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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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
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.
7.4 Other Uses of the RPC Protocol
The intended use of this protocol is for calling remote procedures.
Normally, each call message is matched with a reply message.
However, the protocol itself is a message-passing protocol with which
other (non-procedure call) protocols can be implemented. Sun
currently uses, or perhaps abuses, the RPC message protocol for the
batching (or pipelining) and broadcast remote procedure calls.
7.4.1 Batching
Batching is useful when a client wishes to send an arbitrarily large
sequence of call messages to a server. Batching typically uses
reliable byte stream protocols (like TCP) for its transport. In the
case of batching, the client never waits for a reply from the server,
and the server does not send replies to batch calls. A sequence of
batch calls is usually terminated by a legitimate remote procedure
call operation in order to flush the pipeline and get positive
acknowledgement.
7.4.2 Broadcast Remote Procedure Calls
In broadcast protocols, the client sends a broadcast call to the
network and waits for numerous replies. This requires the use of
packet-based protocols (like UDP) as its transport protocol. Servers
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that support broadcast protocols only respond when the call is
successfully processed, and are silent in the face of errors.
Broadcast calls use the Port Mapper RPC service to achieve their
semantics. See Appendix A for more information.
8. THE RPC MESSAGE PROTOCOL
This section defines the RPC message protocol in the XDR data
description language [9].
enum msg_type {
CALL = 0,
REPLY = 1
};
A reply to a call message can take on two forms: The message was
either accepted or rejected.
enum reply_stat {
MSG_ACCEPTED = 0,
MSG_DENIED = 1
};
Given that a call message was accepted, the following is the status
of an attempt to call a remote procedure.
enum accept_stat {
SUCCESS = 0, /* RPC executed successfully */
PROG_UNAVAIL = 1, /* remote hasn't exported program */
PROG_MISMATCH = 2, /* remote can't support version # */
PROC_UNAVAIL = 3, /* program can't support procedure */
GARBAGE_ARGS = 4 /* procedure can't decode params */
};
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 */
};
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Why authentication failed:
enum auth_stat {
AUTH_BADCRED = 1, /* bad credentials (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 */
};
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 credentials) and verf (authentication verifier). The
two authentication parameters are followed by the parameters to the
remote procedure, which are specified by the specific program
protocol.
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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, and
GARBAGE_ARGS 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.
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struct accepted_reply {
opaque_auth verf;
union switch (accept_stat stat) {
case SUCCESS:
opaque results[0];
/*
* procedure-specific results start here
*/
case PROG_MISMATCH:
struct {
unsigned int low;
unsigned int high;
} mismatch_info;
default:
/*
* Void. Cases include PROG_UNAVAIL, PROC_UNAVAIL,
* and GARBAGE_ARGS.
*/
void;
} reply_data;
};
Reply to an RPC call that was rejected by the server:
The call can be rejected for two reasons: either the server is not
running a compatible version of the RPC protocol (RPC_MISMATCH), or
the server refuses to authenticate 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 refused 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;
};
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9. AUTHENTICATION PROTOCOLS
As previously stated, authentication parameters are opaque, but
open-ended to the rest of the RPC protocol. This section defines
some "flavors" of authentication implemented at (and supported by)
Sun. Other sites are free to invent new authentication types, with
the same rules of flavor number assignment as there is for program
number assignment.
9.1 Null Authentication
Often calls must be made where the client does not know its identity
or the server does not care who the client is. In this case, the
flavor value (the discriminant of the opaque_auth's union) of the RPC
message's credentials, verifier, and reply verifier is "AUTH_NULL".
The bytes of the opaque_auth's body are undefined. It is recommended
that the opaque length be zero.
9.2 UNIX Authentication
The client may wish to identify itself as it is identified on a
UNIX(tm) system. The value of the credential's discriminant of an
RPC call message is "AUTH_UNIX". The bytes of the credential's
opaque body encode the the following structure:
struct auth_unix {
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 credentials should be of "AUTH_NULL" (defined
above). Note these credentials are only unique within a particular
domain of machine names, uids, and gids. Inter-domain naming is
beyond the scope of this document.
The value of the discriminant of the reply verifier received in the
reply message from the server may be "AUTH_NULL" 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_UNIX" flavor
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credentials. 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 new
credentials.
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_UNIX" style of credentials.
9.3 DES Authentication
UNIX authentication suffers from three major problems:
(1) The naming is too UNIX oriented.
(2) There is no universal name, uid, and gid space.
(3) There is no verifier, so credentials can easily be faked.
DES authentication attempts to address these problems.
9.3.1 Naming
The first problem is handled by addressing the client by a simple
string of characters instead of by an operating system specific
integer. This string of characters is known as the "netname" or
network name of the client. The server is not allowed to interpret
the contents of the client's name in any other way except to identify
the client. Thus, netnames should be unique for every client in the
Internet.
It is up to each operating system's implementation of DES
authentication to generate netnames for its users that insure this
uniqueness when they call upon remote servers. Operating systems
already know how to distinguish users local to their systems. It is
usually a simple matter to extend this mechanism to the network. For
example, a UNIX user at Sun with a user ID of 515 might be assigned
the following netname: "unix.515@sun.com". This netname contains
three items that serve to insure it is unique. Going backwards,
there is only one naming domain called "sun.com" in the Internet.
Within this domain, there is only one UNIX user with user ID 515.
However, there may be another user on another operating system, for
example VMS, within the same naming domain that, by coincidence,
happens to have the same user ID. To insure that these two users can
be distinguished we add the operating system name. So one user is
"unix.515@sun.com" and the other is "vms.515@sun.com".
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The first field is actually a naming method rather than an operating
system name. It happens that today there is almost a one-to-one
correspondence between naming methods and operating systems. If the
world could agree on a naming standard, the first field could be the
name of that standard, instead of an operating system name.
9.3.2 DES Authentication Verifiers
Unlike UNIX authentication, DES authentication does have a verifier
so the server can validate the client's credential (and vice-versa).
The contents of this verifier is primarily an encrypted timestamp.
The server can decrypt this timestamp, and if it is close to the real
time, then the client must have encrypted it correctly. The only way
the client could encrypt it correctly is to know the "conversation
key" of the RPC session. And if the client knows the conversation
key, then it must be the real client.
The conversation key is a DES [5] key which the client generates and
passes to the server in its first RPC call. The conversation key is
encrypted using a public key scheme in this first transaction. The
particular public key scheme used in DES authentication is Diffie-
Hellman [3] with 192-bit keys. The details of this encryption method
are described later.
The client and the server need the same notion of the current time in
order for all of this to work, perhaps by using the Network Time
Protocol [4]. If network time synchronization cannot be guaranteed,
then the client can determine the server's time before beginning the
conversation using a simpler time request protocol.
The way a server determines if a client timestamp is valid is
somewhat complicated. For any other transaction but the first, the
server just checks for two things:
(1) the timestamp is greater than the one previously seen from the
same client.
(2) the timestamp has not expired.
A timestamp is expired if the server's time is later than the sum of
the client's timestamp plus what is known as the client's "window".
The "window" is a number the client passes (encrypted) to the server
in its first transaction. You can think of it as a lifetime for the
credential.
This explains everything but the first transaction. In the first
transaction, the server checks only that the timestamp has not
expired. If this was all that was done though, then it would be
quite easy for the client to send random data in place of the
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timestamp with a fairly good chance of succeeding. As an added
check, the client sends an encrypted item in the first transaction
known as the "window verifier" which must be equal to the window
minus 1, or the server will reject the credential.
The client too must check the verifier returned from the server to be
sure it is legitimate. The server sends back to the client the
encrypted timestamp it received from the client, minus one second.
If the client gets anything different than this, it will reject it.
9.3.3 Nicknames and Clock Synchronization
After the first transaction, the server's DES authentication
subsystem returns in its verifier to the client an integer "nickname"
which the client may use in its further transactions instead of
passing its netname, encrypted DES key and window every time. The
nickname is most likely an index into a table on the server which
stores for each client its netname, decrypted DES key and window.
Though they originally were synchronized, the client's and server's
clocks can get out of sync again. When this happens the client RPC
subsystem most likely will get back "RPC_AUTHERROR" at which point it
should resynchronize.
A client may still get the "RPC_AUTHERROR" error even though it is
synchronized with the server. The reason is that the server's
nickname table is a limited size, and it may flush entries whenever
it wants. A client should resend its original credential in this
case and the server will give it a new nickname. If a server
crashes, the entire nickname table gets flushed, and all clients will
have to resend their original credentials.
9.3.4 DES Authentication Protocol Specification
There are two kinds of credentials: one in which the client uses its
full network name, and one in which it uses its "nickname" (just an
unsigned integer) given to it by the server. The client must use its
fullname in its first transaction with the server, in which the
server will return to the client its nickname. The client may use
its nickname in all further transactions with the server. There is no
requirement to use the nickname, but it is wise to use it for
performance reasons.
enum authdes_namekind {
ADN_FULLNAME = 0,
ADN_NICKNAME = 1
};
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A 64-bit block of encrypted DES data:
typedef opaque des_block[8];
Maximum length of a network user's name:
const MAXNETNAMELEN = 255;
A fullname contains the network name of the client, an encrypted
conversation key and the window. The window is actually a lifetime
for the credential. If the time indicated in the verifier timestamp
plus the window has past, then the server should expire the request
and not grant it. To insure that requests are not replayed, the
server should insist that timestamps are greater than the previous
one seen, unless it is the first transaction. In the first
transaction, the server checks instead that the window verifier is
one less than the window.
struct authdes_fullname {
string name<MAXNETNAMELEN>; /* name of client */
des_block key; /* PK encrypted conversation key */
opaque window[4]; /* encrypted window */
};
A credential is either a fullname or a nickname:
union authdes_cred switch (authdes_namekind adc_namekind) {
case ADN_FULLNAME:
authdes_fullname adc_fullname;
case ADN_NICKNAME:
int adc_nickname;
};
A timestamp encodes the time since midnight, March 1, 1970.
struct timestamp {
unsigned int seconds; /* seconds */
unsigned int useconds; /* and microseconds */
};
Verifier: client variety.
The window verifier is only used in the first transaction. In
conjunction with a fullname credential, these items are packed into
the following structure before being encrypted:
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struct {
adv_timestamp; -- one DES block
adc_fullname.window; -- one half DES block
adv_winverf; -- one half DES block
}
This structure is encrypted using CBC mode encryption with an input
vector of zero. All other encryptions of timestamps use ECB mode
encryption.
struct authdes_verf_clnt {
des_block adv_timestamp; /* encrypted timestamp */
opaque adv_winverf[4]; /* encrypted window verifier */
};
Verifier: server variety.
The server returns (encrypted) the same timestamp the client gave it
minus one second. It also tells the client its nickname to be used
in future transactions (unencrypted).
struct authdes_verf_svr {
des_block adv_timeverf; /* encrypted verifier */
int adv_nickname; /* new nickname for client */
};
9.3.5 Diffie-Hellman Encryption
In this scheme, there are two constants "BASE" and "MODULUS" [3].
The particular values Sun has chosen for these for the DES
authentication protocol are:
const BASE = 3;
const MODULUS = "d4a0ba0250b6fd2ec626e7efd637df76c716e22d0944b88b"
The way this scheme works is best explained by an example. Suppose
there are two people "A" and "B" who want to send encrypted messages
to each other. So, A and B both generate "secret" keys at random
which they do not reveal to anyone. Let these keys be represented as
SK(A) and SK(B). They also publish in a public directory their
"public" keys. These keys are computed as follows:
PK(A) = ( BASE ** SK(A) ) mod MODULUS
PK(B) = ( BASE ** SK(B) ) mod MODULUS
The "**" notation is used here to represent exponentiation. Now, both
A and B can arrive at the "common" key between them, represented here
as CK(A, B), without revealing their secret keys.
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A computes:
CK(A, B) = ( PK(B) ** SK(A)) mod MODULUS
while B computes:
CK(A, B) = ( PK(A) ** SK(B)) mod MODULUS
These two can be shown to be equivalent:
(PK(B) ** SK(A)) mod MODULUS = (PK(A) ** SK(B)) mod MODULUS
We drop the "mod MODULUS" parts and assume modulo arithmetic to
simplify things:
PK(B) ** SK(A) = PK(A) ** SK(B)
Then, replace PK(B) by what B computed earlier and likewise for PK(A).
((BASE ** SK(B)) ** SK(A) = (BASE ** SK(A)) ** SK(B)
which leads to:
BASE ** (SK(A) * SK(B)) = BASE ** (SK(A) * SK(B))
This common key CK(A, B) is not used to encrypt the timestamps used
in the protocol. Rather, it is used only to encrypt a conversation
key which is then used to encrypt the timestamps. The reason for
doing this is to use the common key as little as possible, for fear
that it could be broken. Breaking the conversation key is a far less
serious offense, since conversations are relatively short-lived.
The conversation key is encrypted using 56-bit DES keys, yet the
common key is 192 bits. To reduce the number of bits, 56 bits are
selected from the common key as follows. The middle-most 8-bytes are
selected from the common key, and then parity is added to the lower
order bit of each byte, producing a 56-bit key with 8 bits of parity.
10. RECORD MARKING STANDARD
When RPC messages are passed on top of a byte stream transport
protocol (like TCP), it is necessary to delimit one message from
another in order to detect and possibly recover from protocol errors.
This is called record marking (RM). Sun uses this RM/TCP/IP
transport for passing RPC messages on TCP streams. One RPC message
fits into one RM record.
A record is composed of one or more record fragments. A record
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fragment is a four-byte header followed by 0 to (2**31) - 1 bytes of
fragment data. The bytes encode an unsigned binary number; as with
XDR integers, the byte order is from highest to lowest. The number
encodes two values -- a boolean which indicates whether the fragment
is the last fragment of the record (bit value 1 implies the fragment
is the last fragment) and a 31-bit unsigned binary value which is the
length in bytes of the fragment's data. The boolean value is the
highest-order bit of the header; the length is the 31 low-order bits.
(Note that this record specification is NOT in XDR standard form!)
11. THE RPC LANGUAGE
Just as there was a need to describe the XDR data-types in a formal
language, there is also need to describe the procedures that operate
on these XDR data-types in a formal language as well. The RPC
Language is an extension to the XDR language, with the addition of
"program", "procedure", and "version" declarations. The following
example is used to describe the essence of the language.
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11.1 An Example Service Described in the RPC Language
Here is an example of the specification of a simple ping program.
program PING_PROG {
/*
* Latest and greatest version
*/
version PING_VERS_PINGBACK {
void
PINGPROC_NULL(void) = 0;
/*
* Ping the client, return the round-trip time
* (in microseconds). Returns -1 if the operation
* timed out.
*/
int
PINGPROC_PINGBACK(void) = 1;
} = 2;
/*
* Original version
*/
version PING_VERS_ORIG {
void
PINGPROC_NULL(void) = 0;
} = 1;
} = 1;
const PING_VERS = 2; /* latest version */
The first version described is PING_VERS_PINGBACK with two
procedures, PINGPROC_NULL and PINGPROC_PINGBACK. PINGPROC_NULL takes
no arguments and returns no results, but it is useful for computing
round-trip times from the client to the server and back again. By
convention, procedure 0 of any RPC protocol should have the same
semantics, and never require any kind of authentication. The second
procedure is used for the client to have the server do a reverse ping
operation back to the client, and it returns the amount of time (in
microseconds) that the operation used. The next version,
PING_VERS_ORIG, is the original version of the protocol and it does
not contain PINGPROC_PINGBACK procedure. It is useful for
compatibility with old client programs, and as this program matures
it may be dropped from the protocol entirely.
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11.2 The RPC Language Specification
The RPC language is identical to the XDR language defined in RFC
1014, except for the added definition of a "program-def" described
below.
program-def:
"program" identifier "{"
version-def
version-def *
"}" "=" constant ";"
version-def:
"version" identifier "{"
procedure-def
procedure-def *
"}" "=" constant ";"
procedure-def:
type-specifier identifier "(" type-specifier
("," type-specifier )* ")" "=" constant ";"
11.3 Syntax Notes
(1) The following keywords are added and cannot be used as
identifiers: "program" and "version";
(2) A version name cannot occur more than once within the scope of a
program definition. Nor can a version number occur more than once
within the scope of a program definition.
(3) A procedure name cannot occur more than once within the scope of
a version definition. Nor can a procedure number occur more than once
within the scope of version definition.
(4) Program identifiers are in the same name space as constant and
type identifiers.
(5) Only unsigned constants can be assigned to programs, versions and
procedures.
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APPENDIX A: PORT MAPPER PROGRAM PROTOCOL
The port mapper program maps RPC program and version numbers to
transport-specific port numbers. This program makes dynamic binding
of remote programs possible.
This is desirable because the range of reserved port numbers is very
small and the number of potential remote programs is very large. By
running only the port mapper on a reserved port, the port numbers of
other remote programs can be ascertained by querying the port mapper.
The port mapper also aids in broadcast RPC. A given RPC program will
usually have different port number bindings on different machines, so
there is no way to directly broadcast to all of these programs. The
port mapper, however, does have a fixed port number. So, to
broadcast to a given program, the client actually sends its message
to the port mapper located at the broadcast address. Each port mapper
that picks up the broadcast then calls the local service specified by
the client. When the port mapper gets the reply from the local
service, it sends the reply on back to the client.
A.1 Port Mapper Protocol Specification (in RPC Language)
const PMAP_PORT = 111; /* portmapper port number */
A mapping of (program, version, protocol) to port number:
struct mapping {
unsigned int prog;
unsigned int vers;
unsigned int prot;
unsigned int port;
};
Supported values for the "prot" field:
const IPPROTO_TCP = 6; /* protocol number for TCP/IP */
const IPPROTO_UDP = 17; /* protocol number for UDP/IP */
A list of mappings:
struct *pmaplist {
mapping map;
pmaplist next;
};
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Arguments to callit:
struct call_args {
unsigned int prog;
unsigned int vers;
unsigned int proc;
opaque args<>;
};
Results of callit:
struct call_result {
unsigned int port;
opaque res<>;
};
Port mapper procedures:
program PMAP_PROG {
version PMAP_VERS {
void
PMAPPROC_NULL(void) = 0;
bool
PMAPPROC_SET(mapping) = 1;
bool
PMAPPROC_UNSET(mapping) = 2;
unsigned int
PMAPPROC_GETPORT(mapping) = 3;
pmaplist
PMAPPROC_DUMP(void) = 4;
call_result
PMAPPROC_CALLIT(call_args) = 5;
} = 2;
} = 100000;
A.2 Port Mapper Operation
The portmapper program currently supports two protocols (UDP and
TCP). The portmapper is contacted by talking to it on assigned port
number 111 (SUNRPC) on either of these protocols.
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The following is a description of each of the portmapper procedures:
PMAPPROC_NULL:
This procedure does no work. By convention, procedure zero of any
protocol takes no parameters and returns no results.
PMAPPROC_SET:
When a program first becomes available on a machine, it registers
itself with the port mapper program on the same machine. The program
passes its program number "prog", version number "vers", transport
protocol number "prot", and the port "port" on which it awaits
service request. The procedure returns a boolean reply whose value
is "TRUE" if the procedure successfully established the mapping and
"FALSE" otherwise. The procedure refuses to establish a mapping if
one already exists for the tuple "(prog, vers, prot)".
PMAPPROC_UNSET:
When a program becomes unavailable, it should unregister itself with
the port mapper program on the same machine. The parameters and
results have meanings identical to those of "PMAPPROC_SET". The
protocol and port number fields of the argument are ignored.
PMAPPROC_GETPORT:
Given a program number "prog", version number "vers", and transport
protocol number "prot", this procedure returns the port number on
which the program is awaiting call requests. A port value of zeros
means the program has not been registered. The "port" field of the
argument is ignored.
PMAPPROC_DUMP:
This procedure enumerates all entries in the port mapper's database.
The procedure takes no parameters and returns a list of program,
version, protocol, and port values.
PMAPPROC_CALLIT:
This procedure allows a client to call another remote procedure on
the same machine without knowing the remote procedure's port number.
It is intended for supporting broadcasts to arbitrary remote programs
via the well-known port mapper's port. The parameters "prog",
"vers", "proc", and the bytes of "args" are the program number,
version number, procedure number, and parameters of the remote
procedure. Note:
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(1) This procedure only sends a reply if the procedure was
successfully executed and is silent (no reply) otherwise.
(2) The port mapper communicates with the remote program using UDP
only.
The procedure returns the remote program's port number, and the reply
is the reply of the remote procedure.
REFERENCES
[1] Birrell, A. D. & Nelson, B. J., "Implementing Remote Procedure
Calls", XEROX CSL-83-7, October 1983.
[2] Cheriton, D., "VMTP: Versatile Message Transaction Protocol",
Preliminary Version 0.3, Stanford University, January 1987.
[3] Diffie & Hellman, "New Directions in Cryptography", IEEE
Transactions on Information Theory IT-22, November 1976.
[4] Mills, D., "Network Time Protocol", RFC-958, M/A-COM Linkabit,
September 1985.
[5] National Bureau of Standards, "Data Encryption Standard", Federal
Information Processing Standards Publication 46, January 1977.
[6] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC-793, Information Sciences
Institute, September 1981.
[7] Postel, J., "User Datagram Protocol", RFC-768, Information
Sciences Institute, August 1980.
[8] Reynolds, J., and Postel, J., "Assigned Numbers", RFC-1010,
Information Sciences Institute, May 1987.
[9] Sun Microsystems, "XDR: External Data Representation Standard",
RFC-1014, June 1987.
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