平行链 A 1000 <— 中继链 rococo DOT —> 平行链 B 1001

1. 开启 A -> B 的 HRMP 通道
2. 在A，B上分别注册中继链上的外部XC-20，如上文的xcDOT
   1. assetManager.registerForeignAsset
   2. assetManager.setAssetUnitsPerSecond
3. 已经从平行链转移一定数量的xcDOT到A链上
4. A -> B 有三种方式：
   1. xTokens.transferMultiasset
      1. 0x1e0101000100000700e40b540201010200a50f0300f24ff3a9cf04c71dbc94d0b566f7a27b94566cac6400000000000000
   2. xTokens.transfer
      1. 0x1e00018080778c30c20fa2ebc0ed18d2cbca1f00e40b5402000000000000000000000001010200a50f0300f24ff3a9cf04c71dbc94d0b566f7a27b94566cac8096980000000000
   3. polkadotXcm.limitedReserveTransferAssets
5. B -> A
   1. xTokens.transfer
      1. 0x1e00018080778c30c20fa2ebc0ed18d2cbca1f0088526a74000000000000000000000001010200a10f0300f24ff3a9cf04c71dbc94d0b566f7a27b94566cac8096980000000000