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

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