Network Working Group M. Eisler, Ed.
Request for Comments: 4506 Network Appliance, Inc.
STD: 67 May 2006
Obsoletes: 1832
Category: Standards Track
XDR: External Data Representation Standard
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.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes the External Data Representation Standard
(XDR) protocol as it is currently deployed and accepted. This
document obsoletes RFC 1832.
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RFC 4506 XDR: External Data Representation Standard May 2006
Table of Contents
1. Introduction ....................................................3
2. Changes from RFC 1832 ...........................................3
3. Basic Block Size ................................................3
4. XDR Data Types ..................................................4
4.1. Integer ....................................................4
4.2. Unsigned Integer ...........................................4
4.3. Enumeration ................................................5
4.4. Boolean ....................................................5
4.5. Hyper Integer and Unsigned Hyper Integer ...................5
4.6. Floating-Point .............................................6
4.7. Double-Precision Floating-Point ............................7
4.8. Quadruple-Precision Floating-Point .........................8
4.9. Fixed-Length Opaque Data ...................................9
4.10. Variable-Length Opaque Data ...............................9
4.11. String ...................................................10
4.12. Fixed-Length Array .......................................11
4.13. Variable-Length Array ....................................11
4.14. Structure ................................................12
4.15. Discriminated Union ......................................12
4.16. Void .....................................................13
4.17. Constant .................................................13
4.18. Typedef ..................................................13
4.19. Optional-Data ............................................14
4.20. Areas for Future Enhancement .............................16
5. Discussion .....................................................16
6. The XDR Language Specification .................................17
6.1. Notational Conventions ....................................17
6.2. Lexical Notes .............................................18
6.3. Syntax Information ........................................18
6.4. Syntax Notes ..............................................20
7. An Example of an XDR Data Description ..........................21
8. Security Considerations ........................................22
9. IANA Considerations ............................................23
10. Trademarks and Owners .........................................23
11. ANSI/IEEE Standard 754-1985 ...................................24
12. Normative References ..........................................25
13. Informative References ........................................25
14. Acknowledgements ..............................................26
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1. Introduction
XDR is a standard for the description and encoding of data. It is
useful for transferring data between different computer
architectures, and it has been used to communicate data between such
diverse machines as the SUN WORKSTATION*, VAX*, IBM-PC*, and Cray*.
XDR fits into the ISO presentation layer and is roughly analogous in
purpose to X.409, ISO Abstract Syntax Notation. The major difference
between these two is that XDR uses implicit typing, while X.409 uses
explicit typing.
XDR uses a language to describe data formats. The language can be
used only to describe data; it is not a programming language. This
language allows one to describe intricate data formats in a concise
manner. The alternative of using graphical representations (itself
an informal language) quickly becomes incomprehensible when faced
with complexity. The XDR language itself is similar to the C
language [KERN], just as Courier [COUR] is similar to Mesa.
Protocols such as ONC RPC (Remote Procedure Call) and the NFS*
(Network File System) use XDR to describe the format of their data.
The XDR standard makes the following assumption: that bytes (or
octets) are portable, where a byte is defined as 8 bits of data. A
given hardware device should encode the bytes onto the various media
in such a way that other hardware devices may decode the bytes
without loss of meaning. For example, the Ethernet* standard
suggests that bytes be encoded in "little-endian" style [COHE], or
least significant bit first.
2. Changes from RFC 1832
This document makes no technical changes to RFC 1832 and is published
for the purposes of noting IANA considerations, augmenting security
considerations, and distinguishing normative from informative
references.
3. Basic Block Size
The representation of all items requires a multiple of four bytes (or
32 bits) of data. The bytes are numbered 0 through n-1. The bytes
are read or written to some byte stream such that byte m always
precedes byte m+1. If the n bytes needed to contain the data are not
a multiple of four, then the n bytes are followed by enough (0 to 3)
residual zero bytes, r, to make the total byte count a multiple of 4.
We include the familiar graphic box notation for illustration and
comparison. In most illustrations, each box (delimited by a plus
sign at the 4 corners and vertical bars and dashes) depicts a byte.
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Ellipses (...) between boxes show zero or more additional bytes where
required.
+--------+--------+...+--------+--------+...+--------+
| byte 0 | byte 1 |...|byte n-1| 0 |...| 0 | BLOCK
+--------+--------+...+--------+--------+...+--------+
|<-----------n bytes---------->|<------r bytes------>|
|<-----------n+r (where (n+r) mod 4 = 0)>----------->|
4. XDR Data Types
Each of the sections that follow describes a data type defined in the
XDR standard, shows how it is declared in the language, and includes
a graphic illustration of its encoding.
For each data type in the language we show a general paradigm
declaration. Note that angle brackets (< and >) denote variable-
length sequences of data and that square brackets ([ and ]) denote
fixed-length sequences of data. "n", "m", and "r" denote integers.
For the full language specification and more formal definitions of
terms such as "identifier" and "declaration", refer to Section 6,
"The XDR Language Specification".
For some data types, more specific examples are included. A more
extensive example of a data description is in Section 7, "An Example
of an XDR Data Description".
4.1. Integer
An XDR signed integer is a 32-bit datum that encodes an integer in
the range [-2147483648,2147483647]. The integer is represented in
two's complement notation. The most and least significant bytes are
0 and 3, respectively. Integers are declared as follows:
int identifier;
(MSB) (LSB)
+-------+-------+-------+-------+
|byte 0 |byte 1 |byte 2 |byte 3 | INTEGER
+-------+-------+-------+-------+
<------------32 bits------------>
4.2. Unsigned Integer
An XDR unsigned integer is a 32-bit datum that encodes a non-negative
integer in the range [0,4294967295]. It is represented by an
unsigned binary number whose most and least significant bytes are 0
and 3, respectively. An unsigned integer is declared as follows:
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unsigned int identifier;
(MSB) (LSB)
+-------+-------+-------+-------+
|byte 0 |byte 1 |byte 2 |byte 3 | UNSIGNED INTEGER
+-------+-------+-------+-------+
<------------32 bits------------>
4.3. Enumeration
Enumerations have the same representation as signed integers.
Enumerations are handy for describing subsets of the integers.
Enumerated data is declared as follows:
enum { name-identifier = constant, ... } identifier;
For example, the three colors red, yellow, and blue could be
described by an enumerated type:
enum { RED = 2, YELLOW = 3, BLUE = 5 } colors;
It is an error to encode as an enum any integer other than those that
have been given assignments in the enum declaration.
4.4. Boolean
Booleans are important enough and occur frequently enough to warrant
their own explicit type in the standard. Booleans are declared as
follows:
bool identifier;
This is equivalent to:
enum { FALSE = 0, TRUE = 1 } identifier;
4.5. Hyper Integer and Unsigned Hyper Integer
The standard also defines 64-bit (8-byte) numbers called hyper
integers and unsigned hyper integers. Their representations are the
obvious extensions of integer and unsigned integer defined above.
They are represented in two's complement notation. The most and
least significant bytes are 0 and 7, respectively. Their
declarations:
hyper identifier; unsigned hyper identifier;
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(MSB) (LSB)
+-------+-------+-------+-------+-------+-------+-------+-------+
|byte 0 |byte 1 |byte 2 |byte 3 |byte 4 |byte 5 |byte 6 |byte 7 |
+-------+-------+-------+-------+-------+-------+-------+-------+
<----------------------------64 bits---------------------------->
HYPER INTEGER
UNSIGNED HYPER INTEGER
4.6. Floating-Point
The standard defines the floating-point data type "float" (32 bits or
4 bytes). The encoding used is the IEEE standard for normalized
single-precision floating-point numbers [IEEE]. The following three
fields describe the single-precision floating-point number:
S: The sign of the number. Values 0 and 1 represent positive and
negative, respectively. One bit.
E: The exponent of the number, base 2. 8 bits are devoted to this
field. The exponent is biased by 127.
F: The fractional part of the number's mantissa, base 2. 23 bits
are devoted to this field.
Therefore, the floating-point number is described by:
(-1)**S * 2**(E-Bias) * 1.F
It is declared as follows:
float identifier;
+-------+-------+-------+-------+
|byte 0 |byte 1 |byte 2 |byte 3 | SINGLE-PRECISION
S| E | F | FLOATING-POINT NUMBER
+-------+-------+-------+-------+
1|<- 8 ->|<-------23 bits------>|
<------------32 bits------------>
Just as the most and least significant bytes of a number are 0 and 3,
the most and least significant bits of a single-precision floating-
point number are 0 and 31. The beginning bit (and most significant
bit) offsets of S, E, and F are 0, 1, and 9, respectively. Note that
these numbers refer to the mathematical positions of the bits, and
NOT to their actual physical locations (which vary from medium to
medium).
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The IEEE specifications should be consulted concerning the encoding
for signed zero, signed infinity (overflow), and denormalized numbers
(underflow) [IEEE]. According to IEEE specifications, the "NaN" (not
a number) is system dependent and should not be interpreted within
XDR as anything other than "NaN".
4.7. Double-Precision Floating-Point
The standard defines the encoding for the double-precision floating-
point data type "double" (64 bits or 8 bytes). The encoding used is
the IEEE standard for normalized double-precision floating-point
numbers [IEEE]. The standard encodes the following three fields,
which describe the double-precision floating-point number:
S: The sign of the number. Values 0 and 1 represent positive and
negative, respectively. One bit.
E: The exponent of the number, base 2. 11 bits are devoted to
this field. The exponent is biased by 1023.
F: The fractional part of the number's mantissa, base 2. 52 bits
are devoted to this field.
Therefore, the floating-point number is described by:
(-1)**S * 2**(E-Bias) * 1.F
It is declared as follows:
double identifier;
+------+------+------+------+------+------+------+------+
|byte 0|byte 1|byte 2|byte 3|byte 4|byte 5|byte 6|byte 7|
S| E | F |
+------+------+------+------+------+------+------+------+
1|<--11-->|<-----------------52 bits------------------->|
<-----------------------64 bits------------------------->
DOUBLE-PRECISION FLOATING-POINT
Just as the most and least significant bytes of a number are 0 and 3,
the most and least significant bits of a double-precision floating-
point number are 0 and 63. The beginning bit (and most significant
bit) offsets of S, E, and F are 0, 1, and 12, respectively. Note
that these numbers refer to the mathematical positions of the bits,
and NOT to their actual physical locations (which vary from medium to
medium).
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The IEEE specifications should be consulted concerning the encoding
for signed zero, signed infinity (overflow), and denormalized numbers
(underflow) [IEEE]. According to IEEE specifications, the "NaN" (not
a number) is system dependent and should not be interpreted within
XDR as anything other than "NaN".
4.8. Quadruple-Precision Floating-Point
The standard defines the encoding for the quadruple-precision
floating-point data type "quadruple" (128 bits or 16 bytes). The
encoding used is designed to be a simple analog of the encoding used
for single- and double-precision floating-point numbers using one
form of IEEE double extended precision. The standard encodes the
following three fields, which describe the quadruple-precision
floating-point number:
S: The sign of the number. Values 0 and 1 represent positive and
negative, respectively. One bit.
E: The exponent of the number, base 2. 15 bits are devoted to
this field. The exponent is biased by 16383.
F: The fractional part of the number's mantissa, base 2. 112 bits
are devoted to this field.
Therefore, the floating-point number is described by:
(-1)**S * 2**(E-Bias) * 1.F
It is declared as follows:
quadruple identifier;
+------+------+------+------+------+------+-...--+------+
|byte 0|byte 1|byte 2|byte 3|byte 4|byte 5| ... |byte15|
S| E | F |
+------+------+------+------+------+------+-...--+------+
1|<----15---->|<-------------112 bits------------------>|
<-----------------------128 bits------------------------>
QUADRUPLE-PRECISION FLOATING-POINT
Just as the most and least significant bytes of a number are 0 and 3,
the most and least significant bits of a quadruple-precision
floating-point number are 0 and 127. The beginning bit (and most
significant bit) offsets of S, E , and F are 0, 1, and 16,
respectively. Note that these numbers refer to the mathematical
positions of the bits, and NOT to their actual physical locations
(which vary from medium to medium).
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The encoding for signed zero, signed infinity (overflow), and
denormalized numbers are analogs of the corresponding encodings for
single and double-precision floating-point numbers [SPAR], [HPRE].
The "NaN" encoding as it applies to quadruple-precision floating-
point numbers is system dependent and should not be interpreted
within XDR as anything other than "NaN".
4.9. Fixed-Length Opaque Data
At times, fixed-length uninterpreted data needs to be passed among
machines. This data is called "opaque" and is declared as follows:
opaque identifier[n];
where the constant n is the (static) number of bytes necessary to
contain the opaque data. If n is not a multiple of four, then the n
bytes are followed by enough (0 to 3) residual zero bytes, r, to make
the total byte count of the opaque object a multiple of four.
0 1 ...
+--------+--------+...+--------+--------+...+--------+
| byte 0 | byte 1 |...|byte n-1| 0 |...| 0 |
+--------+--------+...+--------+--------+...+--------+
|<-----------n bytes---------->|<------r bytes------>|
|<-----------n+r (where (n+r) mod 4 = 0)------------>|
FIXED-LENGTH OPAQUE
4.10. Variable-Length Opaque Data
The standard also provides for variable-length (counted) opaque data,
defined as a sequence of n (numbered 0 through n-1) arbitrary bytes
to be the number n encoded as an unsigned integer (as described
below), and followed by the n bytes of the sequence.
Byte m of the sequence always precedes byte m+1 of the sequence, and
byte 0 of the sequence always follows the sequence's length (count).
If n is not a multiple of four, then the n bytes are followed by
enough (0 to 3) residual zero bytes, r, to make the total byte count
a multiple of four. Variable-length opaque data is declared in the
following way:
opaque identifier;
or
opaque identifier<>;
The constant m denotes an upper bound of the number of bytes that the
sequence may contain. If m is not specified, as in the second
declaration, it is assumed to be (2**32) - 1, the maximum length.
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The constant m would normally be found in a protocol specification.
For example, a filing protocol may state that the maximum data
transfer size is 8192 bytes, as follows:
opaque filedata<8192>;
0 1 2 3 4 5 ...
+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+
| length n |byte0|byte1|...| n-1 | 0 |...| 0 |
+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+
|<-------4 bytes------->|<------n bytes------>|<---r bytes--->|
|<----n+r (where (n+r) mod 4 = 0)---->|
VARIABLE-LENGTH OPAQUE
It is an error to encode a length greater than the maximum described
in the specification.
4.11. String
The standard defines a string of n (numbered 0 through n-1) ASCII
bytes to be the number n encoded as an unsigned integer (as described
above), and followed by the n bytes of the string. Byte m of the
string always precedes byte m+1 of the string, and byte 0 of the
string always follows the string's length. If n is not a multiple of
four, then the n bytes are followed by enough (0 to 3) residual zero
bytes, r, to make the total byte count a multiple of four. Counted
byte strings are declared as follows:
string object;
or
string object<>;
The constant m denotes an upper bound of the number of bytes that a
string may contain. If m is not specified, as in the second
declaration, it is assumed to be (2**32) - 1, the maximum length.
The constant m would normally be found in a protocol specification.
For example, a filing protocol may state that a file name can be no
longer than 255 bytes, as follows:
string filename<255>;
0 1 2 3 4 5 ...
+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+
| length n |byte0|byte1|...| n-1 | 0 |...| 0 |
+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+
|<-------4 bytes------->|<------n bytes------>|<---r bytes--->|
|<----n+r (where (n+r) mod 4 = 0)---->|
STRING
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It is an error to encode a length greater than the maximum described
in the specification.
4.12. Fixed-Length Array
Declarations for fixed-length arrays of homogeneous elements are in
the following form:
type-name identifier[n];
Fixed-length arrays of elements numbered 0 through n-1 are encoded by
individually encoding the elements of the array in their natural
order, 0 through n-1. Each element's size is a multiple of four
bytes. Though all elements are of the same type, the elements may
have different sizes. For example, in a fixed-length array of
strings, all elements are of type "string", yet each element will
vary in its length.
+---+---+---+---+---+---+---+---+...+---+---+---+---+
| element 0 | element 1 |...| element n-1 |
+---+---+---+---+---+---+---+---+...+---+---+---+---+
|<--------------------n elements------------------->|
FIXED-LENGTH ARRAY
4.13. Variable-Length Array
Counted arrays provide the ability to encode variable-length arrays
of homogeneous elements. The array is encoded as the element count n
(an unsigned integer) followed by the encoding of each of the array's
elements, starting with element 0 and progressing through element
n-1. The declaration for variable-length arrays follows this form:
type-name identifier;
or
type-name identifier<>;
The constant m specifies the maximum acceptable element count of an
array; if m is not specified, as in the second declaration, it is
assumed to be (2**32) - 1.
0 1 2 3
+--+--+--+--+--+--+--+--+--+--+--+--+...+--+--+--+--+
| n | element 0 | element 1 |...|element n-1|
+--+--+--+--+--+--+--+--+--+--+--+--+...+--+--+--+--+
|<-4 bytes->|<--------------n elements------------->|
COUNTED ARRAY
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It is an error to encode a value of n that is greater than the
maximum described in the specification.
4.14. Structure
Structures are declared as follows:
struct {
component-declaration-A;
component-declaration-B;
...
} identifier;
The components of the structure are encoded in the order of their
declaration in the structure. Each component's size is a multiple of
four bytes, though the components may be different sizes.
+-------------+-------------+...
| component A | component B |... STRUCTURE
+-------------+-------------+...
4.15. Discriminated Union
A discriminated union is a type composed of a discriminant followed
by a type selected from a set of prearranged types according to the
value of the discriminant. The type of discriminant is either "int",
"unsigned int", or an enumerated type, such as "bool". The component
types are called "arms" of the union and are preceded by the value of
the discriminant that implies their encoding. Discriminated unions
are declared as follows:
union switch (discriminant-declaration) {
case discriminant-value-A:
arm-declaration-A;
case discriminant-value-B:
arm-declaration-B;
...
default: default-declaration;
} identifier;
Each "case" keyword is followed by a legal value of the discriminant.
The default arm is optional. If it is not specified, then a valid
encoding of the union cannot take on unspecified discriminant values.
The size of the implied arm is always a multiple of four bytes.
The discriminated union is encoded as its discriminant followed by
the encoding of the implied arm.
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0 1 2 3
+---+---+---+---+---+---+---+---+
| discriminant | implied arm | DISCRIMINATED UNION
+---+---+---+---+---+---+---+---+
|<---4 bytes--->|
4.16. Void
An XDR void is a 0-byte quantity. Voids are useful for describing
operations that take no data as input or no data as output. They are
also useful in unions, where some arms may contain data and others do
not. The declaration is simply as follows:
void;
Voids are illustrated as follows:
++
|| VOID
++
--><-- 0 bytes
4.17. Constant
The data declaration for a constant follows this form:
const name-identifier = n;
"const" is used to define a symbolic name for a constant; it does not
declare any data. The symbolic constant may be used anywhere a
regular constant may be used. For example, the following defines a
symbolic constant DOZEN, equal to 12.
const DOZEN = 12;
4.18. Typedef
"typedef" does not declare any data either, but serves to define new
identifiers for declaring data. The syntax is:
typedef declaration;
The new type name is actually the variable name in the declaration
part of the typedef. For example, the following defines a new type
called "eggbox" using an existing type called "egg":
typedef egg eggbox[DOZEN];
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Variables declared using the new type name have the same type as the
new type name would have in the typedef, if it were considered a
variable. For example, the following two declarations are equivalent
in declaring the variable "fresheggs":
eggbox fresheggs; egg fresheggs[DOZEN];
When a typedef involves a struct, enum, or union definition, there is
another (preferred) syntax that may be used to define the same type.
In general, a typedef of the following form:
typedef <> identifier;
may be converted to the alternative form by removing the "typedef"
part and placing the identifier after the "struct", "union", or
"enum" keyword, instead of at the end. For example, here are the two