Network socket

Software based endpoint of network communications

A network socket is a software structure within a network node of a computer network that serves as an endpoint for sending and receiving data across the network. The structure and properties of a socket are defined by an application programming interface (API) for the networking architecture. Sockets are created only during the lifetime of a process of an application running in the node. Because of the standardization of the TCP/IP protocols in the development of the Internet, the term network socket is most commonly used in the context of the Internet protocol suite, and is therefore often also referred to as Internet socket. In this context, a socket is externally identified to other hosts by its socket address, which is the triad of transport protocol, IP address, and port number. The term socket is also used for the software endpoint of node internal inter process communication (IPC), which often uses the same API as a network socket.

The use of the term socket in software is analogous to the function of an electrical female connector, a device in hardware for communication between nodes interconnected with an electrical cable. Similarly, the term port is used for external physical endpoints at a node or device. The application programming interface (API) for the network protocol stack creates a handle for each socket created by an application, commonly referred to as a socket descriptor. In Unix like operating systems, this descriptor is a type of file descriptor. It is stored by the application process for use with every read and write operation on the communication channel. At the time of creation with the API, a network socket is bound to the combination of a type of network protocol to be used for transmissions, a network address of the host, and a port number. Ports are numbered resources that represent another type of software structure of the node. They are used as service types, and, once created by a process, serve as an externally (from the network) addressable location component, so that other hosts may establish connections. Network sockets may be dedicated for persistent connections for communication between two nodes, or they may participate in connectionless and multicast communications. In practice, due to the proliferation of the TCP/IP protocols in use on the Internet, the term network socket usually refers to use with the Internet Protocol (IP). It is therefore often also called Internet socket.

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.

The distinctions between a socket (internal representation), socket descriptor (abstract identifier), and socket address (public address) are subtle, and these are not always distinguished in everyday usage. Further, specific definitions of a socket differ between authors. In IETF Request for Comments, Internet Standards, in many textbooks, as well as in this article, the term socket refers to an entity that is uniquely identified by the socket number. In other textbooks, the term socket refers to a local socket address, i. e. a "combination of an IP address and a port number". In the original definition of socket given in RFC 147, as it was related to the ARPA network in 1971, "the socket is specified as a 32 bit number with even sockets identifying receiving sockets and odd sockets identifying sending sockets." Today, however, socket communications are bidirectional. Within the operating system and the application that created a socket, a socket is referred to by a unique integer value called a socket descriptor.

The socket is primarily a concept used in the transport layer of the Internet protocol suite or session layer of the OSI model. Networking equipment such as routers, which operate at the internet layer, and switches, which operate at the link layer, do not require implementations of the transport layer. However, stateful network firewalls, network address translators, and proxy servers keep track of active socket pairs. In multilayer switches and quality of service (QoS) support in routers, packet flows may be identified by extracting information about the socket pairs. Raw sockets are typically available in network equipment and are used for routing protocols such as IGRP and OSPF, and for Internet Control Message Protocol (ICMP).