Introduce XcmFeesToAccount fee manager (#1234)

Combination of paritytech/polkadot#7005, its addon PR paritytech/polkadot#7585 and its companion paritytech/cumulus#2433. This PR introduces a new XcmFeesToAccount struct which implements the `FeeManager` trait, and assigns this struct as the `FeeManager` in the XCM config for all runtimes. The struct simply deposits all fees handled by the XCM executor to a specified account. In all runtimes, the specified account is configured as the treasury account. XCM __delivery__ fees are now being introduced (unless the root origin is sending a message to a system parachain on behalf of the originating chain). # Note for reviewers Most file changes are tests that had to be modified to account for the new fees. Main changes are in: - cumulus/pallets/xcmp-queue/src/lib.rs <- To make it track the delivery fees exponential factor - polkadot/xcm/xcm-builder/src/fee_handling.rs <- Added. Has the FeeManager implementation - All runtime xcm_config files <- To add the FeeManager to the XCM configuration # Important note After this change, instructions that create and send a new XCM (Query*, Report*, ExportMessage, InitiateReserveWithdraw, InitiateTeleport, DepositReserveAsset, TransferReserveAsset, LockAsset and RequestUnlock) will require the corresponding origin account in the origin register to pay for transport delivery fees, and the onward message will fail to be sent if the origin account does not have the required amount. This delivery fee is on top of what we already collect as tx fees in pallet-xcm and XCM BuyExecution fees! Wallet UIs that want to expose the new delivery fee can do so using the formula: ``` delivery_fee_factor * (base_fee + encoded_msg_len * per_byte_fee) ``` where the delivery fee factor can be obtained from the corresponding pallet based on which transport you are using (UMP, HRMP or bridges), the base fee is a constant, the encoded message length from the message itself and the per byte fee is the same as the configured per byte fee for txs (i.e. `TransactionByteFee`). --------- Co-authored-by: Branislav Kontur <bkontur@gmail.com> Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com> Co-authored-by: Giles Cope <gilescope@gmail.com> Co-authored-by: command-bot <> Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com> Co-authored-by: Liam Aharon <liam.aharon@hotmail.com> Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
2026-06-13 22:11:06 +00:00 · 2023-10-18 23:22:25 +08:00
parent 1cf7d3aafa
commit 3dece311be
99 changed files with 2229 additions and 468 deletions
@@ -14,13 +14,15 @@ scale-info = { version = "2.9.0", default-features = false, features = ["derive"
 frame-support = { path = "../../../substrate/frame/support", default-features = false}
 frame-system = { path = "../../../substrate/frame/system", default-features = false}
 sp-io = { path = "../../../substrate/primitives/io", default-features = false}
+sp-core = { path = "../../../substrate/primitives/core", default-features = false }
 sp-runtime = { path = "../../../substrate/primitives/runtime", default-features = false}
 sp-std = { path = "../../../substrate/primitives/std", default-features = false}

 # Polkadot
-polkadot-runtime-common = { path = "../../../polkadot/runtime/common", default-features = false}
-xcm = { package = "staging-xcm", path = "../../../polkadot/xcm", default-features = false}
-xcm-executor = { package = "staging-xcm-executor", path = "../../../polkadot/xcm/xcm-executor", default-features = false}
+polkadot-runtime-common = { path = "../../../polkadot/runtime/common", default-features = false }
+polkadot-runtime-parachains = { path = "../../../polkadot/runtime/parachains", default-features = false }
+xcm = { package = "staging-xcm", path = "../../../polkadot/xcm", default-features = false }
+xcm-executor = { package = "staging-xcm-executor", path = "../../../polkadot/xcm/xcm-executor", default-features = false }

 # Cumulus
 cumulus-primitives-core = { path = "../../primitives/core", default-features = false }
@@ -54,7 +56,9 @@ std = [
 	"frame-system/std",
 	"log/std",
 	"polkadot-runtime-common/std",
+	"polkadot-runtime-parachains/std",
 	"scale-info/std",
+	"sp-core/std",
 	"sp-io/std",
 	"sp-runtime/std",
 	"sp-std/std",
@@ -69,6 +73,7 @@ runtime-benchmarks = [
 	"frame-system/runtime-benchmarks",
 	"pallet-balances/runtime-benchmarks",
 	"polkadot-runtime-common/runtime-benchmarks",
+	"polkadot-runtime-parachains/runtime-benchmarks",
 	"sp-runtime/runtime-benchmarks",
 	"xcm-builder/runtime-benchmarks",
 	"xcm-executor/runtime-benchmarks",
@@ -79,6 +84,7 @@ try-runtime = [
 	"frame-system/try-runtime",
 	"pallet-balances/try-runtime",
 	"polkadot-runtime-common/try-runtime",
+	"polkadot-runtime-parachains/try-runtime",
 	"sp-runtime/try-runtime",
 ]
 bridging = [ "bp-xcm-bridge-hub-router" ]
@@ -22,6 +22,16 @@
 //! Also provides an implementation of `SendXcm` which can be placed in a router tuple for relaying
 //! XCM over XCMP if the destination is `Parent/Parachain`. It requires an implementation of
 //! `XcmExecutor` for dispatching incoming XCM messages.
+//!
+//! To prevent out of memory errors on the `OutboundXcmpMessages` queue, an exponential fee factor
+//! (`DeliveryFeeFactor`) is set, much like the one used in DMP.
+//! The fee factor increases whenever the total size of messages in a particular channel passes a
+//! threshold. This threshold is defined as a percentage of the maximum total size the channel can
+//! have. More concretely, the threshold is `max_total_size` / `THRESHOLD_FACTOR`, where:
+//! - `max_total_size` is the maximum size, in bytes, of the channel, not number of messages.
+//! It is defined in the channel configuration.
+//! - `THRESHOLD_FACTOR` just declares which percentage of the max size is the actual threshold.
+//! If it's 2, then the threshold is half of the max size, if it's 4, it's a quarter, and so on.

 #![cfg_attr(not(feature = "std"), no_std)]

@@ -49,13 +59,15 @@ use frame_support::{
 	traits::{EnsureOrigin, Get},
 	weights::{constants::WEIGHT_REF_TIME_PER_MILLIS, Weight},
 };
-use polkadot_runtime_common::xcm_sender::PriceForParachainDelivery;
+use polkadot_runtime_common::xcm_sender::PriceForMessageDelivery;
+use polkadot_runtime_parachains::FeeTracker;
 use rand_chacha::{
 	rand_core::{RngCore, SeedableRng},
 	ChaChaRng,
 };
 use scale_info::TypeInfo;
-use sp_runtime::RuntimeDebug;
+use sp_core::MAX_POSSIBLE_ALLOCATION;
+use sp_runtime::{FixedU128, RuntimeDebug, Saturating};
 use sp_std::{convert::TryFrom, prelude::*};
 use xcm::{latest::prelude::*, VersionedXcm, WrapVersion, MAX_XCM_DECODE_DEPTH};
 use xcm_executor::traits::ConvertOrigin;
@@ -68,6 +80,19 @@ pub type OverweightIndex = u64;
 const LOG_TARGET: &str = "xcmp_queue";
 const DEFAULT_POV_SIZE: u64 = 64 * 1024; // 64 KB

+/// Constants related to delivery fee calculation
+pub mod delivery_fee_constants {
+	use super::FixedU128;
+
+	/// Fees will start increasing when queue is half full
+	pub const THRESHOLD_FACTOR: u32 = 2;
+	/// The base number the delivery fee factor gets multiplied by every time it is increased.
+	/// Also, the number it gets divided by when decreased.
+	pub const EXPONENTIAL_FEE_BASE: FixedU128 = FixedU128::from_rational(105, 100); // 1.05
+	/// The contribution of each KB to a fee factor increase
+	pub const MESSAGE_SIZE_FEE_BASE: FixedU128 = FixedU128::from_rational(1, 1000); // 0.001
+}
+
 // Maximum amount of messages to process per block. This is a temporary measure until we properly
 // account for proof size weights.
 const MAX_MESSAGES_PER_BLOCK: u8 = 10;
@@ -77,7 +102,7 @@ const MAX_OVERWEIGHT_MESSAGES: u32 = 1000;
 #[frame_support::pallet]
 pub mod pallet {
 	use super::*;
-	use frame_support::pallet_prelude::*;
+	use frame_support::{pallet_prelude::*, Twox64Concat};
 	use frame_system::pallet_prelude::*;

 	#[pallet::pallet]
@@ -109,7 +134,7 @@ pub mod pallet {
 		type ControllerOriginConverter: ConvertOrigin<Self::RuntimeOrigin>;

 		/// The price for delivering an XCM to a sibling parachain destination.
-		type PriceForSiblingDelivery: PriceForParachainDelivery;
+		type PriceForSiblingDelivery: PriceForMessageDelivery<Id = ParaId>;

 		/// The weight information of this pallet.
 		type WeightInfo: WeightInfo;
@@ -374,6 +399,17 @@ pub mod pallet {
 	/// Whether or not the XCMP queue is suspended from executing incoming XCMs or not.
 	#[pallet::storage]
 	pub(super) type QueueSuspended<T: Config> = StorageValue<_, bool, ValueQuery>;
+
+	/// Initialization value for the DeliveryFee factor.
+	#[pallet::type_value]
+	pub fn InitialFactor() -> FixedU128 {
+		FixedU128::from_u32(1)
+	}
+
+	/// The factor to multiply the base delivery fee by.
+	#[pallet::storage]
+	pub(super) type DeliveryFeeFactor<T: Config> =
+		StorageMap<_, Twox64Concat, ParaId, FixedU128, ValueQuery, InitialFactor>;
 }

 #[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, RuntimeDebug, TypeInfo)]
@@ -403,7 +439,7 @@ pub struct InboundChannelDetails {
 }

 /// Struct containing detailed information about the outbound channel.
-#[derive(Clone, Eq, PartialEq, Encode, Decode, TypeInfo)]
+#[derive(Debug, Clone, Eq, PartialEq, Encode, Decode, TypeInfo)]
 pub struct OutboundChannelDetails {
 	/// The `ParaId` of the parachain that this channel is connected with.
 	recipient: ParaId,
@@ -503,56 +539,90 @@ impl<T: Config> Pallet<T> {
 	/// length prefixed and can thus decode each fragment from the aggregate stream. With this,
 	/// we can concatenate them into a single aggregate blob without needing to be concerned
 	/// about encoding fragment boundaries.
+	///
+	/// If successful, returns the number of pages in the outbound queue after enqueuing the new
+	/// fragment.
 	fn send_fragment<Fragment: Encode>(
 		recipient: ParaId,
 		format: XcmpMessageFormat,
 		fragment: Fragment,
 	) -> Result<u32, MessageSendError> {
-		let data = fragment.encode();
+		let encoded_fragment = fragment.encode();

-		// Optimization note: `max_message_size` could potentially be stored in
-		// `OutboundXcmpMessages` once known; that way it's only accessed when a new page is needed.
-
-		let max_message_size =
-			T::ChannelInfo::get_channel_max(recipient).ok_or(MessageSendError::NoChannel)?;
-		if data.len() > max_message_size {
+		let channel_info =
+			T::ChannelInfo::get_channel_info(recipient).ok_or(MessageSendError::NoChannel)?;
+		let max_message_size = channel_info.max_message_size as usize;
+		// Max message size refers to aggregates, or pages. Not to individual fragments.
+		if encoded_fragment.len() > max_message_size {
 			return Err(MessageSendError::TooBig)
 		}

-		let mut s = <OutboundXcmpStatus<T>>::get();
-		let details = if let Some(details) = s.iter_mut().find(|item| item.recipient == recipient) {
+		let mut all_channels = <OutboundXcmpStatus<T>>::get();
+		let channel_details = if let Some(details) =
+			all_channels.iter_mut().find(|channel| channel.recipient == recipient)
+		{
 			details
 		} else {
-			s.push(OutboundChannelDetails::new(recipient));
-			s.last_mut().expect("can't be empty; a new element was just pushed; qed")
+			all_channels.push(OutboundChannelDetails::new(recipient));
+			all_channels
+				.last_mut()
+				.expect("can't be empty; a new element was just pushed; qed")
 		};
-		let have_active = details.last_index > details.first_index;
-		let appended = have_active &&
-			<OutboundXcmpMessages<T>>::mutate(recipient, details.last_index - 1, |s| {
-				if XcmpMessageFormat::decode_with_depth_limit(MAX_XCM_DECODE_DEPTH, &mut &s[..]) !=
-					Ok(format)
-				{
-					return false
-				}
-				if s.len() + data.len() > max_message_size {
-					return false
-				}
-				s.extend_from_slice(&data[..]);
-				true
-			});
-		if appended {
-			Ok((details.last_index - details.first_index - 1) as u32)
+		let have_active = channel_details.last_index > channel_details.first_index;
+		// Try to append fragment to the last page, if there is enough space.
+		// We return the size of the last page inside of the option, to not calculate it again.
+		let appended_to_last_page = have_active
+			.then(|| {
+				<OutboundXcmpMessages<T>>::mutate(
+					recipient,
+					channel_details.last_index - 1,
+					|page| {
+						if XcmpMessageFormat::decode_with_depth_limit(
+							MAX_XCM_DECODE_DEPTH,
+							&mut &page[..],
+						) != Ok(format)
+						{
+							return None
+						}
+						if page.len() + encoded_fragment.len() > max_message_size {
+							return None
+						}
+						page.extend_from_slice(&encoded_fragment[..]);
+						Some(page.len())
+					},
+				)
+			})
+			.flatten();
+
+		let (number_of_pages, last_page_size) = if let Some(size) = appended_to_last_page {
+			let number_of_pages = (channel_details.last_index - channel_details.first_index) as u32;
+			(number_of_pages, size)
 		} else {
 			// Need to add a new page.
-			let page_index = details.last_index;
-			details.last_index += 1;
+			let page_index = channel_details.last_index;
+			channel_details.last_index += 1;
 			let mut new_page = format.encode();
-			new_page.extend_from_slice(&data[..]);
+			new_page.extend_from_slice(&encoded_fragment[..]);
+			let last_page_size = new_page.len();
+			let number_of_pages = (channel_details.last_index - channel_details.first_index) as u32;
 			<OutboundXcmpMessages<T>>::insert(recipient, page_index, new_page);
-			let r = (details.last_index - details.first_index - 1) as u32;
-			<OutboundXcmpStatus<T>>::put(s);
-			Ok(r)
+			<OutboundXcmpStatus<T>>::put(all_channels);
+			(number_of_pages, last_page_size)
+		};
+
+		// We have to count the total size here since `channel_info.total_size` is not updated at
+		// this point in time. We assume all previous pages are filled, which, in practice, is not
+		// always the case.
+		let total_size =
+			number_of_pages.saturating_sub(1) * max_message_size as u32 + last_page_size as u32;
+		let threshold = channel_info.max_total_size / delivery_fee_constants::THRESHOLD_FACTOR;
+		if total_size > threshold {
+			let message_size_factor = FixedU128::from((encoded_fragment.len() / 1024) as u128)
+				.saturating_mul(delivery_fee_constants::MESSAGE_SIZE_FEE_BASE);
+			Self::increase_fee_factor(recipient, message_size_factor);
 		}
+
+		Ok(number_of_pages)
 	}

 	/// Sends a signal to the `dest` chain over XCMP. This is guaranteed to be dispatched on this
@@ -1004,9 +1074,8 @@ impl<T: Config> XcmpMessageHandler for Pallet<T> {
 				// Record the fact we received it.
 				match status.binary_search_by_key(&sender, |item| item.sender) {
 					Ok(i) => {
-						let count = status[i].message_metadata.len();
-						if count as u32 >= suspend_threshold && status[i].state == InboundState::Ok
-						{
+						let count = status[i].message_metadata.len() as u32;
+						if count >= suspend_threshold && status[i].state == InboundState::Ok {
 							status[i].state = InboundState::Suspended;
 							let r = Self::send_signal(sender, ChannelSignal::Suspend);
 							if r.is_err() {
@@ -1015,7 +1084,7 @@ impl<T: Config> XcmpMessageHandler for Pallet<T> {
 								);
 							}
 						}
-						if (count as u32) < drop_threshold {
+						if count < drop_threshold {
 							status[i].message_metadata.push((sent_at, format));
 						} else {
 							debug_assert!(
@@ -1023,6 +1092,13 @@ impl<T: Config> XcmpMessageHandler for Pallet<T> {
 								"XCMP channel queue full. Silently dropping message"
 							);
 						}
+						// Update the delivery fee factor, if applicable.
+						if count > suspend_threshold {
+							let message_size_factor =
+								FixedU128::from((data_ref.len() / 1024) as u128)
+									.saturating_mul(delivery_fee_constants::MESSAGE_SIZE_FEE_BASE);
+							Self::increase_fee_factor(sender, message_size_factor);
+						}
 					},
 					Err(_) => status.push(InboundChannelDetails {
 						sender,
@@ -1120,6 +1196,21 @@ impl<T: Config> XcmpMessageSource for Pallet<T> {
 				result.push((para_id, page));
 			}

+			let max_total_size = match T::ChannelInfo::get_channel_info(para_id) {
+				Some(channel_info) => channel_info.max_total_size,
+				None => {
+					log::warn!("calling `get_channel_info` with no RelevantMessagingState?!");
+					MAX_POSSIBLE_ALLOCATION // We use this as a fallback in case the messaging state is not present
+				},
+			};
+			let threshold = max_total_size.saturating_div(delivery_fee_constants::THRESHOLD_FACTOR);
+			let remaining_total_size: usize = (first_index..last_index)
+				.map(|index| OutboundXcmpMessages::<T>::decode_len(para_id, index).unwrap())
+				.sum();
+			if remaining_total_size <= threshold as usize {
+				Self::decrease_fee_factor(para_id);
+			}
+
 			*status = OutboundChannelDetails {
 				recipient: para_id,
 				state: outbound_state,
@@ -1172,7 +1263,7 @@ impl<T: Config> SendXcm for Pallet<T> {
 			MultiLocation { parents: 1, interior: X1(Parachain(id)) } => {
 				let xcm = msg.take().ok_or(SendError::MissingArgument)?;
 				let id = ParaId::from(*id);
-				let price = T::PriceForSiblingDelivery::price_for_parachain_delivery(id, &xcm);
+				let price = T::PriceForSiblingDelivery::price_for_delivery(id, &xcm);
 				let versioned_xcm = T::VersionWrapper::wrap_version(&d, xcm)
 					.map_err(|()| SendError::DestinationUnsupported)?;
 				Ok(((id, versioned_xcm), price))
@@ -1198,3 +1289,27 @@ impl<T: Config> SendXcm for Pallet<T> {
 		}
 	}
 }
+
+impl<T: Config> FeeTracker for Pallet<T> {
+	type Id = ParaId;
+
+	fn get_fee_factor(id: Self::Id) -> FixedU128 {
+		<DeliveryFeeFactor<T>>::get(id)
+	}
+
+	fn increase_fee_factor(id: Self::Id, message_size_factor: FixedU128) -> FixedU128 {
+		<DeliveryFeeFactor<T>>::mutate(id, |f| {
+			*f = f.saturating_mul(
+				delivery_fee_constants::EXPONENTIAL_FEE_BASE.saturating_add(message_size_factor),
+			);
+			*f
+		})
+	}
+
+	fn decrease_fee_factor(id: Self::Id) -> FixedU128 {
+		<DeliveryFeeFactor<T>>::mutate(id, |f| {
+			*f = InitialFactor::get().max(*f / delivery_fee_constants::EXPONENTIAL_FEE_BASE);
+			*f
+		})
+	}
+}
@@ -85,8 +85,10 @@ parameter_types! {
 	pub const MaxReserves: u32 = 50;
 }

+pub type Balance = u64;
+
 impl pallet_balances::Config for Test {
-	type Balance = u64;
+	type Balance = Balance;
 	type RuntimeEvent = RuntimeEvent;
 	type DustRemoval = ();
 	type ExistentialDeposit = ExistentialDeposit;
@@ -196,6 +198,22 @@ impl<RuntimeOrigin: OriginTrait> ConvertOrigin<RuntimeOrigin>
 	}
 }

+parameter_types! {
+	/// The asset ID for the asset that we use to pay for message delivery fees.
+	pub FeeAssetId: AssetId = Concrete(RelayChain::get());
+	/// The base fee for the message delivery fees.
+	pub const BaseDeliveryFee: Balance = 300_000_000;
+	/// The fee per byte
+	pub const ByteFee: Balance = 1_000_000;
+}
+
+pub type PriceForSiblingParachainDelivery = polkadot_runtime_common::xcm_sender::ExponentialPrice<
+	FeeAssetId,
+	BaseDeliveryFee,
+	ByteFee,
+	XcmpQueue,
+>;
+
 impl Config for Test {
 	type RuntimeEvent = RuntimeEvent;
 	type XcmExecutor = xcm_executor::XcmExecutor<XcmConfig>;
@@ -205,7 +223,7 @@ impl Config for Test {
 	type ControllerOrigin = EnsureRoot<AccountId>;
 	type ControllerOriginConverter = SystemParachainAsSuperuser<RuntimeOrigin>;
 	type WeightInfo = ();
-	type PriceForSiblingDelivery = ();
+	type PriceForSiblingDelivery = PriceForSiblingParachainDelivery;
 }

 pub fn new_test_ext() -> sp_io::TestExternalities {
@@ -324,13 +324,14 @@ fn xcmp_queue_consumes_dest_and_msg_on_ok_validate() {
 	let dest = (Parent, X1(Parachain(5555)));
 	let mut dest_wrapper = Some(dest.into());
 	let mut msg_wrapper = Some(message.clone());
-	assert!(<XcmpQueue as SendXcm>::validate(&mut dest_wrapper, &mut msg_wrapper).is_ok());
-
-	// check wrapper were consumed
-	assert_eq!(None, dest_wrapper.take());
-	assert_eq!(None, msg_wrapper.take());

 	new_test_ext().execute_with(|| {
+		assert!(<XcmpQueue as SendXcm>::validate(&mut dest_wrapper, &mut msg_wrapper).is_ok());
+
+		// check wrapper were consumed
+		assert_eq!(None, dest_wrapper.take());
+		assert_eq!(None, msg_wrapper.take());
+
 		// another try with router chain with asserting sender
 		assert_eq!(
 			Err(SendError::Transport("NoChannel")),
@@ -370,3 +371,74 @@ fn xcmp_queue_send_xcm_works() {
 			.any(|(para_id, _)| para_id == &sibling_para_id));
 	})
 }
+
+#[test]
+fn verify_fee_factor_increase_and_decrease() {
+	use cumulus_primitives_core::AbridgedHrmpChannel;
+	use sp_runtime::FixedU128;
+
+	let sibling_para_id = ParaId::from(12345);
+	let destination = (Parent, Parachain(sibling_para_id.into())).into();
+	let xcm = Xcm(vec![ClearOrigin; 100]);
+	let versioned_xcm = VersionedXcm::from(xcm.clone());
+	let mut xcmp_message = XcmpMessageFormat::ConcatenatedVersionedXcm.encode();
+	xcmp_message.extend(versioned_xcm.encode());
+
+	new_test_ext().execute_with(|| {
+		let initial = InitialFactor::get();
+		assert_eq!(DeliveryFeeFactor::<Test>::get(sibling_para_id), initial);
+
+		// Open channel so messages can actually be sent
+		ParachainSystem::open_custom_outbound_hrmp_channel_for_benchmarks_or_tests(
+			sibling_para_id,
+			AbridgedHrmpChannel {
+				max_capacity: 10,
+				max_total_size: 1000,
+				max_message_size: 104,
+				msg_count: 0,
+				total_size: 0,
+				mqc_head: None,
+			},
+		);
+
+		// Fee factor is only increased in `send_fragment`, which is called by `send_xcm`.
+		// When queue is not congested, fee factor doesn't change.
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 104
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 208
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 312
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 416
+		assert_eq!(DeliveryFeeFactor::<Test>::get(sibling_para_id), initial);
+
+		// Sending the message right now is cheap
+		let (_, delivery_fees) = validate_send::<XcmpQueue>(destination, xcm.clone())
+			.expect("message can be sent; qed");
+		let Fungible(delivery_fee_amount) = delivery_fees.inner()[0].fun else { unreachable!("asset is fungible; qed"); };
+		assert_eq!(delivery_fee_amount, 402_000_000);
+
+		let smaller_xcm = Xcm(vec![ClearOrigin; 30]);
+
+		// When we get to half of `max_total_size`, because `THRESHOLD_FACTOR` is 2,
+		// then the fee factor starts to increase.
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 520
+		assert_eq!(DeliveryFeeFactor::<Test>::get(sibling_para_id), FixedU128::from_float(1.05));
+
+		for _ in 0..12 { // We finish at size 929
+			assert_ok!(send_xcm::<XcmpQueue>(destination, smaller_xcm.clone()));
+		}
+		assert!(DeliveryFeeFactor::<Test>::get(sibling_para_id) > FixedU128::from_float(1.88));
+
+		// Sending the message right now is expensive
+		let (_, delivery_fees) = validate_send::<XcmpQueue>(destination, xcm.clone())
+			.expect("message can be sent; qed");
+		let Fungible(delivery_fee_amount) = delivery_fees.inner()[0].fun else { unreachable!("asset is fungible; qed"); };
+		assert_eq!(delivery_fee_amount, 758_030_955);
+
+		// Fee factor only decreases in `take_outbound_messages`
+		for _ in 0..5 { // We take 5 100 byte pages
+			XcmpQueue::take_outbound_messages(1);
+		}
+		assert!(DeliveryFeeFactor::<Test>::get(sibling_para_id) < FixedU128::from_float(1.72));
+		XcmpQueue::take_outbound_messages(1);
+		assert!(DeliveryFeeFactor::<Test>::get(sibling_para_id) < FixedU128::from_float(1.63));
+	});
+}