Sockets

Figure 3: A socket connection.

• Sockets are used to provide the capability of making connections from one application running on one machine to another running on a different machine.
• A socket abstraction consists of the data structure that holds the information needed for communication, and the system calls that manipulate the socket structure.
• Once a socket is created, it can be used to wait for an incoming connection (passive socket), or can be used to initiate connection (active socket).
• A client can establish an active connection to a remote server by creating an instance of a socket. To establish a server connection and bind it to a particular port number, we should create an instance of a server socket.
• A server socket listens on a TCP port for a connection from a client (passive socket). When a client connects to that port, the server accepts the connection (see Fig. 3).
• Once the connection is established, the client and server can read from and write to the socket using input and output streams. Streams are ordered sequences of data that have a source (input stream), or destination (output stream). Once the client or server finishes using the socket, the socket structure is deallocated.