XDR is a standard for describing and encoding data and a library of functions for external data representation, allowing programmers to encode data structures in a machine-independent way. Classic or 64-bit offset netCDF employs an extended form of XDR for representing information in the header part and the data parts. This extended XDR is used to write portable data that can be read on any other machine for which the library has been implemented.
The cost of using a canonical external representation for data varies according to the type of data and whether the external form is the same as the machine's native form for that type.
For some data types on some machines, the time required to convert data to and from external form can be significant. The worst case is reading or writing large arrays of floating-point data on a machine that does not use IEEE floating-point as its native representation.