[Review] MIP-69: Bridge Fees, MD-69: Bridge Fees

.github/CODEOWNERS

@@ -16,3 +16,4 @@
 /MD/md-15/ @l-monninger @apenzk
 /MIP/mip-39/ @franck44
 /MIP/mip-53/ @l-monninger
+/MD/md-69/ @franck44

MD/md-69/README.md

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+# MD-69: Bridge fees 
+- **Description**: This MD provides some background on the bridge fees mechanism requirements.
+- **Authors**: [Franck Cassez](franck.cassez@movementlabs.xyz)
+
+## Overview
+
+To use the Movement Mainnet, users need to pay for _transaction fees_ in \$L2MOVE tokens. As the \$MOVE token is minted on Ethereum (\$L1MOVE), users need to transfer some \$L1MOVE tokens from the Ethereum network to the Movement Mainnet (to get \$L2MOVE), and back.
+This asset transfer capability is provided by the Movement Mainnet **native bridge**, which is a [lock/mint bridge](https://chain.link/education-hub/cross-chain-bridge#types-of-cross-chain-bridges) described in [MIP-58](tdc).  
+
+This bridge (transfer) operation requires several steps, two of which are transactions:
+
+1. initiate a transfer on source chain;
+2. finalize the transfer on the target chain.
+
+Step (1) is performed by the user on the network (Ethereum or Movement), and step (2) is performed by the bridge operator via a _relayer_ (**ADD link to relayer MIP here**). 
+As a result, the operator has to cover the transaction fees for the _finalize_ step.
+When transferring from Ethereum to Movement, the fees are expected to be very low so we can consider that the operator can cover them using a pool of transaction fees collected from the users on the Movement Mainnet.
+In the other direction, bridging from Movement to Ethereum, the Ethereum fees are expected to be higher, and the operator may not be able to cover them all. 
+
+> [!IMPORTANT] 
+> We need to define a mechanism to collect the Ethereum fees from the users.
+
+
+> [!TIP] 
+> This MD describes the requirements for this fee collection mechanism when bridging from Movement Mainnet to Ethereum.
+
+## Desiderata
+
+### D1: User bridges from Movement Mainnet to Ethereum
+
+**User Journey**: A user can initiate a _bridge_ transaction on the Movement Mainnet.
+
+**Description**: When a user initiates a bridge transaction from the Movement Mainnet to Ethereum, the user pays the Movement Mainnet transaction fees in \$L2MOVE tokens. They request a transfer of \$L2MOVE to \$L1MOVE.
+
+**Justification**: The user should be able to transfer their tokens from the Movement Mainnet to Ethereum. 
+
+### D2: Operator covers the Ethereum transaction fees 
+
+**User Journey**: Once initiated, the bridge transaction is completed by the operator of the Movement Mainnet via a _relayer_. 
+
+**Description**: The operator must cover the Ethereum transaction fees for the completion step. 
+
+**Justification**: Only the operator can finalize the transfer on the Ethereum network. 
+
+
+### D3: Operator collects the Ethereum transaction fees from the user
+
+**User Journey**: The operator collects the Ethereum transaction fees from the user in \$MOVE.
+
+**Description**: The operator collects the Ethereum transaction fees from the user in \$MOVE tokens.
+
+**Justification**: We must collect the fees in \$MOVE tokens.
+The fees cannot be collected in \$ETH as 
+
+1) we cannot ask the user to approve the relayer to transfer \$ETH to them; and 
+2) even if we could, the user may not have enough \$ETH and they initiated the transfer on the Movement Mainnet.
+
+### D4: The operator does not run a deficit
+
+**User Journey**: The operator pays the Ethereum transaction fees in \$ETH and charges the user for bridge fees in \$MOVE. The operator must not run a deficit
+
+**Description**: The operator must ensure that the fees collected from the users are sufficient to cover the Ethereum transaction fees to complete transfers. This implies that 
+
+1) we accurately estimate the Ethereum fees in advance, and 
+2) we also estimate the ratio \$MOVE/\$ETH.
+
+**Justification**: The operator must not run a deficit. We must decide how much the user is charged when they initiate a transfer at time $t$ from the Movement Mainnet to Ethereum. 
+The counterpart complete transactions will be executed on the Ethereum at a later time $t' > t$. To cover the transaction fees, we have to 
+
+1) estimate the Ethereum fees at $t'$, and 
+2) the ratio \$MOVE/\$ETH at $t'$.