addresses An application can communicate with a remote process by exchanging data with TCP/IP by knowing the combination of protocol type, IP address, and port number. This combination is often known as a socket address. It is the network-facing access handle to the network socket. The remote process establishes a network socket in its own instance of the protocol stack, and uses the networking API to connect to the application, presenting its own socket address for use by the application. Implementation A protocol stack, usually provided by the operating system (rather than as a separate library, for instance), is a set of services that allow processes to communicate over a network using the protocols that the stack implements. The operating system forwards the payload of incoming IP packets to the corresponding application by extracting the socket address information from the IP and transport protocol headers and stripping the headers from the application data. 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. Socket often refers specifically to an internet socket or TCP socket. An internet socket is minimally characterized by the following: local socket address, consisting of the local IP address and (for TCP and UDP, but not IP) a port number protocol: