An RTP Extension for Reliable User-Data Transmission over VoIP Traffic

Covert channels are those breaking the restrictions of the legitimate channel and transmitting covert messages secretly. According to the modulation methods, the covert channels are separated into two types, which are the covert storage channel and the covert timing channel. Since the covert storage channel modulates a covert message into the shared storage space, both capacity and throughput exceed the covert timing channel, which guarantees the effectiveness of covert communication. In this paper, a covert storage channel over RTP is proposed and evaluated. Since mobile multimedia communication requires low transmission latency, the RTP based on UDP is widely utilized by VoIP applications. Through modifying the packet layout of RTP packets and embedding secret message sections into the target position, the covert message could be delivered to the receiver. However, since the RTP is not a reliable protocol, the transmission procedure could be infected by the network noise. To solve this, a transmission control mechanism with retransmission is designed for the scheme. Besides, the secret message is also encrypted with a secure encryption algorithm, which prevents message leakage. To evaluate the scheme, several experiments are deployed, and the scheme is proved to be reliable and secure.