What Is a Socket?
Jun 27, · A network socket is the endpoint of communications between processes on a computer network such as the Internet. It is uniquely identified by an IP address, a port number, and a communications protocol. For instance, a TCP protocol connection may be made to port on a computer located at IP address , which would be called "TCP socket " on that computer. Input from a network socket; OutputStream objects represent data sinks, places we can write data to. For example: FileOutputStream for saving to files; thismestory.com is a PrintStream, an OutputStream that prints readable representations of various types; Output to a network socket.
Network communication is inherently concurrent, so building clients and servers will require us to reason about their concurrent behavior and to implement them with thread safety. We must also design the wire protocol that clients and servers use to communicate, just as we design the operations that clients of an ADT use to work with it.
Some of the operations with sockets are blocking : they block the progress of a thread until they can return a result. In this pattern there are two kinds of processes: clients and servers.
A client initiates the communication by connecting to a server. The client sends requests to the server, and the server sends replies back.
Finally, the client disconnects. A server might handle connections from many clients concurrently, and clients might also connect to multiple servers. Many Internet applications work this way: web browsers are clients for web servers, an email how to connect to mysql remotely like Outlook is a client for a mail server, etc. A network interface is identified by an IP address.
IPv4 addresses are bit numbers written in four 8-bit parts. For example as of this writing :. Every address whose first octet is 18 is on the MIT network.
Technically, any address whose first octet is is a loopback address, but You can ask Google for your current IP address. In general, as you carry around your laptop, how to become a pi in canada time you connect your machine to the network it will be assigned a new IP address. Hostnames are names that can be translated into IP addresses. A single hostname can map to different IP addresses at different times; and multiple hostnames can map to the same IP address.
For example:. You can translate this name to an IP address yourself using dighostor nslookup on the command line, e. Try using one of the commands above to find google. What do you see? When you want to talk to a server running on your own machine, talk to localhost. A single machine might have multiple server applications that clients wish to connect to, so we need a way to direct traffic on the same network interface to different processes.
Network interfaces have multiple ports identified by a bit number from 0 which is reserved, so we effectively start at 1 to A server process binds to a particular port — it is now listening on that port. Clients have to know what are the four components of a network socket port number the server is listening on. There are some well-known ports which are reserved for system-level processes and provide standard ports for certain services.
When the port is not a standard port, it is specified as part of the address. A socket represents one end of the connection between client and server. A listening socket is used by a server process to wait for connections what are the four components of a network socket remote clients.
In Java, use ServerSocket to make a listening socket, and use its accept method to listen to it. A connected socket can send and receive messages to and from the process on the other end of the connection.
It is identified by both the local IP address and port number plus the remote address and port, which allows a server to differentiate between concurrent connections from different IPs, or from the same IP on different remote ports.
In Java, clients use a Socket constructor to establish a socket connection to a server. Servers obtain a connected socket as a Socket object returned from ServerSocket. The data that clients and servers exchange over the network is sent in chunks. These are rarely just byte-sized chunks, although they might be. The network chops that chunk up into packets, and each packet is routed how much to cats sleep over the network.
At the other end, the receiver reassembles the packets together into a stream of bytes. The result is a bursty kind of data transmission — the data may already be there when you want to read them, or you may have to wait for them to arrive and be reassembled. When data arrive, they go into a bufferan array in memory that holds the data until you read it. You start the server running on port Seuss Did Technical Writing? The data going into or coming out of a socket is a stream of bytes.
In Java, InputStream objects represent sources of data flowing into your program. OutputStream objects represent data sinks, places we can write data to. With sockets, remember that the output of one process is the input of another process. Blocking means that a thread waits without doing further work until an event occurs. We can use this term to describe methods and method calls: if a method is a blocking methodthen a call to that method can blockwaiting until some event occurs before it returns to the caller.
If data or for writespace is already available in the buffer, the call might return very quickly. The operating system takes care of the details of delaying that thread until read or write can succeed. This statement has the form:. Notice how what is the best canned tuna brand ServerSocket.
While the client-specific thread is working with that client perhaps blocked in a read or a writeanother thread perhaps the main thread is blocked waiting to accept a new connection. Unfortunately, their multithreaded Knock Knock Server implementation creates that new thread by subclassing Thread. Which of these is it necessary for a client to know in order to connect to and communicate with a server?
In the EchoClient example, which of these might block? And in EchoServerwhich of these might block? Since BufferedReader. Now that we have our client and server connected up with sockets, what do they pass back and forth over those sockets?
A protocol is a set of messages that can be exchanged by two communicating parties. You can use a program called Telnet to check them out. You'll be using Telnet on the problem set, so try these out now. The GET command gets a web page. So for many web sites, you might need to speak HTTP 1. This time, your request must end with a blank line. HTTP version 1. You will also more than likely what are the four components of a network socket that telnet does not exit after making this request — this time, the server keeps the connection open so you can make another request right away.
Simple Mail Transfer Protocol SMTP is the protocol for sending email different protocols are used for client programs that retrieve email from your inbox. Because the email system was designed in how to take a loan out on your 401k time before spam, modern email communication is fraught with traps and heuristics designed to prevent abuse. But we can still try to speak SMTP.
Recall that the well-known SMTP port is 25, and dmz-mailsec-scanner When designing a wire protocol, apply the same rules of thumb you use for designing the operations of an abstract data type:. Keep the number of different messages small.
The set of messages must be adequate for clients to make the requests they need what are the four components of a network socket make and for servers to deliver the results. Just as we demand representation independence from our types, we should aim for platform-independence in our protocols. HTTP can be spoken by any web server and any web browser on any operating system. The protocol should be easy for clients and servers to generate and parse.
Simpler code for reading and writing the protocol whether written with a parser generator like ANTLR, with regular expressions, what are the four components of a network socket. Consider the ways a broken or malicious client or server could stuff garbage data into the protocol to break the process on the other end.
Security vulnerabilities are a more serious example. For example, protocols that allow a client to send requests with arbitrary amounts of data require careful handling on the server to avoid running out of buffer space, or worse.
Ready for change : for example, HTTP includes the ability to specify a version number, so clients and servers can agree with one another which version of the protocol they will use. If we need to make changes to the protocol in the future, older clients or servers can continue to work by announcing the version they will use.
Serialization is the process of transforming data structures in memory into a format that can be easily stored or transmitted. Rather than invent a new format for serializing your data between clients and servers, use an existing one.
For example, here is a very small part of the HTTP 1. The grammar is not enough: it fills a similar role to method signatures when defining an ADT. We still need the specifications:. What are the preconditions of a message? For example, if a particular field in a what can i do for under eye puffiness is a string of digits, is any number valid? Or must it be the ID number of a record known to the server?
Under what circumstances can a message be sent? Are certain messages only valid when sent in a certain sequence? What are the postconditions? What action will the server take based on a message?
What server-side data will be mutated? What reply will the server send back to the client?
As you can see, there are four components comprising the address: , , , and 6. Each component ranges from 0 through (inclusive) and accounts for 8 bits of the address. May 09, · A socket connecting to the network is created at each end of the communication. Each socket has a specific address. This address is composed of an IP address and a port number. Socket are generally employed in client server applications. The server creates a socket, attaches it to a network port addresses then waits for the client to contact it. May 24, · This is a unique number comprised of four 8-bit numbers separated by dots. This number identifies your computer when it’s connected to a network using the .
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. 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.
It is therefore often also called Internet socket. 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. 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:. 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. In the original definition of socket given in RFC , as it was related to the ARPA network in , "the socket is specified as a 32 bit number with even sockets identifying receiving sockets and odd sockets identifying sending sockets.
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. On Unix-like operating systems and Microsoft Windows , the command-line tools netstat or ss  are used to list established sockets and related information.
This example, modeled according to the Berkeley socket interface, sends the string "Hello, world! It illustrates the creation of a socket getSocket , connecting it to the remote host, sending the string, and finally closing the socket:. Other socket types are implemented over other transport protocols, such as Systems Network Architecture  and Unix domain sockets for internal inter-process communication. Computer processes that provide application services are referred to as servers , and create sockets on startup that are in the listening state.
These sockets are waiting for initiatives from client programs. A TCP server may serve several clients concurrently by creating a unique dedicated socket for each client connection in a new child process or processing thread for each client. These are in the 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 or port number is different; i. UDP sockets do not have a established state , because the protocol is connectionless.
A UDP server process handles incoming datagrams from all remote clients sequentially through the same socket. UDP sockets are not identified by the remote address, but only by the local address, although each message has an associated remote address that can be retrieved from each datagram with the networking application programming interface API. 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.
Most modern implementations of sockets are based on Berkeley sockets , and other stacks such as Winsock The socket is primarily a concept used in the Transport layer of the Internet model 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. From Wikipedia, the free encyclopedia. Endpoint of network communications. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
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