External Data Representation

eXternal Data Representation (XDR) is an IETF standard from 1995. It allows data to be wrapped in an architecture independent manner so data can be transferred between heterogeneous computer systems. Converting from the local representation to XDR is called encoding. Converting from XDR to the local representation is called decoding. XDR is implemented as a software library of functions that is portable between different operating systems and is also independent of the transport layer.

The XDR data format is in use by many systems, including:

• Network File System (protocol)
• Open Network Computing Remote Procedure Call
• EMC's NetWorker backup software (in its newer releases)
• NetCDF (a scientific data format)
• The R language and environment for statistical computing
• High Level Architecture (simulation)
• The HTTP-NG Binary Wire Protocol
• The SpiderMonkey JavaScript engine, to serialize/deserialize compiled JavaScript code
• The Ganglia distributed monitoring system
• The sFlow network monitoring standard
• The libvirt virtualization library, API and UI

XDR data types

• boolean
• int (32 bit integer)
• unsigned int (unsigned 32 bit integer)
• hyper (64 bit integer)
• unsigned hyper (unsigned 64 bit integer)
• float
• double
• quadruple (new in RFC1832)
• enumeration
• structure
• string
• fixed length array
• variable length array
• union - discriminated union
• fixed length opaque data
• variable length opaque data
• void - zero byte quantity
• optional - Optional data is notated similarly to C pointers, but is represented as the data type "pointed to" with a boolean "present or not" flag.

XDR uses a base unit of 4 bytes. This means that data of types smaller than that occupy four bytes each after encoding. Variable-length types like string and opaque are padded to a total divisible by four bytes.

See also

• Structured Data eXchange Format (SDXF).
• Multipurpose Internet Mail Extensions (MIME) used on top of the SMTP and HTTP application protocols
• Remote Procedure Call
• Abstract Syntax Notation One

External links

The XDR standard exists in three different versions in the following RFC's...

• RFC 4506 2006 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..
• RFC 1832 1995 version. Added Quadruple precision floating point to RFC 1014
• RFC 1014 Original 1987 version.

• jsxdrapi.c, the main source file of SpiderMonkey that uses XDR
• http://www.cs.rpi.edu/~hollingd/netprog/notes/xdr/xdr.pdf
• The GNU Libc implementation of rpcgen, the XDR parser.
• Mu Dynamics Research Labs racc grammar for XDR
• IvmaiAsn ASN1/ECN/XDR Tools (a collection of tools containing an XDR/RPC-to-ASN.1 converter)