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rfc:rfc4506

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.

Eisler Standards Track [Page 1] 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

Eisler Standards Track [Page 2] RFC 4506 XDR: External Data Representation Standard May 2006

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.

Eisler Standards Track [Page 3] RFC 4506 XDR: External Data Representation Standard May 2006

 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:

Eisler Standards Track [Page 4] RFC 4506 XDR: External Data Representation Standard May 2006

       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;

Eisler Standards Track [Page 5] RFC 4506 XDR: External Data Representation Standard May 2006

      (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).

Eisler Standards Track [Page 6] RFC 4506 XDR: External Data Representation Standard May 2006

 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).

Eisler Standards Track [Page 7] RFC 4506 XDR: External Data Representation Standard May 2006

 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).

Eisler Standards Track [Page 8] RFC 4506 XDR: External Data Representation Standard May 2006

 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<m>;
    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.

Eisler Standards Track [Page 9] RFC 4506 XDR: External Data Representation Standard May 2006

 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<m>;
    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

Eisler Standards Track [Page 10] RFC 4506 XDR: External Data Representation Standard May 2006

 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<m>;
    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

Eisler Standards Track [Page 11] RFC 4506 XDR: External Data Representation Standard May 2006

 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.

Eisler Standards Track [Page 12] RFC 4506 XDR: External Data Representation Standard May 2006

         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];

Eisler Standards Track [Page 13] RFC 4506 XDR: External Data Representation Standard May 2006

 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 <<struct, union, or enum definition>> 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
 ways to define the type "bool":
       typedef enum {    /* using typedef */
          FALSE = 0,
          TRUE = 1
       } bool;
       enum bool {       /* preferred alternative */
          FALSE = 0,
          TRUE = 1
       };
 This syntax is preferred because one does not have to wait until the
 end of a declaration to figure out the name of the new type.

4.19. Optional-Data

 Optional-data is one kind of union that occurs so frequently that we
 give it a special syntax of its own for declaring it.  It is declared
 as follows:
       type-name *identifier;
 This is equivalent to the following union:
       union switch (bool opted) {
       case TRUE:
          type-name element;
       case FALSE:
          void;
       } identifier;

Eisler Standards Track [Page 14] RFC 4506 XDR: External Data Representation Standard May 2006

 It is also equivalent to the following variable-length array
 declaration, since the boolean "opted" can be interpreted as the
 length of the array:
       type-name identifier<1>;
 Optional-data is not so interesting in itself, but it is very useful
 for describing recursive data-structures such as linked-lists and
 trees.  For example, the following defines a type "stringlist" that
 encodes lists of zero or more arbitrary length strings:
      struct stringentry {
         string item<>;
         stringentry *next;
      };
      typedef stringentry *stringlist;
 It could have been equivalently declared as the following union:
       union stringlist switch (bool opted) {
       case TRUE:
          struct {
             string item<>;
             stringlist next;
          } element;
       case FALSE:
          void;
       };
 or as a variable-length array:
      struct stringentry {
         string item<>;
         stringentry next<1>;
      };
      typedef stringentry stringlist<1>;
 Both of these declarations obscure the intention of the stringlist
 type, so the optional-data declaration is preferred over both of
 them.  The optional-data type also has a close correlation to how
 recursive data structures are represented in high-level languages
 such as Pascal or C by use of pointers.  In fact, the syntax is the
 same as that of the C language for pointers.

Eisler Standards Track [Page 15] RFC 4506 XDR: External Data Representation Standard May 2006

4.20. Areas for Future Enhancement

 The XDR standard lacks representations for bit fields and bitmaps,
 since the standard is based on bytes.  Also missing are packed (or
 binary-coded) decimals.
 The intent of the XDR standard was not to describe every kind of data
 that people have ever sent or will ever want to send from machine to
 machine.  Rather, it only describes the most commonly used data-types
 of high-level languages such as Pascal or C so that applications
 written in these languages will be able to communicate easily over
 some medium.
 One could imagine extensions to XDR that would let it describe almost
 any existing protocol, such as TCP.  The minimum necessary for this
 is support for different block sizes and byte-orders.  The XDR
 discussed here could then be considered the 4-byte big-endian member
 of a larger XDR family.

5. Discussion

 (1) Why use a language for describing data?  What's wrong with
     diagrams?
 There are many advantages in using a data-description language such
 as XDR versus using diagrams.  Languages are more formal than
 diagrams and lead to less ambiguous descriptions of data.  Languages
 are also easier to understand and allow one to think of other issues
 instead of the low-level details of bit encoding.  Also, there is a
 close analogy between the types of XDR and a high-level language such
 as C or Pascal.  This makes the implementation of XDR encoding and
 decoding modules an easier task.  Finally, the language specification
 itself is an ASCII string that can be passed from machine to machine
 to perform on-the-fly data interpretation.
 (2) Why is there only one byte-order for an XDR unit?
 Supporting two byte-orderings requires a higher-level protocol for
 determining in which byte-order the data is encoded.  Since XDR is
 not a protocol, this can't be done.  The advantage of this, though,
 is that data in XDR format can be written to a magnetic tape, for
 example, and any machine will be able to interpret it, since no
 higher-level protocol is necessary for determining the byte-order.
 (3) Why is the XDR byte-order big-endian instead of little-endian?
     Isn't this unfair to little-endian machines such as the VAX(r),
     which has to convert from one form to the other?

Eisler Standards Track [Page 16] RFC 4506 XDR: External Data Representation Standard May 2006

 Yes, it is unfair, but having only one byte-order means you have to
 be unfair to somebody.  Many architectures, such as the Motorola
 68000* and IBM 370*, support the big-endian byte-order.
 (4) Why is the XDR unit four bytes wide?
 There is a tradeoff in choosing the XDR unit size.  Choosing a small
 size, such as two, makes the encoded data small, but causes alignment
 problems for machines that aren't aligned on these boundaries.  A
 large size, such as eight, means the data will be aligned on
 virtually every machine, but causes the encoded data to grow too big.
 We chose four as a compromise.  Four is big enough to support most
 architectures efficiently, except for rare machines such as the
 eight-byte-aligned Cray*.  Four is also small enough to keep the
 encoded data restricted to a reasonable size.
 (5) Why must variable-length data be padded with zeros?
 It is desirable that the same data encode into the same thing on all
 machines, so that encoded data can be meaningfully compared or
 checksummed.  Forcing the padded bytes to be zero ensures this.
 (6) Why is there no explicit data-typing?
 Data-typing has a relatively high cost for what small advantages it
 may have.  One cost is the expansion of data due to the inserted type
 fields.  Another is the added cost of interpreting these type fields
 and acting accordingly.  And most protocols already know what type
 they expect, so data-typing supplies only redundant information.
 However, one can still get the benefits of data-typing using XDR.
 One way is to encode two things: first, a string that is the XDR data
 description of the encoded data, and then the encoded data itself.
 Another way is to assign a value to all the types in XDR, and then
 define a universal type that takes this value as its discriminant and
 for each value, describes the corresponding data type.

6. The XDR Language Specification

6.1. Notational Conventions

 This specification uses an extended Back-Naur Form notation for
 describing the XDR language.  Here is a brief description of the
 notation:
 (1) The characters '|', '(', ')', '[', ']', '"', and '*' are special.
 (2) Terminal symbols are strings of any characters surrounded by
 double quotes.  (3) Non-terminal symbols are strings of non-special
 characters.  (4) Alternative items are separated by a vertical bar

Eisler Standards Track [Page 17] RFC 4506 XDR: External Data Representation Standard May 2006

 ("|").  (5) Optional items are enclosed in brackets.  (6) Items are
 grouped together by enclosing them in parentheses.  (7) A '*'
 following an item means 0 or more occurrences of that item.
 For example, consider the following pattern:
       "a " "very" (", " "very")* [" cold " "and "]  " rainy "
       ("day" | "night")
 An infinite number of strings match this pattern.  A few of them are:
       "a very rainy day"
       "a very, very rainy day"
       "a very cold and  rainy day"
       "a very, very, very cold and  rainy night"

6.2. Lexical Notes

 (1) Comments begin with '/*' and terminate with '*/'.  (2) White
 space serves to separate items and is otherwise ignored.  (3) An
 identifier is a letter followed by an optional sequence of letters,
 digits, or underbar ('_').  The case of identifiers is not ignored.
 (4) A decimal constant expresses a number in base 10 and is a
 sequence of one or more decimal digits, where the first digit is not
 a zero, and is optionally preceded by a minus-sign ('-').  (5) A
 hexadecimal constant expresses a number in base 16, and must be
 preceded by '0x', followed by one or hexadecimal digits ('A', 'B',
 'C', 'D', E', 'F', 'a', 'b', 'c', 'd', 'e', 'f', '0', '1', '2', '3',
 '4', '5', '6', '7', '8', '9').  (6) An octal constant expresses a
 number in base 8, always leads with digit 0, and is a sequence of one
 or more octal digits ('0', '1', '2', '3', '4', '5', '6', '7').

6.3. Syntax Information

    declaration:
         type-specifier identifier
       | type-specifier identifier "[" value "]"
       | type-specifier identifier "<" [ value ] ">"
       | "opaque" identifier "[" value "]"
       | "opaque" identifier "<" [ value ] ">"
       | "string" identifier "<" [ value ] ">"
       | type-specifier "*" identifier
       | "void"
    value:
         constant
       | identifier

Eisler Standards Track [Page 18] RFC 4506 XDR: External Data Representation Standard May 2006

    constant:
       decimal-constant | hexadecimal-constant | octal-constant
    type-specifier:
         [ "unsigned" ] "int"
       | [ "unsigned" ] "hyper"
       | "float"
       | "double"
       | "quadruple"
       | "bool"
       | enum-type-spec
       | struct-type-spec
       | union-type-spec
       | identifier
    enum-type-spec:
       "enum" enum-body
    enum-body:
       "{"
          ( identifier "=" value )
          ( "," identifier "=" value )*
       "}"
    struct-type-spec:
       "struct" struct-body
    struct-body:
       "{"
          ( declaration ";" )
          ( declaration ";" )*
       "}"
    union-type-spec:
       "union" union-body
    union-body:
       "switch" "(" declaration ")" "{"
          case-spec
          case-spec *
          [ "default" ":" declaration ";" ]
       "}"
    case-spec:
      ( "case" value ":")
      ( "case" value ":") *
      declaration ";"

Eisler Standards Track [Page 19] RFC 4506 XDR: External Data Representation Standard May 2006

    constant-def:
       "const" identifier "=" constant ";"
    type-def:
         "typedef" declaration ";"
       | "enum" identifier enum-body ";"
       | "struct" identifier struct-body ";"
       | "union" identifier union-body ";"
    definition:
         type-def
       | constant-def
    specification:
         definition *

6.4. Syntax Notes

 (1) The following are keywords and cannot be used as identifiers:
 "bool", "case", "const", "default", "double", "quadruple", "enum",
 "float", "hyper", "int", "opaque", "string", "struct", "switch",
 "typedef", "union", "unsigned", and "void".
 (2) Only unsigned constants may be used as size specifications for
 arrays.  If an identifier is used, it must have been declared
 previously as an unsigned constant in a "const" definition.
 (3) Constant and type identifiers within the scope of a specification
 are in the same name space and must be declared uniquely within this
 scope.
 (4) Similarly, variable names must be unique within the scope of
 struct and union declarations.  Nested struct and union declarations
 create new scopes.
 (5) The discriminant of a union must be of a type that evaluates to
 an integer.  That is, "int", "unsigned int", "bool", an enumerated
 type, or any typedefed type that evaluates to one of these is legal.
 Also, the case values must be one of the legal values of the
 discriminant.  Finally, a case value may not be specified more than
 once within the scope of a union declaration.

Eisler Standards Track [Page 20] RFC 4506 XDR: External Data Representation Standard May 2006

7. An Example of an XDR Data Description

 Here is a short XDR data description of a thing called a "file",
 which might be used to transfer files from one machine to another.
       const MAXUSERNAME = 32;     /* max length of a user name */
       const MAXFILELEN = 65535;   /* max length of a file      */
       const MAXNAMELEN = 255;     /* max length of a file name */
       /*
        * Types of files:
        */
       enum filekind {
          TEXT = 0,       /* ascii data */
          DATA = 1,       /* raw data   */
          EXEC = 2        /* executable */
       };
       /*
        * File information, per kind of file:
        */
       union filetype switch (filekind kind) {
       case TEXT:
          void;                           /* no extra information */
       case DATA:
          string creator<MAXNAMELEN>;     /* data creator         */
       case EXEC:
          string interpretor<MAXNAMELEN>; /* program interpretor  */
       };
       /*
        * A complete file:
        */
       struct file {
          string filename<MAXNAMELEN>; /* name of file    */
          filetype type;               /* info about file */
          string owner<MAXUSERNAME>;   /* owner of file   */
          opaque data<MAXFILELEN>;     /* file data       */
       };
 Suppose now that there is a user named "john" who wants to store his
 lisp program "sillyprog" that contains just the data "(quit)".  His
 file would be encoded as follows:

Eisler Standards Track [Page 21] RFC 4506 XDR: External Data Representation Standard May 2006

     OFFSET  HEX BYTES       ASCII    COMMENTS
     ------  ---------       -----    --------
      0      00 00 00 09     ....     -- length of filename = 9
      4      73 69 6c 6c     sill     -- filename characters
      8      79 70 72 6f     ypro     -- ... and more characters ...
     12      67 00 00 00     g...     -- ... and 3 zero-bytes of fill
     16      00 00 00 02     ....     -- filekind is EXEC = 2
     20      00 00 00 04     ....     -- length of interpretor = 4
     24      6c 69 73 70     lisp     -- interpretor characters
     28      00 00 00 04     ....     -- length of owner = 4
     32      6a 6f 68 6e     john     -- owner characters
     36      00 00 00 06     ....     -- length of file data = 6
     40      28 71 75 69     (qui     -- file data bytes ...
     44      74 29 00 00     t)..     -- ... and 2 zero-bytes of fill

8. Security Considerations

 XDR is a data description language, not a protocol, and hence it does
 not inherently give rise to any particular security considerations.
 Protocols that carry XDR-formatted data, such as NFSv4, are
 responsible for providing any necessary security services to secure
 the data they transport.
 Care must be take to properly encode and decode data to avoid
 attacks.  Known and avoidable risks include:
  • Buffer overflow attacks. Where feasible, protocols should be

defined with explicit limits (via the "<" [ value ] ">" notation

      instead of "<" ">") on elements with variable-length data types.
      Regardless of the feasibility of an explicit limit on the
      variable length of an element of a given protocol, decoders need
      to ensure the incoming size does not exceed the length of any
      provisioned receiver buffers.
  • Nul octets embedded in an encoded value of type string. If the

decoder's native string format uses nul-terminated strings, then

      the apparent size of the decoded object will be less than the
      amount of memory allocated for the string.  Some memory
      deallocation interfaces take a size argument.  The caller of the
      deallocation interface would likely determine the size of the
      string by counting to the location of the nul octet and adding
      one.  This discrepancy can cause memory leakage (because less
      memory is actually returned to the free pool than allocated),
      leading to system failure and a denial of service attack.
  • Decoding of characters in strings that are legal ASCII

characters but nonetheless are illegal for the intended

      application.  For example, some operating systems treat the '/'

Eisler Standards Track [Page 22] RFC 4506 XDR: External Data Representation Standard May 2006

      character as a component separator in path names.  For a
      protocol that encodes a string in the argument to a file
      creation operation, the decoder needs to ensure that '/' is not
      inside the component name.  Otherwise, a file with an illegal
      '/' in its name will be created, making it difficult to remove,
      and is therefore a denial of service attack.
  • Denial of service caused by recursive decoder or encoder

subroutines. A recursive decoder or encoder might process data

      that has a structured type with a member of type optional data
      that directly or indirectly refers to the structured type (i.e.,
      a linked list).  For example,
            struct m {
              int x;
              struct m *next;
            };
      An encoder or decoder subroutine might be written to recursively
      call itself each time another element of type "struct m" is
      found.  An attacker could construct a long linked list of
      "struct m" elements in the request or response, which then
      causes a stack overflow on the decoder or encoder.  Decoders and
      encoders should be written non-recursively or impose a limit on
      list length.

9. IANA Considerations

 It is possible, if not likely, that new data types will be added to
 XDR in the future.  The process for adding new types is via a
 standards track RFC and not registration of new types with IANA.
 Standards track RFCs that update or replace this document should be
 documented as such in the RFC Editor's database of RFCs.

10. Trademarks and Owners

 SUN WORKSTATION  Sun Microsystems, Inc.
 VAX              Hewlett-Packard Company
 IBM-PC           International Business Machines Corporation
 Cray             Cray Inc.
 NFS              Sun Microsystems, Inc.
 Ethernet         Xerox Corporation.
 Motorola 68000   Motorola, Inc.
 IBM 370          International Business Machines Corporation

Eisler Standards Track [Page 23] RFC 4506 XDR: External Data Representation Standard May 2006

11. ANSI/IEEE Standard 754-1985

 The definition of NaNs, signed zero and infinity, and denormalized
 numbers from [IEEE] is reproduced here for convenience.  The
 definitions for quadruple-precision floating point numbers are
 analogs of those for single and double-precision floating point
 numbers and are defined in [IEEE].
 In the following, 'S' stands for the sign bit, 'E' for the exponent,
 and 'F' for the fractional part.  The symbol 'u' stands for an
 undefined bit (0 or 1).
 For single-precision floating point numbers:
  Type                  S (1 bit)   E (8 bits)    F (23 bits)
  ----                  ---------   ----------    -----------
  signalling NaN        u           255 (max)     .0uuuuu---u
                                                  (with at least
                                                   one 1 bit)
  quiet NaN             u           255 (max)     .1uuuuu---u
  negative infinity     1           255 (max)     .000000---0
  positive infinity     0           255 (max)     .000000---0
  negative zero         1           0             .000000---0
  positive zero         0           0             .000000---0
 For double-precision floating point numbers:
  Type                  S (1 bit)   E (11 bits)   F (52 bits)
  ----                  ---------   -----------   -----------
  signalling NaN        u           2047 (max)    .0uuuuu---u
                                                  (with at least
                                                   one 1 bit)
  quiet NaN             u           2047 (max)    .1uuuuu---u
  negative infinity     1           2047 (max)    .000000---0
  positive infinity     0           2047 (max)    .000000---0
  negative zero         1           0             .000000---0
  positive zero         0           0             .000000---0

Eisler Standards Track [Page 24] RFC 4506 XDR: External Data Representation Standard May 2006

 For quadruple-precision floating point numbers:
  Type                  S (1 bit)   E (15 bits)   F (112 bits)
  ----                  ---------   -----------   ------------
  signalling NaN        u           32767 (max)   .0uuuuu---u
                                                  (with at least
                                                   one 1 bit)
  quiet NaN             u           32767 (max)   .1uuuuu---u
  negative infinity     1           32767 (max)   .000000---0
  positive infinity     0           32767 (max)   .000000---0
  negative zero         1           0             .000000---0
  positive zero         0           0             .000000---0
 Subnormal numbers are represented as follows:
  Precision            Exponent       Value
  ---------            --------       -----
  Single               0              (-1)**S * 2**(-126) * 0.F
  Double               0              (-1)**S * 2**(-1022) * 0.F
  Quadruple            0              (-1)**S * 2**(-16382) * 0.F

12. Normative References

 [IEEE]  "IEEE Standard for Binary Floating-Point Arithmetic",
         ANSI/IEEE Standard 754-1985, Institute of Electrical and
         Electronics Engineers, August 1985.

13. Informative References

 [KERN]  Brian W. Kernighan & Dennis M. Ritchie, "The C Programming
         Language", Bell Laboratories, Murray Hill, New Jersey, 1978.
 [COHE]  Danny Cohen, "On Holy Wars and a Plea for Peace", IEEE
         Computer, October 1981.
 [COUR]  "Courier: The Remote Procedure Call Protocol", XEROX
         Corporation, XSIS 038112, December 1981.
 [SPAR]  "The SPARC Architecture Manual: Version 8", Prentice Hall,
         ISBN 0-13-825001-4.
 [HPRE]  "HP Precision Architecture Handbook", June 1987, 5954-9906.

Eisler Standards Track [Page 25] RFC 4506 XDR: External Data Representation Standard May 2006

14. Acknowledgements

 Bob Lyon was Sun's visible force behind ONC RPC in the 1980s.  Sun
 Microsystems, Inc., is listed as the author of RFC 1014.  Raj
 Srinivasan and the rest of the old ONC RPC working group edited RFC
 1014 into RFC 1832, from which this document is derived.  Mike Eisler
 and Bill Janssen submitted the implementation reports for this
 standard.  Kevin Coffman, Benny Halevy, and Jon Peterson reviewed
 this document and gave feedback.  Peter Astrand and Bryan Olson
 pointed out several errors in RFC 1832 which are corrected in this
 document.

Editor's Address

 Mike Eisler
 5765 Chase Point Circle
 Colorado Springs, CO 80919
 USA
 Phone: 719-599-9026
 EMail: email2mre-rfc4506@yahoo.com
 Please address comments to: nfsv4@ietf.org

Eisler Standards Track [Page 26] RFC 4506 XDR: External Data Representation Standard May 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 The IETF invites any interested party to bring to its attention any
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Acknowledgement

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Eisler Standards Track [Page 27]

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