rfc1832.txt

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Network Working Group                                      R. Srinivasan
Request for Comments: 1832                              Sun Microsystems
Category: Standards Track                                    August 1995


               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.

ABSTRACT

   This document describes the External Data Representation Standard
   (XDR) protocol as it is currently deployed and accepted.

TABLE OF CONTENTS

   1. INTRODUCTION                                              2
   2. BASIC BLOCK SIZE                                          2
   3. XDR DATA TYPES                                            3
   3.1 Integer                                                  3
   3.2 Unsigned Integer                                         4
   3.3 Enumeration                                              4
   3.4 Boolean                                                  4
   3.5 Hyper Integer and Unsigned Hyper Integer                 4
   3.6 Floating-point                                           5
   3.7 Double-precision Floating-point                          6
   3.8 Quadruple-precision Floating-point                       7
   3.9 Fixed-length Opaque Data                                 8
   3.10 Variable-length Opaque Data                             8
   3.11 String                                                  9
   3.12 Fixed-length Array                                     10
   3.13 Variable-length Array                                  10
   3.14 Structure                                              11
   3.15 Discriminated Union                                    11
   3.16 Void                                                   12
   3.17 Constant                                               12
   3.18 Typedef                                                13
   3.19 Optional-data                                          14
   3.20 Areas for Future Enhancement                           15
   4. DISCUSSION                                               15
   5. THE XDR LANGUAGE SPECIFICATION                           17
   5.1 Notational Conventions                                  17



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   5.2 Lexical Notes                                           17
   5.3 Syntax Information                                      18
   5.4 Syntax Notes                                            19
   6. AN EXAMPLE OF AN XDR DATA DESCRIPTION                    20
   7. TRADEMARKS AND OWNERS                                    21
   APPENDIX A: ANSI/IEEE Standard 754-1985                     22
   APPENDIX B: REFERENCES                                      24
   Security Considerations                                     24
   Author's Address                                            24

1. INTRODUCTION

   XDR is a standard for the description and encoding of data.  It is
   useful for transferring data between different computer
   architectures, and 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 only
   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 [1], just as Courier [4] 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 to be 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 [2], or least
   significant bit first.

2. 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.




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RFC 1832       XDR: External Data Representation Standard    August 1995


   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.
   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)>----------->|

3. 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
   variablelength sequences of data and 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 5:
   "The XDR Language Specification".

   For some data types, more specific examples are included.  A more
   extensive example of a data description is in section 6:  "An Example
   of an XDR Data Description".

3.1 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------------>







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RFC 1832       XDR: External Data Representation Standard    August 1995


3.2. Unsigned Integer

   An XDR unsigned integer is a 32-bit datum that encodes a nonnegative
   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:

         unsigned int identifier;

           (MSB)                   (LSB)
            +-------+-------+-------+-------+
            |byte 0 |byte 1 |byte 2 |byte 3 |             UNSIGNED INTEGER
            +-------+-------+-------+-------+
            <------------32 bits------------>

3.3 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 other integer than those that
   have been given assignments in the enum declaration.

3.4 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;

3.5 Hyper Integer and Unsigned Hyper Integer

   The standard also defines 64-bit (8-byte) numbers called hyper
   integer and unsigned hyper integer.  Their representations are the
   obvious extensions of integer and unsigned integer defined above.



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   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;

        (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

3.6 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 [3].  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



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   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).

   The IEEE specifications should be consulted concerning the encoding
   for signed zero, signed infinity (overflow), and denormalized numbers
   (underflow) [3].  According to IEEE specifications, the "NaN" (not a
   number) is system dependent and should not be interpreted within XDR
   as anything other than "NaN".

3.7 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 [3].  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