Network File System Version 4 Working Group                   R. Thurlow
Internet-Draft                                          Sun Microsystems
Intended status: Draft Standard
Obsoletes: 1831
Expires: December 12, 2008                               June 10, April 30, 2009                               October 30, 2008

      RPC: Remote Procedure Call Protocol Specification Version 2

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   Discussion and suggestions for improvement are requested.  This
   document will expire in December, 2008. April, 2009. Distribution of this draft is


   This document describes the ONC (Open Network Computing) Remote
   Procedure Call (ONC RPC Version 2) protocol as it is currently
   deployed and accepted.  It is meant to supersede  This document obsoletes [RFC1831].

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . 1
   2.  Changes since RFC 1831 . . . . . . . . . . . . . . . . . . . 1
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . 1
   4.  The RPC Model  . . . . . . . . . . . . . . . . . . . . . . . 1
   5.  Transports and Semantics . . . . . . . . . . . . . . . . . . 1
   6.  Binding and Rendezvous Independence  . . . . . . . . . . . . 1
   7.  Authentication . . . . . . . . . . . . . . . . . . . . . . . 1
   8.  RPC Protocol Requirements  . . . . . . . . . . . . . . . . . 1
   8.1.  RPC Programs and Procedures  . . . . . . . . . . . . . . . 1
   8.2.  Authentication, Integrity and Privacy  . . . . . . . . . . 1
   8.3.  Program Number Assignment  . . . . . . . . . . . . . . . . 1
   8.4.  Other Uses of the RPC Protocol . . . . . . . . . . . . . . 1
   8.4.1.  Batching . . . . . . . . . . . . . . . . . . . . . . . . 1
   8.4.2.  Broadcast Remote Procedure Calls . . . . . . . . . . . . 1
   9.  The RPC Message Protocol . . . . . . . . . . . . . . . . . . 1
   10.  Authentication Protocols  . . . . . . . . . . . . . . . . . 1
   10.1.  Null Authentication . . . . . . . . . . . . . . . . . . . 1
   11.  Record Marking Standard . . . . . . . . . . . . . . . . . . 1
   12.  The RPC Language  . . . . . . . . . . . . . . . . . . . . . 1
   12.1.  An Example Service Described in the RPC Language  . . . . 1
   12.2.  The RPC Language Specification  . . . . . . . . . . . . . 1
   12.3.  Syntax Notes  . . . . . . . . . . . . . . . . . . . . . . 1
   13.  IANA Considerations . . . . . . . . . . . . . . . . . . . . 1
   13.1.  Numbering Requests to IANA  . . . . . . . . . . . . . . . 1
   13.2.  Protecting Past Assignments . . . . . . . . . . . . . . . 1
   13.3.  RPC Number Assignment . . . . . . . . . . . . . . . . . . 1
   13.3.1.  To be assigned by IANA  . . . . . . . . . . . . . . . . 1
   13.3.2.  Defined by local administrator  . . . . . . . . . . . . 1
   13.3.3.  Transient block . . . . . . . . . . . . . . . . . . . . 1
   13.3.4.  Reserved block  . . . . . . . . . . . . . . . . . . . . 1
   13.3.5.  RPC Number Sub-Blocks . . . . . . . . . . . . . . . . . 1
   13.4.  RPC Authentication Flavor Number Assignment . . . . . . . 1
   13.4.1.  Assignment Policy . . . . . . . . . . . . . . . . . . . 1
   13.4.2.  Auth Flavors vs. Pseudo-flavors . . . . . . . . . . . . 1
   14.  Security Considerations . . . . . . . . . . . . . . . . . . 1
   15.  Appendix A: System Authentication . . . . . . . . . . . . . 1
   16.  Appendix B: Requesting RPC program or authentication
        numbers . . . . . . . . . . . . . . . . . . . . . . . . . . 1
   17.  Full Copyright Statement  . . . . . . . . . . . . . . . . . 1
   18.  Intellectual property . . . . . . . . . . . . . . . . . . . 1
   19.  Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . 1
   20.  Normative References  . . . . . . . . . . . . . . . . . . . 1
   21.  Informative References  . . . . . . . . . . . . . . . . . . 1
   22.  Author's Address  . . . . . . . . . . . . . . . . . . . . . 1

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 [RFC4506].  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 [XRPC] is recommended as an
   excellent background for the remote procedure call concept.

2.  Changes since RFC 1831

   This document is intended to replace obsoletes RFC 1831 as the authoritative document
   describing RPC, without introducing any over-the-wire protocol
   changes.  The main changes from RFC 1831 are:

   o    Addition of an Appendix which describes how an implementor can
        request new RPC program numbers and authentication flavor
        numbers from IANA, rather than from Sun Microsystems

   o    Addition of an "IANA Considerations" section which describes
        past program and authentication flavor number assignment policy
        and how IANA is intended to assign them in future

   o    Clarification of the RPC Language Specification to match current

   o    Enhancement of the "Security Considerations" section to reflect
        experience with strong security flavors

   o    Specification of new authentication errors that are in common
        use in modern RPC implementations

   o    Updates for the latest IETF intellectual property statements

3.  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

   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.

4.  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:

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

   o    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

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

   o    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.

5.  Transports and Semantics

   The RPC protocol can be implemented on several different transport
   protocols.  The scope of the definition of the RPC protocol excludes
   how a message is passed from one process to another, and includes
   only the 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 [RFC793] 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, 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 [RFC768], 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

   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] is perhaps a natural transport for RPC.  ONC RPC
   currently 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.  The mechanism by which future transports having different
   structural characteristics should be used to transfer ONC RPC
   messages should be specified by means of a standards-track RFC, once
   such additional transports are defined.

6.  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

   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.

7.  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 8.2: "Authentication,
   Integrity and Privacy".

8.  RPC Protocol Requirements

   The RPC protocol must provide for the following:

   o    Unique specification of a procedure to be called.

   o    Provisions for matching response messages to request messages.

   o    Provisions for authenticating the caller to service and vice-

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

   o    RPC protocol mismatches.

   o    Remote program protocol version mismatches.

   o    Protocol errors (such as misspecification of a procedure's

   o    Reasons why remote authentication failed.

   o    Any other reasons why the desired procedure was not called.

8.1.  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 (IANA).  Once
   implementors have a program number, they can implement their remote
   program; the first implementation would most likely have the version
   number 1 but MUST NOT be the number zero.  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:

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

   o    The remote program is not available on the remote system.

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

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

   o    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.)

8.2.  Authentication, Integrity and Privacy

   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

         enum auth_flavor {
            AUTH_NONE       = 0,
            AUTH_SYS        = 1,
            AUTH_SHORT      = 2,
            AUTH_DH         = 3,
            RPCSEC_GSS      = 6
            /* 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.

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

   As demonstrated by  RPCSEC_GSS, it is possible for an "auth_flavor"
   to also support integrity and privacy.

8.3.  Program Number Assignment

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

              0x00000000                Reserved
              0x00000001 - 0x1fffffff   To be assigned by IANA
              0x20000000 - 0x3fffffff   Defined by local administrator
                                        (some blocks assigned here)
              0x40000000 - 0x5fffffff   Transient
              0x60000000 - 0x7effffff   Reserved
              0x7f000000 - 0x7fffffff   Assignment outstanding
              0x80000000 - 0xffffffff   Reserved

   The first group is a range of numbers administered by IANA 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 methods
   described in Appendix B.  The third group is for applications that
   generate program numbers dynamically.  The final groups are reserved
   for future use, and should not be used.

8.4.  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.

8.4.1.  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

8.4.2.  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.

9.  The RPC Message Protocol

   This section defines the RPC message protocol in the XDR data
   description language [RFC4506].

         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  /* e.g. 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                 */
             * AUTH_KERB errors; deprecated. See [RFC2695]
            AUTH_KERB_GENERIC = 8,  /* kerberos generic error */
            AUTH_TIMEEXPIRE = 9,    /* time of credential expired */
            AUTH_TKT_FILE = 10,     /* problem with ticket file */
            AUTH_DECODE = 11,       /* can't decode authenticator */
            AUTH_NET_ADDR = 12,     /* wrong net address in ticket */
             * RPCSEC_GSS GSS related errors
            RPCSEC_GSS_NOCRED = 13,  /* no credentials for user */
            RPCSEC_GSS_FAILED = 14   /* GSS failure, creds deleted */

   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

   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

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

10.  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 methods described in Appendix B.  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 (AUTH_SYS) is described in Appendix A.  Implementors
   MAY include AUTH_SYS in their implementations to support existing
   applications.  See "Security Considerations" for information about
   other, more secure, authentication flavors.

10.1.  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

11.  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

   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!)

12.  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 keywords
   "program" and "version" are reserved in the RPC Language, and
   implementations of XDR compilers MAY reserve these keywords even when
   provided pure XDR, non-RPC, descriptions.  The following example is
   used to describe the essence of the language.

12.1.  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 {
               PINGPROC_NULL(void) = 0;
                * Ping the client, return the round-trip time
                * (in microseconds). Returns -1 if the operation
                * timed out.

               PINGPROC_PINGBACK(void) = 1;
            } = 2;

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

         const PING_VERS = 2;      /* latest version */

   The first version described is PING_VERS_PINGBACK with two
   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.

12.2.  The RPC Language Specification

   The RPC language is identical to the XDR language defined in RFC
   4506, except for the added definition of a "program-def" described

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

         "version" identifier "{"
             procedure-def *
         "}" "=" constant ";"
         proc-return identifier "(" proc-firstarg
           ("," type-specifier )* ")" "=" constant ";"

      proc-return: "void" | type-specifier

      proc-firstarg: "void" | type-specifier

12.3.  Syntax Notes

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

   o    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.

   o    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.

   o    Program identifiers are in the same name space as constant and
        type identifiers.

   o    Only unsigned constants can be assigned to programs, versions
        and procedures.

   o    Current RPC language compilers do not generally support more
        than one type-specifier in procedure argument lists; the usual
        practice is to wrap arguments into a structure.

13.  IANA Considerations

   The assignment of RPC program numbers and authentication flavor
   numbers has in the past been performed by Sun Microsystems, Inc
   (Sun).  This is inappropriate for an IETF standards-track protocol,
   as such work is done well by the Internet Assigned Numbers Authority
   (IANA).  This document proposes the transfer of authority over RPC
   program numbers and authentication flavor numbers described here from
   Sun Microsystems, Inc. to IANA and proposes how IANA will maintain
   and assign RPC program numbers and authentication flavor numbers.
   Users of RPC protocols will benefit by having an independent body
   responsible for RPC number assignments.

13.1.  Numbering Requests to IANA

   Appendix B of this document describes the information to be sent to
   IANA to request one or more RPC numbers and the rules that apply.
   IANA should review this part of store the document as well. request for documentary purposes, and put the
   following information into the public registry:

   o    The short description of purpose and use

   o    The program number(s) assigned

   o    The short identifier string(s)

13.2.  Protecting Past Assignments

   Sun has made assignments in both number spaces since the original
   deployment of RPC.  The assignments made by Sun Microsystems are
   still valid, and will be preserved.  Sun will communicate all current
   assignments in both number spaces to IANA before final handoff of
   number assignment is done.  At the time of submission of this draft,
   current assignments were listed at:

13.3.  RPC Number Assignment

   Future IANA practice should deal with the following partitioning of
   the 32-bit number space as listed in Section 8.3.  Detailed
   information for the administration of the partitioned blocks in
   Section 8.3. is given below.

13.3.1.  To be assigned by IANA

   The first block will be administered by IANA, with previous
   assignments by Sun protected.  Previous assignments were restricted
   to the range decimal 100000-399999 (0x000186a0 to 0x00061a7f),
   therefore IANA should begin assignments at decimal 400000.
   Individual numbers should be grated on a first-come, first-served
   basis, and blocks should be granted under rules related to the size
   of the block.

13.3.2.  Defined by local administrator

   The "Defined by local administrator" block is available for any local
   administrative domain to use, in a similar manner to IP address
   ranges reserved for private use.  The expected use would be through
   the establishment of a local domain "authority" for assigning numbers
   from this range.  This authority would establish any policies or
   procedures to be used within that local domain for use or assignment
   of RPC numbers from the range.  The local domain should be
   sufficiently isolated that it would be unlikely that RPC applications
   developed by other local domains could communicate with the domain.
   This could result in RPC number contention, which would cause one of
   the applications to fail.  In the absence of a local administrator,
   this block can be utilized in a "Private Use" manner per [RFC5226].

13.3.3.  Transient block

   The "Transient" block can be used by any RPC application on a "as
   available" basis.  This range is intended for services that can
   communicate a dynamically selected RPC program number to clients of
   the service.  Any mechanism can be used to communicate the number.
   Examples include shared memory when the client and server are located
   on the same system, or a network message (either RPC or otherwise)
   that disseminates the selected number.

   The transient block is not administered.  An RPC service uses this
   range by selecting a number in the transient range and attempting to
   register that number with the local system's RPC bindery (see the
   RPCBPROC_SET or PMAPPROC_SET procedures in "Binding Protocols for ONC
   RPC", [RFC1833]).  If successful, no other RPC service was using that
   number and the RPC Bindery has assigned that number to the requesting
   RPC application.  The registration is valid until the RPC Bindery
   terminates, which normally would only happen if the system reboots
   causing all applications, including the RPC service using the
   transient number, to terminate.  If the transient number registration
   fails, another RPC application is using the number and the requestor
   must select another number and try again.  To avoid conflicts, the
   recommended method is to select a number randomly from the transient

13.3.4.  Reserved block

   The "Reserved" blocks are available for future use.  RPC applications
   must not use numbers in these ranges unless their use is allowed by
   future action by the IESG.

13.3.5.  RPC Number Sub-Blocks

   RPC numbers are usually assigned for specific RPC services.  Some
   applications, however, require multiple RPC numbers for a service.
   The most common example is an RPC service that needs to have multiple
   instances of the service active simultaneously at a specific site.
   RPC does not have an "instance identifier" in the protocol, so either
   a mechanism must be implemented to multiplex RPC requests amongst
   various instances of the service, or unique RPC numbers must be used
   by each instance.

   In these cases, the RPC protocol used with the various numbers may be
   different or the same.  The numbers may be assigned dynamically by
   the application, or as part of a site-specific administrative
   decision.  If possible, RPC services that dynamically assign RPC
   numbers should use the "Transient" RPC number block defined in
   section 2.  If not possible, RPC number sub-blocks may be requested.

   Assignment of RPC Number Sub-Blocks is controlled by the size of the
   sub-block being requested.  "Specification Required" and "IESG
   Approval" are used as defined by [RFC5226] Section 4.1.

   Size of sub-block        Assignment Method         Authority
   -----------------        -----------------         ---------
   Up to 100 numbers        First Come First Served   IANA
   Up to 1000 numbers       Specification Required    IANA
   More than 1000 numbers   IESG Approval required    IESG

   Note: sub-blocks can be any size.  The limits given above are
   maximums and smaller size sub-blocks are allowed.

   Sub-blocks sized up to 100 numbers may be assigned by IANA on a First
   Come First Served basis.  The RPC Service Description included in the
   range must include an indication of how the sub-block is managed.  At
   minimum, the statement should indicate whether the sub-block is used
   with a single RPC protocol or multiple RPC protocols, and whether the
   numbers are dynamically assigned or statically (through
   administrative action) assigned.

   Sub-blocks of up to 1000 numbers must be documented in detail.  The
   documentation must describe the RPC protocol or protocols that are to
   be used in the range.  It must also describe how the numbers within
   the sub-block are to be assigned or used.

   Sub-blocks sized over 1000 numbers must be documented as described
   above, however an Internet Draft and the assignment must be submitted as an
   Informational or standards-track RFC.  If accepted as either, IANA
   will assign approved by the requested number sub-block. IESG.  It is
   expected that this will be rare.

   In order to avoid multiple requests of large blocks of numbers the
   following rule is proposed.

   Requests up to and including 100 RPC numbers are handled via the
   First Come First Served assignment method.  This 100 number
   threshhold applies to the total number of RPC numbers assigned to an
   individual or entity. For example, if an individual or entity first
   requests say 70 numbers, and then later requests 40 numbers, then the
   request for the 40 numbers will be assigned via the Specification
   Required method.  As long as the total number of numbers assigned
   does not exceed 1000, IANA is free to waive the Specification
   Required assignment for incremental requests of less than 100

   If an individual or entity has under 1000 numbers and later requests
   an additional set of numbers such that the individual or entity would
   be granted over 1000 numbers, then the individual or entity will have the additional request submitted to the IESG.  IESG is free to waive the will
   require IESG Action Required assignment method for incremental requests of
   less than 1000 numbers. Approval.

13.4.  RPC Authentication Flavor Number Assignment

   The second number space is the authentication mechanism identifier,
   or "flavor", number.  This number is used to distinguish between
   various authentication mechanisms which can be optionally used with
   an RPC message.  An authentication identifier is used in the "flavor"
   field of the "opaque_auth" structure.

13.4.1.  Assignment Policy

   Appendix B of this document describes the information to be sent to
   IANA to request one or more RPC auth numbers and the rules that
   apply.  IANA should store the request for documentary purposes, and
   put the following information into the public registry:

   o    The short identifier string(s)

   o    The auth number(s) assigned

   o    The short description of purpose and use

13.4.2.  Auth Flavors vs. Pseudo-flavors

   Recent progress in RPC security has moved away from new auth flavors
   as used by AUTH_DH [DH], and focused on using the existing RPCSEC_GSS
   [RFC2203] flavor and inventing novel GSS-API mechanisms which can be
   used with it.  Even though RPCSEC_GSS is an assigned authentication
   flavor, use of a new RPCSEC_GSS mechanism with NFS ([RFC1094]
   [RFC1813] and [RFC3530]) will require the registration of 'pseudo-
   flavors' which are used to negotiate security mechanisms in an
   unambiguous way, as defined by [RFC2623].  Existing pseudo-flavors
   have been granted in the decimal range 390000-390255.  New pseudo-
   flavor requests should be granted by IANA within this block on a
   First Come First Served basis.

   For non-pseudo-flavor requests, IANA should begin granting RPC
   authentication flavor numbers at 400000 on a First Come First Served
   basis to avoid conflicts with currently granted numbers.

   For authentication flavors or RPCSEC_GSS mechanisms to be used on the
   Internet, it is strongly advised that an informational or standards-
   track RFC be published describing the authentication mechanism
   behaviour and parameters.

14.  Security Considerations

   AUTH_SYS as described in Appendix A is known to be insecure due to
   the lack of a verifier to permit the credential to be validated.
   AUTH_SYS SHOULD NOT be used for services which permit clients to
   modify data.  AUTH_SYS MUST NOT be specified as RECOMMENDED or
   REQUIRED for any standards-track RPC service.

   [RFC2203] defines a new security flavor, RPCSEC_GSS, which permits
   GSS-API [RFC2743] mechanisms to be used for securing RPC.  All non-
   trivial RPC programs developed in future should implement
   RPCSEC_GSS-based security appropriately.  [RFC2623] describes how
   this was done for a widely deployed RPC program.

15.  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).

16.  Appendix B: Requesting RPC program or authentication numbers

   RPC numbers which must be unique across all networks are assigned by
   the Internet Assigned Number Authority.  To apply for a single number
   or a block of numbers, electronic mail must be sent to IANA
   <> with the following information:

   o    The type of number(s) (program number or authentication flavor
        number) sought

   o    How many numbers are sought

   o    The name of person or company which will use the number

   o    An "identifier string" which associates the number with a

   o    Email address of the contact person for the service which will
        be using the number.

   o    A short description of the purpose and use of the number

   o    If an authentication flavor number is sought, and the number
        will be a 'pseudo-flavor' intended for use with RPCSEC_GSS and
        NFS, mappings analogous to those in Section 4.2 of [RFC2623] are

   Specific numbers cannot be requested.  Numbers are assigned on a
   First Come First Served basis.

   For all RPC authentication flavor numbers to used on the Internet, it
   is strongly advised that an informational or standards-track RFC be
   published describing the authentication mechanism behaviour and

17.  Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

18.  Intellectual property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

19.  Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).

20.  Normative References

   Eisler, M., "XDR: External Data Representation Standard", RFC 4506,

21.  Informative References

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

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

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

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

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

   Sun Microsystems, Inc., "NFS: Network File System Protocol
   Specification", RFC 1094, March 1989.

   Callaghan, B., Pawlowski, B., Staubach, P., "NFS Version 3 Protocol
   Specification", RFC 1813, June 1995.

   R. Srinivasan, "RPC: Remote Procedure Call Protocol Specification
   Version 2", RFC 1831, August 1995.

   R. Srinivasan, "Binding Protocols for ONC RPC Version 2", RFC 1833,
   August 1995.

   Bradner, S., "Key words for use in RFCs to Indicate Requirement
   Eisler, M., Chiu, A., Ling, L., "RPCSEC_GSS Protocol Specification",
   RFC 2203, September 1997

   Eisler, M., "NFS Version 2 and Version 3 Security Issues and the NFS
   Protocol's Use of RPCSEC_GSS and Kerberos V5", RFC 2623, June 1999.

   Chiu, A., "Authentication Mechanisms for ONC RPC", RFC 2695,
   September 1999.

   Linn. J., "Generic Security Service Application Program Interface
   Version 2, Update 1", RFC 2743, January 2000.

   Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame, C.,
   Eisler, M., Noveck, D., "Network File System (NFS) version 4
   Protocol", RFC 3530, April 2003.

   Narten, T. and Alvestrand, H., "Guidelines for Writing an IANA
   Considerations Section in RFCs", RFC 5226, May 2008.

22.  Author's Address

   Address comments related to this memorandum to:

   Robert Thurlow
   Sun Microsystems, Inc.
   500 Eldorado Boulevard, UBRM05-171
   Broomfield, CO 80021

   Phone: 877-718-3419