rpc.rfc.ms

RTEMS (Real-Time Executive for Multiprocessor Systems) is a free open source real-time operating sys

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.OH 'Remote Procedure Calls: Protocol Specification''Page %'
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.SH
\&Remote Procedure Calls: Protocol Specification
.LP
.NH 0
\&Status of this Memo
.LP
Note: This chapter specifies a protocol that Sun Microsystems, Inc.,
and others are using.  
It has been designated RFC1050 by the ARPA Network
Information Center.
.LP
.NH 1
\&Introduction
.LP
This chapter specifies  a  message protocol  used in implementing
Sun's Remote Procedure Call (RPC) package.  (The message protocol is
specified with the External Data Representation (XDR) language.
See the
.I "External Data Representation Standard: Protocol Specification"
for the details.  Here, we assume that  the  reader is familiar  
with XDR and do not attempt to justify it or its uses).  The paper
by Birrell and Nelson [1]  is recommended as an  excellent background
to  and justification of RPC.
.NH 2
\&Terminology
.LP
This chapter discusses servers, services, programs, procedures,
clients, and versions.  A server is a piece of software where network
services are implemented.  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 the
\fIPort Mapper Program Protocol\fP\, below, for an example).  Network
clients are pieces of software that initiate remote procedure calls
to services.  A server may support more than one version of a remote
program in order to be forward compatible with changing protocols.
.LP
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
IO and have procedures like "read" and "write".  A client machine of
the network file service would call the procedures associated with
the two programs of the service on behalf of some user on the client
machine.
.NH 2
\&The RPC Model
.LP
The remote procedure call model 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 result
register).  It then transfers control to the procedure, and
eventually gains back control.  At that point, the results of the
procedure are extracted from the well-specified location, and the
caller continues execution.
.LP
The remote procedure call is similar, in that one thread of control
logically winds through two processes\(emone is the caller's process,
the other is a server's process.  That is, the caller process sends a
call message to the server process and waits (blocks) for a reply
message.  The call message contains the procedure's parameters, among
other things.  The reply message contains the procedure's results,
among other things.  Once the reply message is received, the results
of the procedure are extracted, and caller's execution is resumed.
.LP
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.
.LP
Note that 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 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 task to process an
incoming request, so that the server can be free to receive other
requests.
.NH 2
\&Transports and Semantics
.LP
The RPC protocol is independent of transport protocols.  That is, RPC
does not care how a message is passed from one process to another.
The protocol deals only with specification and interpretation of
messages.
.LP
It is important to point out that RPC does not try to implement any
kind of reliability and that the application must 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/IP[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/IP[7], it must implement
is own retransmission and time-out policy as the RPC layer does not
provide this service.
.LP
Because of transport independence, the RPC protocol does not attach
specific semantics to the remote procedures or their execution.
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/IP.  If an
application retransmits RPC messages after short time-outs, the only
thing it can infer if it receives no reply is that the procedure was
executed zero or more times.  If it does receive a reply, then it can
infer that the procedure was executed at least once.
.LP
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
request.  The main use of this transaction is by the client RPC layer
in matching replies to requests.  However, a client application may
choose to reuse its previous transaction ID when retransmitting a
request.  The server application, knowing this fact, may choose to
remember this ID after granting a request and not regrant requests
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.
.LP
On the other hand, if using a reliable transport such as TCP/IP, 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.
.LP
There are other possibilities for transports besides datagram- or
connection-oriented protocols.  For example, a request-reply protocol
such as VMTP[2] is perhaps the most natural transport for RPC.
.SH
.I
NOTE:  At Sun, RPC is currently implemented on top of both TCP/IP
and UDP/IP transports.
.LP
.NH 2
\&Binding and Rendezvous Independence
.LP
The act of binding a client to a service is NOT part of the remote
procedure call specification.  This important and necessary function
is left up to some higher-level software.  (The software may use RPC
itself\(emsee the \fIPort Mapper Program Protocol\fP\, below).
.LP
Implementors should 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 network makes RPC useful, using RPC to accomplish this
task.
.NH 2
\&Authentication
.LP
The RPC protocol provides the fields necessary for a client to
identify itself to a service and vice-versa.  Security and access
control mechanisms can be built on top of the 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 can be found in the
\fIAuthentication Protocols\fP\,
below.
.KS
.NH 1
\&RPC Protocol Requirements
.LP
The RPC protocol must provide for the following:
.IP  1.
Unique specification of a procedure to be called.
.IP  2.
Provisions for matching response messages to request messages.
.KE
.IP  3.
Provisions for authenticating the caller to service and vice-versa.
.LP
Besides these requirements, features that detect the following are
worth supporting because of protocol roll-over errors, implementation
bugs, user error, and network administration:
.IP  1.
RPC protocol mismatches.
.IP  2.
Remote program protocol version mismatches.
.IP  3.
Protocol errors (such as misspecification of a procedure's parameters).
.IP  4.
Reasons why remote authentication failed.
.IP  5.
Any other reasons why the desired procedure was not called.
.NH 2
\&Programs and Procedures
.LP
The RPC call message has three unsigned fields:  remote program
number, remote program version number, and remote procedure number.
The three fields uniquely identify the procedure to be called.
Program numbers are administered by some central authority (like
Sun).  Once an implementor has a program number, he can implement his
remote program; the first implementation would most likely have the
version number of 1.  Because most new protocols evolve into better,
stable, and mature protocols, 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.
.LP
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".
.LP
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.
.LP
The reply message to a request  message  has enough  information to
distinguish the following error conditions:
.IP  1.
The remote implementation of RPC does speak protocol version 2.
The lowest and highest supported RPC version numbers are returned.
.IP  2.
The remote program is not available on the remote system.
.IP  3.
The remote program does not support the requested version number.
The lowest and highest supported remote program version numbers are
returned.
.IP  4.
The requested procedure number does not exist.  (This is usually a
caller side protocol or programming error.)
.IP  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.)
.NH 2
\&Authentication
.LP
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:
.DS
.ft CW
.vs 11
enum auth_flavor {
    AUTH_NULL        = 0,
    AUTH_UNIX        = 1,
    AUTH_SHORT       = 2,
    AUTH_DES         = 3
    /* \fIand more to be defined\fP */
};

struct opaque_auth {
    auth_flavor flavor;
    opaque body<400>;
};
.DE
.LP
In simple English, any
.I opaque_auth 
structure is an 
.I auth_flavor 
enumeration followed by bytes which are  opaque to the RPC protocol
implementation.
.LP
The interpretation and semantics  of the data contained  within the
authentication   fields  is specified  by  individual,  independent
authentication  protocol specifications.   (See 
\fIAuthentication Protocols\fP\,
below, for definitions of the various authentication protocols.)
.LP
If authentication parameters were   rejected, the  response message
contains information stating why they were rejected.
.NH 2
\&Program Number Assignment
.LP
Program numbers are given out in groups of
.I 0x20000000 
(decimal 536870912) according to the following chart:
.TS
box tab (&) ;
lfI lfI
rfL cfI .
Program Numbers&Description
_
.sp .5
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
.TE
.LP
The first group is a range of numbers administered by Sun
Microsystems 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.  The third
group is for applications that generate program numbers dynamically.
The final groups are reserved for future use, and should not be used.
.NH 2
\&Other Uses of the RPC Protocol
.LP
The intended use of this protocol is for calling remote procedures.
That is, each call message is matched with a response message.
However, the protocol itself is a message-passing protocol with which
other (non-RPC) protocols can be implemented.  Sun currently uses, or
perhaps abuses, the RPC message protocol for the following two
(non-RPC) protocols:  batching (or pipelining) and broadcast RPC.
These two protocols are discussed but not defined below.
.NH 3
\&Batching
.LP
Batching allows a client to send an arbitrarily large sequence of
call messages to a server; batching typically uses reliable byte
stream protocols (like TCP/IP) 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 requests.  A sequence of batch
calls is usually terminated by a legitimate RPC in order to flush the
pipeline (with positive acknowledgement).
.NH 3
\&Broadcast RPC
.LP
In broadcast RPC-based protocols, the client sends a broadcast packet
to the network and waits for numerous replies.  Broadcast RPC uses
unreliable, packet-based protocols (like UDP/IP) as its transports.
Servers that support broadcast protocols only respond when the
request is successfully processed, and are silent in the face of
errors.  Broadcast RPC uses the Port Mapper RPC service to achieve
its semantics.  See the \fIPort Mapper Program Protocol\fP\, below,
for more information.
.KS
.NH 1
\&The RPC Message Protocol
.LP
This section defines the RPC message protocol in the XDR data
description language.  The message is defined in a top-down style.
.ie t .DS
.el .DS L
.ft CW
enum msg_type {
	CALL  = 0,
	REPLY = 1
};

.ft I
/*
* A reply to a call message can take on two forms:
* The message was either accepted or rejected.
*/
.ft CW
enum reply_stat {
	MSG_ACCEPTED = 0,
	MSG_DENIED   = 1
};

.ft I
/*
* Given that a call message was accepted,  the following is the
* status of an attempt to call a remote procedure.
*/
.ft CW
enum accept_stat {
	SUCCESS       = 0, /* \fIRPC executed successfully       \fP*/
	PROG_UNAVAIL  = 1, /* \fIremote hasn't exported program  \fP*/
	PROG_MISMATCH = 2, /* \fIremote can't support version #  \fP*/
	PROC_UNAVAIL  = 3, /* \fIprogram can't support procedure \fP*/
	GARBAGE_ARGS  = 4  /* \fIprocedure can't decode params   \fP*/
};
.DE
.ie t .DS
.el .DS L
.ft I
/*
* Reasons why a call message was rejected:
*/
.ft CW
enum reject_stat {
	RPC_MISMATCH = 0, /* \fIRPC version number != 2          \fP*/
	AUTH_ERROR = 1    /* \fIremote can't authenticate caller \fP*/
};

.ft I
/*
* Why authentication failed:
*/
.ft CW
enum auth_stat {
	AUTH_BADCRED      = 1,  /* \fIbad credentials \fP*/
	AUTH_REJECTEDCRED = 2,  /* \fIclient must begin new session \fP*/
	AUTH_BADVERF      = 3,  /* \fIbad verifier \fP*/
	AUTH_REJECTEDVERF = 4,  /* \fIverifier expired or replayed  \fP*/
	AUTH_TOOWEAK      = 5   /* \fIrejected for security reasons \fP*/
};
.DE
.KE
.ie t .DS
.el .DS L
.ft I
/*
* 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 \fIREPLY\fP  message always
* matches  that of the initiating \fICALL\fP   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.
*/
.ft CW
struct rpc_msg {
	unsigned int xid;
	union switch (msg_type mtype) {
		case CALL:
			call_body cbody;
		case REPLY:  
			reply_body rbody;
	} body;
};
.DE
.ie t .DS