A Socket is software abstraction of a wall receptacle - something you plug a data process into. In the socket paradigm developed in the Berkeley Software Distribution of Unix, often called BSD Unix, a socket exist at two end points of a communication - one at your end and another at the other end that you want to communicate with. The receptacle box, wire and cabling behind the socket is a sophisticated collection of network library routines, data structures, communications processing code, network routers, network device drivers, operating systems code, and physical network devices and communications media. Most people never see a socket or even know that they are actually in place - mostly they see network applications that uses the socket. Network programmers touch and use sockets via Applications Programming Interface procedure calls from a library of network services procedures like socket(), connect(), send(), recv(), shutdown(), and close(). The casual user gets somewhat near a socket when presenting a URL to be used by a browser or network application like telnet.

What do sockets do
The question really applies to the so-called reliable packet or byte-stream transmission protocol like TCP itself. TCP belongs to the Internet Protocol (IP) protocol family. There is a companion record or packet oriented protocol called SSP in the Xerox Network Systems (XNS) protocol family. There are sockets for lower layer services which are inherently un-reliable or more well know as best-effort services like UDP (the User Datagram Protocol). The network program PING uses a simpler form of un-reliable sockets. A reliable socket provides the following services:

A by-directional communications stream.
Assured delivery and persistence and notification of delivery failure.
Preserved integrity of the data (no additions or subtractions) and order of transmission.
Transparency of communicated data - binary is as good as displayable data.
Automatic invocation of flow control.
Data stream isolation in the presence of other communications streams.
Support for client/server session services.
Sharing of physical network resources.

What do sockets NOT do

Encapsulate a "file" transfer.
Provide security services - encryption, authentication, confidentiality.

Why use sockets
Sockets provide their real value in that they place a layer of abstraction between an application need and the underlying physical services over which an application gets deployment of a need. This is in contrast to other kinds of communications services like serial async protocols often used when people connect with a BBS. A protocol like zmodem provides a version of assured delivery, works at doing transparency, but falls short in the other service areas. It is possible to embed async services in an application. This makes a copy of the logic and precludes reasonably sharing of well developed and near-universal code. Sockets really flex their muscle in the area of inter networking, from which the Internet takes its name. The network layer (IP) or (XNS) supports data multiplexing over a transmission media AND routing of session data between physically distinct network. The choice to use sockets or intervening services that embed the use of sockets is often one of religion. Appropriate design and use of socket level communications also provides speed and responsiveness to network enabled applications.


©2006 Ziost Technologies. All rights reserved. Terms of Use \| Privacy Statement \| Links