use relaychain asset as fee

xtokens/README.md

@@ -28,3 +28,49 @@ Integration tests could be done manually after integrating orml-xtokens into run
   - Sending the tx from parachain A.
   - Set the destination as Parachain B.
   - Set the currency ID as parachain C token.
+
+
+#### Transfer multiple currencies
+
+- Transfer relay chain tokens to relay chain, and use relay chain token as fee
+- Transfer relay chain tokens to parachain, and use relay chain token as fee
+- Transfer tokens issued by parachain A, from parachain A to parachain B, and use parachain A token as fee
+- Transfer tokens issued by parachain B, from parachain A to parachain B, and use parachain B token as fee
+- Transfer tokens issued by parachain C, from parachain A to parachain B, and use parachain C token as fee
+- Transfer tokens issued by parachain B, from parachain A to parachain B, and use relay chain token as fee
+
+Notice, in the case of parachain A transfer parachain B token to parachain B, and use relay chain token as fee. Because fee asset is relaychain token, and non fee asset is parachain B token, this is two different chain. We call chain of fee asset as fee_reserve, and chain of non fee asset as non_fee_reserve. And in this case fee_reserve location is also refer to destination parachain.
+
+The current implementation is sent two xcm from sender parachain. first xcm sent to fee reserve chain which will also route xcm message to destination parachain. second xcm directly sent to destination parachain. 
+
+the fee amount in fee asset is split into two parts. 
+1. fee asset sent to fee reserve chain = fee_amount - min_xcm_fee
+2. fee asset sent to dest reserve chain = min_xcm_fee
+
+Parachains should implements config `MinXcmFee` in `xtokens` module config:
+
+```rust
+parameter_type_with_key! {
+	pub ParachainMinFee: |location: MultiLocation| -> u128 {
+		#[allow(clippy::match_ref_pats)] // false positive
+		match (location.parents, location.first_interior()) {
+			(1, Some(Parachain(parachains::statemine::ID))) => 4_000_000_000,
+			_ => u128::MAX,
+		}
+	};
+}
+```
+
+If Parachain don't want have this case, can simply return Max value:
+
+```rust
+parameter_type_with_key! {
+	pub ParachainMinFee: |_location: MultiLocation| -> u128 {
+		u128::MAX
+	};
+}
+```
+
+Notice the implementation for now also rely on `SelfLocation` which already in `xtokens` config. The `SelfLocation` current is `(1, Parachain(id))` refer to sender parachain. If parachain set `SelfLocation` to (0, Here), it'll be error in this case.
+
+We use `SelfLocation` to fund fee to sender's parachain sovereign account on destination parachain, which asset is originated from sender account on sender parachain. This means if user setup too much fee, the fee will not returned to user, instead deposit to sibling parachain sovereign account on destination parachain.