Socket API

[TCP Client Server Sockets - Dartmouth University]

- Socket API

The application programming interface (API) that programs use to communicate with the protocol stack, using network sockets, is called a socket API. Development of application programs that utilize this API is called socket programming or network programming. Internet socket APIs are usually based on the Berkeley sockets standard. In the Berkeley sockets standard, sockets are a form of file descriptor, due to the Unix philosophy that "everything is a file", and the analogies between sockets and files. Both have functions to read, write, open, and close. In practice the differences strain the analogy, and different interfaces (send and receive) are used on a socket. In inter-process communication, each end generally has its own socket. 

In the standard Internet protocols TCP and UDP, a socket address is the combination of an IP address and a port number, much like one end of a telephone connection is the combination of a phone number and a particular extension. Sockets need not have a source address, for example, for only sending data, but if a program binds a socket to a source address, the socket can be used to receive data sent to that address. Based on this address, Internet sockets deliver incoming data packets to the appropriate application process.

- History and Implementations of Socket API

Berkeley sockets originated with the 4.2BSD Unix operating system, released in 1983, as a programming interface. Not until 1989, however, could the University of California, Berkeley release versions of the operating system and networking library free from the licensing constraints of AT&T Corporation's proprietary Unix. All modern operating systems implement a version of the Berkeley socket interface. It became the standard interface for applications running in the Internet. The BSD sockets API is written in the C programming language. Most other programming languages provide similar interfaces, typically written as a wrapper library based on the C API.

- Socket API Functions and Applications

A Unix Socket is used in a client-server application framework. A server is a process that performs some functions on request from a client. Most of the application-level protocols like FTP, SMTP, and POP3 make use of sockets to establish connection between client and server and then for exchanging data.

The right Flow diagram of client-server transaction using sockets with the Transmission Control Protocol (TCP).

The Berkeley socket API typically provides the following functions:

• socket() creates a new socket of a certain type, identified by an integer number, and allocates system resources to it.
• bind() is typically used on the server side, and associates a socket with a socket address structure, i.e. a specified local IP address and a port number.
• listen() is used on the server side, and causes a bound TCP socket to enter listening state.
• connect() is used on the client side, and assigns a free local port number to a socket. In case of a TCP socket, it causes an attempt to establish a new TCP connection.
• accept() is used on the server side. It accepts a received incoming attempt to create a new TCP connection from the remote client, and creates a new socket associated with the socket address pair of this connection.
• send(), recv(), sendto(), and recvfrom() are used for sending and receiving data. The standard functions write() and read() may also be used.
• close() causes the system to release resources allocated to a socket. In case of TCP, the connection is terminated.
• gethostbyname() and gethostbyaddr() are used to resolve host names and addresses. IPv4 only.
• select() is used to suspend, waiting for one or more of a provided list of sockets to be ready to read, ready to write, or that have errors.
• poll() is used to check on the state of a socket in a set of sockets. The set can be tested to see if any socket can be written to, read from or if an error occurred.
• getsockopt() is used to retrieve the current value of a particular socket option for the specified socket.
• setsockopt() is used to set a particular socket option for the specified socket.

[More to come ...]