute_with(|| {
    let rocs: Asset = (rocs_id.clone(), rocs_amount).into();
    let pens: Asset = (pens_id, pens_amount).into();
    let assets: Assets = vec![rocs.clone(), pens.clone()].into();

    // XCM to be executed at dest (Westend Asset Hub)
    let xcm_on_dest =
        Xcm(vec![DepositAsset { assets: Wild(All), beneficiary: beneficiary.clone() }]);

    // XCM to be executed at Rococo Asset Hub
    let context = PenpalUniversalLocation::get();
    let reanchored_assets = assets.clone().reanchored(&local_asset_hub, &context).unwrap();
    let reanchored_dest = destination.clone().reanchored(&local_asset_hub, &context).unwrap();
    let reanchored_rocs_id = rocs_id.clone().reanchored(&local_asset_hub, &context).unwrap();

    // from AHR, both ROCs and PENs are local-reserve transferred to Westend Asset Hub
    let assets_filter = vec![
        AssetTransferFilter::ReserveDeposit(reanchored_assets.clone().into())
    ];
    // we want to pay with ROCs on WAH
    let remote_fees = Some(AssetTransferFilter::ReserveDeposit(
        AssetFilter::Wild(AllOf { id: reanchored_rocs_id.into(), fun: WildFungibility::Fungible }))
    );
    let xcm_on_ahr = Xcm(vec![
        InitiateAssetsTransfer {
            dest: reanchored_dest,
            assets: assets_filter,
            remote_fees: Some(),
            remote_xcm: xcm_on_dest,
        },
    ]);

    // pay remote fees with ROCs
    let remote_fees = Some(
        AssetTransferFilter::ReserveWithdraw(
            AssetFilter::Wild(AllOf { id: rocs_id.into(), fun: WildFungibility::Fungible })
        )
    );
    // XCM to be executed locally
    let xcm = Xcm::<penpal_runtime::RuntimeCall>(vec![
        // Withdraw both ROCs and PENs from origin account
        WithdrawAsset(assets.clone().into()),
        // Execute the transfers while paying remote fees with ROCs
        InitiateAssetsTransfer {
            dest: local_asset_hub,
            assets: vec![
                // ROCs are reserve-withdrawn on AHR
                ReserveWithdraw(rocs.into()),
                // PENs are teleported to AHR
                Teleport(pens.into()),
            ],
            remote_fees,
            remote_xcm: xcm_on_ahr,
        },
    ]);

    <Penpal as PenpalPallet>::PolkadotXcm::execute(
        signed_origin,
        bx!(xcm::VersionedXcm::V4(xcm.into())),
        Weight::MAX,
    ).unwrap();
})
}

Drawbacks

No drawbacks identified.

Testing, Security, and Privacy

There should be no security risks related to the new instruction from the XCVM perspective. It follows the same pattern as with single-type asset transfers, only now it allows combining multiple types at once.

Improves security by enabling enforcement of single asset for buying execution, which minimizes the potential free/unpaid work that a receiving chain has to do. It does so, by making the required execution fee payment, part of the instruction logic through the remote_fees: Option<AssetTransferFilter> parameter, which will make sure the remote XCM starts with a single-asset-holding-loading-instruction, immediately followed by a BuyExecution using said asset.

Performance, Ergonomics, and Compatibility

This brings no impact to the rest of the XCM spec. It is a new, independent instruction, no changes to existing instructions.

Enhances the exposed functionality of Polkadot. Will allow multi-chain transfers that are currently forced to happen in multiple programs per asset per "hop", to be possible in a single XCM program.

Performance

No performance changes/implications.

Ergonomics

The proposal enhances developers' and users' cross-chain asset transfer capabilities. This enhancement is optimized for XCM programs transferring multiple assets, needing to run their logic across multiple chains.

Compatibility

Does this proposal break compatibility with existing interfaces, older versions of implementations? Summarize necessary migrations or upgrade strategies, if any.

This enhancement is compatible with all existing XCM programs and versions.

New (XCMv5) programs using this instruction shall be best-effort downgraded to an older XCM version, but cannot guarantee success. A program where the new instruction is used to initiate multiple types of asset transfers, cannot be downgraded to older XCM versions, because there is no equivalent capability there. Such conversion attempts will explicitly fail.

Prior Art and References

None.

Unresolved Questions

• Inner BuyExecution specifies WeightLimit::Unlimited, thus being limited only by the asset "amount". This was a deliberate decision for enhancing UX - in practice, people/dApps care abount limiting fee asset amount not the used weight.
• When setting remote_fees == None, should we append UnpaidExecution or do nothing?

Future Directions and Related Material

None.