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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`

Start Date | 11 July 2024
Description | Add new XCM instruction: `InitiateAssetsTransfer` for mixing asset transfer types in same XCM
Authors | Adrian Catangiu

## 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

A new instruction `InitiateAssetsTransfer` is introduced that initiates an assets transfer from the chain it is executed on, to another chain. The executed transfer is point-to-point (chain-to-chain) with all of the transfer properties specified in the instruction parameters. The instruction also allows specifying another XCM program to be executed on the remote chain. If a transfer requires going through multiple hops, an XCM program can compose this instruction to be used at every chain along the path, on each hop describing that specific leg of the transfer.

Note: Transferring assets that require different paths (chains along the way) is not supported within same XCM because of the async nature of cross chain messages. This new instruction, however, enables initiating transfers for multiple assets that take the same path even if they require different transfer types along that path.

The usage and composition model of `InitiateAssetsTransfer` is the same as with existing `DepositReserveAsset`, `InitiateReserveWithdraw` and `InitiateTeleport` instructions. The main difference comes from the ability to handle assets that have different point-to-point transfer type between A and B. The other benefit is that it also allows specifying remote fee payment and transparently appends the required remote fees logic to the remote XCM.

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).
    ///
    /// If a transfer requires going through multiple hops, an XCM program can compose this instruction
    /// to be used at every chain along the path, describing that specific leg of the transfer.
    ///
    /// Parameters:
    /// - `dest`: The location of the transfer next hop.
    /// - `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*.
    ///
    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. If `remote_fees` is set to `None`, the first instruction in the onward XCM shall be a `UnpaidExecution` instruction. The rest of the assets shall be handled by subsequent instructions, thus also finally allowing single asset buy execution barrier security recommendation.

The `BuyExecution` appended to the onward XCM specifies `WeightLimit::Unlimited`, thus being limited only by the `remote_fees` asset "amount". This is a deliberate decision for enhancing UX - in practice, people/dApps care about limiting the amount of fee asset used and not the actually used weight.

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 multiple types of transfers along the path:

1. 1st leg of the transfer: Penpal -> Rococo Asset Hub:
   - teleport PENs
   - reserve withdraw ROCs
2. 2nd leg of the transfer: Rococo Asset Hub -> Westend Asset Hub:
   - reserve deposit both PENs and ROCs

    #![allow(unused)]
    fn main() {
    Penpal::execute_with(|| {
        let destination = Location::new(2, (GlobalConsensus(Westend), Parachain(1000)).into());
        let rocs_id: AssetId = Parent.into();
        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

None.

## Future Directions and Related Material

None.