Dynamically available total-order broadcast (TOB) protocols tolerate fluctuating participation, e.g., as high as 99% of their participants going offline, which is especially useful in permissionless blockchain environments. However, dynamically available TOB protocols are synchronous protocols, and they lose their safety guarantees during periods of asynchrony. This is a major issue in practice. In this paper, we propose a simple but effective mechanism for tolerating bounded periods of asynchrony in dynamically available TOB protocols that ensure safety deterministically. We propose to trade off assumptions limiting the online/offline churn rate in exchange for tolerating bounded asynchronous periods through the use of a configurable message-expiration period. In practice, this allows picking a small synchrony bound $\delta$, and therefore obtain a fast protocol in the common case, knowing that the protocol tolerates occasional periods of duration at most $\pi>\delta$ during which the bound does not hold. We show how to apply this idea to a state-of-the-art protocol to make it tolerate bounded periods of asynchrony.

翻译：动态可用的全序广播协议能够容忍参与程度的波动，例如高达99%的参与者离线，这在无许可区块链环境中尤为有用。然而，动态可用的全序广播协议属于同步协议，在异步期间会失去其安全性保证。这在实践中是一个重大问题。本文提出一种简单但有效的机制，用于在动态可用的全序广播协议中容忍有界的异步期，并确定性地确保安全性。我们建议通过限制在线/离线更替率的假设，来交换对有限异步期的容忍能力，具体方法是使用可配置的消息过期周期。在实践中，这允许选择一个较小的同步界δ，从而在常见情况下获得快速的协议，同时知道协议能够容忍偶尔出现的时长至多为π > δ的异步期（在此期间该界不成立）。我们展示了如何将该思想应用于一种最先进的协议，使其能够容忍有界的异步期。