VMS Help TCPIP Services, Programming Interfaces, RPC XDR Routines, xdrrec_create, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The stream descriptor for xdrs initializes the maximum allowable
    size for a request recvsize and reply sendsize, the addresses of
    the routine to perform the read (readit) and write (writeit), and
    the TCP handle used for network I/O.