A network socket is an endpoint of a connection in a computer network. In Internet Protocol (IP) networks, these are often called Internet sockets. It is a handle (abstract reference) that a program can pass to the networking application programming interface (API) to use the connection for receiving and sending data. Sockets are often represented internally as integers.
A socket API is an application programming interface, usually provided by the operating system, that allows application programs to control and use network sockets. Internet socket APIs are usually based on the Berkeley sockets standard. In the Berkeley sockets standard, sockets are a form of file descriptor (a file handle), 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 mean the analogy is strained, and one instead uses different interfaces (send and receive) on a socket. In inter-process communication, each end generally has its own socket, but these may use different APIs: they are abstracted by the network protocol.
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 an address, for example, for only sending data, but if a program binds a socket to an 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 or thread.
An Internet socket is characterized by at least the following:
- local socket address, consisting of the local IP address and a port number
- protocol: A transport protocol, e.g., TCP, UDP, raw IP. This means that TCP port 53 and UDP port 53 are distinct sockets.
A socket that has been connected to another socket, e.g., during the establishment of a TCP connection, also has a remote socket address.
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 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.
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.
This example, modeled according to the Berkeley socket interface, sends the string "Hello, world!" via TCP to port 80 of the host with address 126.96.36.199. It illustrates the creation of a socket (getSocket), connecting it to the remote host, sending the string, and finally closing the socket:
Socket socket = getSocket(type = "TCP") connect(socket, address = "188.8.131.52", port = "80") send(socket, "Hello, world!") close(socket)
Several types of Internet socket are available:
- Datagram sockets, also known as connectionless sockets, which use User Datagram Protocol (UDP).
- Stream sockets, also known as connection-oriented sockets, which use Transmission Control Protocol (TCP), Stream Control Transmission Protocol (SCTP) or Datagram Congestion Control Protocol (DCCP).
- Raw sockets (or Raw IP sockets), typically available in routers and other network equipment. Here the transport layer is bypassed, and the packet headers are made accessible to the application.
Socket states in the client-server model
Computer processes that provide application services are referred to as servers, and create sockets on start up that are in listening state. These sockets are waiting for initiatives from client programs.
A TCP server may serve several clients concurrently, by creating a child process for each client and establishing a TCP connection between the child process and the client. Unique dedicated sockets are created for each connection. These are in established state when a socket-to-socket virtual connection or virtual circuit (VC), also known as a TCP session, is established with the remote socket, providing a duplex byte stream.
A server may create several concurrently established TCP sockets with the same local port number and local IP address, each mapped to its own server-child process, serving its own client process. They are treated as different sockets by the operating system, since the remote socket address (the client IP address and/or port number) are different; i.e. since they have different socket pair tuples.
A UDP socket cannot be in an established state, since UDP is connectionless. Therefore, netstat does not show the state of a UDP socket. A UDP server does not create new child processes for every concurrently served client, but the same process handles incoming data packets from all remote clients sequentially through the same socket. It implies that UDP sockets are not identified by the remote address, but only by the local address, although each message has an associated remote address.
Communicating local and remote sockets are called socket pairs. Each socket pair is described by a unique 4-tuple consisting of source and destination IP addresses and port numbers, i.e. of local and remote socket addresses. As seen in the discussion above, in the TCP case, each unique socket pair 4-tuple is assigned a socket number, while in the UDP case, each unique local socket address is assigned a socket number.
Sockets are usually implemented by an application programming interface (API) library. Most implementations are based on Berkeley sockets, for example Winsock introduced in 1991. Other API implementations exist, such as the STREAMS-based Transport Layer Interface (TLI).
Development of application programs that utilize this API is called socket programming or network programming.
In 1983, Berkeley sockets, also known as the BSD socket API, originated with the 4.2BSD Unix operating system (released in 1983) as an API. Only in 1989, however, could UC Berkeley release versions of its operating system and networking library free from the licensing constraints of AT&T's copyright-protected Unix.
Sockets in network equipment
The socket is primarily a concept used in the Transport Layer of the Internet model. Networking equipment such as routers and switches do not require implementations of the Transport Layer, as they operate on the Link Layer level (switches) or at the Internet Layer (routers). However, stateful network firewalls, network address translators, and proxy servers keep track of active socket pairs. Also in fair queuing, layer 3 switching 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 in Internet Control Message Protocol (ICMP).
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