CCS Concepts: • Computing methodologies → Machine learning; Learning paradigms; Distributed artificial intelligence; Distributed algorithms; • Computer systems organization → Peer-to-peer architectures.

1 INTRODUCTION

2 BACKGROUND AND PRELIMINARIES

2.1 Federated Learning

2.2 Peer-to-Peer Systems (P2P)

2.3 Blockchain

3 RESEARCH METHOD

3.1 Research Questions

3.2 Search Process

3.3 Inclusion/Exclusion Criteria

3.4 Data Extraction

4 A TAXONOMY OF DECENTRALIZED FL APPROACHES

4.1 Traditional Distributed FL (TD-FL)

4.2 Blockchain-based FL (BC-FL)

5 CHALLENGES OF DECENTRALIZED FL

5.1 Security against Adversarial Attacks

5.2 Lack of Incentive Mechanisms

6 FUTURE DIRECTIONS

7 CONCLUSION