Internet DRAFT - draft-liman-tld-names
draft-liman-tld-names
Individual Submission L-J. Liman
Internet-Draft Netnod
Updates: 1123 (if approved) J. Abley
Intended status: Standards Track ICANN
Expires: June 1, 2012 November 29, 2011
Top Level Domain Name Specification
draft-liman-tld-names-06
Abstract
The syntax for allowed Top-Level Domain (TLD) labels in the Domain
Name System (DNS) is not clearly applicable to the encoding of
Internationalised Domain Names (IDNs) as TLDs.
This document provides a concise specification of TLD label syntax
based on existing syntax documentation, extended minimally to
accommodate IDNs.
This document updates RFC1123.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on June 1, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. TLD Label Syntax Specification . . . . . . . . . . . . . . . . 7
5. Policy Considerations . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 13
A.1. draft-liman-tld-names-06 . . . . . . . . . . . . . . . . . 13
A.2. draft-liman-tld-names-05 . . . . . . . . . . . . . . . . . 13
A.3. draft-liman-tld-names-04 . . . . . . . . . . . . . . . . . 13
A.4. draft-liman-tld-names-03 . . . . . . . . . . . . . . . . . 13
A.5. draft-liman-tld-names-02 . . . . . . . . . . . . . . . . . 13
A.6. draft-liman-tld-names-01 . . . . . . . . . . . . . . . . . 13
A.7. draft-liman-tld-names-00 . . . . . . . . . . . . . . . . . 13
A.8. Version Identification Tag . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
The syntax of TLD labels ("TLD DNS-Labels", as defined in Section 2)
is specified in [RFC1123], where such labels are asserted to be
"alphabetic" within a section of that document entitled "DISCUSSION".
This can be interpreted as requiring that the hyphen character ("-")
and numeric digits be excluded from TLD DNS-Labels. Such a
restriction would not accommodate the US-ASCII encoding of
Internationalised Domain Names (IDNs), as specified in [RFC5890]. A
more detailed discussion of the existing specifications can be found
in Section 3.
This document extends the syntax of allowable TLD DNS-Labels to
support IDNs, but places some restrictions on the choice of IDN
labels. These restrictions are intended to be consistent with the
existing specification for US-ASCII TLD DNS-Labels. See Section 4
for the updated specification.
This document focuses narrowly on the issue of allowable DNS-Labels
in TLDs and does not (and is not intended to) make any other changes
or clarifications to existing domain name syntax rules.
It is carefully noted that the specification in this document is not
the only factor in choosing suitable TLD DNS-Labels, and that many
considerations external to the IETF are included in that wider
policy. See Section 5 for more discussion of policy considerations.
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2. Definitions
The term DNS-Label is used in this document to have precisely the
same meaning as the term "label", as introduced in [RFC1034], section
3.1. A DNS-Label denotes one node in a DNS tree. A DNS-Label is
zero to 63 octets in length. The term "DNS-Label" refers exclusively
to the "wire format" of the label, and not to any presentation format
of the label.
A Top-Level Domain (TLD) DNS-Label is the right-most ("highest-
level") DNS-Label in a fully-qualified domain name.
The terms A-Label and U-Label are used in this document as defined in
[RFC5890].
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3. Background
[RFC0952] defines a host name as follows:
'A "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.' [Unnumbered section titled "ASSUMPTIONS", first
paragraph]
[RFC1123] reaffirms this definition, but makes one change 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.' [Section 2.1]
In addition, the DISCUSSION section of Section 2.1 says:
'However, a valid host name can never have the dotted-decimal form
#.#.#.#, since at least the highest-level component label will be
alphabetic.' [Section 2.1]
Some implementers may have understood the above phrase 'will be
alphabetic' to be a protocol restriction.
Neither [RFC0952] nor [RFC1123] explicitly states the reasons for
these restrictions. It might be supposed that human factors were a
consideration; [RFC1123] appears to suggest that one of the reasons
was to prevent confusion between dotted-decimal IPv4 addresses and
host domain names. In any case, it is reasonable to believe that the
restrictions have been assumed in some deployed software, and that
changes to the rules should be undertaken with caution.
The Internationalised Domain Names in Applications 2008 specification
(IDNA2008) [RFC5891] [RFC5892] provides a protocol for encoding
Unicode strings in DNS-Labels. The Unicode string used by
applications is known as a U-Label; its corresponding encoding in the
DNS is known as an A-Label. The terms A-Label and U-Label are used
in this document as defined in [RFC5890]. Valid A-Labels always
contain non-alphabetic characters.
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In order to accommodate the wish to express TLD names in scripts
other than Latin (or rather, the US-ASCII subset of Latin), it is
necessary to allow non-alphabetic characters in the corresponding TLD
DNS-Labels. To minimize changes, the U-label form of a TLD name is
restricted in ways functionally compatible with the restrictions
(from [RFC0952] and [RFC1123]) on US-ASCII TLD names, by applying
rules analogous to those already imposed on US-ASCII TLD DNS-Labels
to TLD U-labels.
However, deployed software that checks DNS top-level labels for
conformance with an alphabetic restriction will not recognize such
corresponding A-Labels (i.e., U-labels represented in their US-ASCII
form).
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4. TLD Label Syntax Specification
This document relaxes the existing specification to allow TLD DNS-
Labels to be well-formed A-Labels, but places restrictions on their
corresponding U-Labels. That is, not every well-formed A-Label is a
valid TLD DNS-Label.
The ABNF expression that matches a valid TLD DNS-Label is as follows:
tld-dns-label = 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
A Restricted-A-Label is a DNS-Label which satisfies all the following
conditions:
1. the DNS-Label is a valid A-Label according to [RFC5890];
2. the derived property value of all code points, as defined by
[RFC5890], is PVALID;
3. the general category of all code points, is one of { Ll, Lo, Lm,
Mn, Mc }.
This new specification reflects current practice in registration of
TLD names by the IANA, extended to accommodate IDNs.
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5. Policy Considerations
This document provides a technical specification that limits the set
of TLD DNS-Labels that are available for assignment; it does not aim
to encapsulate the full policy framework within which TLD names are
chosen.
At the time of writing, the policy under which TLD names are chosen
is developed and maintained by ICANN in consultation with a wide base
of stakeholders. As the Internet continues to grow to serve new user
communities, applications and services, it is to be expected that the
corresponding policy will be changed accordingly.
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6. IANA Considerations
While this document makes no requests of the IANA, management of the
root zone is an IANA function. This document expands the set of
strings permitted for delegation from the root zone, and hence
establishes new limits for the corresponding IANA policy.
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7. Security Considerations
This document is believed to have limited security implications.
General discussion about the security effects of internationalized
labels can be found in [RFC5890], section 4. Those considerations
apply equally to TLD labels.
The creation of new TLDs has the potential to conflict with software
which (for example) predates and correspondingly does not accommodate
new TLD names. Such software problems might in turn lead to security
vulnerabilities, e.g. in the case where a DNS name specified by a
user is truncated or otherwise misinterpreted, causing an application
to interact with a different remote host from that which the user
intended. It should be noted that this is not a new phenomenon, and
has been observed following the creation of new (US-ASCII) TLD names
prior to the publication of this document.
The issue that some Unicode characters can be confused with each
other is discussed at length in the Security Considerations section
of [RFC5890].
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8. Acknowledgements
Tina Dam, Patrik Faltstrom, John Klensin, Thomas Narten and Andrew
Sullivan contributed text to this document, and their contributions
are hereby acknowledged.
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9. References
9.1. Normative References
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
9.2. Informative References
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
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Appendix A. Change History
This section (and sub-sections) should be removed before publication.
A.1. draft-liman-tld-names-06
Add Mc as an allowable code-point, required for names in Devanagari
script.
A.2. draft-liman-tld-names-05
New affiliation and address for Liman, due to company merger.
A.3. draft-liman-tld-names-04
Removed subjective and unverified statements regarding deployed
software. Replaced with more generic text. Polishing a few
expressions to make them less obtrusive. Removed confusing paragraph
after ABNF table. Updated some references that are now published as
RFCs.
A.4. draft-liman-tld-names-03
More wordsmithing, and explanatory text. Work on the IANA and the
security considerations sections.
A.5. draft-liman-tld-names-02
Wordsmithing and rearrangement of text following discussions with Joe
Abley, Tina Dam, Thomas Narten and Andrew Sullivan. Incorporated
revised ABNF and associated specification from Patrik Faltstrom.
Tightened definitions and introduced the term "DNS-Label" to avoid
ambiguity with various other uses of the word "label".
A.6. draft-liman-tld-names-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.
A.7. draft-liman-tld-names-00
First cut. Prompted by Olafur Gudmundsson and Tina Dam.
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A.8. Version Identification Tag
$Id: draft-liman-tld-names.xml,v 1.40 2011/04/12 08:20:42 liman Exp $
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Authors' Addresses
Lars-Johan Liman
Netnod Internet Exchange
Box 30194
SE-104 25 Stockholm
Sweden
Email: liman@netnod.se
URI: http://www.netnod.se/
Joe Abley
ICANN
4676 Admiralty Way
Suite 330
Marina del Rey 90292
USA
Email: joe.abley@icann.org
URI: http://www.icann.org/
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