rfc1014.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 1,119 行 · 第 1/3 页

Network Working Group                             Sun Microsystems, Inc.
Request for Comments: 1014                                     June 1987


               XDR: External Data Representation Standard

STATUS OF THIS MEMO

   This RFC describes a standard that Sun Microsystems, Inc., and others
   are using, one we wish to propose for the Internet's consideration.
   Distribution of this memo is unlimited.

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 Sun 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)



SUN Microsystems                                                [Page 1]

RFC 1014              External Data Representation             June 1987


   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.
   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------------>





SUN Microsystems                                                [Page 2]

RFC 1014              External Data Representation             June 1987


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;









SUN Microsystems                                                [Page 3]

RFC 1014              External Data Representation             June 1987


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.
   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













SUN Microsystems                                                [Page 4]

RFC 1014              External Data Representation             June 1987


   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).

   The EEE 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 used externally.

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








SUN Microsystems                                                [Page 5]

RFC 1014              External Data Representation             June 1987


   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).

   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 used externally.

3.8 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

3.9 Variable-length Opaque Data

   The standard also provides for variable-length (counted) opaque data,



SUN Microsystems                                                [Page 6]

RFC 1014              External Data Representation             June 1987


   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.
   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.

3.10 String