rfc1057.txt

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Network Working Group                             Sun Microsystems, Inc.
Request For Comments: 1057                                     June 1988
Obsoletes: RFC 1050


                       RPC: Remote Procedure Call
                         Protocol Specification
                               Version 2

STATUS OF THIS MEMO

   This RFC describes a standard that Sun Microsystems and others are
   using, and is one we wish to propose for the Internet's
   consideration.  This memo is not an Internet standard at this time.
   Distribution of this memo is unlimited.

1. INTRODUCTION

   This document specifies version two of the message protocol used in
   Sun's Remote Procedure Call (RPC) package.  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 sends 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 (see
   Appendix A for an example).  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



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   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.  On the other hand, it may
   simplify software to separate client and server functionality into
   separate libraries or programs.

3. THE RPC MODEL

   The Sun 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 Sun 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



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   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 insecure networks, authentication may be necessary.

   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.  On the other hand, 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, through a mechanism such as the one
   described in Appendix A.

   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 layer 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



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   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 is by the client RPC layer
   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 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.  The Sun RPC
   package currently uses both TCP and UDP transport protocols, with
   experimentation underway on others such as ISO TP4 and TP0.

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.  (The software may use RPC itself; see Appendix A.)

   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".





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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 some central authority (like
   Sun).  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 make speaking old and new
   protocols through the same server process possible.

   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 speak protocol version



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   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 credentials 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_NULL       = 0,
            AUTH_UNIX       = 1,
            AUTH_SHORT      = 2,
            AUTH_DES        = 3
            /* 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 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.



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RFC 1057            Remote Procedure Call, Version 2           June 1988


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 Sun
          20000000 - 3fffffff   defined by user
          40000000 - 5fffffff   transient
          60000000 - 7fffffff   reserved
          80000000 - 9fffffff   reserved
          a0000000 - bfffffff   reserved
          c0000000 - dfffffff   reserved
          e0000000 - ffffffff   reserved