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Architecture

Summary

The Token Bridge xApp implements a bridge that is capable of sending tokens across blockchains.
Features:
  • Ensures that circulating token supply remains constant across all chains.

Limitations

The Token Bridge xApp only supports standard, non-rebasing ERC20-compliant tokens.

Protocol

Handling Messages

  • the BridgeRouter contract only accepts messages from other remote BridgeRouter contracts, which are registered by each BridgeRouter
    • therefore, every message received follows the same "rules" that the local BridgeRouter expects
    • for example, any tokens sent in a message are ensured to be valid, because the remote BridgeRouter sending the message should locally enforce that a user has custody before sending the message to the remote chain
  • the messages from remote BridgeRouter contracts must be sent via Nomad, dispatched by a local Replica contract, which are registered with the UsingNomad contract
    • thus, the BridgeRouter depends on the UsingNomad contract for a valid registry of local Replicas
  • if another chain has sent a token that's "native" to this chain, we send that token from the Router contract's escrow to the recipient on this chain
  • if we're receiving a token that's not "native" to this chain,
    • we check whether a representation token contract has already been deployed by the Router contract on this chain; if not, we deploy that representation token contract and add its address to the token registry
    • we mint representation tokens on this chain and send them to the recipient

Dispatching Messages

  • TODO: describe rules — person must approve token to Router on local chain (if it's a native token) proving they have ownership over that token and can send to the native chain
  • sending tokens
    • the user uses ERC-20 approve to grant allowance for the tokens being sent to the local BridgeRouter contract
    • the user calls send on the local BridgeRouter to transfer the tokens to a remote
  • if the token being sent is "native" to the BridgeRouter's chain, the BridgeRouter contract holds the token in escrow
  • if the token being sent is not "native" to the chain, then the local token is a representation token contract deployed by the BridgeRouter in the first place; the BridgeRouter contract burns the tokens before sending them to another chain

Message Format

  • TODO: specify how messages are encoded for this application

Smart Contracts

TODO: Update links + combine with Smart Contracts section
BridgeRouter (code)
  • Receives incoming messages from local Replica contracts sending tokens from another chain
  • Dispatches outgoing messages to local Home contract in order to send tokens to other chains
  • Manages a registry of representation ERC-20 token contracts that it deploys on its local chain
  • Maintains a registry of remote BridgeRouter contracts to
    • authenticate that incoming messages come from a remote BridgeRouter contract
    • properly address outgoing messages to remote BridgeRouter contracts
TokenRegistry (code)
  • Responsible for deploying and keeping track of representation ERC-20 token contracts on this chain
  • When a new token is transferred, deploys a new representation token contract on this chain, and stores a two-way mapping between the information of the original token contract & the address of the representation on this chain
  • Inherited by the BridgeRouter, who uses this to make sure a representation of the token exists on this chain before minting/burning
BridgeMessage library (code)
  • Library for handling all the nitty gritty of encoding / decoding messages in a standardized way so they can be sent via Nomad

Message Flow

The logical steps and flow of information involved in sending tokens from one chain to another.
  • Chain A
    • User wants to send their tokens to Chain B
      • If it's a native token, the user must first approve tokens to the local BridgeRouter-A
    • User calls send on the local BridgeRouter-A
      • If it's a native token, tokens are pulled from the User's wallet to BridgeRouter-A and held in escrow
      • If it's a non-native token, tokens are burned from User's wallet by BridgeRouter-A
        • Note: BridgeRouter-A can burn non-native tokens because the representative contract for the token on its non-native chain was originally deployed by BridgeRouter-A when it received a message sending the token from another chain. The router has administrative rights on representations
    • BridgeRouter-A constructs a message to BridgeRouter-B
      • BridgeRouter-A keeps a mapping of BridgeRouter contracts on other chains so it knows where to send the message on Chain B
    • BridgeRouter-A calls enqueue on Home-A contract to send the message to Chain B
  • Off-Chain
    • Standard Nomad behavior. Updater → Relayer → Processor
    • Relayers see message on Home-A
    • Relayers pass message to Replica-A on Chain B
  • Chain B
    • After waiting for the acceptance timeout, Replica-A processes the message and dispatches it to BridgeRouter-B
    • BridgeRouter-B keeps a mapping Replica contracts that it trusts on the local chain. It uses this to authenticate that the incoming message came from chain A
    • BridgeRouter-B keeps a mapping of BridgeRouter contracts on other chains, so it can authenticate that this message came from BridgeRouter-A
    • BridgeRouter-B looks for the corresponding ERC-20 token contract in its registry, and deploys a new representative one if it doesn't already exist
    • BridgeRouter-B sends the token to the recipient
      • If it's a native token, BridgeRouter-B sends the tokens from the pool it's holding in escrow
      • If it's a non-native token, BridgeRouter-B mints the token to the recipient (
        • Note: BridgeRouter-B can mint non-native tokens because the representative contract for the token on its non-native chain is deployed by BridgeRouter-B when it received a message sending the token from another chain. The router has administrative rights on representations.

Tracing a Message

Nomad is currently still under active development. Because Nomad batches messages and sends only tree roots, there is no way to track individual messages on-chain once a message is passed to the Home contract. A agent-querying tool could be built to query off-chain agents for individual transactions, but such a tool does not currently exist.
What this means for the token bridge is that there is going to be a state of unknown during the time of send and receipt. You can think of this as snail mail without any tracking but with delivery confirmation. The only things that can be confirmed on-chain are:
  1. 1.
    A transaction was sent on chain A to the BridgeRouter contract
  2. 2.
    The recipient addressed received a token mint on chain B

Pseudo-tracking

  1. 1.
    Start by locating the bridgeRouter contract you are looking for, addresses in the config dir:
  1. 1.
    Verify that a transaction was sent to the BridgeRouter contract on the Home chain
    • Wait time: dependent on block confirmation times for each chain
  2. 2.
    Verify a transaction was sent on the Home contract
    • Wait time: dependent on block confirmation for each chain, but should be shortly after transaction is sent to BridgeRouter contract
    • There is not a way to query for a particular transactions at this time. Cross-check timestamps with BridgeRouter transaction.
  3. 3.
    After acceptance period, verify a transaction was sent on the destination Replica
    • Wait time: acceptance period. Currently 30 minutes
    • Cross-check timestamps
  4. 4.
    Verify a transaction was sent on the destination BridgeRouter
    • Wait time: acceptance period + block confirmation time
  5. 5.
    Verify that the recipient address received a token mint
    1. 1.
      Wait time: block confirmation time for chain A + acceptance period + block confirmation time for chain B

The Token Registry

The Token Bridge xApp relies on a TokenRegistry smart contract. This contract handles mapping canonical Nomad token identifiers to their local representation (or native local deployment) and vice versa.
For most tokens there is a 1:1 relationship between the canonical identifier and the local version. Regrettably, tokens are not always that simple. The token registry allows us to support more complex use cases by adjusting the relationship between identifiers and local tokens.

Custom Tokens

To support teams desiring specific token functionality, the registry supports enrolling "custom" tokens. Governance may instruct the token registry to change the official local representation of a token to a new address. Any ERC20-compliant contract that allows the Bridge Router to mint & burn tokens may be enrolled as a custom token.
When a custom token is enrolled, that token no longer has a 1:1 correspondence between local representation and canonical identifier. Instead, there are >1 local representations. Incoming transfers mint the latest representation (the most recently enrolled custom token), while outgoing transfer may burn any previous representation. This ensures that users' tokens are never invalidated, but new users get the best version.
For convenience, we also expose a migrate function that allows users to immediately exchange any previous representation for the latest representation. This allows a user to upgrade outdated representations and receive the latest without dispatching a cross-chain message.

Dust

We provide the users with dust in order to facilitate their onboarding to the chain (all chains apart from Ethereum).
We provide no guarantees to the end users that they will necessarily receive gas from the Dust feature. We provide it in good faith that it won't be abused and will be used by users who need it.
Dust is donated by people who are incentivized for users to have gas after they bridge. Large $$ values of dust should not be donated at any given time and we are actively talking with teams to avoid doing so.
It is hoped that the gas is used for genuine users, but not a concern if the gas is used improperly, as it should be of negligible importance to the donor.