PBFC: Cross-Layer Flow Control for Parallel Permissioned Blockchains

Abstract

Permissioned blockchains deployed in modern cloud and data-center environments increasingly adopt speculative parallel execution to decouple transaction execution from consensus ordering. This design implicitly relies on the sequencer's canonical transaction instance reaching execution replicas before any competing instance with the same transaction identifier. However, transient congestion can amplify cross-replica delivery skew and violate this timing condition. Byzantine participants can further exploit this fragility by injecting competing instances, causing ordering races that trigger costly block re-execution and severe performance degradation. We propose PBFC, an execution-aware cross-layer flow control framework to enhance the robustness of parallel blockchains under network congestion, without modifying the underlying consensus protocol, by aligning transaction validation semantics with network-layer prioritization. Specifically, PBFC detects anomalies and identifies conflicting transactions, mapping each transaction to its corresponding flows. These flows are aggregated by transaction ID into pertransaction flow sets and dispatched to switches. At the switches, PBFC temporarily prioritizes each flow set within a bounded window, suppressing delivery skew and reducing ordering races. Evaluation shows that PBFC improves committed throughput by up to 25.7% and reduces end-to-end latency by up to 50.9% compared with state-of-the-art congestion-control baselines.