Application Techniques for Checking and Transformation of Names
draft-klensin-name-filters-03

Network Working Group                                         J. Klensin
Internet-Draft                                         September 5, 2003
Expires: March 5, 2004

   User Interface Evaluation and Filtering of Internet Addresses and
              Locators -or- Syntaxes for Common Namespaces
                   draft-klensin-name-filters-03.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on March 5, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). 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, expecially 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Restrictions on domain (DNS) names . . . . . . . . . . . . . .  4
   3.  Restrictions on email addresses  . . . . . . . . . . . . . . .  7
   4.  URLs and URIs  . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.1 URI syntax definitions and issues  . . . . . . . . . . . . . .  9
   4.2 The HTTP URL . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.3 The MAILTO URL . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.4 Guessing domain names in web contexts  . . . . . . . . . . . . 12
   5.  Implications of internationalization . . . . . . . . . . . . . 14
   6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Security considerations  . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
       Normative References . . . . . . . . . . . . . . . . . . . . . 18
       Non-normative References . . . . . . . . . . . . . . . . . . . 20
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 20
       Intellectual Property and Copyright Statements . . . . . . . . 21

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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 URLs and URIs)) 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 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 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
   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.

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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 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 only of
   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  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 Implications of internationalization.

   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

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   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 to trying 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 entitles [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].
   These length-dependent rules were, however, 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 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 that 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
   domains: one two-letter domain that does not appear in the ISO 3166-1
   table 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 check the
   current list of TLDs (both "generic" and country-code-related)
   published by IANA at http://www.iana.org/domain-names.htm.  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

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   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 non-ASCII label is converted to encoded
   "punycode" form (see Implications of internationalization) the length
   of that form may restrict the number of actual characters (in the
   original character set) that can be accomodated.  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.

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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, instead, should be passed to the email system
   for evaluation by the destination host.

   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\@def@example.com

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

      Fred\ Bloggs@example.com

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

      Joe.\\Blow@example.com

   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

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   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

      user+mailbox@example.com

      customer/department=shipping@example.com

      $A12345@example.com

      !def!xyz%abc@example.com

      _somename@example.com

   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.

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4. URLs and URIs

4.1 URI syntax definitions and issues

   The syntax for URLs (Uniform Resource Locators) is specified in  The
   syntax for the more general "URI" (Uniform Resource Identifier) is
   specified in .  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.

      ; / ? : @ & = + $ , ?

   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.

   When it is necessary to encode these, or other, characters, the
   method used is replace it with a percent-sign ("%") followed by two
   hexidecimal digits representing its octet value.   See section 2.4.1
   of  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 [RFC1866] 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

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   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

      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  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
   Restrictions on email addresses above.  It can also support multiple
   addresses and 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

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   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 URI syntax definitions and issues above, when
   the addresses are mapped into the URL form.  Conversely, addresses in
   MAILTO URLs cannot 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.

   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 |
   +----------------------+----------------------+---------------------+
   |    Joe@example.com   | mailto:joe@example.c |                   1 |
   |                      |          om          |                     |
   |                      |                      |                     |
   | user+mailbox@example |        mailto:       |                   2 |
   |         .com         | user%2Bmailbox@examp |                     |
   |                      |        le.com        |                     |
   |                      |                      |                     |
   | customer/department= | mailto:customer%2Fde |                   3 |
   | shipping@example.com | partment=shipping@ex |                     |
   |                      |       ample.com      |                     |
   |                      |                      |                     |
   |  $A12345@example.com | mailto:$A12345@examp |                   4 |
   |                      |        le.com        |                     |
   |                      |                      |                     |
   | !def!xyz%abc@example | mailto:!def!xyz%25ab |                   5 |
   |         .com         |     c@example.com    |                     |
   |                      |                      |                     |
   | _somename@example.co | mailto:_somename@exa |                   4 |

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   |           m          |       mple.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.

   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.  If this approach is 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.

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   As discussed in Restrictions on domain (DNS) names 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 .

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5. Implications of internationalization

   The IETF has adopted a series of proposals ( - ) 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 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 both
   the encoded form (those strings are consistent with the "LDH rule"
   (see Restrictions on domain (DNS) names) 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., . 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.

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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 protoocl 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
   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.  Making the test
   incorrectly, or with obsolete information, can be extremely
   frustrating for potential correspondents or customers and may harm
   desired relationships.

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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.

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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.

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Normative References

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

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

   [RFC1866]  Berners-Lee, T. and D. Connolly, "Hypertext Markup
              Language - 2.0", RFC 1866, November 1995.

   [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., Nielsen, H.,
              Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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

   [RFC2822]  Resnick, P., "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.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications

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              (IDNA)", RFC 3492, March 2003.

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

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Non-normative References

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

   [JET]      Seng, J., "Internationalized Domain Names Registration and
              Administration Guideline for Chinese, Japanese and
              Korean", draft-jseng-idn-admin-03 (work in progress), June
              2003.

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

   [RegRestr]
              Klensin, J., "Registration Restrictions on
              Internationalized Domain Names -- An Overview",
              draft-klensin-reg-guidelines-00 (work in progress), June
              2003.

Author's Address

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

   Phone: +1 617 491 5735
   EMail: john-ietf@jck.com

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