Bridge Messages Pallet

The messages pallet is used to deliver messages from source chain to target chain. Message is (almost) opaque to the module and the final goal is to hand message to the message dispatch mechanism.

Contents

• Overview
• Message Workflow
• Integrating Message Lane Module into Runtime
• Non-Essential Functionality
• Weights of Module Extrinsics

Overview

Message lane is an unidirectional channel, where messages are sent from source chain to the target chain. At the same time, a single instance of messages module supports both outbound lanes and inbound lanes. So the chain where the module is deployed (this chain), may act as a source chain for outbound messages (heading to a bridged chain) and as a target chain for inbound messages (coming from a bridged chain).

Messages module supports multiple message lanes. Every message lane is identified with a 4-byte identifier. Messages sent through the lane are assigned unique (for this lane) increasing integer value that is known as nonce ("number that can only be used once"). Messages that are sent over the same lane are guaranteed to be delivered to the target chain in the same order they're sent from the source chain. In other words, message with nonce N will be delivered right before delivering a message with nonce N+1.

Single message lane may be seen as a transport channel for single application (onchain, offchain or mixed). At the same time the module itself never dictates any lane or message rules. In the end, it is the runtime developer who defines what message lane and message mean for this runtime.

In our Kusama<>Polkadot bridge we are using lane as a channel of communication between two parachains of different relay chains. For example, lane [0, 0, 0, 0] is used for Polkadot <> Kusama Asset Hub communications. Other lanes may be used to bridge other parachains.

Message Workflow

The pallet is not intended to be used by end users and provides no public calls to send the message. Instead, it provides runtime-internal method that allows other pallets (or other runtime code) to queue outbound messages.

The message "appears" when some runtime code calls the send_message() method of the pallet. The submitter specifies the lane that they're willing to use and the message itself. If some fee must be paid for sending the message, it must be paid outside of the pallet. If a message passes all checks (that include, for example, message size check, disabled lane check, ...), the nonce is assigned and the message is stored in the module storage. The message is in an "undelivered" state now.

We assume that there are external, offchain actors, called relayers, that are submitting module related transactions to both target and source chains. The pallet itself has no assumptions about relayers incentivization scheme, but it has some callbacks for paying rewards. See Integrating Messages Module into runtime for details.

Eventually, some relayer would notice this message in the "undelivered" state and it would decide to deliver this message. Relayer then crafts receive_messages_proof() transaction (aka delivery transaction) for the messages module instance, deployed at the target chain. Relayer provides its account id at the source chain, the proof of message (or several messages), the number of messages in the transaction and their cumulative dispatch weight. Once a transaction is mined, the message is considered "delivered".

Once a message is delivered, the relayer may want to confirm delivery back to the source chain. There are two reasons why it would want to do that. The first is that we intentionally limit number of "delivered", but not yet "confirmed" messages at inbound lanes (see What about other Constants in the Messages Module Configuration Trait for explanation). So at some point, the target chain may stop accepting new messages until relayers confirm some of these. The second is that if the relayer wants to be rewarded for delivery, it must prove the fact that it has actually delivered the message. And this proof may only be generated after the delivery transaction is mined. So relayer crafts the receive_messages_delivery_proof() transaction (aka confirmation transaction) for the messages module instance, deployed at the source chain. Once this transaction is mined, the message is considered "confirmed".

The "confirmed" state is the final state of the message. But there's one last thing related to the message - the fact that it is now "confirmed" and reward has been paid to the relayer (or at least callback for this has been called), must be confirmed to the target chain. Otherwise, we may reach the limit of "unconfirmed" messages at the target chain and it will stop accepting new messages. So relayer sometimes includes a nonce of the latest "confirmed" message in the next receive_messages_proof() transaction, proving that some messages have been confirmed.

Integrating Messages Module into Runtime

As it has been said above, the messages module supports both outbound and inbound message lanes. So if we will integrate a module in some runtime, it may act as the source chain runtime for outbound messages and as the target chain runtime for inbound messages. In this section, we'll sometimes refer to the chain we're currently integrating with, as "this chain" and the other chain as "bridged chain".

Messages module doesn't simply accept transactions that are claiming that the bridged chain has some updated data for us. Instead of this, the module assumes that the bridged chain is able to prove that updated data in some way. The proof is abstracted from the module and may be of any kind. In our Substrate-to-Substrate bridge we're using runtime storage proofs. Other bridges may use transaction proofs, Substrate header digests or anything else that may be proved.

IMPORTANT NOTE: everything below in this chapter describes details of the messages module configuration. But if you're interested in well-probed and relatively easy integration of two Substrate-based chains, you may want to look at the bridge-runtime-common crate. This crate is providing a lot of helpers for integration, which may be directly used from within your runtime. Then if you'll decide to change something in this scheme, get back here for detailed information.

General Information

The messages module supports instances. Every module instance is supposed to bridge this chain and some bridged chain. To bridge with another chain, using another instance is suggested (this isn't forced anywhere in the code, though). Keep in mind, that the pallet may be used to build virtual channels between multiple chains, as we do in our Polkadot <> Kusama bridge. There, the pallet actually bridges only two parachains - Kusama Bridge Hub and Polkadot Bridge Hub. However, other Kusama and Polkadot parachains are able to send (XCM) messages to their Bridge Hubs. The messages will be delivered to the other side of the bridge and routed to the proper destination parachain within the bridged chain consensus.

Message submitters may track message progress by inspecting module events. When Message is accepted, the MessageAccepted event is emitted. The event contains both message lane identifier and nonce that has been assigned to the message. When a message is delivered to the target chain, the MessagesDelivered event is emitted from the receive_messages_delivery_proof() transaction. The MessagesDelivered contains the message lane identifier and inclusive range of delivered message nonces.

The pallet provides no means to get the result of message dispatch at the target chain. If that is required, it must be done outside of the pallet. For example, XCM messages, when dispatched, have special instructions to send some data back to the sender. Other dispatchers may use similar mechanism for that.

How to plug-in Messages Module to Send Messages to the Bridged Chain?

The pallet_bridge_messages::Config trait has 3 main associated types that are used to work with outbound messages. The pallet_bridge_messages::Config::TargetHeaderChain defines how we see the bridged chain as the target for our outbound messages. It must be able to check that the bridged chain may accept our message - like that the message has size below maximal possible transaction size of the chain and so on. And when the relayer sends us a confirmation transaction, this implementation must be able to parse and verify the proof of messages delivery. Normally, you would reuse the same (configurable) type on all chains that are sending messages to the same bridged chain.

The last type is the pallet_bridge_messages::Config::DeliveryConfirmationPayments. When confirmation transaction is received, we call the pay_reward() method, passing the range of delivered messages. You may use the pallet-bridge-relayers pallet and its DeliveryConfirmationPaymentsAdapter adapter as a possible implementation. It allows you to pay fixed reward for relaying the message and some of its portion for confirming delivery.

I have a Messages Module in my Runtime, but I Want to Reject all Outbound Messages. What shall I do?

You should be looking at the bp_messages::source_chain::ForbidOutboundMessages structure bp_messages::source_chain. It implements all required traits and will simply reject all transactions, related to outbound messages.

How to plug-in Messages Module to Receive Messages from the Bridged Chain?

The pallet_bridge_messages::Config trait has 2 main associated types that are used to work with inbound messages. The pallet_bridge_messages::Config::SourceHeaderChain defines how we see the bridged chain as the source of our inbound messages. When relayer sends us a delivery transaction, this implementation must be able to parse and verify the proof of messages wrapped in this transaction. Normally, you would reuse the same (configurable) type on all chains that are sending messages to the same bridged chain.

The pallet_bridge_messages::Config::MessageDispatch defines a way on how to dispatch delivered messages. Apart from actually dispatching the message, the implementation must return the correct dispatch weight of the message before dispatch is called.

I have a Messages Module in my Runtime, but I Want to Reject all Inbound Messages. What shall I do?

You should be looking at the bp_messages::target_chain::ForbidInboundMessages structure from the bp_messages::target_chain module. It implements all required traits and will simply reject all transactions, related to inbound messages.

What about other Constants in the Messages Module Configuration Trait?

Two settings that are used to check messages in the send_message() function. The pallet_bridge_messages::Config::ActiveOutboundLanes is an array of all message lanes, that may be used to send messages. All messages sent using other lanes are rejected. All messages that have size above pallet_bridge_messages::Config::MaximalOutboundPayloadSize will also be rejected.

To be able to reward the relayer for delivering messages, we store a map of message nonces range => identifier of the relayer that has delivered this range at the target chain runtime storage. If a relayer delivers multiple consequent ranges, they're merged into single entry. So there may be more than one entry for the same relayer. Eventually, this whole map must be delivered back to the source chain to confirm delivery and pay rewards. So to make sure we are able to craft this confirmation transaction, we need to: (1) keep the size of this map below a certain limit and (2) make sure that the weight of processing this map is below a certain limit. Both size and processing weight mostly depend on the number of entries. The number of entries is limited with the pallet_bridge_messages::ConfigMaxUnrewardedRelayerEntriesAtInboundLane parameter. Processing weight also depends on the total number of messages that are being confirmed, because every confirmed message needs to be read. So there's another pallet_bridge_messages::Config::MaxUnconfirmedMessagesAtInboundLane parameter for that.

When choosing values for these parameters, you must also keep in mind that if proof in your scheme is based on finality of headers (and it is the most obvious option for Substrate-based chains with finality notion), then choosing too small values for these parameters may cause significant delays in message delivery. That's because there are too many actors involved in this scheme: 1) authorities that are finalizing headers of the target chain need to finalize header with non-empty map; 2) the headers relayer then needs to submit this header and its finality proof to the source chain; 3) the messages relayer must then send confirmation transaction (storage proof of this map) to the source chain; 4) when the confirmation transaction will be mined at some header, source chain authorities must finalize this header; 5) the headers relay then needs to submit this header and its finality proof to the target chain; 6) only now the messages relayer may submit new messages from the source to target chain and prune the entry from the map.

Delivery transaction requires the relayer to provide both number of entries and total number of messages in the map. This means that the module never charges an extra cost for delivering a map - the relayer would need to pay exactly for the number of entries+messages it has delivered. So the best guess for values of these parameters would be the pair that would occupy N percent of the maximal transaction size and weight of the source chain. The N should be large enough to process large maps, at the same time keeping reserve for future source chain upgrades.

Non-Essential Functionality

There may be a special account in every runtime where the messages module is deployed. This account, named 'module owner', is like a module-level sudo account - he's able to halt and resume all module operations without requiring runtime upgrade. Calls that are related to this account are:

• fn set_owner(): current module owner may call it to transfer "ownership" to another account;
• fn halt_operations(): the module owner (or sudo account) may call this function to stop all module operations. After this call, all message-related transactions will be rejected until further resume_operations call'. This call may be used when something extraordinary happens with the bridge;
• fn resume_operations(): module owner may call this function to resume bridge operations. The module will resume its regular operations after this call.

If pallet owner is not defined, the governance may be used to make those calls.

Messages Relay

We have an offchain actor, who is watching for new messages and submits them to the bridged chain. It is the messages relay - you may look at the crate level documentation and the code.