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

    This routine is useful for users who want to generate messages
    without using the ONC RPC package. The xdr_callmsg routine
    encodes the following fields: transaction ID, direction, RPC
    version, server program number, server version number, server
    procedure number, and client authentication.

    The pmap_rmtcall and svc_sendreply routines call xdr_callmsg.