Fake web traffic can hide secret chat - tech - 26 May 2009 - New Scientist
Retransmission Stenography makes use of TCP's packet retransmission scheme to send secret messages. The system works by embedding data in packets returned to sender flagged as corrupted. If the original transmitter is looking for markers indicating the presence of these messages, they can compose a clandestine message from a sequence of retransmitted packages. If they keep the retransmission rate to about 1 in 1000 packets then nobody has reason to suspect that secret messages are being transmitted. The developers hope to use this to help people under totalitarian regimes get information out under the noses of government sensors.