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The Unix input/output (I/O) system follows a paradigm usually referred to as Open-Read-Write-Close. Before a user process can perform I/O operations, it calls Open to specify and obtain permissions for the file or device to be used. Once an object has been opened, the user process makes one or more calls to Read or Write data. Read reads data from the object and transfers it to the user process, while Write transfers data from the user process to the object. After all transfer operations are complete, the user process calls Close to inform the operating system that it has finished using that object.
When facilities for InterProcess Communication (IPC) and networking were added to Unix, the idea was to make the interface to IPC similar to that of file I/O. In Unix, a process has a set of I/O descriptors that one reads from and writes to. These descriptors may refer to files, devices, or communication channels (sockets). The lifetime of a descriptor is made up of three phases: creation (open socket), reading and writing (receive and send to socket), and destruction (close socket).
The IPC interface in BSD-like versions of Unix is implemented as a layer over the network TCP and UDP protocols. Message destinations are specified as socket addresses; each socket address is a communication identifier that consists of a port number and an Internet address.
The IPC operations are based on socket pairs, one belonging to a communication process. IPC is done by exchanging some data through transmitting that data in a message between a socket in one process and another socket in another process. When messages are sent, the messages are queued at the sending socket until the underlying network protocol has transmitted them. When they arrive, the messages are queued at the receiving socket until the receiving process makes the necessary calls to receive them.
There are two communication protocols that one can use for socket programming: datagram communication and stream communication.
The datagram communication protocol, known as UDP (user datagram protocol), is a connectionless protocol, meaning that each time you send datagrams, you also need to send the local socket descriptor and the receiving socket's address. As you can tell, additional data must be sent each time a communication is made.
The stream communication protocol is known as TCP (transfer control protocol). Unlike UDP, TCP is a connection-oriented protocol. In order to do communication over the TCP protocol, a connection must first be established between the pair of sockets. While one of the sockets listens for a connection request (server), the other asks for a connection (client). Once two sockets have been connected, they can be used to transmit data in both (or either one of the) directions.