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Publications of year 2022
Thesis
  1. Saswata Paul. Formal Verification of Decentralized Coordination in Autonomous Multi-Agent Aerospace Systems. PhD thesis, Rensselaer Polytechnic Institute, 2022. Keyword(s): formal verification, Athena, distributed systems, distributed computing, air traffic management, autonomous systems, runtime verification, cyber physical systems.
    Abstract:
    As autonomous vehicular technologies such as self-driving cars and uncrewed aircraft systems (UAS) evolve to become more accessible and cost-efficient, autonomous multi-agent systems, that comprise of such entities, will become ubiquitous in the near future. The close operational proximity between such autonomous agents will warrant the need for multi-agent coordination to ensure safe operations. In this thesis, we adopt a formal methods-based approach to investigate multi-agent coordination for safety-critical autonomous multi-agent systems. We explore algorithms that can be used for decentralized multi-agent coordination among autonomous mobile agents by communicating over asynchronous vehicle-to-vehicle (V2V) networks that can be prone to agent failures. In particular, we study two types of distributed algorithms that are useful for decentralized coordination — consensus, which can be used by autonomous agents to agree on a set of compatible operations; and knowledge propagation, which can be used to ensure sufficient situational awareness in autonomous multi-agent systems. We develop the first machine-checked proof of eventual progress for the Synod consensus algorithm, that does not assume a unique leader. To consider agent failures while reasoning about progress, we introduce a novel Failure-Aware Actor Model (FAM). We then propose a formally verified Two-Phase Acknowledge Protocol (TAP) for knowledge propagation that can establish a safe state of knowledge suitable for autonomous vehicular operations. The non-deterministic and dynamic operating conditions of distributed algorithms deployed over asynchronous V2V networks make it challenging to provide appropriate formal guarantees for the algorithms. To address this, we introduce probabilistic correctness properties that can be developed by stochastically modeling the systems. We present a formal proof library that can be used for reasoning about probabilistic properties of distributed algorithms deployed over V2V networks. We also propose a Dynamic Data-Driven Applications Systems (DDDAS)-based approach for the runtime verification of distributed algorithms. This approach uses parameterized proofs, which can be instantiated at runtime, and progress envelopes, which can divide the operational state space into distinct regions where a proof of progress may or may not hold. To motivate our verification of decentralized coordination, we introduce an autonomous air traffic management (ATM) technique for multi-aircraft systems called Decentralized Admission Control (DAC).

    @PhdThesis{paul-phd-2022,
    author = {Saswata Paul},
    title = {Formal Verification of Decentralized Coordination in Autonomous Multi-Agent Aerospace Systems},
    school = {Rensselaer Polytechnic Institute},
    year = 2022,
    keywords = {formal verification, Athena, distributed systems, distributed computing, air traffic management, autonomous systems, runtime verification, cyber physical systems},
    abstract = {As autonomous vehicular technologies such as self-driving cars and uncrewed aircraft systems (UAS) evolve to become more accessible and cost-efficient, autonomous multi-agent systems, that comprise of such entities, will become ubiquitous in the near future. The close operational proximity between such autonomous agents will warrant the need for multi-agent coordination to ensure safe operations. In this thesis, we adopt a formal methods-based approach to investigate multi-agent coordination for safety-critical autonomous multi-agent systems. We explore algorithms that can be used for decentralized multi-agent coordination among autonomous mobile agents by communicating over asynchronous vehicle-to-vehicle (V2V) networks that can be prone to agent failures. In particular, we study two types of distributed algorithms that are useful for decentralized coordination — consensus, which can be used by autonomous agents to agree on a set of compatible operations; and knowledge propagation, which can be used to ensure sufficient situational awareness in autonomous multi-agent systems. We develop the first machine-checked proof of eventual progress for the Synod consensus algorithm, that does not assume a unique leader. To consider agent failures while reasoning about progress, we introduce a novel Failure-Aware Actor Model (FAM). We then propose a formally verified Two-Phase Acknowledge Protocol (TAP) for knowledge propagation that can establish a safe state of knowledge suitable for autonomous vehicular operations. The non-deterministic and dynamic operating conditions of distributed algorithms deployed over asynchronous V2V networks make it challenging to provide appropriate formal guarantees for the algorithms. To address this, we introduce probabilistic correctness properties that can be developed by stochastically modeling the systems. We present a formal proof library that can be used for reasoning about probabilistic properties of distributed algorithms deployed over V2V networks. We also propose a Dynamic Data-Driven Applications Systems (DDDAS)-based approach for the runtime verification of distributed algorithms. This approach uses parameterized proofs, which can be instantiated at runtime, and progress envelopes, which can divide the operational state space into distinct regions where a proof of progress may or may not hold. To motivate our verification of decentralized coordination, we introduce an autonomous air traffic management (ATM) technique for multi-aircraft systems called Decentralized Admission Control (DAC).} 
    }
    


Articles in journal, book chapters
  1. Saswata Paul, Gul A. Agha, Stacy Patterson, and Carlos A. Varela. Eventual Consensus in Synod: Verification using a Failure-Aware Actor Model. Innovations in Systems and Software Engineering - A NASA Journal, 2022. Keyword(s): formal verification, Athena, distributed systems, distributed computing, air traffic management, consensus, actor model.
    Abstract:
    Successfully attaining consensus in the absence of a centralized coordinator is a fundamental problem in distributed multi-agent systems. We analyze progress in the Synod consensus protocol — which does not assume a unique leader — under the assumptions of asynchronous communication and potential agent failures. We identify a set of sufficient conditions under which it is possible to guarantee that a set of agents will eventually attain consensus using Synod. First, a subset of the agents must not permanently fail or exhibit Byzantine failure until consensus is reached, and second, at least one proposal must be eventually uninterrupted by higher-numbered proposals. To formally reason about agent failures, we introduce a failure-aware actor model (FAM). Using FAM, we model the identified conditions and provide a formal proof of eventual progress in Synod. Our proof has been mechanically verified using the Athena proof assistant and, to the best of our knowledge, it is the first machine-checked proof of eventual progress in Synod.

    @article{paul-isse-2022,
    author = {Saswata Paul and Gul A. Agha and Stacy Patterson and Carlos A. Varela},
    title = {Eventual Consensus in Synod: Verification using a Failure-Aware Actor Model},
    journal="Innovations in Systems and Software Engineering - A NASA Journal",
    year="2022",
    publisher="Springer Nature",
    address="Cham",
    pdf="http://wcl.cs.rpi.edu/papers/paul-ISSE-2022.pdf",
    doi="https://doi.org/10.1007/s11334-022-00463-5",
    keywords = {formal verification, Athena, distributed systems, distributed computing, air traffic management, consensus, actor model},
    abstract = {Successfully attaining consensus in the absence of a centralized coordinator is a fundamental problem in distributed multi-agent systems. We analyze progress in the Synod consensus protocol — which does not assume a unique leader — under the assumptions of asynchronous communication and potential agent failures. We identify a set of sufficient conditions under which it is possible to guarantee that a set of agents will eventually attain consensus using Synod. First, a subset of the agents must not permanently fail or exhibit Byzantine failure until consensus is reached, and second, at least one proposal must be eventually uninterrupted by higher-numbered proposals. To formally reason about agent failures, we introduce a failure-aware actor model (FAM). Using FAM, we model the identified conditions and provide a formal proof of eventual progress in Synod. Our proof has been mechanically verified using the Athena proof assistant and, to the best of our knowledge, it is the first machine-checked proof of eventual progress in Synod.} 
    }
    



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