Bridge Hub questions

Tech Talk
5

We are running the whole setup on Rococo/Wococo, where we move (periodically) ROC from Rococo:Rockmine2 (statemine runtime) to Wococo:Wockmint (westmint runtime), so it is more close to the “bridge KSM from Statemine to Statemint”.

Initialize bridge itself (allow tracking Rococo/Wococo finality):

  1. (BridgeHubRococo) initialize bridge - Rococo governance call xcm::Transact with initialize call
  2. (BridgeHubWococo) initialize bridge - Wococo governance call xcm::Transact with initialize call

Steps to allow bridge transfer were:

  1. (Rococo) force_open_hrmp_channel(rockmine2_para_id, bridge_hub_rococo_para_id) and back as governance call
  2. (Statemine) add configuration to allow send assets to Wococo:Wockmint - Rococo governance call xcm::Transact with add_exporter_config call
  3. (Wococo) force_open_hrmp_channel(wockmint_para_id, bridge_hub_wococo_para_id) and back as governance call
  4. (Wockmint) add configuration for trusted UniversalOrigin/Alias mapping - Wococo governance call xcm::Transact with add_universal_alias call
  5. (Wockmint) add configuration for trusted reserve location - Wococo governance call xcm::Transact with add_reserve_location call
  6. (Wockmint) create asset for ROC in pallet_assets::ForeignAssets instance - Wococo governance call xcm::Transact with pallet_assets::force_create call

(looks like too much configuration, but adding new stuff does not require runtime upgrade and governance can decide about everything )

I am not sure what exactly “reserve chain for DOT on Statemine” means, but as it works now:

  • user sends some of his ROCs from Rococo:Rockmine2 to Wococo:Wockmint account with new extrinsic transfer_asset_via_bridge
    • first, ROCs are moved to reserve account on Rococo:Rockmine2
      • it is still open question if reserve account will be sovereign account of local bridge hub parachain (in this case BridgeHubRococo) or sovereign account of target remote parachain (in this case Wococo:Wockmint), but probably it will be SA of target remote parachain and for that we need to finish this PR: https://github.com/paritytech/polkadot/pull/7016
    • second, reserved ROCs are sent with xcm over bridge as ReserveAssetDeposited instruction here
      • it is still open for case when we want to send back foreign assets (we should not do reserve but burn instead from reserve account)
  • so, here is XCM with ReserveAssetDeposited which will come to the local bridge hub (BridgeHubRococo) (captured from logs):
Xcm([
   "UnpaidExecution"{
      "weight_limit":"Unlimited",
      "check_origin":"None"
   },
   "ExportMessage"{
      "network":"Wococo",
      "destination":X1(Parachain(1000)),
      "xcm":"Xcm("[
         "WithdrawAsset(MultiAssets("[
            "MultiAsset"{
               "id":"Concrete(MultiLocation"{
                  "parents":1,
                  "interior":"Here"
               }")",
               "fun":Fungible(50000000000)
            }
         ]"))",
         "BuyExecution"{
            "fees":"MultiAsset"{
               "id":"Concrete(MultiLocation"{
                  "parents":1,
                  "interior":"Here"
               }")",
               "fun":Fungible(50000000000)
            },
            "weight_limit":"Unlimited"
         },
         "ReserveAssetDeposited(MultiAssets("[
            "MultiAsset"{
               "id":"Concrete(MultiLocation"{
                  "parents":2,
                  "interior":X1(GlobalConsensus(Kusama))
               }")",
               "fun":Fungible(100000000)
            }
         ]"))",
         "ClearOrigin",
         "DepositAsset"{
            "assets":"Definite(MultiAssets("[
               "MultiAsset"{
                  "id":"Concrete(MultiLocation"{
                     "parents":2,
                     "interior":X1(GlobalConsensus(Kusama))
                  }")",
                  "fun":Fungible(100000000)
               }
            ]"))",
            "beneficiary":"MultiLocation"{
               "parents":0,
               "interior":X1(AccountId32{
                  "network":"None",
                  "id":[212,53,147,199,21,253,211,28,97,20,26,189,4,169,159,214,130,44,133,88,133,76,205,227,154,86,132,231,165,109,162,125]
               }")"
            }
         }
      ]")"
   }
])
  • realyer relays xcm from BridgeHubRococo to BridgeHubWococo
  • BridgeHubWococo routes xcm over hrmp to the Wococo:Wockmint
  • Wococo:Wockmint’s XcmExecutor is configured (by governance) with trusted UniversalOrigin/Alias and reserve location 
Xcm([
   UniversalOrigin("GlobalConsensus(Rococo))",
   DescendOrigin(X1(Parachain(1000))),
   "WithdrawAsset(MultiAssets("[
      "MultiAsset"{
         "id":"Concrete(MultiLocation"{
            "parents":1,
            "interior":"Here"
         }")",
         "fun":Fungible(50000000000)
      }
   ]"))",
   "BuyExecution"{
      "fees":"MultiAsset"{
         "id":"Concrete(MultiLocation"{
            "parents":1,
            "interior":"Here"
         }")",
         "fun":Fungible(50000000000)
      },
      "weight_limit":"Unlimited"
   },
   "ReserveAssetDeposited(MultiAssets("[
      "MultiAsset"{
         "id":"Concrete(MultiLocation"{
            "parents":2,
            "interior":X1(GlobalConsensus(Kusama))
         }")",
         "fun":Fungible(100000000)
      }
   ]"))",
   "ClearOrigin",
   "DepositAsset"{
      "assets":"Definite(MultiAssets("[
         "MultiAsset"{
            "id":"Concrete(MultiLocation"{
               "parents":2,
               "interior":X1(GlobalConsensus(Kusama))
            }")",
            "fun":Fungible(100000000)
         }
      ]"))",
      "beneficiary":"MultiLocation"{
         "parents":0,
         "interior":X1(AccountId32{
            "network":"None",
            "id":[212,53,147,199,21,253,211,28,97,20,26,189,4,169,159,214,130,44,133,88,133,76,205,227,154,86,132,231,165,109,162,125]
         }")"
      }
   }
])
  • result is that user received ROCs on Wococo:Wockmint in pallet_assets::ForeignAssets instance with assetId: MultiLocation { parents: 2, X1(GlobalConsensus(Kusama)) } Note: Kusama is here because statemine runtime has hard-coded NetworkId as Kusama and we dont have rockmine copy of statemine with Rococo NetworkId

@xlc hope, this answers also to your comment, we actually dont have any high level spec, we have several (now not up-to-date) hackmds based on which this PR is done, so we could consider this post as a latest up-to-date high level spec

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