Optimistic Byzantine Agreement Protocol for Bounded Communication
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Solution Overview
Problem
Scalable Byzantine Agreement State Machine Replication (SMR) systems face challenges in achieving bounded communication costs, especially in asynchronous periods, due to the unbounded communication costs associated with existing 'eventually synchronous' communication models, which are not suitable for geo-replicated systems with hundreds of computing nodes across distant locations.
Innovation Solution
An optimistic Byzantine agreement protocol that first attempts to reach agreement via an efficient deterministic synchronous algorithm and then transitions to an asynchronous algorithm if synchrony is not achieved, ensuring termination with bounded communication costs by minimizing unnecessary transitions to the more expensive asynchronous fallback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If an eventually synchronous communication model is used to reduce communication cost after GST, then communication cost is reduced in synchronous periods, but communication cost becomes unbounded during asynchronous periods
Solution Approach 1:
The protocol dynamically adapts its communication pattern based on the actual network conditions. During synchronous periods, it uses efficient synchronous communication; during asynchronous periods, it automatically transitions to bounded asynchronous communication patterns, ensuring communication cost remains bounded regardless of network conditions
Solution Approach 2:
The protocol changes its operational parameters (communication patterns, timing assumptions) based on the detected network state. It monitors whether the system is in a synchronous or asynchronous period and adjusts its behavior accordingly, ensuring bounded communication cost in both regimes
2Reliability
If a deterministic asynchronous agreement protocol is used, then safety is guaranteed during asynchronous periods, but progress cannot be guaranteed without synchrony assumptions
Solution Approach 1:
The protocol segments the agreement process into distinct phases: a synchronous phase for efficient progress when possible, and an asynchronous phase for safety guarantees. This segmentation allows the system to achieve both progress and safety by operating in the appropriate phase based on network conditions
Solution Approach 2:
The protocol serves multiple functions: it provides fast synchronous agreement when network conditions permit, while simultaneously guaranteeing safety during asynchronous periods. It universally handles both synchronous and asynchronous scenarios within a single unified protocol framework
3Quantity of substance
If threshold signatures and collectors are used to reduce quadratic communication cost, then communication cost is reduced after GST, but the number of views executed prior to GST remains unbounded
Solution Approach 1:
The protocol performs preliminary actions during synchronous periods to advance the agreement state before asynchronous periods begin. This preliminary progress reduces the number of views that need to be executed during subsequent asynchronous periods, bounding the total communication cost
Data Source
AI summary
An optimistic byzantine agreement protocol (the protocol) first tries to reach agreement via an efficient deterministic algorithm (synchronous protocol) that relies on synchrony for termination. If an agreement is not reached (e.g., due to asynchrony), the protocol uses a randomized asynchronous algorithm (asynchronous protocol) for fallback. Although randomized asynchronous algorithms are considered to be costly, the rationale here is to bound communication in non-synchronous runs after an equivalent cost has already paid.