A Session Type System for Asynchronous Unreliable Broadcast Communication

TL;DR

This paper develops a session type system for unreliable broadcast communication, enabling formal verification of protocols like Paxos in distributed systems with message loss.

Contribution

It introduces the Unreliable Broadcast Session Calculus with a sound session type system, including autonomous recovery, for unreliable asynchronous networks.

Findings

Type system ensures safety and progress in unreliable broadcast settings.

Framework successfully models Paxos protocol.

Supports autonomous recovery without acknowledgements.

Abstract

Session types are formal specifications of communication protocols, allowing protocol implementations to be verified by typechecking. Up to now, session type disciplines have assumed that the communication medium is reliable, with no loss of messages. However, unreliable broadcast communication is common in a wide class of distributed systems such as ad-hoc and wireless sensor networks. Often such systems have structured communication patterns that should be amenable to analysis by means of session types, but the necessary theory has not previously been developed. We introduce the Unreliable Broadcast Session Calculus, a process calculus with unreliable broadcast communication, and equip it with a session type system that we show is sound. We capture two common operations, broadcast and gather, inhabiting dual session types. Message loss may lead to non-synchronised session endpoints. To further account for unreliability we provide with an autonomous recovery mechanism that does not require acknowledgements from session participants. Our type system ensures soundness, safety, and progress between the synchronised endpoints within a session. We demonstrate the expressiveness of our framework by implementing Paxos, the textbook protocol for reaching consensus in an unreliable, asynchronous network.