GFW reassembles both IP fragments and TCP segments for HTTP connections, but its overlap-resolution policy diverges from receiver behavior in documented cases: it prefers the original IP fragment in all overlap configurations except when the challenger is simultaneously left-long and right-long (IP2), and prefers a later left-equal TCP segment over the original (TCP5). The paper tests all 18 possible fragment overlap cases and confirms that placing a banned keyword only in the fragment version GFW discards achieves evasion.
From 2013-khattak-towards — Towards Illuminating a Censorship Monitor's Model to Facilitate Evasion
· §5, Table 1 (IP2, TCP5)
· 2013
· Free and Open Communications on the Internet
Implications
Fragment or segment payloads so that banned keywords appear only in the portion GFW discards per its documented overlap policy (left-long+right-long for IP; non-left-equal for TCP); the receiver reconstructs the benign version.
Automating this with Geneva or fragroute is straightforward: the overlap policy is deterministic and the evasion is confirmed; treat it as a known-good technique subject to future GFW normalization upgrades.