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Architecture & Data Flow

This page describes the full architecture of the USSD-RSK bridge: how a user interaction on a feature phone translates into a signed blockchain transaction on the Rootstock network.

Components Overview

The system is built from four distinct components that form a linear pipeline from user input to blockchain state change:

1. GSM Network & USSD Gateway

USSD sessions are stateful, real-time text sessions established over a GSM network. When a user dials a shortcode (e.g., *384#), the telecom carrier routes the session to a configured callback URL via HTTP POST. In this architecture, that gateway is provided by Africa's Talking.

Each HTTP POST from the gateway includes:

• sessionId — a unique string identifying the current USSD session
• phoneNumber — the caller's MSISDN (phone number)
• text — a *-delimited string of all user inputs in the current session

Session state is fully encoded in the text field. There is no server-side session storage required. For example, if a user selected option 2 then entered a recipient address, the text field arrives as 2*0xRecipientAddress.

2. Node.js Relay Server

The Express server is the core bridge component. It is responsible for:

• Parsing the incoming USSD payload and determining which menu level the user is at
• Routing the request to either a read call (balance check) or a write transaction (transfer, loan)
• Signing and broadcasting transactions to the RSK network using ethers.js
• Returning a correctly formatted USSD response string

USSD responses must begin with either CON (continue 🡢 show next menu) or END (terminate the session and display a final message). No other prefixes are valid.

3. Ethers.js + RSK JSON-RPC

The relay server connects to the RSK network using a standard JSON-RPC provider pointed at the RSK public node:

https://public-node.testnet.rsk.co

All transactions are signed by a single relayer wallet whose private key is loaded from an environment variable. This wallet holds tRBTC to pay for gas fees on behalf of users. The wallet interacts with the deployed InclusiveDeFi contract through an ABI-defined interface.

4. InclusiveDeFi Smart Contract

The on-chain component is a Solidity contract that maintains internal balance and loan mappings. It does not use ERC-20 or any external token standard, it uses native tRBTC via msg.value deposits and internal accounting via mapping(address => uint256).

USSD Session Lifecycle

The following diagram shows the full data flow for a balance check request:

User dials *384#
            ↓
GSM Network routes session to Africa's Talking gateway
            ↓
Africa's Talking sends HTTP POST to relay server:
{
text: "",
phoneNumber: "+2348012345678",
sessionId: "ATsession_xyz"
}
            ↓
Relay server parses text === "" → serves main menu:
CON Rootstock DeFi (+2348012345678)
1. My Balance
2. Send Money (P2P)
3. Request Micro-Loan
            ↓
User selects 1
            ↓
Africa's Talking sends HTTP POST:
{ text: "1", phoneNumber: "...", sessionId: "..." }
            ↓
Relay server calls contract.getBalance(wallet.address)
→ RSK JSON-RPC read call (no gas, no transaction)
            ↓
Relay server formats and returns:
END Your Balance: 0.01 tRBTC
            ↓
Session terminates. User sees final balance on screen.

USSD Session Lifecycle — P2P Transfer

A transfer requires three sequential interactions within one USSD session, which is why stateless text accumulation is critical:

Round 1: text = ""        → Main menu shown
Round 2: text = "2"       → Prompt: "Enter Recipient Address:"
Round 3: text = "2*0xABC" → Prompt: "Enter Amount:"
Round 4: text = "2*0xABC*0.005"
         → Relay server:
              1. Parses input[1] = "0xABC" (recipient)
              2. Parses input[2] = "0.005" (amount)
              3. Calls contract.transfer("0xABC", parseEther("0.005"))
              4. Signs & broadcasts tx via RSK JSON-RPC
              5. Awaits tx confirmation
         → Returns: END Transfer Sent! Hash: 0xa924c40e...

nota

Each round trip has a latency budget imposed by the telecom network. Typical USSD gateways require a response within 5–10 seconds. For write operations (transfer, loan), the relay server calls tx.wait() which blocks until the transaction is mined on RSK. Ensure your RSK node connection is reliable and monitor average block confirmation time on the testnet.

Relay Server Architecture

The relay server is a minimal Express application. Its request handler implements a single /ussd POST route that acts as the USSD state machine:

POST /ussd
│
├── text === ""           → Serve main menu (CON)
│
├── input[0] === "1"      → Read: getBalance()          → END
│
├── input[0] === "2"
│   ├── !input[1]         → Prompt: Enter recipient     (CON)
│   ├── !input[2]         → Prompt: Enter amount        (CON)
│   └── input[1] & [2]   → Write: transfer()           → END
│
└── input[0] === "3"      → Write: applyForLoan()       → END

This pattern is intentionally flat and stateless. Every USSD payload contains the complete session history in the text field no database or session store is needed.

Security Considerations

The current proof-of-concept architecture has deliberate simplifications that are acceptable for a guidance demo but must be addressed in production:

Single relayer wallet: All users share one relayer wallet address. This means getBalance() always returns the relay wallet's balance, not the individual user's. In production, you would map each phoneNumber to a unique wallet address stored in a database.

Private key in environment variables: The relayer's private key is stored in .env. This is standard practice for local development but is not acceptable for production. Use a secrets manager (e.g., AWS Secrets Manager, HashiCorp Vault) or an HSM-backed signing service.

No input sanitization: Recipient addresses and amounts from USSD text are passed directly to ethers.js. Production code must validate Ethereum address format and parse amounts safely before calling contract functions.

No loan repayment logic: The current applyForLoan() function issues a loan with no repayment mechanism. This is a demo limitation. A production implementation would require a separate repayment flow and time-based interest logic.

Gas & Fee Model

The relayer wallet pays all gas fees on behalf of users. This is a meta-transaction pattern (without formal EIP-2771 implementation). The implications are:

• Users never need to hold tRBTC to interact with the system
• The relayer wallet must be kept funded with sufficient tRBTC for gas
• Gas costs on RSK are denominated in RBTC and are significantly cheaper than Ethereum mainnet
• Operators must build a fee recovery model (e.g., charging a small service fee per transaction or operating as a subsidized financial inclusion service)

Network Configuration

Parameter	Value
Network	RSK Testnet
Chain ID	31
RPC URL	https://public-node.testnet.rsk.co
Block Explorer	https://explorer.testnet.rootstock.io
Native Currency	tRBTC
Consensus	Merge-mined with Bitcoin (PoW)

RSK is fully EVM-compatible. All standard Ethereum tooling (ethers.js, Hardhat, Ignition) works without modification when pointed at the RSK RPC endpoint.