VMS Help TCPIP Services, Programming Interfaces, Sockets API, setsockopt(), Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This function manipulates options associated with a socket.
    Options can exist at multiple protocol levels. They are always
    present at the uppermost socket level.
    When manipulating socket options, specify the level at which the
    option resides and the name of the option. To manipulate options
    at the socket level, specify the value of level as SOL_SOCKET. To
    manipulate options at any other level, supply the protocol number
    of the appropriate protocol controlling the option. For example,
    to indicate that an option is to be interpreted by TCP, set the
    value for level argument to the protocol number (IPPROTO_TCP) of
    TCP. See the IN.H header file for the various IPPROTO values.