3.4.1 Sockets

The sockets API exported to ns-3 attempts to mimic the standard BSD sockets API. The major difference in the implementation is that while BSD socket calls are synchronous (that is, they do not return control to their caller until they complete), the ns-3 socket API calls return immediately. This is due to the fact that in a simulation environment where one machine is simulating possibly thousands of socket calls across different simulated machines simultaneously, the simulator simply cannot afford to wait for the socket function call to return. The way the software handles the situation instead is by returning immediately, then using callbacks when other portions of the code need to be notified of a socket event. For example, when in the course of the simulation a socket is directed to listen() on a specific port, the caller also provides a callback to handle when the socket receives a connection request. The listen() method returns immediately, and then whenever the socket receives the connection, it invokes the callback to handle the connection. Similar things happen for the other common socket APIs, like send(), connect(), and bind().