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UTF-9 and UTF-18

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This is an old revision of this page, as edited by Plugwash (talk | contribs) at 21:33, 20 October 2005 (→Technical details: i'm pretty sure plane numbers start at 0). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

UTF-9 (9-bit Unicode Transformation Format) and UTF-18 (9-bit Unicode Transformation Format) are two specifications for encoding unicode on systems where the nonet (nine bit group) is a better fit for the native word size than the octet such as the PDP-10. Both encodings were specified in RFC 4042 which was released on april 1st 2005. The encodings suffer from a number of flaws and it is reasonable to assume that they were intended as a joke. However unlike some of the "specifications" given in other April 1st RFCs they are actually techincally possible to implement. They are not endorsed by the Unicode Consortium.

Technical details

UTF-9 uses a system of putting an octet in the low 8 bits of each nonet and using the high bit to indicate continuation. This means that ASCII and LATIN 1 characters take one nonet each, the rest of the BMP characters take two nonets each and non BMP code points take 3. Code points that require multiple nonets are stored with the most significant octet in the first nonet (at least according to the examples in the specification it doesn't actually appear to be stated anywhere).

UTF-18 is the simpler of the two encodings using a single 18 bit integer per code point. This allows representation of 4 planes which are mapped to the 4 planes currently used by unicode (planes 0-2 and 14). This means that the two private use planes (15 and 16) and the currently unused planes (4-13) are not supported. The UTF-18 specification doesn't say why they didn't allow surrogates to be used for theese code points though when talking about UTF-16 ealier in the RFC they said "This transformation format requires complex surrogates to represent codepoints outside the BMP". After complaining about thier complexity it would have looked a bit hypocritical of them to use them in thier new standard.

Problems

Both specifications suffer from the problem that standard communication protocols are simply not built arround nonets and so it would not be possible to exchange text in theese formats without further encoding or specially designed protocols. This alone would probablly be sufficiant reason to consider there use impractical in most cases.

Furthermore both UTF-9 and UTF-18 have specific problems of there own. UTF-9 requires special care when searching as a shorter sequence can be found at the end of a longer sequence and utf-18 cannot represent all unicode code points (though it can represent all the planes that currently have non private use code point assignments) making it a bad choice for a system that may develop in future.

  • RFC 4042: UTF-9 and UTF-18 Efficient Transformation Formats of Unicode
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