Network protocols define the rules of communication between machines. They define how these machines will be recognized on the network and how they will interact. In trading finance, we use the UDP and TCP over the IP protocol. Additionally, we use a software protocol that will define how to communicate with an order and get a price update. The Communication API will set the rules of communication at the software level. The Communication API is given by the entity that you would like to trade with. This document contains all the messages that you will use to receive prices and send orders.
You can find examples of trading API documents at https://en.wikipedia.org/wiki/List_of_electronic_trading_protocols.
Before diving into the trading API, we will need to explain the basics of networking.
