Communication protocol can be defined as sign, position, rule, minimum duration and color of each movement in semaphore flags.
In computing, a communication protocol refers to the set of rules that computers use to communicate with each other. The protocol defines the signals that the computers will give each other, and other details such as how communication begins and/or ends.
Post Office Protocol version 3 (PoP3) is the most common account type for personal e‑mail. Messages are typically deleted from the server when you check your e‑mail.