The paper identifies three distinct GFW resynchronization-state triggers with protocol-specific behavior: (1) a server payload on any non-SYN+ACK packet causes resync on the next SYN+ACK or client ACK-flagged packet for all protocols; (2) a server RST causes resync on the next client packet for all protocols except HTTPS; (3) a SYN+ACK with a corrupted acknowledgment number triggers resync only for FTP. Strategy 1's 50% per-attempt success rate for HTTP is confirmed to result from the 50% probability of the GFW entering the resynchronization state on an injected RST, consistent with Wang et al. [36].

All 25 applicable client-side Geneva strategies failed when mechanically translated to server-side analogs against China's GFW, even when the only structural difference was which endpoint sent the insertion packet. Experiments with the server placed inside China and client outside also failed, indicating the GFW tracks connection initiator identity and processes client versus server packets asymmetrically—meaning server-side circumvention requires a completely independent discovery approach.

§3 evaluation

China's GFW uses distinct, co-located censorship boxes—each with its own independent network stack implementation and bugs—for each application-layer protocol it censors. TCP-level strategies that exploit transport-layer bugs show dramatically different success rates per protocol: Strategy 1 (Simultaneous Open + Injected RST) achieves 89% for DNS but only 14% for HTTPS; Strategy 8 (TCP Window Reduction) achieves 100% for SMTP but only 2–3% for DNS, HTTP, and HTTPS. TTL-limited probes confirm all protocol boxes are co-located at the same network hop.

§6, Table 2 detection

The paper presents 11 purely server-side censorship evasion strategies requiring zero client-side software, successfully bypassing censorship in China, India, Iran, and Kazakhstan across DNS-over-TCP, FTP, HTTP, HTTPS, and SMTP. All strategies manipulate only TCP handshake packets (primarily the SYN+ACK) and were verified against 17 versions of 6 client operating systems (Windows XP–Server 2018, MacOS, iOS, Android, Ubuntu, CentOS) with unmodified clients.

§1, §5, Table 2 defense

TCP Window Reduction (Strategy 8)—reducing the SYN+ACK TCP window to 10 bytes and stripping wscale options, forcing the client to segment its request—achieves 100% evasion success against HTTP in India and Kazakhstan, 100% against HTTP and HTTPS in Iran, and 100% against SMTP in China, because none of these censors can reassemble TCP segments. The strategy is compatible with all 17 tested client OS versions when implemented without SYN+ACK payloads, making it the most broadly deployable server-side strategy found.

§5.1, §5.2, §7, Table 2 defense

Russian TSPU devices directly block ECH by dropping ClientHello messages that contain both an ECH extension and the outer SNI hostname "cloudflare-ech.com" — the static outer SNI Cloudflare advertises in all its ECH configurations. Blocking affects both TLS and QUIC. ECH connections to servers with Cloudflare ECH support but outside Cloudflare's official IP ranges are NOT blocked. TCP segmentation alone or TLS record fragmentation alone did NOT bypass TSPU ECH blocking, but combining both techniques did circumvent it. TSPU has also added TCP reassembly capabilities that defeat previously effective fragmentation-only bypasses.

2025-niere-encrypted §5, Figure 5 dpirst-injectionmiddlebox-interference

The GFI's HTTP and HTTPS filters are now stateful (requiring initial SYN packet with matching sequence numbers) and have been activated on all TCP ports—not only standard ports 80 and 443 as reported by prior studies. This is a significant departure from previous work that found stateless HTTP/HTTPS blocking limited to standard ports. The HTTP filter injects a 403 Forbidden blockpage (not RST packets as used by the GFW), while HTTPS injects a single RST+ACK packet. The GFI also exhibits TCP non-compliance (not requiring a full three-way handshake to trigger filtering), enabling outside-in measurement without in-country servers.

2025-tai-irblock §2.1, §3.2 dpirst-injectionpacket-injectionmiddlebox-interference

China's Great Firewall showed anomalous inconsistency: 13 test vectors produced mixed outcomes—TCP RST injection on some executions and a clean server response on others—with circumvention rates between 10% and 35% across 100 executions per vector. The authors attribute this to heterogeneous GFW infrastructure components applying different HTTP parsing logic, a departure from the GFW's usual consistency.

2024-m-ller-turning §5.2 dpirst-injectionkeyword-filteringmiddlebox-interference

Server-side censorship evasion strategies require zero client-side changes: clients bypass censorship without installing software or even being aware of the evasion, and this approach has been adopted in production tools including Psiphon's packetman. The packet manipulations exploit weaknesses in how censors track or tear down TCP connections, occurring entirely at the server during the three-way handshake.

2023-tran-crowdsourcing §1 Introduction dpirst-injectionmiddlebox-interference

Evasion strategies are strongly censor-specific: TCB Teardown strategies that achieve 80–96% against the GFW fail completely (0%) against Kazakhstan's HTTPS MITM; India's Airtel is defeated uniquely by a 'Stutter Request' (duplicating the PSH/ACK and replacing IP length to 64) at 100% success, which scores only 3% against the GFW. Geneva converged on distinct species for each censor within 4–8 hours of live training.

2019-bock-geneva Table 1, §5.1–5.4 dpirst-injectionmiddlebox-interference

The GFW evolved to create a TCB not only on SYN packets but also on SYN/ACK packets, and enters a 're-synchronization state' upon seeing multiple SYN packets, multiple SYN/ACK packets, or a SYN/ACK with an incorrect acknowledgment number. Once in this state, it re-synchronizes its TCB using the next client-to-server data packet or server SYN/ACK, invalidating prior TCB-creation evasion strategies that assumed the GFW used only the first SYN sequence number.

2017-wang-your §4 dpirst-injectionmiddlebox-interference