RFC 5891

Internet Engineering Task Force (IETF) 🎀

Okay bestie, this is RFC 5891 💖 It's titled Internationalized Domain Names in Applications (IDNA): Protocol, written by J. Klensin for the IETF, and dated August 2010. It's a Standards Track doc, so it's the real deal 👑 Its ISSN is 2070-1721, and its Request for Comments number is 5891. It obsoletes 3490 and 3491, and it updates 3492 ✨

Abstract 🌸

Bestie, this document is the revised protocol definition for Internationalized Domain Names (IDNs) 🌈 The rationale for the changes, how it relates to the older spec, and the important terminology all live in other documents. This one specifies the protocol mechanism, called Internationalized Domain Names in Applications (IDNA), for registering and looking up IDNs in a way that does not require changes to the DNS itself. IDNA is only meant for processing domain names, not free text 🫶

Status of This Memo 📜

This is an Internet Standards Track document ✨

It's a product of the Internet Engineering Task Force (IETF), so it represents the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). For more on Internet Standards, check Section 2 of RFC 5741. Info about the current status, any errata, and how to send feedback is at http://www.rfc-editor.org/info/rfc5891 🔗

Copyright Notice 💜

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as in effect on the date of publication. Please actually read these, because they spell out your rights and restrictions for this document 🙈 Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they're provided without warranty as described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The people who control the copyright in some of that material may not have given the IETF Trust the right to allow modifications outside the IETF Standards Process. Without an adequate license from those copyright holders, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process. The exceptions are formatting it for publication as an RFC or translating it into languages other than English 🌷

Table of Contents 🗂️

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements and Applicability . . . . . . . . . . . . . . . .  5
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  DNS Resource Records . . . . . . . . . . . . . . . . .  6
       3.2.2.  Non-Domain-Name Data Types Stored in the DNS . . . . .  6
   4.  Registration Protocol  . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Input to IDNA Registration . . . . . . . . . . . . . . . .  7
     4.2.  Permitted Character and Label Validation . . . . . . . . .  7
       4.2.1.  Input Format . . . . . . . . . . . . . . . . . . . . .  7
       4.2.2.  Rejection of Characters That Are Not Permitted . . . .  8
       4.2.3.  Label Validation . . . . . . . . . . . . . . . . . . .  8
       4.2.4.  Registration Validation Requirements . . . . . . . . .  9
     4.3.  Registry Restrictions  . . . . . . . . . . . . . . . . . .  9
     4.4.  Punycode Conversion  . . . . . . . . . . . . . . . . . . .  9
     4.5.  Insertion in the Zone  . . . . . . . . . . . . . . . . . . 10
   5.  Domain Name Lookup Protocol  . . . . . . . . . . . . . . . . . 10
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . . 10
     5.3.  A-label Input  . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Validation and Character List Testing  . . . . . . . . . . 11
     5.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 13
     5.6.  DNS Name Resolution  . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Summary of Major Changes from IDNA2003  . . . . . . . 17

1. Introduction ✨

This document is the protocol definition for Internationalized Domain Names in Applications (IDNA), and the version specified here is called IDNA2008 💅 The essential definitions and terminology, plus a road map of all the documents that make up IDNA2008, live in a separate Definitions document [RFC5890]. Appendix A talks about how this specification relates to the earlier version (referred to here as "IDNA2003"). The rationale for these changes, along with lots of explanatory material and advice for zone administrators who support IDNs, is in another doc known informally in this series as the "Rationale document" [RFC5894] 📚

IDNA works by letting applications use certain ASCII [ASCII] string labels (starting with a special prefix) to represent non-ASCII name labels. Lower-layer protocols don't need to be aware of this, so IDNA doesn't change any infrastructure. In particular, IDNA doesn't depend on any changes to DNS servers, resolvers, or DNS protocol elements, because the ASCII name service the existing DNS already provides can be used for IDNA 🦋

IDNA applies only to a specific subset of DNS labels. The base DNS standards [RFC1034] [RFC1035] and their various updates specify how to combine labels into fully-qualified domain names and how to parse labels back out of those names 🧚

This document describes two separate protocols, one for IDN registration (Section 4) and one for IDN lookup (Section 5). These two protocols share some terminology, reference data, and operations 🫶

2. Terminology 💖

As mentioned above, the terminology used in the definition of IDNA is in the Definitions document [RFC5890]. It's worth noting that some of this terminology overlaps with, and is consistent with, what's used in Unicode or other character set standards and in the DNS. Readers are assumed to be familiar with the Definitions document and with the DNS-specific terminology in RFC 1034 [RFC1034] 🌸

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [RFC2119] ⭐

3. Requirements and Applicability 👑

3.1. Requirements 🎀

IDNA makes the following requirements:

1. Whenever a domain name is put into a domain name slot that is not IDNA-aware (see Section 2.3.2.6 of the Definitions document [RFC5890]), it MUST contain only ASCII characters (i.e., its labels must be either A-labels or NR-LDH labels), unless the DNS application is not subject to historical recommendations for "hostname"-style names (see RFC 1034 [RFC1034] and Section 3.2.1).
2. Labels MUST be compared using equivalent forms: either both A-label forms or both U-label forms. Because A-labels and U-labels can be transformed into each other without loss of information, these comparisons are equivalent. However, in practice, comparing U-labels requires first verifying that they actually are U-labels and not just Unicode strings. A pair of A-labels MUST be compared as case-insensitive ASCII (as with all comparisons of ASCII DNS labels). U-labels MUST be compared as-is, without case folding or other intermediate steps. While it isn't necessary to validate labels in order to compare them, a successful comparison does not imply validity. In many cases, not just comparison, validation may be important for other reasons and SHOULD be performed 🔥
3. Labels being registered MUST conform to the requirements of Section 4. Labels being looked up, and the lookup process, MUST conform to the requirements of Section 5 💅

3.2. Applicability 🌈

IDNA applies to all domain names in all domain name slots in protocols, except where it is explicitly excluded. It does not apply to domain name slots that don't use the LDH syntax rules described in the Definitions document [RFC5890] 🦋

Because it uses the DNS, IDNA applies to many protocols that were specified before it was designed. IDNs occupying domain name slots in those older protocols MUST be in A-label form until and unless those protocols and their implementations are explicitly upgraded to be aware of IDNs and to accept the U-label form. IDNs actually appearing in DNS queries or responses MUST be A-labels 😭

IDNA-aware protocols and implementations MAY accept U-labels, A-labels, or both, as those particular protocols specify. IDNA is not defined for extended label types (see RFC 2671 [RFC2671], Section 3) ✨

3.2.1. DNS Resource Records 🧚

IDNA applies only to domain names in the NAME and RDATA fields of DNS resource records whose CLASS is IN. See the DNS specification [RFC1035] for precise definitions of these terms 💜

The application of IDNA to DNS resource records depends entirely on the CLASS of the record, and not on the TYPE, except as noted below. This will remain true even as new TYPEs are defined, unless a new TYPE defines TYPE-specific rules. Special naming conventions for SRV records (and "underscore labels" more generally) are incompatible with IDNA coding, as discussed in the Definitions document [RFC5890], especially Section 2.3.2.3. Of course, underscore labels may be part of a domain that uses IDN labels at higher levels in the tree 🌷

3.2.2. Non-Domain-Name Data Types Stored in the DNS 🙈

Although IDNA enables the representation of non-ASCII characters in domain names, that doesn't mean IDNA enables the representation of non-ASCII characters in other data types stored in domain names. That goes especially for the RDATA field of types that have a structured RDATA format. For example, an email address local part is stored in a domain name in the RNAME field as part of the RDATA of an SOA record (e.g., hostmaster@example.com would be represented as hostmaster.example.com). IDNA doesn't update the existing email standards, which allow only ASCII characters in local parts. Even though work is in progress to define internationalization for email addresses [RFC4952], changing the email address part of the SOA RDATA would require action in, or updates to, other standards, specifically those that specify the format of the SOA RR 🍓

4. Registration Protocol ✨

okay so this section is the model for registering an IDN 💖 The model doesn't care how you build it. Any sequence of steps that gives exactly the same result for all labels counts as a valid implementation 🧚

Note that the registration bit (this section) and the lookup bit (Section 5) are super similar in most ways, but they are not identical. So implementers should carefully follow the steps described in this specification 🎀

4.1. Input to IDNA Registration

Registration stuff, especially what happens with the registrars (often called "registrars") who deal with registrants before the request actually reaches the zone manager ("registry"), is outside the scope of this definition 🌸 It can look really different depending on local needs. By the time a string enters the IDNA registration process described here, it MUST be in Unicode and in Normalization Form C (NFC [Unicode-UAX15]) 💅 Entities responsible for zone files ("registries") MUST accept only the exact string being requested for registration, with no mappings or local adjustments. They MAY accept that input in any of three forms:

1. As a pair of A-label and U-label.
2. As an A-label only.
3. As a U-label only.

The first two forms are RECOMMENDED, because using A-labels avoids any possibility of ambiguity 🦋 The first is normally preferred over the second, because it allows further verification of user intent (see Section 4.2.1) ✨

4.2. Permitted Character and Label Validation

4.2.1. Input Format

If both the U-label and A-label forms are available, the registry MUST ensure that the A-label form is in lowercase 💜 Then it performs a conversion to a U-label, runs the steps and tests described below on that U-label, and verifies that the A-label produced by the step in Section 4.4 matches the one provided as input. On top of that, the U-label that was provided as input and the one obtained by converting the A-label MUST match exactly. If any of these tests fail for some reason, the registration MUST be rejected 🙈

If only an A-label was provided and the conversion to a U-label is not performed, the registry MUST still verify that the A-label is superficially valid. That means checking that it doesn't break any of the rules of Punycode encoding [RFC3492], like the ban on a trailing hyphen-minus, the requirement that all characters be ASCII, and so on 🍓 Strings that look like A-labels (for example, they start with "xn--") and strings supplied to the registry in a context reserved for A-labels (like a field in a form to be filled out), but that are not valid A-labels under this paragraph, MUST NOT be placed in DNS zones that support IDNA 💀

If only an A-label is provided and the conversion to a U-label is not performed, but the superficial tests from the previous paragraph are done, registration procedures MAY, and usually will, skip the tests and actions in the rest of Section 4.2 and in Sections 4.3 and 4.4 🫶

4.2.2. Rejection of Characters That Are Not Permitted

The candidate Unicode string MUST NOT contain characters that appear in the "DISALLOWED" and "UNASSIGNED" lists from the Tables document [RFC5892] 😭

4.2.3. Label Validation

The proposed label (a Unicode string that at least superficially looks like a U-label) then gets checked with tests that need more than one character at a time 👑 Character order is based on the on-the-wire order. That order might not match the display order, so keep that in mind 🌈

4.2.3.1. Hyphen Restrictions

The Unicode string MUST NOT contain "--" (two consecutive hyphens) in the third and fourth character positions, and it MUST NOT start or end with a "-" (hyphen) 🎀

4.2.3.2. Leading Combining Marks

The Unicode string MUST NOT begin with a combining mark or combining character (see Section 2.11 of The Unicode Standard [Unicode] for the exact definition) 🦋

4.2.3.3. Contextual Rules

The Unicode string MUST NOT contain any characters whose validity depends on context, unless a contextual rule positively confirms that validity ✨ To check this, each code point marked CONTEXTJ or CONTEXTO in the Tables document [RFC5892] MUST have a non-null rule. If a code point is missing a rule, the label is invalid. If the rule exists but applying it gives a negative or inconclusive result, the proposed label is invalid 🔥

4.2.3.4. Labels Containing Characters Written Right to Left

If the proposed label contains any characters from scripts written right to left, it MUST meet the Bidi criteria [RFC5893] 🧚

4.2.4. Registration Validation Requirements

Strings that have at least one non-ASCII character, were produced by the steps above, pass all of the tests in Section 4.2.3, and are 63 or fewer characters long in ASCII-compatible encoding (ACE) form (see Section 4.4) count as U-labels 💖

To sum up, Section 4.2 tests for invalid characters, invalid combinations of characters, labels that are invalid even though each character is valid on its own, and labels that don't follow the restrictions for strings containing right-to-left characters 💅

4.3. Registry Restrictions

Besides the rules and tests above, there are lots of reasons a registry could reject a label 🌸 Registries at every level of the DNS, not just the top level, are expected to set policies about label registrations. Those policies are likely to be shaped by local languages and the scripts used to write them, and they may depend on lots of factors, including what characters are in the label. For example, a label could get rejected based on other labels that are already registered 👑 See Section 3.2 of the Rationale document [RFC5894] for more discussion and recommendations about registry policies 🦋

The string produced by the steps in Section 4.2 is checked and processed as appropriate for local registry restrictions. Applying those restrictions may get some labels rejected or put special restrictions on others ✨

4.4. Punycode Conversion

The resulting U-label gets converted to an A-label (defined in Section 2.3.2.1 of the Definitions document [RFC5890]) 🧚 The A-label is the encoding of the U-label using the Punycode algorithm [RFC3492], with the ACE prefix "xn--" added at the beginning of the string. The resulting string must, of course, conform to the length limits imposed by the DNS 🎀 This document doesn't update or change the Punycode algorithm specified in RFC 3492 in any way 💜 RFC 3492 does make a non-normative reference to information about the value and construction of the ACE prefix, which appears in RFC 3490 or Nameprep [RFC3491]. For consistency and reader convenience, IDNA2008 effectively updates that reference to point to this document 🫶 That change doesn't alter the prefix itself. The prefix, "xn--", is the same in both sets of documents 🌈

Except for the maximum string length test on Punycode output, the failure conditions in the Punycode encoding procedure can't happen if the input is a U-label, as determined by the steps in Sections 4.1 through 4.3 above 😭

4.5. Insertion in the Zone

The label gets registered in the DNS by inserting the A-label into a zone 🍓

5. Domain Name Lookup Protocol 🦋

Lookup is different from registration, and different tests get applied on the client 💅 Some validity checks are still needed to avoid serious problems with the protocol, but the lookup-side tests are more permissive and rely on the assumption that names present in the DNS are valid. That assumption is a pretty weak one, though, because wildcards in the DNS might let a string that isn't actually registered in the DNS get looked up successfully 👀

5.1. Label String Input

The user supplies a string in the local character set, for example by typing it, clicking on it, or copying and pasting it from a resource identifier, like a Uniform Resource Identifier (URI) [RFC3986] or an Internationalized Resource Identifier (IRI) [RFC3987], where the domain name gets extracted from it 💖 Alternately, some process that doesn't directly involve the user might read the string from a file or get it some other way. Processing in this step and the one in Section 5.2 are local matters, handled before IDNA is actually invoked 🌸

5.2. Conversion to Unicode

The string gets converted from the local character set into Unicode, if it isn't already in Unicode 🧚 Depending on local needs, this conversion might map some characters to other characters as well as doing coding conversions. Those issues are covered in the mapping-related sections (Sections 4.2, 4.4, 6, and 7.3) of the Rationale document [RFC5894] and in the separate Mapping document [IDNA2008-Mapping] 🎀 The result MUST be a Unicode string in NFC form 🔥

5.3. A-label Input

If the input to this procedure looks like an A-label (it starts with "xn--", case-insensitively), the lookup application MAY try to convert it to a U-label. It first makes sure the A-label is entirely lowercase (lowercasing it if needed), then applies the tests of Section 5.4 and the conversion of Section 5.5 to that form 💜 If the label gets converted to Unicode (meaning U-label form) using the Punycode decoding algorithm, then the processing in those two sections MUST be performed, and the label MUST be rejected if the resulting label isn't identical to the original 🙈 See Section 8.1 of the Rationale document [RFC5894] for more discussion on this topic 🦋

Converting from the A-label and testing that the result is a U-label SHOULD be performed if the domain name will later be shown to the user in native character form (this requires that the lookup application be IDNA-aware) ✨ If those steps aren't performed, the lookup process SHOULD at least test whether the string is actually an A-label, checking it for the invalid formats specified in the Punycode decoding specification 🫶 Applications that aren't IDNA-aware will obviously skip that testing. Others MAY treat the string as opaque to avoid the extra processing, at the expense of giving users less protection and information 🌈

5.4. Validation and Character List Testing ✨

okay so, just like the registration procedure in Section 4, the Unicode string gets checked to make sure every character in it is valid as input to IDNA lookup processing 🔍 As we talked about above and in the Rationale document [RFC5894], the lookup check is more liberal than the registration one. Labels that haven't been fully checked against the applicable rules are called "putative" labels, as covered in Section 2.3.2.1 of the Definitions document [RFC5890]. Putative U-labels with any of the following characteristics MUST be rejected prior to DNS lookup 🚫

• Labels that are not in NFC [Unicode-UAX15].
• Labels containing "--" (two consecutive hyphens) in the third and fourth character positions.
• Labels whose first character is a combining mark (see The Unicode Standard, Section 2.11 [Unicode]).
• Labels containing prohibited code points, i.e., those that are assigned to the "DISALLOWED" category of the Tables document [RFC5892].
• Labels containing code points that are identified in the Tables document as "CONTEXTJ", i.e., requiring exceptional contextual rule processing on lookup, but that do not conform to those rules. Note that this implies that a rule must be defined, not null: a character that requires a contextual rule but for which the rule is null is treated in this step as having failed to conform to the rule.
• Labels containing code points that are identified in the Tables document as "CONTEXTO", but for which no such rule appears in the table of rules. Applications resolving DNS names or carrying out equivalent operations are not required to test contextual rules for "CONTEXTO" characters, only to verify that a rule is defined (although they MAY make such tests to provide better protection or give better information to the user).
• Labels containing code points that are unassigned in the version of Unicode being used by the application, i.e., in the UNASSIGNED category of the Tables document.

Okay, that last one needs a little extra explanation 🥺 This requirement means that the application must use a list of unassigned characters that is matched to the version of Unicode being used for the other requirements in this section. It is not required that the application know which version of Unicode is being used; that information might be part of the operating environment the application is running in 🌸

In addition, the application SHOULD apply the following test 💅

• Verification that the string is compliant with the requirements for right-to-left characters specified in the Bidi document [RFC5893].

This test may be omitted in special circumstances, such as when the lookup application knows that the conditions are enforced elsewhere, because an attempt to look up and resolve such strings will almost certainly lead to a DNS lookup failure except when wildcards are present in the zone. However, applying the test is likely to give much better information about the reason for a lookup failure (information that may be usefully passed to the user when that is feasible) than DNS resolution failure information alone 🧠✨

For all other strings, the lookup application MUST rely on the presence or absence of labels in the DNS to determine the validity of those labels and the validity of the characters they contain. If they are registered, they are presumed to be valid; if they are not, their possible validity is not relevant. While a lookup application may reasonably issue warnings about strings it believes may be problematic, applications that decline to process a string that conforms to the rules above (i.e., does not look it up in the DNS) are not in conformance with this protocol 🙅‍♀️💖

5.5. Punycode Conversion 🎀

The string that has now been validated for lookup is converted to ACE form by applying the Punycode algorithm to the string and then adding the ACE prefix (xn--) 🦋

5.6. DNS Name Resolution 🌈

The A-label resulting from the conversion in Section 5.5 or supplied directly (see Section 5.3) is combined with other labels as needed to form a fully-qualified domain name, which is then looked up in the DNS using normal DNS resolver procedures. The lookup can obviously either succeed (returning information) or fail 💅

6. Security Considerations 🔐

Security Considerations for this version of IDNA are described in the Definitions document [RFC5890], except for the special issues associated with right-to-left scripts and characters. The latter are discussed in the Bidi document [RFC5893] 🧐

In order to avoid intentional or accidental attacks from labels that might be confused with others, special problems in rendering, and so on, the IDNA model requires that registries exercise care and thoughtfulness about what labels they choose to permit. That issue is discussed in Section 4.3 of this document, which in turn points to a somewhat more extensive discussion in the Rationale document [RFC5894]. This is seriously important, bestie 😭 Registries really need to be careful about which labels they allow, because confusable or badly rendered labels can cause real trouble 💀

7. IANA Considerations 📋

IANA actions for this version of IDNA are specified in the Tables document [RFC5892] and discussed informally in the Rationale document [RFC5894]. The components of IDNA described in this document do not require any IANA actions ✨

8. Contributors 🫶

While the listed editor held the pen, the original versions of this document represent the joint work and conclusions of an ad hoc design team consisting of the editor and, in alphabetic order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. This document draws significantly on the original version of IDNA [RFC3490], both conceptually and for specific text. This second-generation version would not have been possible without the work that went into that first version, and especially the contributions of its authors Patrik Faltstrom, Paul Hoffman, and Adam Costello. While Faltstrom was actively involved in the creation of this version, Hoffman and Costello were not, and should not be held responsible for any errors or omissions 🌷

9. Acknowledgments 💜

This revision to IDNA would have been impossible without the accumulated experience since RFC 3490 was published, and the resulting comments and complaints of many people in the IETF, ICANN, and other communities (too many people to list here). Nor would it have been possible without RFC 3490 itself and the efforts of the Working Group that defined it. Those people whose contributions are acknowledged in RFC 3490, RFC 4690 [RFC4690], and the Rationale document [RFC5894] were particularly important 🙌

Specific textual changes were incorporated into this document after suggestions from the other contributors, Stephane Bortzmeyer, Vint Cerf, Lisa Dusseault, Paul Hoffman, Kent Karlsson, James Mitchell, Erik van der Poel, Marcos Sanz, Andrew Sullivan, Wil Tan, Ken Whistler, Chris Wright, and other WG participants and reviewers, including Martin Duerst, James Mitchell, Subramanian Moonesamy, Peter Saint-Andre, Margaret Wasserman, and Dan Winship, who caught specific errors and recommended corrections. Special thanks are due to Paul Hoffman for permission to extract material to form the basis for Appendix A from a draft document that he prepared 🥰

10. References 📚

10.1. Normative References

• [RFC1034]: Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.
• [RFC1035]: Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.
• [RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
• [RFC3492]: Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003.
• [RFC5890]: Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010.
• [RFC5892]: Faltstrom, P., Ed., "The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)", RFC 5892, August 2010.
• [RFC5893]: Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)", RFC 5893, August 2010.
• [Unicode-UAX15]: The Unicode Consortium, "Unicode Standard Annex #15: Unicode Normalization Forms", September 2009, <http://www.unicode.org/reports/tr15/>.

10.2. Informative References

• [ASCII]: American National Standards Institute (formerly United States of America Standards Institute), "USA Code for Information Interchange", ANSI X3.4-1968, 1968. ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet.
• [IDNA2008-Mapping]: Resnick, P. and P. Hoffman, "Mapping Characters in Internationalized Domain Names for Applications (IDNA)", Work in Progress, April 2010.
• [RFC2671]: Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999.
• [RFC3490]: Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003.
• [RFC3491]: Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, March 2003.
• [RFC3986]: Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
• [RFC3987]: Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005.
• [RFC4690]: Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and Recommendations for Internationalized Domain Names (IDNs)", RFC 4690, September 2006.
• [RFC4952]: Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 4952, July 2007.
• [RFC5894]: Klensin, J., "Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale", RFC 5894, August 2010.
• [Unicode]: The Unicode Consortium, "The Unicode Standard, Version 5.0", 2007. Boston, MA, USA: Addison-Wesley. ISBN 0-321-48091-0. This printed reference has now been updated online to reflect additional code points. For code points, the reference at the time this document was published is to Unicode 5.2 🦄

Appendix A. Summary of Major Changes from IDNA2003 ✨

Okay, bestie, this is the glow-up recap 💅 Here's everything that changed from IDNA2003:

1. Update the base character set from Unicode 3.2 to being Unicode version agnostic. 🌈
2. Separate the definitions for the "registration" and "lookup" activities. 🎀
3. Disallow symbol and punctuation characters, except where special exceptions are necessary. 🚫
4. Remove the mapping and normalization steps from the protocol. Applications do them instead, possibly in a local fashion, before invoking the protocol. 🧚
5. Change how the protocol decides which characters are allowed in labels. It used to be "humans decide what the table of code points contains." Now it's "decision about code points are based on Unicode properties plus a small exclusion list created by humans." Way more organized, no cap 📋✨
6. Introduce the brand-new concept of characters that can be used only in specific contexts. 🦋
7. Allow typical words and names in languages such as Dhivehi and Yiddish to be expressed. Finally, representation for those words! 🥹💜
8. Make bidirectional domain names (delimited strings of labels, not just labels standing on their own) display in a less surprising fashion. This works whether they show up in obvious domain name contexts or as part of running text in paragraphs. Iconic 👑
9. Remove the dot separator from the mandatory part of the protocol. 🍓
10. Make some currently valid labels that aren't actually IDNA labels invalid. Sorry, not sorry 🙈

Author's Address

John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA  02140
USA

Phone: +1 617 245 1457
EMail: john+ietf@jck.com

And that's the whole RFC, babe! 🫶 Our girl John wrote the rulebook, and honestly, he did amazing. 💖