RFC 1832 (rfc1832) - Page 2 of 24
XDR: External Data Representation Standard
Alternative Format: Original Text Document
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
