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,
  • 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),
  • 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.
• 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.