I have an assignment where I shall communicate with a server and send messages between client and server. The text says that when a message has been received, the server shall answer with an ACK. I really don't understand what an ACK means and how to send one. I have searched on google and some books but haven't found an answer. Can anyone help me out and point me in the right direction. We're using Berkley sockets and communicating through TCP on Linux.