Distributed Oracle for Estimating Global Network Delay with Known Error Bounds.

Partially synchronous models are often assumed for designing distributed protocols because they capture realistic timing assumptions, such as the asynchronous and synchronous periods that the system can experience. In some of these models, protocols need to estimate network delays. Some protocols fix the global message delay bound for all executions, which leads to sub-optimal solutions in terms of latency, because this bound must be chosen conservatively. And other protocols employ delay estimation mechanisms that only give an upper bound on the delay without quantifying the estimation error. The performance of these protocols depends on how close their estimations are in relation to the actual network delay. For instance, some Byzantine consensus protocols use timeouts based on this estimation. We formalize this problem as the Global Delay Bound Estimation (GDBE) and address it by introducing a distributed oracle that enriches partial synchronous models. This oracle produces estimates of the channel delays that allow processes to derive an efficient global bounded estimate. Oracles and global bounded estimates, provide a framework that facilitates the design of protocols for partially synchronous models and the analysis of their time complexity. We formalize the properties of the oracle and the proposed framework and show that it can be implemented in the presence of crash failures. In contrast, we prove that GDBE cannot be solved in the Byzantine failure model, and show how to circumvent this impossibility using an extra assumption. Finally, we show how to use our framework to implement a view synchronizer thus obtaining an efficient solution for Byzantine consensus.