A second important concept of the Essential Networking Module is the socket. Rogue Wave uses the traditional definition of a socket as an endpoint for communication with another application. When you establish a network connection with a socket, you create a channel for data transmission between your socket and a socket in use by the application to which you're connecting. The sockets are used for sending and receiving data, and they are closed when either side wants to stop communicating. The Berkeley sockets API is a standard C API for network programming that is available on all popular operating systems.