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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Eisler Standards Track [Page 27]