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12. Changes from RFC 2279

This section documents the changes made in RFC 3629 compared to the previous specification in RFC 2279.

Character Range Restriction​

Restricted the range of characters to 0000-10FFFF (the UTF-16 accessible range).

This is a significant change that limits UTF-8 to the Unicode-defined character space, excluding the previously allowed but impractical ranges beyond U+10FFFF.

Normative Source Change​

Made Unicode the source of the normative definition of UTF-8, keeping ISO/IEC 10646 as the reference for characters.

This change recognizes Unicode as the primary authoritative source for UTF-8 encoding rules, while ISO/IEC 10646 remains the reference for the character repertoire.

Terminology Clarification​

Straightened out terminology. UTF-8 now described in terms of an encoding form of the character number. UCS-2 and UCS-4 almost disappeared.

The updated terminology provides clearer and more consistent descriptions of UTF-8 as an encoding form rather than a transformation format.

Invalid Sequence Handling​

Turned the note warning against decoding of invalid sequences into a normative MUST NOT.

Previously, the warning about invalid sequences was informative. In RFC 3629, it became a normative requirement that implementations MUST NOT decode invalid sequences.

BOM Section Addition​

Added a new section about the UTF-8 BOM, with advice for protocols.

Section 6 provides comprehensive guidance on:

  • When to use or forbid the BOM
  • How to handle the BOM in different protocol contexts
  • Recommendations for protocol designers

MIME Registration Change​

Removed suggested UNICODE-1-1-UTF-8 MIME charset registration.

The version-specific charset label was removed in favor of the generic "UTF-8" label that covers all Unicode versions after Amendment 5.

ABNF Syntax Addition​

Added an ABNF syntax for valid UTF-8 octet sequences

Section 4 now includes a formal ABNF grammar that precisely defines valid UTF-8 byte sequences, making it easier for implementers to validate UTF-8 data.

Security Section Expansion​

Expanded Security Considerations section, in particular impact of Unicode normalization

The security section was significantly enhanced to include:

  • Detailed discussion of overlong encoding attacks
  • Buffer overflow risks
  • Canonical equivalence security implications
  • References to Unicode Normalization Forms
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