None of the operational networks tested—T-Mobile, AT&T, the Great Firewall of China, and Iran—classify UDP traffic; the authors describe this as 'a surprisingly easy way to evade their policies.' Iran's censor inspects the entire TCP flow but leaves UDP flows untouched across all tested applications.

Middlebox classification state is ephemeral: the testbed carrier-grade DPI device flushes results after 120 seconds (or 10 seconds after a TCP RST), and the GFC flushes state after 40–240 seconds depending on time of day. A strategically timed pause before the matching payload, or a TTL-limited RST packet, causes the classifier to re-evaluate the connection as unclassified traffic.

§4.3, §5.3 detection

Iran's censor and AT&T's Stream Saver restrict DPI inspection strictly to port 80; traffic on any other TCP port escapes classification entirely. Iran additionally inspects the full flow (not just initial packets), unlike T-Mobile and the testbed device which only inspect the first few packets, making packet-count-based evasion insufficient against Iran on port 80.

§1 (key findings), §6.3 detection

TCP segment splitting and out-of-order delivery evades DPI classification in the testbed, T-Mobile, and Iran, but fails against the GFC—which performs extensive packet validation and correctly reassembles reordered streams—and AT&T, which uses a transparent HTTP proxy that normalizes all traffic before inspection. Payload splitting to one byte in the first packet is sufficient to defeat packet-count-limited classifiers.

§4.3, Table 3 evaluation

lib·erate's TTL-limited inert packet insertion—sending a decoy packet with TTL set to expire at the middlebox but carrying a misclassifying payload—successfully evades classification in a carrier-grade testbed DPI device, T-Mobile's Binge On, and the Great Firewall of China, but fails against Iran's censor and AT&T (Table 3). When bilateral server support is available, inserting a single dummy packet at flow start evades classification in all four deployments.

§4.3, Table 3 evaluation

During the June 2025 Iran shutdown, circumvention tool performance diverged sharply by transport design. Psiphon's multi-protocol architecture sustained 1.5 million concurrent users—roughly one-third of its normal Iranian base. Lantern's "proxyless" protocol (domain-fronting via CDN, ~40% of Lantern's Iranian traffic) showed moderate success. Tor usage collapsed during the blackout but bridge connections surged and rebounded quickly after lifting. BeePass (serving 500k+ daily users at shutdown onset) used live A/B testing of port/obfuscation-prefix combinations to probe the censors' blocking parameters in real time. The Ceno Browser's P2P network grew from 600 active peers on June 13 to ~8,000 by July 11, indicating that decentralized fallback paths stayed up even during peak blocking.

2025-iran-shutdown-measurement §Tool Performance dpithrottling

The June 2025 Iran shutdown—carried out during the Iran-Israel war beginning ~June 19—did not use BGP route withdrawals as in 2019. Instead, authorities applied service-level restrictions at the national border: DNS poisoning of foreign destinations, protocol whitelisting permitting only pre-approved domestic services, and DPI to block circumvention-tool traffic. Iran's international traffic fell roughly 90% while the country's BGP routes remained advertised, making the shutdown invisible to BGP-based monitoring systems. OONI measurement volume, which totalled 121,333 in June 2025, collapsed to under 200 submissions on June 19-20.

2025-iran-shutdown-measurement Executive Summary / §2 dpidns-poisoningport-blockingip-blocking

During the June 2025 shutdown, Iranian authorities blocked international One-Time Password (OTP) SMS delivery, preventing new sign-ins to foreign secure-messaging platforms and VPN services. This forced users toward government-approved domestic platforms that lack security and privacy protections. The blockade of OTPs effectively weaponized account-recovery flows as a secondary shutdown layer, disproportionately affecting users who needed to activate new circumvention tools during the crisis.

2025-iran-shutdown-measurement §Human Rights Implications port-blockingdpikeyword-filtering

Input blocking in Chinese LLM services (DeepSeek, Qwen, Kimi, Doubao) is overwhelmingly consistent: all four services persistently block the exact same queries across all 5 measurement samples in both Simplified and Traditional Chinese. Output blocking is far less consistent, with only 29 out of 349 output-blocked queries blocked across all 5 samples. Baidu-Chat is exceptional: it performs almost no input blocking but instead relies heavily on post-search and output blocking (78.6% of blocks are output-phase).

2026-ablove-characterizing §VI-B keyword-filteringdpi

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

The paper identifies a fundamental architectural vulnerability in single-IP circumvention designs: a relay must generate new observable flows (via DNS or TLS SNI) to reach end services after receiving client connections, creating a detectable server-and-client behavioral contrast. A relay accessing user-facing domains (news, social media) scores high on a Relay Suspicion Score (w=0.9) versus infrastructure domains (w=0.1). The paper argues this host-level signal is censorship-invariant and cannot be concealed by link obfuscation.

2026-almutairi-server §2.4, §4 traffic-shapedpisni-blocking