XDR: External Data Representation Standard – Introduction

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XDR: External Data Representation Standard – 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 quickly becomes incomprehensible when faced with complexity.

The XDR language itself is similar to the C language, just as Courier 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 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, or least significant bit first.


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