Network Working Group J. Klensin

Network Working Group J. Klensin
Request for Comments: 3696 February 2004
Category: Informational

Application Techniques for Checking and Transformation of Names

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

Many Internet applications have been designed to deduce top-level
domains (or other domain name labels) from partial information. The
introduction of new top-level domains, especially non-country-code
ones, has exposed flaws in some of the methods used by these
applications. These flaws make it more difficult, or impossible, for
users of the applications to access the full Internet. This memo
discusses some of the techniques that have been used and gives some
guidance for minimizing their negative impact as the domain name
environment evolves. This document draws summaries of the applicable
rules together in one place and supplies references to the actual
standards.

Klensin Informational [Page 1]

RFC 3696 Checking and Transformation of Names February 2004

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Restrictions on domain (DNS) names . . . . . . . . . . . . . . 3
3. Restrictions on email addresses . . . . . . . . . . . . . . . 5
4. URLs and URIs . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. URI syntax definitions and issues . . . . . . . . . . . 7
4.2. The HTTP URL . . . . . . . . . . . . . . . . . . . . . . 8
4.3. The MAILTO URL . . . . . . . . . . . . . . . . . . . . . 9
4.4. Guessing domain names in web contexts . . . . . . . . . 11
5. Implications of internationalization . . . . . . . . . . . . . 11
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . 15
10. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16

1. Introduction

Designers of user interfaces to Internet applications have often
found it useful to examine user-provided values for validity before
passing them to the Internet tools themselves. This type of test,
most commonly involving syntax checks or application of other rules
to domain names, email addresses, or "web addresses" (URLs or,
occasionally, extended URI forms (see Section 4)) may enable better-
quality diagnostics for the user than might be available from the
protocol itself. Local validity tests on values are also thought to
improve the efficiency of back-office processing programs and to
reduce the load on the protocols themselves. Certainly, they are
consistent with the well-established principle that it is better to
detect errors as early as possible.

The tests must, however, be made correctly or at least safely. If
criteria are applied that do not match the protocols, users will be
inconvenienced, addresses and sites will effectively become
inaccessible to some groups, and business and communications
opportunities will be lost. Experience in recent years indicates
that syntax tests are often performed incorrectly and that tests for
top-level domain names are applied using obsolete lists and
conventions. We assume that most of these incorrect tests are the
result of the inability to conveniently locate exact definitions for
the criteria to be applied. This document draws summaries of the
applicable rules together in one place and supplies references to the

Klensin Informational [Page 2]

RFC 3696 Checking and Transformation of Names February 2004

actual standards. It does not add anything to those standards; it
merely draws the information together into a form that may be more
accessible.

Many experts on Internet protocols believe that tests and rules of
these sorts should be avoided in applications and that the tests in
the protocols and back-office systems should be relied on instead.
Certainly implementations of the protocols cannot assume that the
data passed to them will be valid. Unless the standards specify
particular behavior, this document takes no position on whether or
not the testing is desirable. It only identifies the correct tests
to be made if tests are to be applied.

The sections that follow discuss domain names, email addresses, and
URLs.

2. Restrictions on domain (DNS) names

The authoritative definitions of the format and syntax of domain
names appear in RFCs 1035 [RFC1035], 1123 [RFC1123], and 2181
[RFC2181].

Any characters, or combination of bits (as octets), are permitted in
DNS names. However, there is a preferred form that is required by
most applications. This preferred form has been the only one
permitted in the names of top-level domains, or TLDs. In general, it
is also the only form permitted in most second-level names registered
in TLDs, although some names that are normally not seen by users obey
other rules. It derives from the original ARPANET rules for the
naming of hosts (i.e., the "hostname" rule) and is perhaps better
described as the "LDH rule", after the characters that it permits.
The LDH rule, as updated, provides that the labels (words or strings
separated by periods) that make up a domain name must consist of only
the ASCII [ASCII] alphabetic and numeric characters, plus the hyphen.
No other symbols or punctuation characters are permitted, nor is
blank space. If the hyphen is used, it is not permitted to appear at
either the beginning or end of a label. There is an additional rule
that essentially requires that top-level domain names not be all-
numeric.

When it is necessary to express labels with non-character octets, or
to embed periods within labels, there is a mechanism for keying them
in that utilizes an escape sequence. RFC 1035 [RFC1035] should be
consulted if that mechanism is needed (most common applications,
including email and the Web, will generally not permit those escaped
strings). A special encoding is now available for non-ASCII
characters, see the brief discussion in Section 5.

Klensin Informational [Page 3]

RFC 3696 Checking and Transformation of Names February 2004

Most internet applications that reference other hosts or systems
assume they will be supplied with "fully-qualified" domain names,
i.e., ones that include all of the labels leading to the root,
including the TLD name. Those fully-qualified domain names are then
passed to either the domain name resolution protocol itself or to the
remote systems. Consequently, purported DNS names to be used in
applications and to locate resources generally must contain at least
one period (".") character. Those that do not are either invalid or
require the application to supply additional information. Of course,
this principle does not apply when the purpose of the application is
to process or query TLD names themselves. The DNS specification also
permits a trailing period to be used to denote the root, e.g.,
"a.b.c" and "a.b.c." are equivalent, but the latter is more explicit
and is required to be accepted by applications. This convention is
especially important when a TLD name is being referred to directly.
For example, while ".COM" has become the popular terminology for
referring to that top-level domain, "COM." would be strictly and
technically correct in talking about the DNS, since it shows that
"COM" is a top-level domain name.

There is a long history of applications moving beyond the "one or
more periods" test in an attempt to verify that a valid TLD name is
actually present. They have done this either by applying some
heuristics to the form of the name or by consulting a local list of
valid names. The historical heuristics are no longer effective. If
one is to keep a local list, much more effort must be devoted to
keeping it up-to-date than was the case several years ago.

The heuristics were based on the observation that, since the DNS was
first deployed, all top-level domain names were two, three, or four
characters in length. All two-character names were associated with
"country code" domains, with the specific labels (with a few early
exceptions) drawn from the ISO list of codes for countries and
similar entities [IS3166]. The three-letter names were "generic"
TLDs, whose function was not country-specific, and there was exactly
one four-letter TLD, the infrastructure domain "ARPA." [RFC1591].
However, these length-dependent rules were conventions, rather than
anything on which the protocols depended.

Before the mid-1990s, lists of valid top-level domain names changed
infrequently. New country codes were gradually, and then more
rapidly, added as the Internet expanded, but the list of generic
domains did not change at all between the establishment of the "INT."
domain in 1988 and ICANN's allocation of new generic TLDs in 2000.
Some application developers responded by assuming that any two-letter
domain name could be valid as a TLD, but the list of generic TLDs was
fixed and could be kept locally and tested. Several of these
assumptions changed as ICANN started to allocate new top-level

Klensin Informational [Page 4]

RFC 3696 Checking and Transformation of Names February 2004

domains: one two-letter domain that does not appear in the ISO 3166-1
table [ISO.3166.1988] was tentatively approved, and new domains were
created with three, four, and even six letter codes.

As of the first quarter of 2003, the list of valid, non-country,
top-level domains was .AERO, .BIZ, .COM, .COOP, .EDU, .GOV, .INFO,
.INT, .MIL, .MUSEUM, .NAME, .NET, .ORG, .PRO, and .ARPA. ICANN is
expected to expand that list at regular intervals, so the list that
appears here should not be used in testing. Instead, systems that
filter by testing top-level domain names should regularly update
their local tables of TLDs (both "generic" and country-code-related)
by polling the list published by IANA [DomainList]. It is
likely that the better strategy has now become to make the "at least
one period" test, to verify LDH conformance (including verification
that the apparent TLD name is not all-numeric), and then to use the
DNS to determine domain name validity, rather than trying to maintain
a local list of valid TLD names.

A DNS label may be no more than 63 octets long. This is in the form
actually stored; if a non-ASCII label is converted to encoded
"punycode" form (see Section 5), the length of that form may restrict
the number of actual characters (in the original character set) that
can be accommodated. A complete, fully-qualified, domain name must
not exceed 255 octets.

Some additional mechanisms for guessing correct domain names when
incomplete information is provided have been developed for use with
the web and are discussed in Section 4.4.

3. Restrictions on email addresses

Reference documents: RFC 2821 [RFC2821] and RFC 2822 [RFC2822]

Contemporary email addresses consist of a "local part" separated from
a "domain part" (a fully-qualified domain name) by an at-sign ("@").
The syntax of the domain part corresponds to that in the previous
section. The concerns identified in that section about filtering and
lists of names apply to the domain names used in an email context as
well. The domain name can also be replaced by an IP address in
square brackets, but that form is strongly discouraged except for
testing and troubleshooting purposes.

The local part may appear using the quoting conventions described
below. The quoted forms are rarely used in practice, but are
required for some legitimate purposes. Hence, they should not be
rejected in filtering routines but, should instead be passed to the
email system for evaluation by the destination host.

Klensin Informational [Page 5]

RFC 3696 Checking and Transformation of Names February 2004

The exact rule is that any ASCII character, including control
characters, may appear quoted, or in a quoted string. When quoting
is needed, the backslash character is used to quote the following
character. For example

Abc\@[email protected]

is a valid form of an email address. Blank spaces may also appear,
as in

Fred\ [email protected]

The backslash character may also be used to quote itself, e.g.,

Joe.\\[email protected]

In addition to quoting using the backslash character, conventional
double-quote characters may be used to surround strings. For example

"Abc@def"@example.com

"Fred Bloggs"@example.com

are alternate forms of the first two examples above. These quoted
forms are rarely recommended, and are uncommon in practice, but, as
discussed above, must be supported by applications that are
processing email addresses. In particular, the quoted forms often
appear in the context of addresses associated with transitions from
other systems and contexts; those transitional requirements do still
arise and, since a system that accepts a user-provided email address
cannot "know" whether that address is associated with a legacy
system, the address forms must be accepted and passed into the email
environment.

Without quotes, local-parts may consist of any combination of
alphabetic characters, digits, or any of the special characters

! # $ % & ' * + - / = ? ^ _ ` . { | } ~

period (".") may also appear, but may not be used to start or end the
local part, nor may two or more consecutive periods appear. Stated
differently, any ASCII graphic (printing) character other than the
at-sign ("@"), backslash, double quote, comma, or square brackets may
appear without quoting. If any of that list of excluded characters
are to appear, they must be quoted. Forms such as

[email protected]

Klensin Informational [Page 6]

RFC 3696 Checking and Transformation of Names February 2004

customer/[email protected]

[email protected]

!def!xyz%[email protected]

[email protected]

are valid and are seen fairly regularly, but any of the characters
listed above are permitted. In the context of local parts,
apostrophe ("'") and acute accent ("`") are ordinary characters, not
quoting characters. Some of the characters listed above are used in
conventions about routing or other types of special handling by some
receiving hosts. But, since there is no way to know whether the
remote host is using those conventions or just treating these
characters as normal text, sending programs (and programs evaluating
address validity) must simply accept the strings and pass them on.

In addition to restrictions on syntax, there is a length limit on
email addresses. That limit is a maximum of 64 characters (octets)
in the "local part" (before the "@") and a maximum of 255 characters
(octets) in the domain part (after the "@") for a total length of 320
characters. Systems that handle email should be prepared to process
addresses which are that long, even though they are rarely
encountered.

4. URLs and URIs

4.1. URI syntax definitions and issues

The syntax for URLs (Uniform Resource Locators) is specified in
[RFC1738]. The syntax for the more general "URI" (Uniform Resource
Identifier) is specified in [RFC2396]. The URI syntax is extremely
general, with considerable variations permitted according to the type
of "scheme" (e.g., "http", "ftp", "mailto") that is being used.
While it is possible to use the general syntax rules of RFC 2396 to
perform syntax checks, they are general enough --essentially only
specifying the separation of the scheme name and "scheme specific
part" with a colon (":") and excluding some characters that must be
escaped if used-- to provide little significant filtering or
validation power.

The following characters are reserved in many URIs -- they must be
used for either their URI-intended purpose or must be encoded. Some
particular schemes may either broaden or relax these restrictions
(see the following sections for URLs applicable to "web pages" and
electronic mail), or apply them only to particular URI component
parts.

Klensin Informational [Page 7]

RFC 3696 Checking and Transformation of Names February 2004

; / ? : @ & = + $ , ?

In addition, control characters, the space character, the double-
quote (") character, and the following special characters

< > # %

are generally forbidden and must either be avoided or escaped, as
discussed below.

The colon after the scheme name, and the percent sign used to escape
characters, are specifically reserved for those purposes, although
":" may also be used elsewhere in some schemes.

When it is necessary to encode these, or other, characters, the
method used is to replace it with a percent-sign ("%") followed by
two hexidecimal digits representing its octet value. See section
2.4.1 of [RFC2396] for an exact definition. Unless it is used as a
delimiter of the URI scheme itself, any character may optionally be
encoded this way; systems that are testing URI syntax should be
prepared for these encodings to appear in any component of the URI
except the scheme name itself.

A "generic URI" syntax is specified and is more restrictive, but
using it to test URI strings requires that one know whether or not
the particular scheme in use obeys that syntax. Consequently,
applications that intend to check or validate URIs should normally
identify the scheme name and then apply scheme-specific tests. The
rules for two of those -- HTTP [RFC1738] and MAILTO [RFC2368] URLs --
are discussed below, but the author of an application which intends
to make very precise checks, or to reject particular syntax rather
than just warning the user, should consult the relevant scheme-
definition documents for precise syntax and relationships.

4.2. The HTTP URL

Absolute HTTP URLs consist of the scheme name, a host name (expressed
as a domain name or IP address), and optional port number, and then,
optionally, a path, a search part, and a fragment identifier. These
are separated, respectively, by a colon and the two slashes that
precede the host name, a colon, a slash, a question mark, and a hash
mark ("#"). So we have

http://host:port/path?search#fragment

http://host/path/

http://host/path#fragment

Klensin Informational [Page 8]

RFC 3696 Checking and Transformation of Names February 2004

http://host/path?search

http://host

and other variations on that form. There is also a "relative" form,
but it almost never appears in text that a user might, e.g., enter
into a form. See [RFC2616] for details.

The characters

/ ; ?

are reserved within the path and search parts and must be encoded;
the first of these may be used unencoded, and is often used within
the path, to designate hierarchy.

4.3. The MAILTO URL

MAILTO is a URL type whose content is an email address. It can be
used to encode any of the email address formats discussed in Section
3 above. It can also support multiple addresses and the inclusion of
headers (e.g., Subject lines) within the body of the URL. MAILTO is
authoritatively defined in RFC 2368 [RFC2368]; anyone expecting to
accept and test multiple addresses or mail header or body formats
should consult that document carefully.

In accepting text for, or validating, a MAILTO URL, it is important
to note that, while it can be used to encode any valid email address,
it is not sufficient to copy an email address into a MAILTO URL since
email addresses may include a number of characters that are invalid
in, or have reserved uses for, URLs. Those characters must be
encoded, as outlined in Section 4.1 above, when the addresses are
mapped into the URL form. Conversely, addresses in MAILTO URLs
cannot, in general, be copied directly into email contexts, since few
email programs will reverse the decodings (and doing so might be
interpreted as a protocol violation).

The following characters may appear in MAILTO URLs only with the
specific defined meanings given. If they appear in an email address
(i.e., for some other purpose), they must be encoded:

: The colon in "mailto:"

< > # " % { } | \ ^ ~ `

These characters are "unsafe" in any URL, and must always be
encoded.

Klensin Informational [Page 9]

RFC 3696 Checking and Transformation of Names February 2004

The following characters must also be encoded if they appear in a
MAILTO URL

? & =
Used to delimit headers and their values when these are encoded
into URLs.

Some examples may be helpful:

+-------------------------+-----------------------------+-----------+
| Email address | MAILTO URL | Notes |
+-------------------------+-----------------------------+-----------+
| [email protected] | mailto:[email protected] | 1 |
| | | |
| user+mailbox@example | mailto: | 2 |
| .com | user%2Bmailbox@example | |
| | .com | |
| | | |
| customer/department= | mailto:customer%2F | 3 |
| [email protected] | department=shipping@example | |
| | .com | |
| | | |
| [email protected] | mailto:$A12345@example | 4 |
| | .com | |
| | | |
| !def!xyz%abc@example | mailto:!def!xyz%25abc | 5 |
| .com | @example.com | |
| | | |
| [email protected] | mailto:_somename@example | 4 |
| | .com | |
+-------------------------+-----------------------------+-----------+

Table 1

Notes on Table

1. No characters appear in the email address that require escaping,
so the body of the MAILTO URL is identical to the email address.

2. There is actually some uncertainty as to whether or not the "+"
characters requires escaping in MAILTO URLs (the standards are
not precisely clear). But, since any character in the address
specification may optionally be encoded, it is probably safer to
encode it.

3. The "/" character is generally reserved in URLs, and must be
encoded as %2F.

Klensin Informational [Page 10]

RFC 3696 Checking and Transformation of Names February 2004

4. Neither the "$" nor the "_" character are given any special
interpretation in MAILTO URLs, so need not be encoded.

5. While the "!" character has no special interpretation, the "%"
character is used to introduce encoded sequences and hence it
must always be encoded.

4.4. Guessing domain names in web contexts

Several web browsers have adopted a practice that permits an
incomplete domain name to be used as input instead of a complete URL.
This has, for example, permitted users to type "microsoft" and have
the browser interpret the input as "http://www.microsoft.com/".
Other browser versions have gone even further, trying to build DNS
names up through a series of heuristics, testing each variation in
turn to see if it appears in the DNS, and accepting the first one
found as the intended domain name. Still, others automatically
invoke search engines if no period appears or if the reference fails.
If any of these approaches are to be used, it is often critical that
the browser recognize the complete list of TLDs. If an incomplete
list is used, complete domain names may not be recognized as such and
the system may try to turn them into completely different names. For
example, "example.aero" is a fully-qualified name, since "AERO." is a
TLD name. But, if the system doesn't recognize "AERO" as a TLD name,
it is likely to try to look up "example.aero.com" and
"www.example.aero.com" (and then fail or find the wrong host), rather
than simply looking up the user-supplied name.

As discussed in Section 2 above, there are dangers associated with
software that attempts to "know" the list of top-level domain names
locally and take advantage of that knowledge. These name-guessing
heuristics are another example of that situation: if the lists are
up-to-date and used carefully, the systems in which they are embedded
may provide an easier, and more attractive, experience for at least
some users. But finding the wrong host, or being unable to find a
host even when its name is precisely known, constitute bad
experiences by any measure.

More generally, there have been bad experiences with attempts to
"complete" domain names by adding additional information to them.
These issues are described in some detail in RFC 1535 [RFC1535].

5. Implications of internationalization

The IETF has adopted a series of proposals ([RFC3490] - [RFC3492])
whose purpose is to permit encoding internationalized (i.e., non-
ASCII) names in the DNS. The primary standard, and the group
generically, are known as "IDNA". The actual strings stored in the

Klensin Informational [Page 11]

RFC 3696 Checking and Transformation of Names February 2004

DNS are in an encoded form: the labels begin with the characters
"xn--" followed by the encoded string. Applications should be
prepared to accept and process the encoded form (those strings are
consistent with the "LDH rule" (see Section 2) so should not raise
any separate issues) and the use of local, and potentially other,
characters as appropriate to local systems and circumstances.

The IDNA specification describes the exact process to be used to
validate a name or encoded string. The process is sufficiently
complex that shortcuts or heuristics, especially for versions of
labels written directly in Unicode or other coded character sets, are
likely to fail and cause problems. In particular, the strings cannot
be validated with syntax or semantic rules of any of the usual sorts:
syntax validity is defined only in terms of the result of executing a
particular function.

In addition to the restrictions imposed by the protocols themselves,
many domains are implementing rules about just which non-ASCII names
they will permit to be registered (see, e.g., [JET], [RegRestr]).
This work is still relatively new, and the rules and conventions are
likely to be different for each domain, or at least each language or
script group. Attempting to test for those rules in a client program
to see if a user-supplied name might possibly exist in the relevant
domain would almost certainly be ill-advised.

One quick local test however, may be reasonable: as of the time of
this writing, there should be no instances of labels in the DNS that
start with two characters, followed by two hyphens, where the two
characters are not "xn" (in, of course, either upper or lower case).
Such label strings, if they appear, are probably erroneous or
obsolete, and it may be reasonable to at least warn the user about
them.

There is ongoing work in the IETF and elsewhere to define
internationalized formats for use in other protocols, including email
addresses. Those forms may or may not conform to existing rules for
ASCII-only identifiers; anyone designing evaluators or filters should
watch that work closely.

6. Summary

When an application accepts a string from the user and ultimately
passes it on to an API for a protocol, the desirability of testing or
filtering the text in any way not required by the protocol itself is
hotly debated. If it must divide the string into its components, or
otherwise interpret it, it obviously must make at least enough tests
to validate that process. With, e.g., domain names or email
addresses that can be passed on untouched, the appropriateness of

Klensin Informational [Page 12]

RFC 3696 Checking and Transformation of Names February 2004

trying to figure out which ones are valid and which ones are not
requires a more complex decision, one that should include
considerations of how to make exactly the correct tests and to keep
information that changes and evolves up-to-date. A test containing
obsolete information, can be extremely frustrating for potential
correspondents or customers and may harm desired relationships.

7. Security Considerations

Since this document merely summarizes the requirements of existing
standards, it does not introduce any new security issues. However,
many of the techniques that motivate the document raise important
security concerns of their own. Rejecting valid forms of domain
names, email addresses, or URIs often denies service to the user of
those entities. Worse, guessing at the user's intent when an
incomplete address, or other string, is given can result in
compromises to privacy or accuracy of reference if the wrong target
is found and returned. From a security standpoint, the optimum
behavior is probably to never guess, but instead, to force the user
to specify exactly what is wanted. When that position involves a
tradeoff with an acceptable user experience, good judgment should be
used and the fact that it is a tradeoff recognized.

Some characters have special or privileged meanings on some systems
(i.e., ` on Unix). Applications should be careful to escape those
locally if necessary. By the same token, they are valid, and should
not be disallowed locally, or escaped when transmitted through
Internet protocols, for such reasons if a remote site chooses to use
them.

The presence of local checking does not permit remote checking to be
bypassed. Note that this can apply to a single machine; in
particular, a local MTA should not assume that a local MUA has
properly escaped locally-significant special characters.

8. Acknowledgements

The author would like to express his appreciation for helpful
comments from Harald Alvestrand, Eric A. Hall, and the RFC Editor,
and for partial support of this work from SITA. Responsibility for
any errors remains, of course, with the author.

The first Internet-Draft on this subject was posted in February 2003.
The document was submitted to the RFC Editor on 20 June 2003,
returned for revisions on 19 August, and resubmitted on 5 September
2003.

Klensin Informational [Page 13]

RFC 3696 Checking and Transformation of Names February 2004

9. References

9.1. Normative References

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October
1989.

[RFC1535] Gavron, E., "A Security Problem and Proposed
Correction With Widely Deployed DNS Software", RFC
1535, October 1993.

[RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill,
"Uniform Resource Locators (URL)", RFC 1738, December
1994.

[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.

[RFC2368] Hoffman, P., Masinter, L. and J. Zawinski, "The
mailto URL scheme", RFC 2368, July 1998.

[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic Syntax",
RFC 2396, August 1998.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.

[RFC2821] Klensin, J., Ed., "Simple Mail Transfer Protocol",
RFC 2821, April 2001.

[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC
2822, April 2001.

[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.

Klensin Informational [Page 14]

RFC 3696 Checking and Transformation of Names February 2004

[RFC3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for Internationalized Domain Names in
Applications (IDNA)", RFC 3492, March 2003.

[ASCII] American National Standards Institute (formerly
United States of America Standards Institute), "USA
Code for Information Interchange", ANSI X3.4-1968.
ANSI X3.4-1968 has been replaced by newer versions
with slight modifications, but the 1968 version
remains definitive for the Internet.

[DomainList] Internet Assigned Numbers Authority (IANA), Untitled
alphabetical list of current top-level domains.
http://data.iana.org/TLD/tlds-alpha-by-domain.txt
ftp://data.iana.org/TLD/tlds-alpha-by-domain.txt

9.2. Informative References

[ISO.3166.1988] International Organization for Standardization,
"Codes for the representation of names of countries,
3rd edition", ISO Standard 3166, August 1988.

[JET] Konishi, K., et al., "Internationalized Domain Names
Registration and Administration Guideline for
Chinese, Japanese and Korean", Work in Progress.

[RFC1591] Postel, J., "Domain Name System Structure and
Delegation", RFC 1591, March 1994.

[RegRestr] Klensin, J., "Registration of Internationalized
Domain Names: Overview and Method", Work in Progress,
February 2004.

10. Author's Address

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

Phone: +1 617 491 5735
EMail: [email protected]

Klensin Informational [Page 15]

RFC 3696 Checking and Transformation of Names February 2004

11. Full Copyright Statement

Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
[email protected].

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Klensin Informational [Page 16]