A complete patent invention-disclosure document describes a "Multipath TCP Functionality Restriction" device deployed inline at a network egress point that identifies the MPTCP TCP option (kind=30, MP_CAPABLE subtype) in initial-handshake SYN/SYN-ACK packets and strips or NOPs it in transit, forcing the connection to fall back to ordinary single-path TCP so that conventional TCP-based inspection and attack detection can be applied to traffic that would otherwise be split across multiple paths.
通过修改初始建立多路径TCP连接的握手SYN包，即TCP报文头部选项kind字段value=30(多路径TCP)以及subtype字段value=0(MP_CAPABLE)的SYN包，将选项字段的kind字段值修改为value=1(NOP)，转发成功后，多路径TCP就fallback成正常TCP。
Defense implications
- Do not rely on MPTCP (or any TCP-option-signaled multipath/extension feature) as a censorship-evasion primitive against this class of adversary — a fielded, patented inline technique specifically strips the MP_CAPABLE option from the initial SYN/SYN-ACK to force single-path fallback before further inspection is applied.
Related findings
An internal TSG operations/troubleshooting manual lays out TSG's full traffic pipeline (NIC -> mrzcpd/marsio capture driver -> sapp DPI engine -> firewall/proxy(KNI->TFE)/active-defense/WAN-NAT policy branches) and shows engineers using maat_redis_tool to pull the live Redis-synced blocking policy tables (TSG_SECURITY_COMPILE, TSG_OBJ_IP_ADDR, TSG_OBJ_APP_ID) and filter them by numeric policy/object ID to debug why a block rule isn't firing.
A 2023 MESA Lab monthly report describes the 'TF' project's active-defense work: test cases for serial ('串联') HTTP hijack/tamper and parallel ('并联') DNS race-injection ('DNS抢答'), performance tuning that scaled active-defense capacity from 3 to 5 units, converting two existing TSG boxes to active-defense mode, and rewriting the active-defense flow-control logic to no longer depend on sapp.
TSG implements heuristic tunnel-protocol detection distinguishing Teredo (IPv6-over-UDP, default port 3544) from GTP tunneling based on UDP payload inspection, used to decide 'innermost' session attribution for firewall matching.
An IIE CAS thesis revision memo shows a student's research was retitled from "measuring evasion attacks against censorship middleboxes" to "evasion-attack traffic generation for DPI middleware" after committee feedback that "censorship middlebox" was too sensitive a term; the work builds evasion-generation tooling (citing Geneva's genetic-algorithm and SymTCP/Alembic's symbolic-execution approaches to TCP state desync) and explicitly tests it against "a specific country's DPI middleware" plus open-source DPI/NIDS, then derives defensive recommendations from the vulnerabilities it discovers.
In sapp's inline deployment mode, packet injection is handled by a driver pair (mrzcpd + mrtunnat, an mrzcpd sub-component not previously catalogued) that must reconstruct per-link tunnel state (cached in /run/mrzcpd/mrmonit.tunnat) before forging a packet in the correct direction; the internal troubleshooting guide documents that injection silently fails for GTP/MPLS/GRE-tunneled flows when the reverse-direction tunnel identifier has never been observed on that link, an explicit "asymmetric address layer" edge case.
sapp's internal architecture guide documents three deployment topologies (mirror/passive-tap, inline, dual-arm transparent) and four distinct methods for injecting blocking packets back onto the wire, including a MAC-in-MAC scheme that extracts device/link IDs from mirrored traffic's source MAC field to route an injected packet to the correct physically separate inline device over VXLAN when multiple injection points exist.