Douzragore The above code is sort of brej naive implementation that stores a float sockers a bit number. What are the two types? And they very commonly assume that the result from calls to getaddrinfo succeed and return a valid entry in the linked list. Just about everything on the network deals with client processes talking to server processes and vice-versa. The good news is this: This structure holds socket address information for many types of sockets:. The whole thing could be completely and utterly wrong!
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Use[ edit ] A process can refer to a socket using a socket descriptor, a type of handle. A process first requests that the protocol stack create a socket, and the stack returns a descriptor to the process so it can identify the socket.
The process then passes the descriptor back to the protocol stack when it wishes to send or receive data using this socket. Unlike ports , sockets are specific to one node; they are local resources and cannot be referred to directly by other nodes.
Further, sockets are not necessarily associated with a persistent connection channel for communication between two nodes, nor is there necessarily some single other endpoint. For example, a datagram socket can be used for connectionless communication , and a multicast socket can be used to send to multiple nodes.
However, in practice for internet communication, sockets are generally used to connect to a specific endpoint and often with a persistent connection. Socket addresses[ edit ] In practice, socket usually refers to a socket in an Internet Protocol IP network where a socket may be called an Internet socket , in particular for the Transmission Control Protocol TCP , which is a protocol for one-to-one connections.
In this context, sockets are assumed to be associated with a specific socket address, namely the IP address and a port number for the local node, and there is a corresponding socket address at the foreign node other node , which itself has an associated socket, used by the foreign process. Associating a socket with a socket address is called binding. Note that while a local process can communicate with a foreign process by sending or receiving data to or from a foreign socket address, it does not have access to the foreign socket itself, nor can it use the foreign socket descriptor, as these are both internal to the foreign node.
For example, in a connection between These are referred to locally by numerical socket descriptors, say at one side[A] and at the other[B]. A process on node The protocol stack will then forward data to and from node Assuming that the foreign process at node B already received a socket descriptor , communication can now take place between node A and B.
However, a process on node Implementation[ edit ] A protocol stack , today 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: A transport protocol, e.
A socket that has been connected to another socket, e. Definition[ edit ] 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.
Tools[ edit ] 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[ edit ] This example, modeled according to the Berkeley socket interface, sends the string "Hello, world!
Order and reliability are not guaranteed with datagram sockets, so multiple packets sent from one machine or process to another may arrive in any order or might not arrive at all. Special configuration may be required to send broadcasts on a datagram socket. A stream socket provides a sequenced and unique flow of error-free data without record boundaries, with well-defined mechanisms for creating and destroying connections and reporting errors. A stream socket transmits data reliably , in order, and with out-of-band capabilities.
Raw sockets Allow direct sending and receiving of IP packets without any protocol-specific transport layer formatting. With other types of sockets, the payload is automatically encapsulated according to the chosen transport layer protocol e.
TCP, UDP , and the socket user is unaware of the existence of protocol headers that are broadcast with the payload. When reading from a raw socket, the headers are usually included. When transmitting packets from a raw socket, the automatic addition of a header is optional. Most socket application programming interfaces APIs , for example those based on Berkeley sockets , support raw sockets. One use case for raw sockets is the implementation of new transport-layer protocols in user space.
Socket states in the client-server model[ edit ] 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.
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[ edit ] 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. Today most implementations of sockets are based on Berkeley sockets , for the internet, such as Winsock 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.
Beej's Guide to Network Programming