# Graph Aware Sync Protocol

Ragnar Friedman <ragnar.friedman@proton.me>

## Abstract

GASP is designed to synchronize transaction data between two parties in a blockchain environment. It ensures the legitimacy and completeness of transaction data using a recursive reconciliation method.

## Participants

* **Alice**: Initiates the sync process.
* **Bob**: Responds and participates in the sync process.

## Protocol Steps

1. **Initialization**
   * Alice starts by sending a bloom filter containing all current spendable TXID+VOUTs as elements.
2. **Receiving and Building List**
   * Bob receives the filter and builds a list of his items that are not members of the set.
3. **Transaction Verification**
   * Bob sends an INV (Inventory) message to Alice for each item not in the set.
   * The INV includes:
     * Output, transaction, and associated merkle proof.
     * Any metadata (such as labels, descriptions, or local timestamps) associated with transactions or outputs.
     * A list of VOUTs spent by its inputs and associated metadata hashes.
     * The preimage for each hash, including all metadata and the merkle proof or broadcast response.
4. **Alice's Response**
   * For each INV, Alice responds with a list of input transactions she does not know about.
   * If Alice has the transaction but the metadata hash differs, she requests updated metadata.
   * If Alice lacks the transaction, she requests the entire transaction.
5. **Recursive Transaction Sync**
   * Bob responds to Alice's requests with an INV containing the encompassing transaction, done recursively.
     * When including the full transaction, all metadata is provided.
     * When only metadata is included, it encompasses the list of spent VOUTs and associated metadata hashes.
6. **Error Handling and Recovery**
   * In case of errors, affected transactions are ignored and not synced.
   * If errors prevent a party from fully anchoring transactions back to the blockchain, these transactions are ignored.
   * Failures experienced by one party are not communicated to the other due to the declaratory nature of the protocol.
7. **Finalization of Sync**
   * The process continues until there are no more INVs for Bob to send.
   * Once complete, all of Bob's records are considered synced with Alice.
8. **Role Reversal**
   * The roles reverse, with Bob sending Alice a bloom filter.
   * The parties then exchange data in the other direction, following the same steps.

## Security Measures

* Verification of merkle proofs and the longest chain of block headers.
* Recursively requesting information until all inputs are fully proven.
* Invalidating transactions that cannot be linked back to a valid proof.

## Notes

* The protocol is adaptable to various blockchain environments.
* The recursive nature ensures thorough and complete data synchronization.
* The protocol emphasizes security, efficiency, and data integrity.

## Conclusion

GASP offers a robust and secure method for synchronizing transaction data between parties in a blockchain network, leveraging recursive data exchange and thorough verification mechanisms.