Protocol Channels
  Covert channel techniques are used by attackers to transfer data in a way prohibited by the security policy. There are two main categories of covert channels: timing channels and storage channels. This paper introduces a new storage channel technique called a protocol channel. A protocol channel switches one of at least two protocols to send a bit combination to a destination. The main goal of a protocol channel is that packets containing covert information look equal to all other packets within a network, what makes a protocol channel hard to detect. 
Steffen Wendzel