A Canonical Standard

The XDR approach to standardizing data representations is canonical.

That is, XDR defines representations for a single byte (most significant bit first), a single floating-point representation (IEEE), and so on. Any program running on any machine can use XDR to create portable data by translating its local representation to the XDR standards. Similarly, any program running on any machine can read portable data by translating the XDR standard representations to its local equivalents. The canonical standard completely decouples programs that create or send portable data from those that use or receive portable data.

The advent of a new machine or new language has no effect upon the community of existing portable data creators and users. A new machine can be programmed to convert both the standard representations and its local representations regardless of the local representations of other machines. Conversely, the local representations of the new machine are also irrelevant to existing programs running on other machines. These existing programs can immediately read portable data produced by the new machine, because such data conforms to canonical standards.

Strong precedents exist for XDR's canonical approach. All protocols below layer five of the ISO model, including Transmission Control Protocol (TCP), Internet Protocol (IP), User Datagram Protocol (UDP), and Ethernet, are canonical protocols. The advantage of any canonical approach is simplicity. XDR fits into the ISO presentation layer and is roughly analogous in purpose to X.409, ISO Abstract Syntax Notation. The major difference here is that XDR uses implicit typing, while X.409 uses explicit typing. With XDR, a single set of conversion routines need only be written once.

The time spent converting to and from a canonical representation is insignificant, especially in networking applications. When preparing a data structure for transfer, traversing the elements of the structure requires more time than converting the data. In networking applications, additional time is required to move the data down through the sender's protocol layers, across the network, and up through the receiver's protocol layers. Every machine must traverse and copy data structures, regardless of whether conversion is required.