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

 rqst
    A pointer to an svc_req structure with the requested program
    number, procedure number, version number, and credentials passed
    by the client.
 msg
    A pointer to an rpc_msg structure with members that make up the
    RPC message.