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Versions: (draft-conroy-enum-experiences) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 5483

ENUM                                                           L. Conroy
Internet-Draft                                                      RMRL
Intended status: Informational                               K. Fujiwara
Expires: September 6, 2007                                          JPRS
                                                           March 5, 2007


               ENUM Implementation Issues and Experiences
                  <draft-ietf-enum-experiences-06.txt>

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 6, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).













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Abstract

   This document captures experience in implementing systems based on
   the ENUM protocol, and experience of ENUM data that have been created
   by others.  As such, it is advisory, and produced as a help to others
   in reporting what is "out there" and the potential pitfalls in
   interpreting the set of documents that specify the protocol.


Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Document Goal  . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Changes since last version . . . . . . . . . . . . . . . .  4
   3.  Character Sets and ENUM  . . . . . . . . . . . . . . . . . . .  5
     3.1.  Character Sets - Non-ASCII considered harmful  . . . . . .  5
     3.2.  Case Sensitivity . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  RegExp field delimiter . . . . . . . . . . . . . . . . . .  8
     3.4.  RegExp Meta-character Issue  . . . . . . . . . . . . . . .  9
   4.  ORDER/PRIORITY Processing  . . . . . . . . . . . . . . . . . . 11
     4.1.  Order/Priority values - general processing . . . . . . . . 11
     4.2.  NAPTRs with identical ORDER/PRIORITY values  . . . . . . . 13
       4.2.1.  Compound NAPTRs and implicit ORDER/REFERENCE Values  . 14
     4.3.  Compound NAPTR Processing  . . . . . . . . . . . . . . . . 14
     4.4.  Processing Order value across Domains  . . . . . . . . . . 15
   5.  Non-Terminal NAPTR Processing  . . . . . . . . . . . . . . . . 17
     5.1.  Non-Terminal NAPTRs - necessity  . . . . . . . . . . . . . 17
     5.2.  Non-Terminal NAPTRs - future implementation  . . . . . . . 18
       5.2.1.  Non-Terminal NAPTRs - general  . . . . . . . . . . . . 18
       5.2.2.  Non-Terminal NAPTRs - loop detection and response  . . 19
     5.3.  Interpretation of RFC 3403 and RFC 3761  . . . . . . . . . 19
       5.3.1.  Flags field content with Non-Terminal NAPTRs . . . . . 20
       5.3.2.  Services field content with Non-Terminal NAPTRs  . . . 20
       5.3.3.  Regular Expression and Replacement field content
               with non-terminal NAPTRs . . . . . . . . . . . . . . . 21
   6.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . . 24
     6.1.  Services field syntax  . . . . . . . . . . . . . . . . . . 24
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     10.2. Informative References . . . . . . . . . . . . . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
   Intellectual Property and Copyright Statements . . . . . . . . . . 32





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1.  Terminology

   This document is Advisory, and does not specify a standard of any
   kind.  Note that recommendations here contain the words "must",
   "require", "should", and "may".  This particular document does not
   form a standard and so these terms DO NOT hold their normative
   definitions.  The proposals include these terms from observation of
   behaviour and for internal consistency, where Client and Server
   recommendations have to match.










































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2.  Introduction

2.1.  Document Goal

   This document has been in place for a considerable period.  The goal
   has always been to advise implementers on the pitfalls that they may
   find.  It highlights areas where ENUM implementations have differed
   over interpretation of the standards documents, or have outright
   failed to implement some features as specified.  It does mention
   potential choices that can be made, in an attempt to help to foster
   interworking between components that use this protocol.  The reader
   is reminded that others may make different choices.

   It also covers topics that may be included in updates or
   clarifications to the applicable standards for ENUM.  The text here
   is not, in any way, to be treated as a standard.  However, those
   topics that impinge on candidates for standards update are marked in
   the text.

   The ENUM protocol ([1]) and the Dynamic Delegation Discovery System
   (DDDS, [2] [3] [4] [5] [6]) are defined elsewhere, and those
   documents alone form the normative definition of the ENUM system.
   Unfortunately, this document cannot provide an overview of the
   specifications, so the reader is assumed to have read and understood
   the complete set of ENUM normative documents.

2.2.  Changes since last version

   Since the last version of this document, a separate draft [20] has
   been produced to specify the requirement for EDNS0 support when
   processing ENUM queries and responses.  The advisory text covering
   the remaining topics can be ascertained by a thorough analysis of all
   of the existing DNS standard documents, and so is not required
   separately here.  Thus, the section in the previous version of this
   document that covered DNS issues has been removed entirely.  In this
   version we have tried to bracket text impinging on potential updates
   to standards inside <STD> and </STD> tags.  Similarly, text merely
   reiterating behaviour specified in standards is marked with <STBO>
   and </STBO>.  Some observations from trials have been clarified, and
   where possible we have bracketed advisory observations with <ADV> and
   </ADV> pairs.  Finally, where a common approach may lead to easier
   interoperation, we have made recommendations and bracketed those with
   <ITW> and </ITW> pairs.








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3.  Character Sets and ENUM

3.1.  Character Sets - Non-ASCII considered harmful

   RFC 3761 and RFC 3403 ([1] and [2]) specify respectively that ENUM
   and NAPTRs support Unicode using the UTF-8 encoding specified in [7].
   This raises an issue where implementations use "single byte" string
   processing routines.  If there are multi-byte characters within an
   ENUM NAPTR, incorrect processing may well result from these "UTF-8
   unaware" systems.

   The UTF-8 encoding has a "US-ASCII equivalent range", so that all
   characters in US-ASCII [21] from 0x00 to 0x7F hexadecimal have an
   identity map to the UTF-8 encoding; the encodings are the same.  In
   UTF-8, characters with Unicode code points above this range will be
   encoded using more than one byte, all of which will be in the range
   0x80 to 0xFF hexadecimal.  Thus it is important to consider the
   different fields of a NAPTR and whether or not multi-byte characters
   can or should appear in them.

   In addition, characters in the "non-printable" portion of US-ASCII
   (0x00 to 0x1F hexadecimal, plus 0x7F hexadecimal) are "difficult".
   Although NAPTRs are processed by machine, they may sometimes need to
   be written in a "human readable" form.  Similarly, if NAPTR content
   is shown to an end user so that he or she may choose, it is important
   that the content is "human readable".  Thus it is unwise to use non-
   printable characters within the US-ASCII range; the ENUM client may
   have good reason to reject NAPTRs that include these characters as
   they cannot readily be presented to an end-user.

   There are two numeric fields in a NAPTR; the ORDER and PREFERENCE/
   PRIORITY fields.  As these contain binary values, no risk is involved
   as string processing should not be applied to them.  The "string
   based" fields are the Flags, Services, and RegExp fields.  The
   Replacement field holds a domain name encoded according to the
   standard DNS mechanism [8][9].  With the introduction of
   Internationalized Domain Name (IDN) support, this domain name MUST be
   further encoded using Punycode [10].  As this holds a domain name
   that is not subject to replacement or modification (other than
   Punycode processing), it is not of concern here.

   Taking the string fields in turn, the Flags field contains characters
   that indicate the disposition of the NAPTR.  This may be empty, in
   which case the NAPTR is "non-terminal", or it may include a flag
   character as specified in RFC 3761.  These characters all fall into
   the US-ASCII equivalent range, so multi-byte characters cannot occur.

   The Services field includes the DDDS Application identifier ("E2U")



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   used for ENUM, the '+' character used to separate tokens, and a set
   of ENUMservice identifiers, any of which may include the ':'
   separator character.  In section 2.4.2 of RFC 3761 these identifiers
   are specified as 1*32 ALPHA/DIGIT, so there is no possibility of non-
   ASCII characters in the Services field.

   The RegExp field is more complex.  It forms a sed-like substitution
   expression, defined in [2], and consists of two sub-fields:

   o  the POSIX Extended Regular Expression (ERE) sub-field [11]

   o  a replacement (repl) sub-field [2].

   Additionally, RFC 3403 specifies that a flag character may be
   appended, but the only flag currently defined there (the 'i' case
   insensitivity flag) is not appropriate for ENUM - see later in this
   document.

   The ERE sub-field matches against the "Application Unique String";
   for ENUM, this is defined in RFC 3761 to consist of digit characters,
   with an initial '+' character.  It is similar to a global-number-
   digits production of a tel: URI, as specified in [12], but with
   visual-separators removed.  In short, it is a telephone number (see
   [13]) in restricted format.  All of these characters fall into the
   US-ASCII equivalent range of UTF-8 encoding, as do the characters
   significant to the ERE processing.  Thus, for ENUM, there will be no
   multi-byte characters within this sub-field.

   The repl sub-field can include a mixture of explicit text used to
   construct a URI and characters significant to the substitution
   expression, as defined in RFC 3403.  Whilst the latter set all fall
   into the US-ASCII equivalent range of UTF-8 encoding, this might not
   be the case for all conceivable text used to construct a URI.
   Presence of multi-byte characters could complicate URI generation and
   processing routines.

   URI generic syntax is defined in [14] as a sequence of characters
   chosen from a limited subset of the repertoire of US-ASCII
   characters.  The current URIs use the standard URI character
   "escaping" rules specified in the URI generic syntax, and so any
   multi-byte characters will be pre-processed; they will not occur in
   the explicit text used to construct a URI within the repl sub-field.
   However, the Internationalized Resource Identifier (IRI) is defined
   in [15] as extending the syntax of URIs, and specifies a mapping from
   an IRI to a URI.  IRI syntax allows characters with multi-byte UTF-8
   encoding.

   Given that this is the only place within an ENUM NAPTR where such



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   multi-byte encodings might reasonably be found, a simple solution is
   to use the mapping method specified in section 3.1 of [15] to convert
   any IRI into its equivalent URI.

   This process consists of two elements; the domain part of an IRI MUST
   be processed using Punycode if it has a non-ASCII domain name, and
   the remainder MUST be processed using the extended "escaping" rules
   specified in the IRI document if it contains characters outside the
   normal URI repertoire.  Using this process, there will be no non-
   ASCII characters in any part of any URI, even if it has been
   converted from an IRI that contains such characters.

   Taking into account the existing client base, it is RECOMMENDED that:

   <STD>

     Spec    All ENUMservice registrations should REQUIRE that any
             static text in the repl sub-field is encoded using only
             characters in the US-ASCII equivalent range that are
             "printable".  If any of the static text characters do fall
             outside this range then they MUST be pre-processed using an
             IRI/URI-specific "escape" mechanism such as that specified
             in section 3.1 of [15] to re-encode them only using US-
             ASCII equivalent printable characters (those in the range
             U+0020 to U+007E).

   </STD>

   Finally, the majority of ENUM clients in use today do not support
   multi-byte encodings of the Unicode Consortium's Universal Character
   Set (UCS).  This is a reasonable choice, particularly for "small
   footprint" implementations, and they may not be able to support NAPTR
   content that is non-printable as they need to present the content to
   an end user for selection.  Thus, it is advised that:

   <ADV>

     Client  ENUM clients have been known to discard NAPTRs in which
             they detect characters not in the US-ASCII "printable"
             range (0x20 to 0x7E hexadecimal).

   </ADV>

   ENUM zone provisioning systems should consider this.  It is
   RECOMMENDED that:

   <ITW>




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     Server  ENUM zone provisioning systems should not use unescaped
             non-ASCII characters in the NAPTRs they generate unless it
             is clear that all ENUM clients they are designed to support
             will be able correctly to process such characters.

   </ITW>

3.2.  Case Sensitivity

   The only place where NAPTR field content is case sensitive is in any
   static text in the repl sub-field of the RegExp field.  Everywhere
   else, case insensitive processing can be used.

   The case insensitivity flag ('i') could be added at the end of the
   RegExp field.  However, in ENUM, the ERE sub-field operates on a
   string defined as the '+' character, followed by a sequence of digit
   characters.  Thus this flag is redundant for E2U NAPTRs, as it does
   not act on the repl sub-field contents.

   To avoid the confusion that this generates, It is RECOMMENDED that:

   <STD>

     Server  When populating ENUM zones with NAPTRs, provisioning
             systems should not use the 'i' RegExp field flag, as it has
             no effect and some ENUM clients do not expect it.

     Client  ENUM clients should not assume that the delimiter is the
             last character of the field.

   </STD>

3.3.  RegExp field delimiter

   It is not possible to select a delimiter character that cannot appear
   in one of the sub-fields.  Some old clients are "hardwired" to expect
   the character '!' as a delimiter.  This is used in an example in RFC
   3403.

   It is RECOMMENDED that:

   <ITW>

     Server  ENUM zone provisioning systems should use '!'  (U+0021) as
             their RegExp delimiter character.

   </ITW>




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   From experience, a number of different client implementations are
   still "hardwired" to expect this character as a delimiter.
   Administrators of ENUM Provisioning systems are advised that:

   <ADV>

     Client  ENUM clients have been known to discard NAPTRs that do not
             use '!' as a RegExp delimiter.

   </ADV>

   The '!' character cannot appear in the ERE sub-field.  It may appear
   in the content of some URIs, as it is a valid character (e.g. in http
   URLs).

   <STBO>

   Thus, it is further RECOMMENDED that:

     Server  ENUM zone provisioning systems must ensure that, if the
             RegExp delimiter is a character in the static text of the
             repl sub-field, it must be "escaped" using the escaped-
             delimiter production of the BNF specification shown in
             section 3.2 of RFC 3402 (i.e. "\!", U+005C U+0021).

   Finally, in keeping with RFC 3402:

     Client  ENUM clients should discard NAPTRs that have more or less
             than 3 "unescaped" instances of the delimiter character
             within the RegExp field.

   </STBO>

3.4.  RegExp Meta-character Issue

   <STBO>

   In ENUM, the ERE sub-field may include a literal character '+', as
   the Application Unique String on which it operates includes this.
   However, if it is present, then '+' must be "escaped" using a single
   backslash character as '+' is a meta-character in POSIX Extended
   Regular Expression syntax.

   The following NAPTR example is incorrect:

   * IN NAPTR 100 10 "u" "E2U+sip" "!^+46555(.*)$!sip:\1@example.net!" .

   This example MUST be written as:



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   * IN NAPTR 100 10 "u" "E2U+sip" "!^\+46555(.*)$!sip:\1@example.net!"
   .

   Thus, it is RECOMMENDED that:

     Server  If present in the ERE sub-field of an ENUM NAPTR, '+' must
             be written as "\+" (i.e.  U+005C U+002B).

   </STBO>










































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4.  ORDER/PRIORITY Processing

4.1.  Order/Priority values - general processing

   RFC 3761 and RFC 3403 state that the ENUM client MUST sort the NAPTRs
   using the ORDER field value ("lowest value is first") and SHOULD
   order the NAPTRs using the PREFERENCE/PRIORITY field value as the
   minor sort term (again, lowest value first).  The NAPTRs in the
   sorted list must be processed in order.  Subsequent NAPTRs with less
   preferred ORDER values must only be dealt with once the current ones
   with a "winning" ORDER value have been processed.

   However, this stated behaviour is a simplification; ENUM clients may
   not behave this way in practice, and so there is a conflict between
   the specification and practice.  For example, ENUM clients will be
   incapable of using most NAPTRs as they do not support the ENUMservice
   (and the URI generated by those NAPTRs).  As such, they will discard
   the "unusable" NAPTRs and continue with processing the "next best"
   NAPTR in the list.

   The end user may have pre-specified his or her own preference for
   services to be used.  Thus, an end user may specify that he or she
   would prefer to use contacts with a "sip" ENUMservice, and then those
   with "email:mailto" service, and is not interested in any other
   options.  Thus the sorted list as proposed by the Registrant (and
   published via ENUM) may be reordered.  For example, a NAPTR with a
   "sip" ENUMservice may have a "losing" ORDER field value, and yet is
   chosen before a NAPTR with an "h323" ENUMservice and a "winning"
   ORDER value.  This may occur even if the node the end user controls
   is capable of handling other ENUMservices.

   ENUM clients may also include the end user "in the decision loop",
   offering the end user the choice from a list of possible NAPTRs.
   Given that the ORDER field value is the major sort term, one would
   expect a conforming ENUM client to present only those NAPTRs with a
   "winning" ORDER field value as choices.  However, if all the options
   presented had been rejected, then the ENUM client might offer those
   with the "next best" ORDER field value, and so on.  As this may be
   confusing for the end user, some clients simply offer all of the
   available NAPTRs as options to the end user for his or her selection
   "in one go".

   In summary, some ENUM clients will take into account the Services
   field value along with the ORDER and PREFERENCE/PRIORITY field
   values, and may consider the preferences of the end user.

   The Registrant and the ENUM zone provisioning system he or she uses
   must be aware of this and should not rely on ENUM clients taking



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   account of the value of the ORDER and the PREFERENCE/PRIORITY fields.

   Specifically, it is unsafe to assume that a ENUM client will not
   consider another NAPTR until it has discarded one with a "winning"
   ORDER value.  The instruction (in RFC 3403 section 4.1 and section 8)
   may or may not be followed strictly by different ENUM clients for
   perfectly justifiable reasons.

   To avoid the risk of variable behaviour, it is RECOMMENDED that:

   <STD>

     Server  ENUM zone provisioning systems should not use different
             ORDER values for NAPTRs in a Resource Record Set (RRSet).

   </STD>

   From experience, incorrect ORDER values in ENUM zones is a major
   source of problems.  Although it is by no means required, it is
   further RECOMMENDED that:

   <ITW>

     Server  ENUM zone provisioning systems should use a value of 100 as
             the default ORDER value to be used with all NAPTRs.

   </ITW>

   When populating an RRSet with NAPTRs, it is RECOMMENDED that:

   <ADV>

     Server  A Registrant should not expect the ENUM client to ignore
             NAPTRs with higher ORDER field values - the "winning" ones
             may have been discarded.

     Server  A Registrant should not expect ENUM clients to conform to
             the ORDER and PREFERENCE/PRIORITY sort order he or she has
             specified for NAPTRs; end users may have their own
             preferences for ENUMservices.

   </ADV>

   Where the ENUM Client presents a list of possible URLs to the end
   user for his or her choice, it should attempt to keep as close as
   possible to the ORDER and PREFERENCE/PRIORITY values specified by the
   Registrant.  To improve interworking whilst still accepting that end
   users with ENUM clients have valid reasons for preferring particular



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   URL schemes regardless of the preference specified in an RRSet, it is
   RECOMMENDED that:

   <ITW>

     Client  Each ENUM client may reorder the NAPTRs it receives only to
             match an explicit preference pre-specified by its end user.

     Client  ENUM clients that offer a list of contacts to the end user
             for his or her choice may present all NAPTRs, not just the
             ones with the highest currently unprocessed ORDER field
             value.

     Server  A Registrant should not assume which NAPTR choices will be
             presented to an end user "at once".

   </ITW>

   <STBO>

   The impact of this is that a Registrant should place into his or her
   zone only contacts that he or she is willing to support; even those
   with the "least preferred" ORDER and PREFERENCE/PRIORITY values may
   be selected by an end user.

   Finally, we have noticed a number of ENUM domains with NAPTRs that
   have identical PREFERENCE/PRIORITY field values and different ORDER
   values.  This may be the result of an ENUM zone provisioning system
   "bug" or a misunderstanding over the uses of the two fields.

   To clarify, the ORDER field value is the major sort term, and the
   PREFERENCE/PRIORITY field value is the minor sort term.  Thus one
   should expect to have a set of NAPTRs in a zone with identical ORDER
   field values and different PREFERENCE/PRIORITY field values.

   </STBO>

4.2.  NAPTRs with identical ORDER/PRIORITY values

   From experience, there are zones that hold discrete NAPTRs with
   identical ORDER and identical PREFERENCE/PRIORITY field values.  This
   will lead to indeterminate client behaviour and so should not occur.
   However, in the spirit of being liberal in what is allowed, it is
   RECOMMENDED that:

   <ITW>





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     Client  ENUM clients should accept all NAPTRs with identical ORDER
             and identical PREFERENCE/PRIORITY field values, and process
             them in the sequence in which they appear in the DNS
             response.

             (There is no benefit in further randomising the order in
             which these are processed, as intervening DNS Servers may
             do this already).

   Conversely, populating the records with these identical values is
   unwise, as it may lead to indeterminate client behaviour, and so it
   is RECOMMENDED that:

     Server  When populating ENUM RRSets with NAPTRs, ENUM zone
             provisioning systems should not have more than one NAPTR
             with the same ORDER and the same PREFERENCE/PRIORITY field
             values in any given RRSet, as ENUM clients may reject the
             response, and the sequence in which these NAPTRs are
             delivered to the client may vary.

   </ITW>

4.2.1.  Compound NAPTRs and implicit ORDER/REFERENCE Values

   There is one special case in which one could derive a set of NAPTRs
   with identical ORDER and identical PREFERENCE/PRIORITY fields.  This
   will not exist explicitly in the RRSet delivered to the client, but
   may occur whilst processing a "Compound" NAPTR, and is dealt with
   next.

4.3.  Compound NAPTR Processing

   With RFC 3761, it is possible to have more than one ENUMservice
   associated with a single NAPTR.  Of course, the different
   ENUMservices share the same RegExp field and so generate the same
   URI.  Such a "compound" NAPTR could well be used to indicate, for
   example, a mobile phone that supports both "voice:tel" and "sms:tel"
   ENUMservices.

   This compound NAPTR may be reconstructed into a set of NAPTRs each
   holding a single ENUMservice.  However, in this case the members of
   this set all logically hold the same ORDER and PREFERENCE/PRIORITY
   field values.

   In this case, it is RECOMMENDED that:

   <ITW>




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     Client  ENUM clients receiving compound NAPTRs (i.e. ones with more
             than one ENUMservice) should process these ENUMservices
             using a left-to-right sort ordering, so that the first
             ENUMservice to be processed will be the leftmost one, and
             the last will be the rightmost one.

     Server  An ENUM zone provisioning system should assume that, if it
             generates compound NAPTRs, the ENUMservices will normally
             be processed in left to right order within such NAPTRs.

   </ITW>

   As a final point on ENUM client processing of compound NAPTRs, it is
   quite possible that the client is incapable of processing one of the
   ENUMservices indicated.

   <STBO>

   To clarify, it is RECOMMENDED that:

     Client  When an ENUM client encounters a compound NAPTR and cannot
             process one of the ENUMservices within it, that ENUM client
             should ignore it and continue with the next ENUMservice
             within this NAPTR's Services field, discarding the NAPTR
             only if it cannot handle any of the ENUMservices contained.

   </STBO>

4.4.  Processing Order value across Domains

   Using a different ORDER field value in different domains is
   unimportant for most queries.  However, DDDS includes a mechanism for
   continuing a search for NAPTRs in another domain by including a
   reference to that other domain in a "non-terminal" NAPTR.  The
   treatment of non-terminal NAPTRs is covered in the next section, but
   if these are supported then it does have a bearing on the way that
   ORDER and PREFERENCE/PRIORITY field values are processed.

   Two main questions remain from the specifications of DDDS and RFC
   3671:

   o  If there is a different (lower) order field value in a domain
      referred to by a non-terminal NAPTR, then does this mean that the
      ENUM client discards any remaining NAPTRs in the referring RRSet?

   o  Conversely, if the domain referred to by a non-terminal NAPTR
      contains entries that only have a higher ORDER field value, then
      does the ENUM client ignore those NAPTRs in the referenced domain?



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   Whilst one interpretation of section 1.3 of RFC 3761 is that the
   answer to both questions is "yes", this is not the way that those
   examples of non-terminal NAPTRs that do exist (and those ENUM clients
   that support them) seem to be designed.

   It is RECOMMENDED that:

   <STD>

     Server  ENUM zone provisioning systems should assume that, once a
             non-terminal NAPTR has been selected for processing, the
             ORDER field value in a domain referred to by that non-
             terminal NAPTR will be considered only within the context
             of that referenced domain (i.e. the ORDER value will be
             used only to sort within the current RRSet, and will not be
             used in the processing of NAPTRs in any other RRSet).



     Client  ENUM clients should consider the ORDER field value only
             when sorting NAPTRs within a single RRSet.  The ORDER field
             value should not be taken into account when processing
             NAPTRs across a sequence of DNS queries created by
             traversal of non-terminal NAPTR references.

   </STD>

























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5.  Non-Terminal NAPTR Processing

5.1.  Non-Terminal NAPTRs - necessity

   Consider an ENUM RRSet that contains a non-terminal NAPTR record.
   This non-terminal NAPTR "points to" another domain that has a set of
   NAPTRs.  In effect, this is similar to the non-terminal NAPTR being
   replaced by the NAPTRs contained in the domain to which it points.

   It is possible to have a non-terminal NAPTR in a domain that is,
   itself, pointed to by another non-terminal NAPTR.  Thus a set of
   domains forms a "chain", and the list of NAPTRs to be considered is
   the set of all NAPTRs contained in all of the domains in that chain.

   For an ENUM management system to support non-terminal NAPTRs, it is
   necessary for it to be able to analyse, validate and (where needed)
   correct not only the NAPTRs in its current ENUM domain but also those
   "pointed to" by non-terminal NAPTRs in other domains.  If the domains
   pointed to have non-terminal NAPTRs of their own, the management
   system will have to check each of the referenced domains in turn, as
   their contents forms part of the result of a query on the "main" ENUM
   domain.  The domain content in the referenced domains may well not be
   under the control of the ENUM management system, and so it may not be
   possible to correct any errors in those RRSets.  This is both complex
   and prone to error in the management system design, and any reported
   errors in validation may well be non-intuitive for users.

   For an ENUM client, supporting non-terminal NAPTRs can also be
   difficult.  Processing non-terminal NAPTRs causes a set of sequential
   DNS queries that can take an indeterminate time, and requires extra
   resources and complexity to handle fault conditions like non-terminal
   loops.  The indeterminacy of response time makes ENUM supported
   Telephony Applications difficult (such as in an "ENUM-aware" PBX),
   whilst the added complexity and resources needed makes support
   problematic in embedded devices like "ENUM-aware" mobile phones.

   Given that, in principle, a non-terminal NAPTR can be replaced by the
   NAPTRs in the domain to which it points, support of non-terminal
   NAPTRs is not needed and non-terminal NAPTRs may not be useful.

   Furthermore, some existing ENUM clients do not support non-terminal
   NAPTRs and ignore them if received.

   <ITW>

   To avoid interoperability problems, some kind of acceptable
   requirement is needed on non-terminal NAPTRs.  Given the lack of
   current support and the issues raised, we propose that in general one



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   should not use non-terminal NAPTRs in ENUM.

   Thus, it is RECOMMENDED that:

     Server  ENUM zone provisioning systems should not generate non-
             terminal NAPTRs (i.e.  NAPTRs with an empty Flags field)
             unless it is clear that all ENUM clients they are designed
             to support can process these.

   </ITW>

   ENUM zone administrators are advised that:

   <ADV>

     Client  ENUM clients have been known to discard non-terminal NAPTRs
             (i.e. they may only support ENUM NAPTRs with a Flags field
             value of "u").

   </ADV>

5.2.  Non-Terminal NAPTRs - future implementation

   The following specific issues need to be considered if non-terminal
   NAPTRs are to be supported in the future.  These issues are gleaned
   from experience, and indicate the kinds of conditions that should be
   considered before support for non-terminal NAPTRs is contemplated.
   Note that these issues are in addition to the point just mentioned on
   ENUM provisioning or management system complexity and the potential
   for that management system to have no control over the zone contents
   to which non-terminal NAPTRs in "its" managed zones refer.

5.2.1.  Non-Terminal NAPTRs - general

   As mentioned earlier, a non-terminal NAPTR in one RRSet refers to the
   NAPTRs contained in another domain.  The NAPTRs in the domain
   referred to by the non-terminal NAPTR may have a different ORDER
   value from that in the referring non-terminal NAPTR.  See Section 4.4
   for details.

   In addition, to Clarify, it is RECOMMENDED that:

   <STBO>

     Client  If all NAPTRs in a domain traversed as a result of a
             reference in a non-terminal NAPTR have been discarded, then
             the ENUM client should continue its processing with the
             next NAPTR in the "referring" RRSet (i.e. the one including



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             the non-terminal NAPTR that caused the traversal).

   </STBO>

5.2.2.  Non-Terminal NAPTRs - loop detection and response

   Where a "chain" of non-terminal NAPTRs refers back to a domain
   already traversed in the current query, this implies a "non-terminal
   loop".  To ensure consistent behaviour, it is RECOMMENDED that:

   <ITW>

     Client  ENUM clients should consider that processing a chain of
             more than 5 "non-terminal" NAPTRs in a single ENUM query
             indicates that a loop may have been detected, and act
             accordingly.

     Server  When populating a set of domains with NAPTRs, ENUM zone
             provisioning systems should not configure non-terminal
             NAPTRs so that more than 5 such NAPTRs will be processed in
             an ENUM query.

   </ITW>

   From experience, those ENUM client implementations that support Non-
   Terminal NAPTRs have differed in what they should do on encountering
   a Non-Terminal loop.  To avoid this confusion, it is RECOMMENDED
   that:

   <STD>

     Client  Where a domain is about to be entered as the result of a
             reference in a non-terminal NAPTR, and the ENUM client has
             detected a potential "non-terminal loop", then the client
             should discard the non-terminal NAPTR from its processing
             and continue with the next NAPTR in its list.  It should
             not make the DNS query indicated by that non-terminal
             NAPTR.

   </STD>

5.3.  Interpretation of RFC 3403 and RFC 3761

   The set of specifications defining DDDS and its applications are
   complex and multi-layered.  This reflects the flexibility that the
   system provides, but it does mean that some of the specifications
   need clarification as to their interpretation, particularly where
   non-terminal rules are concerned.



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5.3.1.  Flags field content with Non-Terminal NAPTRs

   <STBO>

   RFC 3761, section 2.4.1 states that the only flag character valid for
   use with the "E2U" DDDS Application is 'u'.  The flag 'u' is defined
   (in RFC 3404 [5], section 4.3) thus: 'The "u" flag means that the
   output of the Rule is a URI'.

   RFC 3761 section 2.4.1 also states that an empty Flags field
   indicates a non-terminal NAPTR.  This is also the case for other DDDS
   Application specifications, such as that specified in RFC 3404.  One
   could well argue that this is a feature potentially common to all
   DDDS Applications, and so should have been specified in RFC 3402 or
   RFC 3403.

   </STBO>

5.3.2.  Services field content with Non-Terminal NAPTRs

   Furthermore, RFC 3761 section 3.1.1 states that any ENUMservice
   Specification requires definition of the URI that is the expected
   output of this ENUMservice.  This means that, at present, there is no
   way to specify an ENUMservice that is non-terminal.  Such a non-
   terminal NAPTR has, by definition, no URI as its expected output,
   instead returning a key (DNS domain name) that is to be used in the
   "next round" of DDDS processing.

   This in turn means that there can be no valid (non-empty) Services
   field content for a NAPTR to be used with the "E2U" DDDS application.
   Section 2.4.2 of RFC 3761 specifies the syntax for this field
   content, and requires at least one element of type <servicespec>
   (i.e. at least one ENUMservice identifier).  Given that there can be
   no definition of a non-terminal ENUMservice (and so no such
   Registered ENUMservice identifier), this syntax cannot be met with a
   non-terminal NAPTR.

   A reasonable interpretation of the specifications in their current
   state is that the Services field must also be empty; this appears to
   be the approach taken by those clients that do either process non-
   terminal NAPTRs or check the validity of the fields.  To ensure
   consistent behaviour, it is RECOMMENDED that:

   <STD>







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     Client  ENUM clients should ignore any content of the Services
             field when encountering a non-terminal NAPTR with an empty
             Flags field.

     Server  ENUM zone provisioning systems should ensure that the
             Services field of any non-terminal NAPTR (with an empty
             Flags field) is also empty.

   </STD>

5.3.3.  Regular Expression and Replacement field content with non-
        terminal NAPTRs

   The descriptive text in section 4.1 of RFC 3403 is intended to
   explain how the fields are to be used in a NAPTR.  However, the
   descriptions associated with the RegExp and Replacement elements have
   led to some confusion over which of these should be considered when
   dealing with non-terminal NAPTRs.

   RFC 3403 is specific; these two elements are mutually exclusive.
   This means that if the RegExp element is not empty then the
   Replacement element must be empty, and vice versa.  However, is does
   not specify which is used with terminal and non-terminal rules.

   The descriptive text of section 4.1 of RFC 3403 for the NAPTR
   Replacement element shows that this element holds an uncompressed
   domain name.  Thus it is clear that this element cannot be used to
   deliver the terminal string for any DDDS application that does not
   have a domain name as its intended terminal output.

   However, the first paragraph of descriptive text for the NAPTR RegExp
   element has led to some confusion.  It appears that the RegExp
   element is to be used to find "the next domain name to lookup".  This
   might be interpreted as meaning that a client program processing the
   DDDS application could need to examine each non-terminal NAPTR to
   decide whether the RegExp element or instead the Replacement element
   were to be used to construct the key (a domain name) to be used next
   in non-terminal rule processing.

   Given that a NAPTR holding a terminal rule (a "terminal NAPTR") must
   use the Substitution expression field to generate the expected output
   of that DDDS application, the RegExp element is also used in such
   rules.  Indeed, unless that DDDS application has a domain name as its
   terminal output, the RegExp element is the only possibility.

   Thus from the descriptive text of this section, a Replacement element
   can be used only in NAPTRs holding a non-terminal rule (a "non-
   terminal NAPTR") unless that DDDS Application has a domain name as



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   its terminal output, whilst the alternative RegExp element may be
   used either to generate a domain name as the next key to be used in
   the non-terminal case, or to generate the output of the DDDS
   application.

   Note that each DDDS Application is free to specify the set of flags
   to be used with that application.  This includes specifying whether a
   particular flag is associated with a terminal or non-terminal rule,
   and also to specify the interpretation of an empty Flags field (i.e.
   whether this is to be interpreted as a terminal or non-terminal rule,
   and if it is terminal, then the expected output).  ENUM (as specified
   in section 2.4.1 of RFC 3761) specifies only the 'u' flag, with an
   empty Flags field indicating a non-terminal NAPTR.

   The general case in which a client program must check which of the
   two elements to use in non-terminal NAPTR processing complicates
   implementation, and this interpretation has NOT been made in current
   ENUM examples "out in the wild".  It would be useful to define
   exactly when a client program can expect to process the RegExp
   element and when to expect to process the Replacement element, if
   only to improve robustness.

   In keeping with current implementations, we suggest that a non-
   terminal NAPTR with an empty Flags field must be provisioned using
   the (non-empty) Replacement element to hold the domain name that
   forms the "next key" output from this non-terminal rule.

   Thus it is RECOMMENDED that:

   <STD>

     Client  ENUM clients receiving a non-terminal NAPTR with an empty
             Flags field must treat the Replacement field as holding the
             domain name to be used in the next round of the ENUM query.
             An ENUM client must discard such a non-terminal NAPTR if
             the Replacement field is empty or does not contain a valid
             domain name.  By definition, it follows that the RegExp
             field will be empty in such a non-terminal NAPTR, and
             should be ignored by ENUM clients

     Server  with an empty Flags field into an ENUM zone must ensure
             that the "target" domain name is set into the Replacement
             field of this NAPTR.  It must not use the RegExp field in
             such a non-terminal NAPTR.

   </STD>

   <STD>



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   In the future, it would be possible to update RFC 3761 (sections
   3.1.1 and 2.4.1) to add a new flag to indicate a non-terminal NAPTR,
   and to change the ENUMservice template to permit specification of an
   ENUMservice that operates with this new flag in non-terminal NAPTRs.
   In doing this, it would be possible to include a syntactically valid
   non-empty Services field in such non-terminal NAPTRs.  To
   differentiate from the case of an empty Flags field, this new flag
   could also indicate that the RegExp field was to be non-empty, and to
   be processed - by implication, this would mean that the Replacement
   field would be empty.  However, such a change would require an update
   to RFC 3761, and so will have to wait.

   </STD>






































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6.  Backwards Compatibility

6.1.  Services field syntax

   RFC 3761 is the current standard for the syntax for NAPTRs supporting
   the ENUM DDDS application.  This obsoletes the original specification
   that was given in RFC 2916.  There has been a change to the syntax of
   the Services field of the NAPTR that reflects a refinement of the
   concept of ENUM processing.

   As defined in RFC 3403, there is now a single identifier that
   indicates the DDDS Application.  In the obsolete specification (RFC
   2915), there were zero or more "Resolution Service" identifiers (the
   equivalent of the DDDS Application).  The same identifier string is
   defined in both RFC 3761 and in the old RFC 2916 specifications for
   the DDDS identifier or the Resolution Service; "E2U".

   Also, RFC 3761 defines at least one but potentially several
   ENUMservice sub-fields; in the obsolete specification, only one
   "protocol" sub-field was allowed.

   In many ways, the most important change for implementations is that
   the order of the sub-fields has been reversed.  RFC 3761 specifies
   that the DDDS Application identifier is the leftmost sub-field,
   followed by one or more ENUMservice sub-fields, each separated by the
   '+' character delimiter.  RFC 2916 specified that the protocol sub-
   field was the leftmost, followed by the '+' delimiter, in turn
   followed by the "E2U" resolution service tag.

   RFC 2915 and RFC 2916 have been obsoleted by RFC 3401 - RFC 3404 and
   by RFC 3761.  Thus it is RECOMMENDED that:

   <STD>

     Server  ENUM zone provisioning systems must not generate NAPTRs
             according to the syntax defined in RFC 2916.  All zones
             must hold ENUM NAPTRs according to RFC 3761 (and
             ENUMservice specifications according to the framework
             specified there).

   </STD>

   However, RFC 3824 [16] suggests that ENUM clients should be prepared
   to accept NAPTRs with the obsolete syntax.  Thus, an ENUM client
   implementation may have to deal with both forms.

   It is RECOMMENDED that:




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   <STD>

     Client  ENUM clients must support ENUM NAPTRs according to RFC 3761
             syntax.  ENUM clients should also support ENUM NAPTRs
             according to the obsolete syntax of RFC 2916; there are
             still zones that hold "old" syntax NAPTRs.

   </STD>

   This need not be difficult.  For example, an implementation could
   process the Services field into a set of tokens, and expect exactly
   one of these tokens to be "E2U".  In this way, the ENUM client might
   be designed to handle both the old and the current forms without
   added complexity.

   There is one subtle implication of this scheme.  It is RECOMMENDED
   that:

   <STD>

     Spec    Registrations for an ENUMservice with the type string of
             "E2U" and an empty sub-type string must not be accepted.

   </STD>



























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7.  Security Considerations

   This document does not specify any standard.  It does however make
   some recommendations, and so the implications of following those
   suggestions have to be considered.

   In addition to these issues, those in the basic use of ENUM (and
   specified in the normative documents for this protocol) should be
   considered as well; this document does not negate those in any way.

   The clarifications throughout this document are intended only as
   that; clarifications of text in the normative documents.  They do not
   appear to have any security implications above those mentioned in the
   normative documents.

   The suggestions in Section 3, Section 4, and Section 6 do not appear
   to have any security considerations (either positive or negative).

   The suggestions in Section 5.2.2 are a valid approach to a known
   security threat.  It does not open an advantage to an attacker in
   causing excess processing or memory usage in the client.  It does,
   however, mean that an ENUM client will traverse a "tight loop" of
   non-terminal NAPTRs in two domains 5 times before the client detects
   this as a loop; this does introduce slightly higher processing load
   than would be provided using other methods, but avoids the risks they
   incur.

























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8.  IANA Considerations

   This document is only advisory, and does not include any IANA
   considerations other than the proposals labelled as "Spec".  This is
   the suggestion (at the end of Section 6.1) that no-one should specify
   an ENUMservice with the identifying tag "E2U".













































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9.  Acknowledgements

   We would like to thank the various development teams who implemented
   ENUM (both creation systems and clients) and who read the normative
   documents differently - without these differences it would have been
   harder for us all to develop robust clients and suitably conservative
   management systems.  We would also thank those who allowed us to
   check their implementations to explore behaviour; their trust and
   help were much appreciated.

   In particular, thanks to Richard Stastny for his hard work on a
   similar task TS 102 172 [22] under the aegis of ETSI, and for
   supporting some of the ENUM implementations that exist today.

   Finally, thanks for the dedication of Michael Mealling in giving us
   such detailed DDDS specifications, without which the ENUM development
   effort would have had a less rigourous framework on which to build.
   This document reflects how complex a system it is: Without the
   intricacy of RFC 3401 - RFC 3404 and the work that went into them, it
   could have been quite different.































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10.  References

10.1.  Normative References

   [1]   Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
         Identifiers (URI) Dynamic Delegation  Discovery System (DDDS)
         Application (ENUM)", RFC 3761, April 2004.

   [2]   Mealling, M., "Dynamic Delegation Discovery System (DDDS)  Part
         Three: The Domain Name System (DNS) Database", RFC 3403,
         October 2002.

   [3]   Mealling, M., "Dynamic Delegation Discovery System (DDDS)  Part
         One: The Comprehensive DDDS", RFC 3401, October 2002.

   [4]   Mealling, M., "Dynamic Delegation Discovery System (DDDS)  Part
         Two: The Algorithm", RFC 3402, October 2002.

   [5]   Mealling, M., "Dynamic Delegation Discovery System (DDDS)  Part
         Four: The Uniform Resource Identifiers (URI)", RFC 3404,
         October 2002.

   [6]   Mealling, M., "Dynamic Delegation Discovery System (DDDS)  Part
         Five: URI.ARPA Assignment Procedures", RFC 3405, October 2002.

   [7]   Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, RFC 3629, November 2003.

   [8]   Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
         RFC 1034, November 1987.

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

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

   [11]  Institute of Electrical and Electronics Engineers, "Information
         Technology - Portable Operating System Interface (POSIX) - Part
         2: Shell and Utilities (Vol. 1)", IEEE Standard 1003.2,
         January 1993.

   [12]  Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
         December 2004.

   [13]  ITU-T, "The International Public Telecommunication Number
         Plan", Recommendation E.164, February 2005.



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   [14]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", RFC 3986,
         January 2005.

   [15]  Duerst, M. and M. Suignard, "Internationalized Resource
         Identifiers (IRIs)", RFC 3987, January 2005.

   [16]  Peterson, J., Liu, H., Yu, J., and B. Campbell, "Using E.164
         numbers with the Session Initiation Protocol (SIP)", RFC 3824,
         June 2004.

   [17]  Braden, R., "Requirements for Internet Hosts -- Application and
         Support", RFC 1123, October 1989.

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

   [19]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
         August 1999.

10.2.  Informative References

   [20]  Conroy, L. and J. Reid, "ENUM Requirement for EDNS0 Support",
         draft-ietf-enum-edns0-00.txt (work in progress),
         September 2006.

   [21]  American National Standards Institute, "Coded Character Set --
         7-bit American Standard Code for Information Interchange",
         ANSI X3.4, 1986.

   [22]  ETSI, "Minimum Requirements for Interoperability of European
         ENUM Implementations", ETSI TS 102 172, October 2004.

   [23]  Atkins, D. and R. Austein, "Threat Analysis of the Domain Name
         System (DNS)", RFC 3833, August 2004.

   [24]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "DNS Security Introduction and Requirements", RFC 4033,
         March 2005.

   [25]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Resource Records for the DNS Security Extensions", RFC 4034,
         March 2005.

   [26]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Protocol Modifications for the DNS Security Extensions",
         RFC 4035, March 2005.




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Authors' Addresses

   Lawrence Conroy
   Roke Manor Research
   Roke Manor
   Old Salisbury Lane
   Romsey
   United Kingdom

   Phone: +44-1794-833666
   Email: lconroy@insensate.co.uk
   URI:   http://www.sienum.co.uk


   Kazunori Fujiwara
   Japan Registry Service Co., Ltd.
   Chiyoda First Bldg. East 13F
   3-8-1 Nishi-Kanda Chiyoda-ku
   Tokyo 101-0165
   JAPAN

   Email: fujiwara@jprs.co.jp
   URI:   http://jprs.jp/en/




























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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
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