Cross-chain Flow

Overview

MAP Protocol v2 supports various cross-chain operations including asset transfers, liquidity management, and asset migration. This document describes the flow mechanisms for each operation type.

Cross-out Flow

Cross-out enables users to transfer assets from a source chain to a target chain.

Flow Diagram

┌─────────────────────────────────────────────────────────────────────────────┐
│                            Cross-out Flow                                    │
└─────────────────────────────────────────────────────────────────────────────┘

  User Initiates           Observer Monitors         Consensus Vote
      │                          │                        │
      ▼                          ▼                        ▼
┌──────────┐             ┌──────────┐             ┌──────────┐
│  Source  │────────────▶│  Build   │────────────▶│  2/3+    │
│  Deposit │             │ TxInItem │             │ Consensus│
└──────────┘             └──────────┘             └──────────┘
                                                        │
      ┌─────────────────────────────────────────────────┘
      │
      ▼
┌──────────┐             ┌──────────┐             ┌──────────┐
│ Generate │────────────▶│   TSS    │────────────▶│  Target  │
│ TxOutItem│             │  Sign    │             │  Execute │
└──────────┘             └──────────┘             └──────────┘

Process Steps

Step 1: User Initiates Cross-chain

• Contract chains (EVM): User calls Gateway.crossOut(), assets transfer to Vault, CrossOut event emitted with sender, receiver, token, amount, target chain, and memo

• Non-contract chains (Bitcoin): User sends assets to Vault address with cross-chain instructions in memo/OP_RETURN

Step 2: Observer Monitoring

Maintainer Observer monitors source chain blocks/logs, filters Vault-related transactions, parses memo for cross-chain info, and builds TxInItem after sufficient confirmations.

Step 3: Consensus Voting

Each Maintainer submits observation to Relay contract. Request confirmed after 2/3+ consensus reached.

Step 4: Generate Outbound Transaction

Relay contract generates TxOutItem, calculates target chain gas fee, deducts cross-chain fee, and emits TxOut event.

Step 5: TSS Signing

• Contract chains: Maintainers coordinate TSS signing via P2P, submit signature to Relay Chain, Relayer submits to target Gateway

• Non-contract chains: Maintainers build native transaction, TSS sign, broadcast directly to target chain

Step 6: Completion Confirmation

Observer monitors target chain, detects outbound tx on-chain, submits ObservedTxOut, updates status to complete.

Gas Handling

Fee Type	Payer	Description
Source chain Gas	User	Paid when initiating transaction
Target chain Gas	Deducted from amount	Based on GasService estimate
Cross-chain fee	Deducted from amount	Incentivizes Maintainers and LPs

Cross-in Flow

Cross-in is the target chain receiving assets, the execution phase of cross-out.

Contract Chain Cross-in

1. Relayer gets TSS signature from Relay Chain

2. Submits to target Gateway.executeIn() with TxOutItem data + signature

3. Gateway verifies signature, checks order uniqueness, transfers from Vault to target address

Non-contract Chain Cross-in

1. Maintainers build and sign native transaction via TSS

2. Broadcast directly to target chain network

Add Liquidity

LP Providers deposit assets to Vault to earn cross-chain fee returns.

Flow

LP Provider Deposit ──▶ Observer Detects ──▶ 2/3+ Consensus ──▶ Mint LP Tokens
       │                      │                    │                  │
       ▼                      ▼                    ▼                  ▼
  Gateway.addLiquidity()   Parse memo         Vote confirm      Send to receiver
  or Vault transfer        (LP address)                         on MAP Relay Chain

LP Token Properties

Property	Description
Minting location	MAP Relay Chain
Valuation	Shares based on deposited asset value
Returns	Cross-chain fee sharing

Remove Liquidity

LP Providers redeem LP tokens to withdraw original assets.

Flow

Burn LP Tokens ──▶ Calculate Amount ──▶ TSS Sign ──▶ Transfer from Vault
      │                  │                 │               │
      ▼                  ▼                 ▼               ▼
  MAP Relay Chain   Generate TxOutItem  Maintainers    To LP address
                    (chain + address)   collaborate    on target chain

Asset Migration

During TSS key rotation (Churn), assets migrate from old Vault to new Vault.

Contract Chain Migration

Assets custodied in Gateway contracts only require TSS public key update, no actual transfer needed.

New Vault KeyGen ──▶ Old Vault Signs ──▶ Update Gateway ──▶ Complete
      │                    │                   │               │
      ▼                    ▼                   ▼               ▼
  Get new TSS key    Sign switch msg     updateTssKey()    Old Vault Retired
                     (key + epoch)       on each chain

Advantages: No asset transfer, one transaction per chain, no service interruption

Non-contract Chain Migration

Assets held at TSS addresses require actual transfers to new Vault.

Query Old Vault ──▶ Batch Generate ──▶ TSS Sign ──▶ Observer Confirm ──▶ Complete
      │                  │                │              │                │
      ▼                  ▼                ▼              ▼                ▼
  Get all UTXOs/    Split into       Old Vault      Monitor tx        Old Vault
  balances          multiple txs     Maintainers    on-chain          Retired

Migration Comparison

Chain Type	Method	Description
EVM chains	Key switch	One tx updates Gateway
Solana	Key switch	One tx updates Gateway
Bitcoin	Asset transfer	Batch migrate UTXOs
Dogecoin	Asset transfer	Similar to Bitcoin

Refund Mechanism

When cross-chain fails, assets are refunded to users.

Trigger Conditions

Scenario	Description
Execution failure	Gateway execution fails
Insufficient amount	Too low after gas deduction
Chain paused	Target chain unavailable
Retired Vault deposit	User mistakenly transfers to old Vault

Refund Flow

Detect Refund Need ──▶ Generate Refund Tx ──▶ TSS Sign ──▶ Execute ──▶ Confirm
       │                      │                  │            │           │
       ▼                      ▼                  ▼            ▼           ▼
  Failure/insufficient   Target = source    Maintainers   Submit to   Status =
                         Address = sender   collaborate   source chain Refunded

Refund Fees

Fee	Description
Refund Gas	Deducted from refund amount
Original fee	Not refunded (resources consumed)

Failure Retry

Contract Chains

Signatures can be resubmitted without cancelling original transaction. System monitors and resubmits with adjusted gas if needed.

Non-contract Chains

Failed transactions may need RBF (Replace-By-Fee) or double-spend cancellation, then rebuild with higher fees.

Retry Parameters

Parameter	Description
Max retries	Configurable, default 3
Interval	Exponential backoff
Gas adjustment	May increase on retry

Status Tracking

Status	Description
Pending	Submitted, awaiting confirmation
Observed	Observer detected
Voting	Consensus in progress
Signed	TSS signing complete
Executed	Target chain confirmed
Completed	Fully done
Refunding	Refund in progress
Refunded	Refund complete
Failed	Terminal failure