Network socket
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 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 often 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, which often uses the same API as a network socket.
Use
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 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 sockets 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 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 in the Internet, the term network socket usually refers to use with the Internet Protocol. It is therefore often also called Internet socket.
Socket 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, 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 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 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 a port number
- protocol: A transport protocol, e.g., TCP, UDP, raw IP. This means that endpoints with TCP port 53 and UDP port 53 are distinct sockets, while IP does not have ports.
- A socket that has been connected to another socket, e.g., during the establishment of a TCP connection, also has a remote socket address.
Definition
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.
Tools
On Unix-like operating systems and Microsoft Windows, the command-line tools netstat or ss are used to list established sockets and related information.Example
This example, modeled according to the Berkeley socket interface, sends the string "Hello, world!" via TCP to port 80 of the host with address 1.2.3.4. It illustrates the creation of a socket, connecting it to the remote host, sending the string, and finally closing the socket:Socket mysocket = getSocket
connect
send
close
Types
Several types of Internet socket are available:;Datagram sockets
;Stream sockets
;Raw sockets
Other socket types are implemented over other transport protocols, such as Systems Network Architecture and Unix domain sockets for internal inter-process communication.
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, 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 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.
Socket pairs
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.History
The term socket dates to RFC 147, where it was used in ARPANET. Today most implementations of sockets are based on Berkeley sockets, for the internet, such as Winsock. Other API implementations exist, such as the STREAMS-based Transport Layer Interface.In 1983, Berkeley sockets, also known as the BSD socket API, originated with the 4.2BSD Unix operating system 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.
In 1987, the TLI was the networking application programming interface provided by AT&T UNIX System V Release 3. and continued into Release 4.
Other early implementations were written for TOPS-20, MVS, VM, IBM-DOS.