Individual submission L-J. Liman
Internet-Draft Autonomica
Intended status: Informational October 26, 2009
Expires: April 29, 2010
Top Level Domain Name Specification
draft-liman-tld-names-01
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Abstract
The precise syntax allowed in top-level domain name labels has been
the subject to some debate. RFC 1123, for example, makes the
statement that top-level domain names are "alphabetic". This
document updates the definition of allowable top-level domain names
in order to support internationalized domain names (IDNs), as encoded
by the IDNA protocols. This document focuses narrowly on the issue
of IDNs and does not make any other changes or clarifications to
existing domain name syntax rules.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Other Limitations on Top Level Domain Labels . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. To Do . . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 11
B.1. draft-liman-tld-named-01 . . . . . . . . . . . . . . . . . 11
B.2. draft-liman-tld-named-00 . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The precise syntax allowed in top-level domain (TLD) name labels has
been the subject to some debate. RFC 1123 [RFC1123], for example,
states that TLD names must be "alphabetic", which is interpreted as
excluding the hyphen (or dash) character. This document updates the
definition of allowable top-level domain names to support
internationalized domain names that consist of Unicode letters, as
encoded by the IDNA protocols [RFCXXX]. In particular, this document
clarifies that ASCII TLDs beginning with the IDN A-label prefix
(currently "xn--"), as encoded by IDNA, are permissible as DNS TLD
names as long as they are made from Unicode letters. This document
focuses narrowly on the issue of allowable ASCII labels encoded by
the IDNA protocols and does not (and is not intended to) make any
other changes or clarifications to existing domain name syntax rules.
1.1. Terminology
The terminology used in this document is as defined in RFC 0952
[RFC0952] and RFC 1035 [RFC1035].
1.2. Requirements Language
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 RFC 2119 [RFC2119].
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2. Background
RFC 0952 [RFC0952] states (among other things) that a host name is;
... a text string up to 24 characters drawn from the alphabet
(A-Z), digits (0-9), minus sign (-), and period (.). Note that
periods are only allowed when they serve to delimit components of
"domain style names". (See RFC-921, "Domain Name System
Implementation Schedule", for background). No blank or space
characters are permitted as part of a name. No distinction is
made between upper and lower case. The first character must be an
alpha character. The last character must not be a minus sign or
period.
RFC 1123 [RFC1123] reaffirms this definition, making two additional
changes to the syntax:
The syntax of a legal Internet host name was specified in RFC-952
[DNS:4]. One aspect of host name syntax is hereby changed: the
restriction on the first character is relaxed to allow either a
letter or a digit. Host software MUST support this more liberal
syntax.
and
However, a valid host name can never have the dotted-decimal form
#.#.#.#, since at least the highest-level component label will be
alphabetic.
The restrictions on host names and specifically TLD names have always
been, at least in part, driven by human factors considerations.
Underscores in host names are avoided because they are
indistinguishable from hyphens when seen on a page or written in
longhand, and to some extent because of early internationalization
issues. The original "no leading digits" rule was driven by wanting
to make sure that even imprecise programming or human thought errors
didn't confuse addresses with names.
The wish to express TLD names in other scripts than Latin makes it
necessary to relax the the rules for TLD names. However, the old
motivations for keeping the TLD names alphabetical still hold, and
furthermore, certain characteristics of some IDN names with digits in
them make them unsuitable as DNS labels. The problem is referred to
as "jumping digits", and is described in draft-ietf-idnabis-bidi.
In order to keep changes to existing specifications to a minimum but
to still allow for IDN TLD names, this document hereby changes the
existing specification to allow for IDN TLD names in the "A-label
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form" as specified by the IDNA-2008 specifications, i.e., an ASCII-
compatible-encoding, using reversible Punycode conversion from valid
IDN labels, with IDN A-label prefix (currently "xn--"), but requiring
that the native-character ("Unicode") form consist of letters only.
Hence, the ABNF expression that matches a valid TLD label is as
follows:
tldlabel = traditional-tld-label / idn-label
traditional-tld-label = 1*63(ALPHA)
idn-label = Restricted-A-label
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
Restricted-A-label is an A-label as defined in draft-ietf-idna-defs
converted from (and convertible to) a U-label that is consistent with
the definition in draft-ietf-idna-defs and that is further restricted
to contain only Unicode characters of General Category "L". Note
that "L" contains several sub-categories. The list is:
; Letter
L = Ll / Lm / Lo / Lt / Lu
Ll = Lowercase-Letter
Lm = Modifier-Letter
Lo = Other-Letter
Lt = Titlecase-Letter
Lu = Uppercase-Letter
although IDNA prohibits (categorizes as DISALLOWED) all characters in
the last two categories and several of the characters that fall into
the other categories.
This new specification reflects current practice in registration of
TLD names by the IANA, and allows for IDNs.
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3. Other Limitations on Top Level Domain Labels
It should be noted that there are many issues that must be considered
in making any changes to current restrictions on DNS labels,
especially at the top level. DNS software is widely deployed, and
some of that software contains embedded assumptions that may not hold
if DNS names are used at the top level that differ from the older
rules. For example, when TLDs longer than 3 characters became
available (e.g., .info, .museum, etc.), some deployed systems did not
process such DNS names properly. This document does not take the
position that no problems will result when IDN TLDs are created, but
does recognize that relaxing the syntax of allowed TLDs is necessary
in order to allow deployment of IDNs to happen.
It is also carefully noted that the above specification is not the
only limiting factor on TLD labels. There may be other entities than
the IETF that have influence over TLD names, and which may decide to
restrict the names further. The above technical specification is
just one limiting factor.
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4. IANA Considerations
This memo changes the specifications for TLD names registered by the
IANA, and the IANA is requested to change its registration process to
use the above specification.
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5. Security Considerations
This document is believed to have limited security consequences.
It may introduce stability issues where names registered under this
new specification may inter-operate badly with old software written
to enforce a strict interpretation of the old specification. This
might also open up attack vectors (e.g. form names being truncated).
However, it is believed that such software is scarce on the Internet,
and since TLD names that do not adhere to a strict interpretation of
the old specification are already used (including test IDNs) without
apparent problems, it is believed that this change of the
specification will not create major stability or security problems on
the Internet.
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6. References
6.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
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Appendix A. To Do
1. Clean up references. Check situation with references to Internet
Drafts. Are they/will they be published as RFCs before this
draft?
2. Verify quotations.
3. Get rid of the term "jumping digits" and replace with appropriate
wording. Also mention additional reasons not to have digits that
relate to Input Method Editors and localization.
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Appendix B. Change History
B.1. draft-liman-tld-named-01
Substantial comments and improvements supplied by Thomas Narten and
John Klensin. Decided to go for a minimal change approach. Also
noted that U-labels have to be letters due to jumping digit problem.
Rewritten major parts.
B.2. draft-liman-tld-named-00
First cut. Prompted by Olafur Gudmundsson and Tina Dam.
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Author's Address
Lars-Johan Liman
Autonomica AB
Franzengatan 5
SE-112 51 Stockholm
Sweden
Email: liman@autonomica.se
URI: http://www.autonomica.se/
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