rfc1831.txt

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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



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RFC 1831        Remote Procedure Call Protocol Version 2     August 1995


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.






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RFC 1831        Remote Procedure Call Protocol Version 2     August 1995


   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.






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RFC 1831        Remote Procedure Call Protocol Version 2     August 1995


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



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RFC 1831        Remote Procedure Call Protocol Version 2     August 1995


   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.









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RFC 1831        Remote Procedure Call Protocol Version 2     August 1995


   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
   ([e-mail]).  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.




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