Network Working Group                                      R. Srinivasan
Request for Comments: 1831                              Sun Microsystems
Category: Standards Track                                    August 1995

RPC: Remote Procedure Call Protocol Specification Version 2

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

ABSTRACT

   This document describes the ONC Remote Procedure Call (ONC RPC
   Version 2) protocol as it is currently deployed and accepted.  "ONC"
   stands for "Open Network Computing".

TABLE OF CONTENTS

      1. INTRODUCTION                                              2
      2. TERMINOLOGY                                               2
      3. THE RPC MODEL                                             2
      4. TRANSPORTS AND SEMANTICS                                  4
      5. BINDING AND RENDEZVOUS INDEPENDENCE                       5
      6. AUTHENTICATION                                            5
      7. RPC PROTOCOL REQUIREMENTS                                 5
      7.1 RPC Programs and Procedures                              6
      7.2 Authentication                                           7
      7.3 Program Number Assignment                                8
      7.4 Other Uses of the RPC Protocol                           8
      7.4.1 Batching                                               8
      7.4.2 Broadcast Remote Procedure Calls                       8
      8. THE RPC MESSAGE PROTOCOL                                  9
      9. AUTHENTICATION PROTOCOLS                                 12
      9.1 Null Authentication                                     13
      10. RECORD MARKING STANDARD                                 13
      11. THE RPC LANGUAGE                                        13
      11.1 An Example Service Described in the RPC Language       13
      11.2 The RPC Language Specification                         14
      11.3 Syntax Notes                                           15
      APPENDIX A: SYSTEM AUTHENTICATION                           16
      REFERENCES                                                  17
      Security Considerations                                     18
      Author's Address                                            18

1. INTRODUCTION

   This document specifies version two of the message protocol used in
   ONC Remote Procedure Call (RPC).  The message protocol is specified
   with the eXternal Data Representation (XDR) language [9].  This
   document assumes that the reader is familiar with XDR.  It does not
   attempt to justify remote procedure calls systems or describe their
   use.  The paper by Birrell and Nelson [1] is recommended as an
   excellent background for the remote procedure call concept.

2. TERMINOLOGY

   This document discusses clients, calls, servers, replies, services,
   programs, procedures, and versions.  Each remote procedure call has
   two sides: an active client side that makes the call to a server,
   which sends back a reply.  A network service is a collection of one
   or more remote programs.  A remote program implements one or more
   remote procedures; the procedures, their parameters, and results are
   documented in the specific program's protocol specification.  A
   server may support more than one version of a remote program in order
   to be compatible with changing protocols.

   For example, a network file service may be composed of two programs.
   One program may deal with high-level applications such as file system
   access control and locking.  The other may deal with low-level file
   input and output and have procedures like "read" and "write".  A
   client of the network file service would call the procedures
   associated with the two programs of the service on behalf of the
   client.

   The terms client and server only apply to a particular transaction; a
   particular hardware entity (host) or software entity (process or
   program) could operate in both roles at different times.  For
   example, a program that supplies remote execution service could also
   be a client of a network file service.

3. THE RPC MODEL

   The ONC RPC protocol is based on the remote procedure call model,
   which is similar to the local procedure call model.  In the local
   case, the caller places arguments to a procedure in some well-
   specified location (such as a register window).  It then transfers
   control to the procedure, and eventually regains control.  At that
   point, the results of the procedure are extracted from the well-
   specified location, and the caller continues execution.

   The remote procedure call model is similar.  One thread of control
   logically winds through two processes: the caller's process, and a
   server's process.  The caller process first sends a call message to
   the server process and waits (blocks) for a reply message.  The call
   message includes the procedure's parameters, and the reply message
   includes the procedure's results.  Once the reply message is
   received, the results of the procedure are extracted, and caller's
   execution is resumed.

   On the server side, a process is dormant awaiting the arrival of a
   call message.  When one arrives, the server process extracts the
   procedure's parameters, computes the results, sends a reply message,
   and then awaits the next call message.

   In this model, only one of the two processes is active at any given
   time.  However, this model is only given as an example.  The ONC RPC
   protocol makes no restrictions on the concurrency model implemented,
   and others are possible.  For example, an implementation may choose
   to have RPC calls be asynchronous, so that the client may do useful
   work while waiting for the reply from the server.  Another
   possibility is to have the server create a separate task to process
   an incoming call, so that the original server can be free to receive
   other requests.

   There are a few important ways in which remote procedure calls differ
   from local procedure calls:

      1. Error handling: failures of the remote server or network must
      be handled when using remote procedure calls.

      2. Global variables and side-effects: since the server does not
      have access to the client's address space, hidden arguments cannot
      be passed as global variables or returned as side effects.

      3. Performance:  remote procedures usually operate one or more
      orders of magnitude slower than local procedure calls.

      4. Authentication: since remote procedure calls can be transported
      over unsecured networks, authentication may be necessary.
      Authentication prevents one entity from masquerading as some other
      entity.

   The conclusion is that even though there are tools to automatically
   generate client and server libraries for a given service, protocols
   must still be designed carefully.

4. TRANSPORTS AND SEMANTICS

   The RPC protocol can be implemented on several different transport
   protocols.  The RPC protocol does not care how a message is passed
   from one process to another, but only with specification and
   interpretation of messages.  However, the application may wish to
   obtain information about (and perhaps control over) the transport
   layer through an interface not specified in this document.  For
   example, the transport protocol may impose a restriction on the
   maximum size of RPC messages, or it may be stream-oriented like TCP
   with no size limit.  The client and server must agree on their
   transport protocol choices.

   It is important to point out that RPC does not try to implement any
   kind of reliability and that the application may need to be aware of
   the type of transport protocol underneath RPC.  If it knows it is
   running on top of a reliable transport such as TCP [6], then most of
   the work is already done for it.  On the other hand, if it is running
   on top of an unreliable transport such as UDP [7], it must implement
   its own time-out, retransmission, and duplicate detection policies as
   the RPC protocol does not provide these services.

   Because of transport independence, the RPC protocol does not attach
   specific semantics to the remote procedures or their execution
   requirements.  Semantics can be inferred from (but should be
   explicitly specified by) the underlying transport protocol.  For
   example, consider RPC running on top of an unreliable transport such
   as UDP.  If an application retransmits RPC call messages after time-
   outs, and does not receive a reply, it cannot infer anything about
   the number of times the procedure was executed.  If it does receive a
   reply, then it can infer that the procedure was executed at least
   once.

   A server may wish to remember previously granted requests from a
   client and not regrant them in order to insure some degree of
   execute-at-most-once semantics.  A server can do this by taking
   advantage of the transaction ID that is packaged with every RPC
   message.  The main use of this transaction ID is by the client RPC
   entity in matching replies to calls.  However, a client application
   may choose to reuse its previous transaction ID when retransmitting a
   call.  The server may choose to remember this ID after executing a
   call and not execute calls with the same ID in order to achieve some
   degree of execute-at-most-once semantics.  The server is not allowed
   to examine this ID in any other way except as a test for equality.

   On the other hand, if using a "reliable" transport such as TCP, the
   application can infer from a reply message that the procedure was
   executed exactly once, but if it receives no reply message, it cannot

   assume that the remote procedure was not executed.  Note that even if
   a connection-oriented protocol like TCP is used, an application still
   needs time-outs and reconnection to handle server crashes.

   There are other possibilities for transports besides datagram- or
   connection-oriented protocols.  For example, a request-reply protocol
   such as VMTP [2] is perhaps a natural transport for RPC.  ONC RPC
   uses both TCP and UDP transport protocols.  Section 10 (RECORD
   MARKING STANDARD) describes the mechanism employed by ONC RPC to
   utilize a connection-oriented, stream-oriented transport such as TCP.

5. BINDING AND RENDEZVOUS INDEPENDENCE

   The act of binding a particular client to a particular service and
   transport parameters is NOT part of this RPC protocol specification.
   This important and necessary function is left up to some higher-level
   software.

   Implementors could think of the RPC protocol as the jump-subroutine
   instruction ("JSR") of a network; the loader (binder) makes JSR
   useful, and the loader itself uses JSR to accomplish its task.
   Likewise, the binding software makes RPC useful, possibly using RPC
   to accomplish this task.

6. AUTHENTICATION

   The RPC protocol provides the fields necessary for a client to
   identify itself to a service, and vice-versa, in each call and reply
   message.  Security and access control mechanisms can be built on top
   of this message authentication.  Several different authentication
   protocols can be supported.  A field in the RPC header indicates
   which protocol is being used. More information on specific
   authentication protocols is in section 9: "Authentication Protocols".

7. RPC PROTOCOL REQUIREMENTS

   The RPC protocol must provide for the following:

      (1) Unique specification of a procedure to be called.
      (2) Provisions for matching response messages to request messages.
      (3) Provisions for authenticating the caller to service and
          vice-versa.

   Besides these requirements, features that detect the following are
   worth supporting because of protocol roll-over errors, implementation
   bugs, user error, and network administration:

      (1) RPC protocol mismatches.
      (2) Remote program protocol version mismatches.
      (3) Protocol errors (such as misspecification of a procedure's
          parameters).
      (4) Reasons why remote authentication failed.
      (5) Any other reasons why the desired procedure was not called.

7.1 RPC Programs and Procedures

   The RPC call message has three unsigned integer fields -- remote
   program number, remote program version number, and remote procedure
   number -- which uniquely identify the procedure to be called.
   Program numbers are administered by a central authority
   (rpc@sun.com).  Once implementors have a program number, they can
   implement their remote program; the first implementation would most
   likely have the version number 1.  Because most new protocols evolve,
   a version field of the call message identifies which version of the
   protocol the caller is using.  Version numbers enable support of both
   old and new protocols through the same server process.

   The procedure number identifies the procedure to be called.  These
   numbers are documented in the specific program's protocol
   specification.  For example, a file service's protocol specification
   may state that its procedure number 5 is "read" and procedure number
   12 is "write".

   Just as remote program protocols may change over several versions,
   the actual RPC message protocol could also change.  Therefore, the
   call message also has in it the RPC version number, which is always
   equal to two for the version of RPC described here.

   The reply message to a request message has enough information to
   distinguish the following error conditions:

      (1) The remote implementation of RPC does not support protocol
      version 2.  The lowest and highest supported RPC version numbers
      are returned.

      (2) The remote program is not available on the remote system.

      (3) The remote program does not support the requested version
      number.  The lowest and highest supported remote program version
      numbers are returned.

      (4) The requested procedure number does not exist.  (This is
      usually a client side protocol or programming error.)

      (5) The parameters to the remote procedure appear to be garbage
      from the server's point of view.  (Again, this is usually caused
      by a disagreement about the protocol between client and service.)

7.2 Authentication

   Provisions for authentication of caller to service and vice-versa are
   provided as a part of the RPC protocol.  The call message has two
   authentication fields, the credential and verifier.  The reply
   message has one authentication field, the response verifier.  The RPC
   protocol specification defines all three fields to be the following
   opaque type (in the eXternal Data Representation (XDR) language [9]):

      enum auth_flavor {
         AUTH_NONE       = 0,
         AUTH_SYS        = 1,
         AUTH_SHORT      = 2
         /* and more to be defined */
      };

      struct opaque_auth {
         auth_flavor flavor;
         opaque body<400>;
      };

   In other words, any "opaque_auth" structure is an "auth_flavor"
   enumeration followed by up to 400 bytes which are opaque to
   (uninterpreted by) the RPC protocol implementation.

   The interpretation and semantics of the data contained within the
   authentication fields is specified by individual, independent
   authentication protocol specifications.  (Section 9 defines the
   various authentication protocols.)

   If authentication parameters were rejected, the reply message
   contains information stating why they were rejected.

7.3 Program Number Assignment

   Program numbers are given out in groups of hexadecimal 20000000
   (decimal 536870912) according to the following chart:

              0 - 1fffffff   defined by rpc@sun.com
       20000000 - 3fffffff   defined by user
       40000000 - 5fffffff   transient
       60000000 - 7fffffff   reserved
       80000000 - 9fffffff   reserved
       a0000000 - bfffffff   reserved
       c0000000 - dfffffff   reserved
       e0000000 - ffffffff   reserved

   The first group is a range of numbers administered by rpc@sun.com and
   should be identical for all sites.  The second range is for
   applications peculiar to a particular site.  This range is intended
   primarily for debugging new programs.  When a site develops an
   application that might be of general interest, that application
   should be given an assigned number in the first range.  Application
   developers may apply for blocks of RPC program numbers in the first
   range by sending electronic mail to "rpc@sun.com".  The third group
   is for applications that generate program numbers dynamically.  The
   final groups are reserved for future use, and should not be used.

7.4 Other Uses of the RPC Protocol

   The intended use of this protocol is for calling remote procedures.
   Normally, each call message is matched with a reply message.
   However, the protocol itself is a message-passing protocol with which
   other (non-procedure call) protocols can be implemented.

7.4.1 Batching

   Batching is useful when a client wishes to send an arbitrarily large
   sequence of call messages to a server.  Batching typically uses
   reliable byte stream protocols (like TCP) for its transport.  In the
   case of batching, the client never waits for a reply from the server,
   and the server does not send replies to batch calls.  A sequence of
   batch calls is usually terminated by a legitimate remote procedure
   call operation in order to flush the pipeline and get positive
   acknowledgement.

7.4.2 Broadcast Remote Procedure Calls

   In broadcast protocols, the client sends a broadcast call to the
   network and waits for numerous replies.  This requires the use of
   packet-based protocols (like UDP) as its transport protocol.  Servers

   that support broadcast protocols usually respond only when the call
   is successfully processed and are silent in the face of errors, but
   this varies with the application.

   The principles of broadcast RPC also apply to multicasting - an RPC
   request can be sent to a multicast address.

8. THE RPC MESSAGE PROTOCOL

   This section defines the RPC message protocol in the XDR data
   description language [9].

      enum msg_type {
         CALL  = 0,
         REPLY = 1
      };

   A reply to a call message can take on two forms: The message was
   either accepted or rejected.

      enum reply_stat {
         MSG_ACCEPTED = 0,
         MSG_DENIED   = 1
      };

   Given that a call message was accepted, the following is the status
   of an attempt to call a remote procedure.

      enum accept_stat {
         SUCCESS       = 0, /* RPC executed successfully             */
         PROG_UNAVAIL  = 1, /* remote hasn't exported program        */
         PROG_MISMATCH = 2, /* remote can't support version #        */
         PROC_UNAVAIL  = 3, /* program can't support procedure       */
         GARBAGE_ARGS  = 4, /* procedure can't decode params         */
         SYSTEM_ERR    = 5  /* errors like memory allocation failure */
      };

   Reasons why a call message was rejected:

      enum reject_stat {
         RPC_MISMATCH = 0, /* RPC version number != 2          */
         AUTH_ERROR = 1    /* remote can't authenticate caller */
      };

   Why authentication failed:

      enum auth_stat {
         AUTH_OK           = 0,  /* success                          */

         /*
          * failed at remote end
          */
         AUTH_BADCRED      = 1,  /* bad credential (seal broken)     */
         AUTH_REJECTEDCRED = 2,  /* client must begin new session    */
         AUTH_BADVERF      = 3,  /* bad verifier (seal broken)       */
         AUTH_REJECTEDVERF = 4,  /* verifier expired or replayed     */
         AUTH_TOOWEAK      = 5,  /* rejected for security reasons    */
         /*
          * failed locally
          */
         AUTH_INVALIDRESP  = 6,  /* bogus response verifier          */
         AUTH_FAILED       = 7   /* reason unknown                   */
      };

   The RPC message:

   All messages start with a transaction identifier, xid, followed by a
   two-armed discriminated union.  The union's discriminant is a
   msg_type which switches to one of the two types of the message.  The
   xid of a REPLY message always matches that of the initiating CALL
   message.  NB: The xid field is only used for clients matching reply
   messages with call messages or for servers detecting retransmissions;
   the service side cannot treat this id as any type of sequence number.

      struct rpc_msg {
         unsigned int xid;
         union switch (msg_type mtype) {
         case CALL:
            call_body cbody;
         case REPLY:
            reply_body rbody;
         } body;
      };

   Body of an RPC call:

   In version 2 of the RPC protocol specification, rpcvers must be equal
   to 2.  The fields prog, vers, and proc specify the remote program,
   its version number, and the procedure within the remote program to be
   called.  After these fields are two authentication parameters:  cred
   (authentication credential) and verf (authentication verifier).  The
   two authentication parameters are followed by the parameters to the
   remote procedure, which are specified by the specific program
   protocol.

   The purpose of the authentication verifier is to validate the
   authentication credential.  Note that these two items are

   historically separate, but are always used together as one logical
   entity.

      struct call_body {
         unsigned int rpcvers;       /* must be equal to two (2) */
         unsigned int prog;
         unsigned int vers;
         unsigned int proc;
         opaque_auth  cred;
         opaque_auth  verf;
         /* procedure specific parameters start here */
      };

   Body of a reply to an RPC call:

      union reply_body switch (reply_stat stat) {
      case MSG_ACCEPTED:
         accepted_reply areply;
      case MSG_DENIED:
         rejected_reply rreply;
      } reply;

   Reply to an RPC call that was accepted by the server:

   There could be an error even though the call was accepted.  The first
   field is an authentication verifier that the server generates in
   order to validate itself to the client.  It is followed by a union
   whose discriminant is an enum accept_stat.  The SUCCESS arm of the
   union is protocol specific.  The PROG_UNAVAIL, PROC_UNAVAIL,
   GARBAGE_ARGS, and SYSTEM_ERR arms of the union are void.  The
   PROG_MISMATCH arm specifies the lowest and highest version numbers of
   the remote program supported by the server.

      struct accepted_reply {
         opaque_auth verf;
         union switch (accept_stat stat) {
         case SUCCESS:
            opaque results[0];
            /*
             * procedure-specific results start here
             */
          case PROG_MISMATCH:
             struct {
                unsigned int low;
                unsigned int high;
             } mismatch_info;
          default:
             /*

              * Void.  Cases include PROG_UNAVAIL, PROC_UNAVAIL,
              * GARBAGE_ARGS, and SYSTEM_ERR.
              */
             void;
          } reply_data;
      };

   Reply to an RPC call that was rejected by the server:

   The call can be rejected for two reasons: either the server is not
   running a compatible version of the RPC protocol (RPC_MISMATCH), or
   the server rejects the identity of the caller (AUTH_ERROR). In case
   of an RPC version mismatch, the server returns the lowest and highest
   supported RPC version numbers.  In case of invalid authentication,
   failure status is returned.

      union rejected_reply switch (reject_stat stat) {
      case RPC_MISMATCH:
         struct {
            unsigned int low;
            unsigned int high;
         } mismatch_info;
      case AUTH_ERROR:
         auth_stat stat;
      };

9. AUTHENTICATION PROTOCOLS

   As previously stated, authentication parameters are opaque, but
   open-ended to the rest of the RPC protocol.  This section defines two
   standard "flavors" of authentication.  Implementors are free to
   invent new authentication types, with the same rules of flavor number
   assignment as there is for program number assignment.  The "flavor"
   of a credential or verifier refers to the value of the "flavor" field
   in the opaque_auth structure. Flavor numbers, like RPC program
   numbers, are also administered centrally, and developers may assign
   new flavor numbers by applying through electronic mail to
   "rpc@sun.com".  Credentials and verifiers are represented as variable
   length opaque data (the "body" field in the opaque_auth structure).

   In this document, two flavors of authentication are described.  Of
   these, Null authentication (described in the next subsection) is
   mandatory - it must be available in all implementations.  System
   authentication is described in Appendix A.  It is strongly
   recommended that implementors include System authentication in their
   implementations.  Many applications use this style of authentication,
   and availability of this flavor in an implementation will enhance
   interoperability.

9.1 Null Authentication

   Often calls must be made where the client does not care about its
   identity or the server does not care who the client is.  In this
   case, the flavor of the RPC message's credential, verifier, and reply
   verifier is "AUTH_NONE".  Opaque data associated with "AUTH_NONE" is
   undefined.  It is recommended that the length of the opaque data be
   zero.

10. RECORD MARKING STANDARD

   When RPC messages are passed on top of a byte stream transport
   protocol (like TCP), it is necessary to delimit one message from
   another in order to detect and possibly recover from protocol errors.
   This is called record marking (RM).  One RPC message fits into one RM
   record.

   A record is composed of one or more record fragments.  A record
   fragment is a four-byte header followed by 0 to (2**31) - 1 bytes of
   fragment data.  The bytes encode an unsigned binary number; as with
   XDR integers, the byte order is from highest to lowest.  The number
   encodes two values -- a boolean which indicates whether the fragment
   is the last fragment of the record (bit value 1 implies the fragment
   is the last fragment) and a 31-bit unsigned binary value which is the
   length in bytes of the fragment's data.  The boolean value is the
   highest-order bit of the header; the length is the 31 low-order bits.
   (Note that this record specification is NOT in XDR standard form!)

11. THE RPC LANGUAGE

   Just as there was a need to describe the XDR data-types in a formal
   language, there is also need to describe the procedures that operate
   on these XDR data-types in a formal language as well.  The RPC
   Language is an extension to the XDR language, with the addition of
   "program", "procedure", and "version" declarations.  The following
   example is used to describe the essence of the language.

11.1 An Example Service Described in the RPC Language

   Here is an example of the specification of a simple ping program.

   program PING_PROG {
         /*
          * Latest and greatest version
          */
         version PING_VERS_PINGBACK {
            void
            PINGPROC_NULL(void) = 0;

            /*
             * Ping the client, return the round-trip time
             * (in microseconds). Returns -1 if the operation
             * timed out.
             */
            int
            PINGPROC_PINGBACK(void) = 1;
         } = 2;

         /*
          * Original version
          */
         version PING_VERS_ORIG {
            void
            PINGPROC_NULL(void) = 0;
         } = 1;
      } = 1;

      const PING_VERS = 2;      /* latest version */

   The first version described is PING_VERS_PINGBACK with two
   procedures, PINGPROC_NULL and PINGPROC_PINGBACK.  PINGPROC_NULL takes
   no arguments and returns no results, but it is useful for computing
   round-trip times from the client to the server and back again.  By
   convention, procedure 0 of any RPC protocol should have the same
   semantics, and never require any kind of authentication.  The second
   procedure is used for the client to have the server do a reverse ping
   operation back to the client, and it returns the amount of time (in
   microseconds) that the operation used.  The next version,
   PING_VERS_ORIG, is the original version of the protocol and it does
   not contain PINGPROC_PINGBACK procedure. It is useful for
   compatibility with old client programs, and as this program matures
   it may be dropped from the protocol entirely.

11.2 The RPC Language Specification

   The RPC language is identical to the XDR language defined in RFC
   1014, except for the added definition of a "program-def" described
   below.

   program-def:
      "program" identifier "{"
         version-def
         version-def *
      "}" "=" constant ";"

   version-def:
      "version" identifier "{"

          procedure-def
          procedure-def *
      "}" "=" constant ";"

   procedure-def:
      type-specifier identifier "(" type-specifier
        ("," type-specifier )* ")" "=" constant ";"

11.3 Syntax Notes

   (1) The following keywords are added and cannot be used as
   identifiers: "program" and "version";

   (2) A version name cannot occur more than once within the scope of a
   program definition. Nor can a version number occur more than once
   within the scope of a program definition.

   (3) A procedure name cannot occur more than once within the scope of
   a version definition. Nor can a procedure number occur more than once
   within the scope of version definition.

   (4) Program identifiers are in the same name space as constant and
   type identifiers.

   (5) Only unsigned constants can be assigned to programs, versions and
   procedures.

APPENDIX A: SYSTEM AUTHENTICATION

   The client may wish to identify itself, for example, as it is
   identified on a UNIX(tm) system.  The flavor of the client credential
   is "AUTH_SYS".  The opaque data constituting the credential encodes
   the following structure:

      struct authsys_parms {
         unsigned int stamp;
         string machinename<255>;
         unsigned int uid;
         unsigned int gid;
         unsigned int gids<16>;
      };

   The "stamp" is an arbitrary ID which the caller machine may generate.
   The "machinename" is the name of the caller's machine (like
   "krypton").  The "uid" is the caller's effective user ID.  The "gid"
   is the caller's effective group ID.  The "gids" is a counted array of
   groups which contain the caller as a member.  The verifier
   accompanying the credential should have "AUTH_NONE" flavor value
   (defined above).  Note this credential is only unique within a
   particular domain of machine names, uids, and gids.

   The flavor value of the verifier received in the reply message from
   the server may be "AUTH_NONE" or "AUTH_SHORT".  In the case of
   "AUTH_SHORT", the bytes of the reply verifier's string encode an
   opaque structure.  This new opaque structure may now be passed to the
   server instead of the original "AUTH_SYS" flavor credential.  The
   server may keep a cache which maps shorthand opaque structures
   (passed back by way of an "AUTH_SHORT" style reply verifier) to the
   original credentials of the caller.  The caller can save network
   bandwidth and server cpu cycles by using the shorthand credential.

   The server may flush the shorthand opaque structure at any time.  If
   this happens, the remote procedure call message will be rejected due
   to an authentication error.  The reason for the failure will be
   "AUTH_REJECTEDCRED".  At this point, the client may wish to try the
   original "AUTH_SYS" style of credential.

   It should be noted that use of this flavor of authentication does not
   guarantee any security for the users or providers of a service, in
   itself.  The authentication provided by this scheme can be considered
   legitimate only when applications using this scheme and the network
   can be secured externally, and privileged transport addresses are
   used for the communicating end-points (an example of this is the use
   of privileged TCP/UDP ports in Unix systems - note that not all
   systems enforce privileged transport address mechanisms).

REFERENCES

   [1]  Birrell, A. D.  & Nelson, B. J., "Implementing Remote Procedure
        Calls", XEROX CSL-83-7, October 1983.

   [2]  Cheriton, D., "VMTP: Versatile Message Transaction Protocol",
        Preliminary Version 0.3, Stanford University, January 1987.

   [3]  Diffie & Hellman, "New Directions in Cryptography", IEEE
        Transactions on Information Theory IT-22, November 1976.

   [4]  Mills, D., "Network Time Protocol", RFC 1305, UDEL,
        March 1992.

   [5]  National Bureau of Standards, "Data Encryption Standard",
        Federal Information Processing Standards Publication 46, January
        1977.

   [6]  Postel, J., "Transmission Control Protocol - DARPA Internet
        Program Protocol Specification", STD 7, RFC 793, USC/Information
        Sciences Institute, September 1981.

   [7]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
        USC/Information Sciences Institute, August 1980.

   [8]  Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
        RFC 1700, USC/Information Sciences Institute, October 1994.

   [9]  Srinivasan, R., "XDR: External Data Representation Standard",
        RFC 1832, Sun Microsystems, Inc., August 1995.

   [10] Miller, S., Neuman, C., Schiller, J., and  J. Saltzer, "Section
        E.2.1: Kerberos  Authentication and Authorization System",
        M.I.T. Project Athena, Cambridge, Massachusetts, December 21,
        1987.

   [11] Steiner, J., Neuman, C., and J. Schiller, "Kerberos: An
        Authentication Service for Open Network Systems", pp. 191-202 in
        Usenix Conference Proceedings, Dallas, Texas, February 1988.

   [12] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
        Service (V5)", RFC 1510, Digital Equipment Corporation,
        USC/Information Sciences Institute, September 1993.

Security Considerations

Security issues are not discussed in this memo.

Author's Address

   Raj Srinivasan
   Sun Microsystems, Inc.
   ONC Technologies
   2550 Garcia Avenue
   M/S MTV-5-40
   Mountain View, CA  94043
   USA

   Phone: 415-336-2478
   Fax:   415-336-6015
   EMail: raj@eng.sun.com