A covert channel is used to communicate secretly. Whereas encryption only protects the communication from being decoded by unauthorized parties, covert channels aim to hide the very existence of the communication. Initially, covert channels were identified as a security threat on monolithic systems such as mainframes. More recently, focus has shifted towards covert channels in computer network protocols. The huge amount of data and the vast number of different protocols in the internet make it an ideal high-bandwidth vehicle for covert communication. Some of the most common protocols that are used to create a covert channel are as follows.
(The following are referenced from: https://holdmybeersecurity.com/2017/10/15/part-3-how-to-red-team-setting-up-environment/)