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

    This function returns the SDC associated with a socket. Normally,
    socket descriptors are used either as file descriptors or with
    one of the functions that takes an explicit socket descriptor as
    its argument. Sockets are implemented using TCP/IP socket device
    channels. This function returns the SDC used by a given socket
    descriptor so you can use the TCP/IP facilities directly by means
    of various I/O system services ($QIO).