In computing, the Windows Sockets API (WSA), which was later shortened to Winsock, is a technical specification that defines how Windows network software should access network services, especially TCP/IP.
It defines a standard interface between a Windows TCP/IP client application (such as an FTP client or a web browser) and the underlying TCP/IP protocol stack. The nomenclature is based on the Berkeley sockets API model used in BSD for communications between programs. Initially, all the participating developers resisted the shortening of the name to Winsock for a long time, since there was much confusion among users between the API and the DLL library file (winsock.dll) which only exposed the common WSA interfaces to applications above it. Users would commonly believe that only making sure the DLL file was present on a system would provide full TCP/IP protocol support.
The Windows Sockets API specification defines two interfaces: the API used by application developers, and the SPI, which provides a means for network software developers to add new protocol modules to the system. Each interface represents a contract. The API guarantees that a conforming application will function correctly with a conformant protocol implementation from any network software vendor. The SPI contract guarantees that a conforming protocol module may be added to Windows and will thereby be usable by an API-conformant application. Although these contracts were important when Windows Sockets was first released, as network environments required multi-protocol support (see above) they are now of only academic interest. Included in the Windows Sockets API version 2.0 are functions to use IPX/SPX, but no commercial application is known to exist which utilises this transport, since the protocol was all but obsolete already at the time WSA 2.0 shipped. Microsoft has shipped the TCP/IP protocol stack with all recent versions of Windows, and there are no significant independent alternatives. Nor has there been significant interest in implementing protocols other than the TCP/IP family (includes UDP and RTSP), IrDA, and Bluetooth.
