Centralization in Block Building and Proposer-Builder Separation

TL;DR

This paper uses mathematical models to analyze whether centralization in block building and proposer-builder separation in blockchain systems is inevitable or can be mitigated, focusing on heterogeneity and competition effects.

Contribution

It provides a rigorous, model-based analysis of how heterogeneity and competition influence centralization in blockchain block-building and proposer separation.

Findings

Heterogeneous rewards lead to stake concentration among top producers.

Reinvestment of rewards accelerates stake centralization.

Proposer-builder separation can reduce reward heterogeneity depending on ecosystem competitiveness.

Abstract

The goal of this paper is to rigorously interrogate conventional wisdom about centralization in block-building (due to, e.g., MEV and private order flow) and the outsourcing of block-building by validators to specialists (i.e., proposer-builder separation): 1. Does heterogeneity in skills and knowledge across block producers inevitably lead to centralization? 2. Does proposer-builder separation eliminate heterogeneity and preserve decentralization among proposers? This paper develops mathematical models and results that offer answers to these questions: 1. In a game-theoretic model with endogenous staking, heterogeneous block producer rewards, and staking costs, we quantify the extent to which heterogeneous rewards lead to concentration in the equilibrium staking distribution. 2. In a stochastic model in which heterogeneous block producers repeatedly reinvest rewards into staking, we quantify, as a function of the block producer heterogeneity, the rate at which stake concentrates on the most sophisticated block producers. 3. In a model with heterogeneous proposers and specialized builders, we quantify, as a function of the competitiveness of the builder ecosystem, the extent to which proposer-builder separation reduces the heterogeneity in rewards across different proposers. Our models and results take advantage of connections to contest design, P\'olya urn processes, and auction theory.