Abstract: Protocols for State-Machine-Replication (sometimes called `blockchain’ protocols)  generally make use of rotating leaders to drive consensus. In typical protocols (henceforth called `single-sender’ protocols), the leader is a single processor responsible for making and disseminating proposals to others. Since the leader acts as a bottleneck, apparently limiting throughput, a recent line of research has investigated the use of `multi-sender’ protocols in which many processors distribute proposals in parallel. Examples include DAG-based protocols such as DAG-Rider, Bullshark, Sailfish, Cordial Miners, Mysticeti, and variants such as Autobahn. However, existing models do not allow for a formal analysis to determine whether these protocols can actually handle higher throughputs than single-sender protocols such as PBFT, Tendermint, and HotStuff.

In this paper, we describe a very simple model that allows for such an analysis. For any given protocol, the model allows one to calculate latency as a function of  network bandwidth, network delays, the number of processors $n$, and the incoming transaction rate. Each protocol has a latency bottleneck: an incoming transaction rate at which latency becomes unbounded over the protocol execution, i.e., a maximum throughput that the protocol can handle without unbounded latency.  

With the aim of building to an analysis for state-of-the-art State-Machine-Replication (SMR) protocols, we begin by considering protocols for simpler primitives, such as Best-effort Broadcast and Reliable Broadcast. For Best-effort Broadcast, we establish a tight lower bound on latency for single-sender and multi-sender protocols when blocks are distributed without the use of techniques such as erasure coding. Perhaps unsurprisingly, a key difference between the single-sender and multi-sender approaches in this case is a factor $n$ in the point at which the latency bottleneck appears. However, for other primitives such as Reliable Broadcast,  our results may be more surprising: the factor $n$ difference now disappears, and maximum throughput for the two approaches differs by a constant factor, while multi-sender approaches will generally have latency that grows more quickly with $n$. For state-of-the-art SMR protocols, the picture that emerges is one with seemingly inherent trade-offs. If one compares single-sender protocols that use pipelining and erasure coding, such as DispersedSimplex, with DAG-based protocols such as Sailfish or Bullshark, the former are seen to have lower latency for a wide range of throughputs, while the benefit of the latter protocols is that they have  a latency bottleneck which is higher by a constant factor. 

Joint work with Kartik Nayak and Nibesh Strestha. Experiments to be added soon!!

Here is the pdf.