Cross-layer RTT discrepancy (RTTdiff) is a protocol-agnostic fingerprint that exploits an inherent architectural property of all proxy setups: transport-layer sessions terminate at the proxy while application-layer sessions remain end-to-end. Evaluation across 10 proxy protocols—including VMess, Shadowsocks, VLESS, Trojan, XTLS-Vision, and obfs4-wrapped SOCKS—shows near-identical detection rates for all except obfs4, confirming the fingerprint is not tied to any specific obfuscation scheme. At FPR=0.01, per-website detection rates exceed 70% across all tested client and proxy location combinations.

Per-flow RTTdiff detection rates are only ~20% because the majority of proxy flows connect to CDN-cached content (Cloudflare, Google, Fastly) that sits within 5ms of the proxy, suppressing the discrepancy. However, aggregating across flows per website visit yields detection rates exceeding 70%—and from the abstract, approximately 80% of top-5K domains generate at least one detectable flow—with half of those detections made within the first 60 packets. This means an adversary can reliably expose client and proxy IPs after just a few website visits.

§VI-C-2, Table II detection

The paper evaluates two short-term mitigations—TCP delayed ACK on the proxy server and connection multiplexing—but finds both are limited: delayed ACK produces atypical ACK timing that may itself be fingerprintable, and multiplexing only adds entropy without eliminating the RTTdiff signal. Critically, obfs4 and ScrambleSuit's delay-based timing obfuscation are described as 'fundamentally limited' because they manipulate inter-arrival times without eliminating the underlying transport/application-layer session misalignment. The paper concludes no existing obfuscation scheme provides a principled defense against timing-based proxy fingerprinting.

§VII, §I defense

IMAP/SSL traffic on port 993 constitutes less than 1% of total ISP traffic but accounts for nearly one third of all false positives in the RTTdiff exploit, because IMAP's non-RESTful multi-connection pattern violates the request-response correlation assumption. The overall per-flow FPR is bounded at 0.6–0.7% (on par with GFW's estimated FPR against fully-encrypted proxies), but implementing a pre-filter to whitelist IMAP traffic reduces the FPR by approximately one third, making the fingerprint substantially more precise.

§VI-C-3, Table III detection

Proxy users who resolve DNS locally (at the client) are approximately twice as susceptible to RTTdiff fingerprinting compared to users who resolve DNS at the proxy, across all tested client/proxy location combinations. Local DNS returns IPs optimally reachable from the client's region, which may be geographically distant from the proxy, increasing the proxy-to-server path distance and thus the RTTdiff discrepancy.

§VI-C-2 detection

A three-stage detection pipeline exploiting the "dual-role" behavioral fingerprint of single-IP circumvention relays achieved 23.2% recall (96/414 ground-truth relays) with a 0.18% false-positive rate against 97,651 benign TLS servers, for an overall accuracy of 99.5%. The ground-truth set covered OpenVPN, WireGuard, and SOCKS relays identified in a 17 TB single-day backbone trace (WIDE Project, April 9, 2025).

2026-almutairi-server §3.4, Table 1 traffic-shapedpiflow-correlation

Multi-censor simulations show that single-censor-optimized distribution strategies perform suboptimally in realistic multi-region deployments. When two networks have different censor strategies (e.g., one optimal, one zig-zag), the distributor cannot detect that a proxy is blocked until all censors have blocked it; this leaves clients without reachable proxies despite the proxy appearing "available" from the distributor's view. The authors conclude that "single-censor evaluation does not accurately predict more realistic deployment performance." A zig-zag censor in one region with 0.25 weight caused 44.4% collateral damage while reducing proxy lifetime to a median of 4 steps.

2026-fares-game §6, Table 4 traffic-shapeflow-correlation

The zig-zag traffic analysis attack (confirmed supported in Geedge TSG leak) rapidly enumerates all static proxy pools. With ζ_watch ∈ {4, 6} steps and a best-quality classifier (ρ_TP=0.99, ρ_FP=0.001), almost total proxy enumeration and user blockage occurs well before step 300. Even ζ_watch=2 leaves ~50% of users blocked. Collateral damage is high across all settings when ζ_watch ≥ 4: eventually ~50% of innocent servers are also blocked. However, Snowflake-style ephemeral proxies resist zig-zag effectively: reachability remains above 95% after 360 steps because churn prevents the censor from expanding its known proxy set beyond agents' direct assignments.

2026-fares-game §5.1, Fig 3, Fig 4 traffic-shapeflow-correlation

PushProxy decouples upstream (XOR-obfuscated UDP) from downstream (FCM push notifications), implementing triangular routing that prevents per-flow traffic analysis: a network adversary with limited visibility cannot correlate upload and download flows since they use different transport protocols and paths. Median TTFB was 572ms versus 492ms (Shadowsocks) and 508ms (OpenVPN), while performance remained stable during Chinese peak hours (20:00–02:00 GMT+8) when Shadowsocks download times increased from 3s to over 100s.

2023-xue-use §4, §5.2, Figure 7–8 flow-correlationtraffic-shape

CRON restricts multi-hop covert circuits (N≥1 relays) to delay-tolerant traffic only, because establishing multiple simultaneous WebRTC video calls is 'highly atypical in normal user profiles' and would trigger S1 behavioral anomaly detection. Real-time interactive tunneling is limited to direct circuits (N=0) within pre-existing calls, and active mode introduces only bounded variability in call times and frequency to stay within plausible user-profile ranges.

2020-barradas-towards §4.1 traffic-shapeflow-correlation

Even when individual WebRTC flows pass traffic analysis, a censor can identify CRON users via three long-term statistical attack types: S1 (simultaneous video calls, atypical for normal users), S2 (sudden connections to previously unknown parties), and S3 (calls at anomalous times, frequencies, or durations). Relay nodes in multi-hop circuits are particularly exposed via S1 because conducting multiple simultaneous video calls is highly atypical in normal user profiles.

2020-barradas-towards §4.1 traffic-shapeflow-correlationml-classifier