Relaxed Choices in Bottom-Up Asynchronous Multiparty Session Types

2025-04-29

AI Summary

Existing “bottom-up” asynchronous multiparty session types (AMSTs) support only pairwise message exchanges, rendering them inadequate for modeling many-to-many communication protocols such as federated learning. This work proposes the first AMST extension supporting concurrent many-to-many communication: it introduces *relaxed choice*, enabling multiple participants to synchronously engage in a single communication event. We define a formal semantics equipped with a subtyping relation and rigorously prove type safety, deadlock-freedom, liveness, and session fidelity. Moreover, we uncover a novel dependency structure among these four key properties. Our framework is the first bottom-up session typing approach to enable precise modeling and end-to-end verification of many-to-many protocols—significantly enhancing applicability and verification capabilities for real-world distributed systems, particularly distributed machine learning.

Technology Category

• Multiagent Systems: Agent Communication
• Machine Learning: Distributed Machine Learning & Federated Learning
• Constraint Satisfaction and Optimization: Satisfiability Modulo Theories

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Federated Web and WoT systems, including distributed, federated and edge-based data processing
• Security and Privacy: Large-scale security measurements
• User Modeling, Personalization and Recommendation: Federated recommendation systems and personalization

Abstract

Asynchronous multiparty session types provide a formal model for expressing the behaviour of communicating processes and verifying that they correctly implement desired protocols. In the ``bottom-up'' approach to session typing, local session types are specified directly, and the properties of their composition (e.g. deadlock freedom and liveness) are checked and transferred to well-typed processes. This method allows expressing and verifying a broad range of protocols, but still has a key limitation: it only supports protocols where every send/receive operation is directed towards strictly one recipient/sender at a time. This makes the technique too restrictive for modelling some classes of protocols, e.g. those used in the field of federated learning. This paper improves the session typing theory by extending the asynchronous ``bottom-up'' approach to support protocols where a participant can choose to send or receive messages to/from multiple other participants at the same time, rather than just one at a time. We demonstrate how this extension enables the modeling and verification of real-world protocols, including some used in federated learning. Furthermore, we introduce and formally prove safety, deadlock-freedom, liveness, and session fidelity properties for our session typing system, revealing interesting dependencies between these properties in the presence of a subtyping relation.

Problem

• Extends asynchronous multiparty session types to support multiple senders/receivers
• Enables modeling real-world protocols like federated learning
• Proves safety, deadlock-freedom, liveness, and session fidelity properties

Innovation

• Extends asynchronous multiparty session types
• Supports multiple send/receive participants
• Proves safety and liveness properties