Understanding Polygon POS Bridge Implementation

Key Terminology

Bridge Types in Polygon

Polygon offers two bridge solutions for asset transfer between chains:

1. Plasma Bridge

   • More secure due to Plasma's exit mechanism
   • 7-day withdrawal period from Polygon to Ethereum

2. PoS Bridge

   • Faster transactions (single checkpoint period)
   • Preferred for most use cases

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How PoS Bridge Works

The PoS Bridge consists of several smart contracts:

1. Token Mapping Creation

To enable cross-chain transfers:

1. Submit mapping request via Polygon Mapper
2. Polygon team adds the RootToken-ChildToken relationship in Registry.sol

function mapToken(
  address _rootToken,
  address _childToken,
  bool _isERC721
) external onlyGovernance {
  require(_rootToken != address(0x0) && _childToken != address(0x0), "INVALID_TOKEN_ADDRESS");
  rootToChildToken[_rootToken] = _childToken;
  childToRootToken[_childToken] = _rootToken;
  isERC721[_rootToken] = _isERC721;
  IWithdrawManager(contractMap[WITHDRAW_MANAGER]).createExitQueue(_rootToken);
  emit TokenMapped(_rootToken, _childToken);
}

2. Ethereum to Polygon Transfers

Process flow:

1. User initiates transfer via DepositManager
2. Tokens get locked on Ethereum
3. StateSender notifies Polygon chain
4. After checkpoint period, tokens minted on Polygon

Key components:

• DepositManager.sol (Ethereum)
• ChildChain.sol (Polygon)
• ChildToken contract (implements deposit/withdraw)

// Deposit function in ChildERC20
function deposit(address user, bytes calldata depositData) external {
  uint256 amount = abi.decode(depositData, (uint256));
  _totalSupply = _totalSupply.add(amount);
  _balances[user] = _balances[user].add(amount);
  emit Transfer(address(0), user, amount);
}

Polygon to Ethereum Transfers

The withdrawal process mirrors the deposit mechanism:

1. Tokens burned on Polygon
2. StateSender notifies Ethereum
3. After checkpoint, tokens released on Ethereum

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FAQ Section

Q: How long does a PoS Bridge transfer take?
A: Typically completes within one checkpoint period (~10-30 mins).

Q: Can any token use the PoS Bridge?
A: Yes, after proper mapping approval by Polygon team.

Q: What's the difference between Plasma and PoS Bridge?
A: Plasma offers higher security but longer withdrawal times (7 days vs minutes).

Q: Who maintains the bridge contracts?
A: Polygon team deploys and manages core bridge components.

Q: Is there a transaction limit for PoS Bridge?
A: Yes, maximum ERC20 deposit amounts are enforced for security.

Q: Can I map my own token without Polygon's help?
A: No, token mapping requires Polygon team approval for security reasons.