External Data Representation
External Data Representation (XDR) is a standard data serialization format, for uses such as computer network protocols. It allows data to be transferred between different kinds of 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 which is portable between different operating systems and is also independent of the transport layer.
XDR uses a base unit of 4 bytes, serialized in big-endian order; smaller data types still occupy four bytes each after encoding. Variable-length types such as string and opaque are padded to a total divisible by four bytes. Floating-point numbers are represented in IEEE 754 format.
The XDR data format is in use by many systems, including:
- Network File System (protocol)
- NDMP Network Data Managerment Protocol
- Open Network Computing Remote Procedure Call
- Legato NetWorker backup software (later sold by EMC)
- NetCDF (a scientific data format)
- The R language and environment for statistical computing
- The HTTP-NG Binary Wire Protocol
- The Ganglia distributed monitoring system
- The sFlow network monitoring standard
- The libvirt virtualization library, API and UI
- The Firebird (database server) for Remote Binary Wire Protocol
XDR data types 
- int – 32-bit integer
- unsigned int – unsigned 32-bit integer
- hyper – 64-bit integer
- unsigned hyper – unsigned 64-bit integer
- IEEE float
- IEEE double
- quadruple (new in RFC1832)
- 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.
See also 
- Structured Data eXchange Format (SDXF)
- Remote Procedure Call
- Abstract Syntax Notation One
- Data Format Description Language
- Comparison of data serialization formats
The XDR standard exists in three different versions in the following RFCs:
- 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.
- Cisco's XDR: Technical Notes
- jsxdrapi.c, the main source file of SpiderMonkey that uses XDR
- protocol.cpp main xdr source file used in Firebird remote protocol
- 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)
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