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 Srinivasan Standards Track [Page 1] RFC 1832 XDR: External Data Representation Standard August 1995 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. Srinivasan Standards Track [Page 2] 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------------> Srinivasan Standards Track [Page 3] 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. Srinivasan Standards Track [Page 4] RFC 1832 XDR: External Data Representation Standard August 1995 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 Srinivasan Standards Track [Page 5] RFC 1832 XDR: External Data Representation Standard August 1995 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 Srinivasan Standards Track [Page 6] RFC 1832 XDR: External Data Representation Standard August 1995 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". 3.8 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 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 Srinivasan Standards Track [Page 7] RFC 1832 XDR: External Data Representation Standard August 1995 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). 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 [5], [6]. 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". 3.9 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 3.10 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. Srinivasan Standards Track [Page 8] RFC 1832 XDR: External Data Representation Standard August 1995 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 |