The Discriminative Power of Cross-layer RTTs in Fingerprinting Proxy Traffic

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 traffic-shapeflow-correlation generic

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 traffic-shape generic

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 traffic-shapeflow-correlation genericcn

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 traffic-shapedns-poisoning generic

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.

§I, §VI-C detection traffic-shapeflow-correlation genericcnru