Open HRMP channel between Crab and Karura

Summary

We propose to open a bi-directional channel between Kurara and Crab. Initially, the main usecase will be to transfer Crab and KAR between the two chains and bring the s2s bridge ability to Karura.

Proposal

There is no on-chain proposal yet; #TBD.

Technical details:

The procedure for opening the channels is as follows:

  1. Karura propose to request to open Karura to Crab HRMP channel
  2. Wait until proposal on step 1 get approved & encated
  3. Crab propose to accept the request and request to open Crab to Karura HRMP channel(Batch call)
  4. Wait until proposal on step 3 get approved & encated
  5. Karura propose to accept Crab to Karura HRMP channel
  6. Wait until proposals on step 5 are approved & enacted
  7. Wait for another session on Kusama for the change to be effective
  8. XCM based crosschain transfer will be possible at this stage.

The extrinsics that need to be executed on the relay chain , are:

  • For step 1: hrmp.hrmpInitOpenChannel(recipient: 2105, proposedMaxCapacity: 1000, proposedMaxMessageSize: 102400) , which hex-encoded is 0x3c0039080000e803000000900100
  • For step 3: first call is hrmp.hrmpAcceptOpenChannel(sender: 2000) , second call is hrmp.hrmpInitOpenChannel(recipient: 2000, proposedMaxCapacity: 1000, proposedMaxMessageSize: 102400) , the batch call hex-encoded is 0x1802083c01d00700003c00d0070000e803000000900100
  • For step 5: hrmp.hrmpAcceptOpenChannel(sender: 2105) , which hex-encoded is 0x3c0139080000

The proposedMaxCapacity and proposedMaxMessageSize are set to the values of Kusama’s configuration.activeConfig.hrmpChannelMaxCapacity and configuration.activeConfig.hrmpChannelMaxMessageSize values, respectively.

These extrinsics need to be called with the parachain’s sovereign account as origin. To achieve this, on the Crab side we will use PolkadotXcm pallet to send xcm message to the relaychain, by executing the following extrinsic from the parachain. Karura should make a open request call and a accept request call with its orml-xcm pallet.

ormlXcm.send(
  dest: XcmVersionedMultiLocation
  {
    V1: {
      parents: 1
      interior: Here
    }
  },
  message: XcmVersionedXcm
  {
    V2: [
      {
        WithdrawAsset: [
          {
            id: {
              Concrete: {
                parents: 0,
                interior: 'Here'
              }
            },
            fun: {
              Fungible: '1000000000000'
            }
          }
        ]
      },
      {
        BuyExecution: {
          fees: {
            id: {
              Concrete: {
                parents: 0,
                interior: 'Here'
              }
            },
            fun: {
              Fungible: '1000000000000'
            }
          },
          weightLimit: 'Unlimited'
        }
      },
      {
        Transact: {
          originType: 'Native',
          requireWeightAtMost: '1000000000',
          call: {
            encoded: 0x1802083c01d00700003c00d0070000e803000000900100
          }
        }
      },
      {
        DepositAsset: {
          assets: {
            Wild: 'All'
          },
          maxAssets: 1,
          beneficiary: {
            parents: 0,
            interior: {
              X1: {
                AccountId32: {
                  network: 'Any',
                  id : 0x7061726127080000000000000000000000000000000000000000000000000000,
                }
              }
            }
          }
        }
      }
    ]
  }
})

As a prerequisite, the parachain’s sovereign account must contain at least 10 KSM to be locked as collateral (5 for each channel direction), plus some KSM to pay for xcm execution fees.

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