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.
XDR was developed in the mid 1980s at Sun Microsystems, and first widely published in 1987. XDR became an IETF standard in 1995.
The XDR data format is in use by many systems, including:
Network File System (protocol)
ZFS File System
NDMP Network Data Management 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 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
The Firebird (database server) for Remote Binary Wire Protocol
boolean
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)
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.
