In Iran in 2013, censors dropped or throttled certain TCP connections after 60 seconds, severely disrupting circumvention protocols like obfs4 that fuse session state with a single long-lived TCP connection, while short-lived HTTP connections were largely unaffected. obfs4 has no session concept independent of the underlying TCP connection; when that connection is terminated, all end-to-end state is lost and a new session must restart from scratch.

The dnstt DNS-over-HTTPS tunnel, built on a KCP Turbo Tunnel session layer, achieved download speeds of 130 KB/s using Google and Cloudflare DoH resolvers and 30 KB/s using Quad9, compared to iodine's maximum of 2 KB/s over the same operators' UDP DNS resolvers — a 15–65× improvement. DNS-over-HTTPS hides message contents from the censor, removing the two main classical DNS tunnel detection vectors: unusual DNS message structure and plaintext tunnel domain names in queries.

§3.4 evaluation

Simultaneous upload and download of a 10 MB file took 10.6 s over TCP-encapsulated QUIC, 23.3 s over traditional meek, and 34.9 s over meek with encapsulated QUIC (Table 1), showing that naively adding a QUIC session layer to meek degraded throughput by approximately 50% relative to unmodified meek. Performance was sensitive to HTTP body size limits and request-thread count, but the root cause remained uncertain.

Table 1, §3.2 evaluation

Geddes et al. demonstrated that acknowledgement packets in covert-channel circumvention systems can be identified through timing characteristics and selectively interfered with to disrupt the tunnel [§4.3, CCS 2013]. A Turbo Tunnel session layer adds fixed-overhead headers and periodic ACK/keepalive traffic that may produce distinctive timing patterns absent in legitimate flows, potentially increasing susceptibility to traffic-shape classifiers.

§2 detection

Turbo Tunnel inserts an interior session/reliability protocol (KCP or QUIC) between the obfuscation layer and user streams, decoupling end-to-end session state from any single transport connection. A session survives TCP termination, proxy rotation, or unreliable carriers by retransmitting lost packets over a new connection bearing the same session identifier. The pattern was implemented in obfs4, meek, and Snowflake, with Turbo Tunnel–enabled Snowflake shipping in Tor Browser alpha releases 9.5a13 (desktop) and 10.0a1 (Android).

§1–§2 defense

TSPU devices perform in-line packet manipulation — they can inject RST packets, drop traffic, and throttle connections — rather than routing traffic to an out-of-band sniffer that votes to block. The inline placement means TSPU can act on the first-packet payload and impose latency on all matching flows, not only on those selected by sampling. Blocking decisions are therefore applied with high recall at the ISP edge, and circumvention tools that rely on short observation windows (e.g. only obfuscating the first N bytes) are vulnerable to continued inline inspection of subsequent traffic.

2024-xue-tspu-russia §3–§4 dpirst-injectionthrottlingsni-blocking

Hop-by-hop bottleneck localization showed that in more than 71% of measured paths the first lossy hop is located deep inside China (beyond the border), with only 34.45% of bottleneck hops coinciding with the GFW hop as detected by RST injection probing — suggesting Chinese ISP infrastructure underprovisioning rather than GFW intervention as the primary cause.

2020-zhu-characterizing §5, Figure 13 throttlingrst-injection

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

Article 19 documents that Iran's National Information Network (NIN / SHOMA) was designed with explicit reference to China's Great Firewall as a model, with institutional mirroring: Iran's Supreme Council of Cyberspace parallels China's Cyberspace Administration of China, and both governments share a "cyber sovereignty" doctrine used to justify domestic content controls and cross-border technology transfer. The report frames Iran's filtering infrastructure as deliberately architected to replicate GFW capabilities, not as an independently developed system.

2026-article19-tightening-the-net §2, §3 dpisni-blockingdns-poisoningip-blockingbgp-hijackthrottling

Article 19 documents that Iran combines technical filtering with formal coercion of major foreign platforms (including Telegram, Instagram, and WhatsApp) to comply with content removal orders under threat of full blocking. The report notes that Iran's 2022 Women Life Freedom protests accelerated platform blocking when foreign operators refused compliance, demonstrating that the censorship system operates in two modes: coerce-and-allow for compliant platforms, block for non-compliant ones. Domain fronting via these platforms is therefore subject to sudden revocation if political conditions change.

2026-article19-tightening-the-net §6 ip-blockingdns-poisoningthrottling

The report maps specific Belt and Road Initiative Digital Silk Road projects through which Chinese technology vendors have transferred censorship and surveillance infrastructure to Iran, including fiber backbone investments, data-center co-location agreements, and equipment supply chains. Specific vendors named include Huawei and ZTE as network infrastructure providers, with the report noting that equipment exports include filtering-capable hardware that Iran's ISPs have deployed at network choke points.

2026-article19-tightening-the-net §4, §5 dpisni-blockingthrottling