What is a TCP/IP Socket Connection ?

A socket programming interface provides the routines required for interprocess communication between applications, either on the local system or spread in a distributed, TCP/IP based network environment. Once a peer-to-peer connection is established, a socket descriptor is used to uniquely identify the connection. The socket descriptor itself is a task specific numerical value.

One end of a peer-to-peer connection of a TCP/IP based distributed network application described by a socket is uniquely defined by
  • Internet address

    for example (in an IPv4 network) or FF01::101 (in an IPv6 network).

  • Communication protocol
    • User Datagram Protocol (UDP)
    • Transmission Control Protocol (TCP)
  • Port
    A numerical value, identifying an application. We distinguish between
    • "well known" ports, for example port 23 for Telnet
    • user defined ports

Socket applications were usually C or C++ applications using a variation of the socket API originally defined by the Berkeley Software Distribution (BSD). The JAVA language also provides a socket API. JAVA based Client/Server applications exploit those socket services.

Socket programming interfaces have been standardized for ease of portability by The Open Group for example.

Besides TCP/IP based sockets, UNIX systems provide socket interfaces for interprocess communication (IPC) within the local UNIX host itself. Those UNIX sockets use the local file system for interprocess communication.

z/VSE provides TCP/IP based socket services. They can be used for IPC too, although they are primarily aimed for network communication only.