Is Custom Congestion Control a Bad Idea for Circumvention Tools?
Authors: Wayne Wang (University of Michigan), Diwen Xue (University of Michigan), Piyush Kumar (University of Michigan), Ayush Mishra (ETH Zürich), Anonymous, Roya Ensafi (University of Michigan)
Year: 2025
Issue: 1
Pages: 1–6
Abstract: Circumvention proxies often have to operate under adverse network conditions, especially over cross-border links with high packet loss. These scenarios motivated the development of proxies that implement custom congestion control algorithms (CCAs) that aggressively sustain high sending rates by disregarding standard congestion responses. In this paper, we argue that such custom CCAs are fundamentally at odds with the core principles of censorship circumvention. Using Hysteria and TCP-Brutal as case studies, we demonstrate how these custom CCAs produce traffic patterns that significantly deviate from standard TCP and QUIC behaviors, and further develop simple, threshold-based classifiers to show how a censor can distinguish such proxy traffic by its lack of response to congestion signals. We emphasize that any performance optimizations must be grounded in standard protocol behaviors to maintain the indistinguishability required for effective censorship circumvention.
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