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RFC 5483 - ENUM Implementation Issues and Experiences


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Network Working Group                                          L. Conroy
Request for Comments: 5483                                          RMRL
Category: Informational                                      K. Fujiwara
                                                                    JPRS
                                                              March 2009

               ENUM Implementation Issues and Experiences

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) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document captures experiences in implementing systems based on
   the ENUM protocol and experiences of ENUM data that have been created
   by others.  As such, it clarifies the ENUM and Dynamic Delegation
   Discovery System standards.  Its aim is to help others by reporting
   both what is "out there" and potential pitfalls in interpreting the
   set of documents that specify the ENUM protocol.  It does not revise
   the standards but is intended to provide technical input to future
   revisions of those documents.

Table of Contents

   1. Introduction ....................................................3
      1.1. Document Goal ..............................................3
      1.2. Terminology ................................................3
   2. Character Sets and ENUM .........................................4
      2.1. Character Sets - Non-ASCII Considered Harmful ..............4
           2.1.1. Non-ASCII in the Regular Expression Field ...........5
           2.1.2. Non-ASCII Support - Conclusions .....................6
      2.2. Case Sensitivity ...........................................7
      2.3. Regexp Field Delimiter .....................................7
      2.4. Regexp Meta-Character Issue ................................8
   3. Unsupported NAPTRs ..............................................8
      3.1. Non-Compliant Client Behaviour .............................9
   4. ENUM NAPTR Processing ..........................................10
      4.1. Common Non-Compliant Client Behaviour .....................11
           4.1.1. Example ............................................11
      4.2. Order/Priority Values - Processing Sequence ...............12
      4.3. Use of Order and Preference Fields ........................13
      4.4. NAPTRs with Identical ORDER/PRIORITY Values ...............14
           4.4.1. Compound NAPTRs and Implicit
                  ORDER/REFERENCE Values .............................14
      4.5. Processing Order Value across Domains .....................15
   5. Non-Terminal NAPTR Processing ..................................16
      5.1. Non-Terminal NAPTRs - Necessity ...........................16
      5.2. Non-Terminal NAPTRs - Considerations ......................17
           5.2.1. Non-Terminal NAPTRs - General ......................17
           5.2.2. Non-Terminal NAPTRs - Loop Detection and Response ..17
           5.2.3. Field Content in Non-Terminal NAPTRs ...............17
   6. Backwards Compatibility ........................................20
      6.1. Services Field Syntax .....................................20
   7. Collected Implications for ENUM Provisioning ...................21
   8. Collected Implications for ENUM Clients ........................23
      8.1. Non-Terminal NAPTR Processing .............................25
   9. Security Considerations ........................................26
   10. Acknowledgements ..............................................27
   11. References ....................................................27
      11.1. Normative References .....................................27
      11.2. Informative References ...................................29

1.  Introduction

1.1.  Document Goal

   The goal of this document is to clarify the ENUM and Dynamic
   Delegation Discovery System (DDDS) standards.  It does not itself
   revise ENUM or DDDS standards but is intended to provide technical
   input to future revisions of those documents.  It also serves 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.

   As well as providing clarifications to standards text, this document
   also mentions potential choices that can be made, in an attempt to
   help foster interworking between components that use this protocol.
   The reader is reminded that others may make different choices.

   The core specifications for the E.164 Number Mapping (ENUM) protocol
   [RFC3761] and the Dynamic Delegation Discovery System (DDDS)
   [RFC3403] [RFC3401] [RFC3402] [RFC3404] [RFC3405] are defined
   elsewhere.  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.

   The Domain Name System (DNS) is ENUM's database.  ENUM uses the NAPTR
   (Naming Authority Pointer) resource record type to store its DDDS
   rules into DNS domains.  ENUM relies on DNS services.  Thus, it is
   also important for ENUM implementers to carry out a thorough analysis
   of all of the existing DNS standard documents to understand what
   services are provided to ENUM and what load ENUM provisioning and
   queries will place on the DNS.

   A great deal of the rationale for making the choices listed in this
   document is available to those who explore the standards.  The trick
   of course is in understanding those standards and the subtle
   implications that are involved in some of their features.  In almost
   all cases, the choices presented here are merely selections from
   values that are permissible within the standards.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Character Sets and ENUM

2.1.  Character Sets - Non-ASCII Considered Harmful

   [RFC3403] and [RFC3761] specify respectively that NAPTR resource
   records and ENUM support Unicode using the UTF-8 encoding defined in
   [RFC3629].  This raises an issue when 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 [ASCII] 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.  Specifically, if NAPTR content
   is shown to an end user so that he or she may choose, it is
   imperative that the content is human-readable.  Thus, it is unwise to
   use non-printable characters even if they lie 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
   because string processing should not be applied to them.  The string-
   based fields are the Flags, Services, and Regexp fields.  The
   Replacement field holds an uncompressed domain name, encoded
   according to the standard DNS mechanism [RFC1034][RFC1035].  The
   Internationalised Domain Name (IDN) can be supported (as specified in
   [RFC3490], [RFC3491], and [RFC3492]).  Any such IDN MUST be further
   encoded using Punycode [RFC3492].  As the Replacement field 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 [RFC3761].  These characters all fall into
   the printable US-ASCII equivalent range, so multi-byte characters
   cannot occur.

   The Services field includes the DDDS Application identifier ("E2U")
   used for ENUM, a set of Enumservice identifiers, any of which may
   embed the ':' separator character, together with the '+' character
   used to separate Enumservices from one another and from this DDDS
   Application identifier.  In Section 2.4.2 of [RFC3761], Enumservice
   identifier tokens are specified as 1*32 ALPHA/DIGIT, so there is no
   possibility of non-ASCII characters in the Services field.

2.1.1.  Non-ASCII in the Regular Expression Field

   The Regexp field is more complex.  It forms a sed-like substitution
   expression, defined in [RFC3402], and consists of two sub-fields:

   o  a POSIX Extended Regular Expression (ERE) sub-field
      [IEEE.1003-2.1992]

   o  a replacement (Repl) sub-field [RFC3402].

   Additionally, [RFC3402] 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 Section 2.2.

   The ERE sub-field matches against the "Application Unique String";
   for ENUM, this is defined in [RFC3761] 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
   [RFC3966], but with visual-separators removed.  In short, it is a
   telephone number (see [E.164]) 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.

   Strictly, the ERE might include other characters.  The ERE could
   include choice elements matching against different items, some of
   which might not be an ENUM Application Unique String.  Those
   alternative matching elements might conceivably include non-ASCII
   characters.  As an operational issue, it is not reasonable to include
   such constructs, as ENUM NAPTRs match against telephone numbers.

   In the normal situation in which E2U NAPTRs are provisioned in ENUM
   domains, there will be no multi-byte characters within this sub-
   field, as the ERE will be intended to match against telephone
   numbers.  ENUM clients must be able to handle NAPTRs that do contain
   such multi-byte characters (as the standard does not preclude them),
   but there is no operational reason for these ever being provisioned

   in ENUM domains.  If NAPTRs provisioned in ENUM domains are
   encountered containing such multi-byte characters, these could
   reasonably be discarded.

   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 [RFC3403].  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 [RFC3986] 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
   character will be pre-processed; they will not occur in the explicit
   text used to construct a URI within the Repl sub-field.

2.1.1.1.  Impact of Future Support for IRIs

   As currently specified, ENUM only permits URIs to be generated in the
   Regexp field.  However, even if this were to be extended in future
   revisions of the ENUM specification to allow the use of
   Internationalised Resource Identifiers (IRIs), defined in [RFC3987],
   further support for non-ASCII characters may be avoided.  IRIs are
   defined as extending the syntax of URIs, and RFC 3987 specifies a
   mapping from IRIs to URIs.  IRI syntax allows characters with multi-
   byte UTF-8 encoding.

   Given that this is the only place within an ENUM NAPTR where such
   multi-byte encodings might reasonably be found, a simple solution is
   to use the mapping method specified in Section 3.1 of [RFC3987] 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 [RFC3987] 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.

2.1.2.  Non-ASCII Support - Conclusions

   From the analysis just given, the only place within an ENUM NAPTR
   where non-ASCII characters might be found is the Regexp field.  It is
   possible to remove any requirement to process characters outside the

   US-ASCII equivalent range by adding very few operational
   restrictions.  There is no obvious benefit in providing characters
   outside this range.  Handling multi-byte characters complicates
   development and operation of client programs, and many existing
   programs do not include such support.

   As the gain from permitting characters outside the US-ASCII
   equivalent range is unclear, and the costs of multi-byte character
   processing are very clear, ENUM NAPTRs SHOULD NOT include characters
   outside the printable US-ASCII equivalent range.

2.2.  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.  This flag is redundant for E2U NAPTRs, as it does not
   act on the Repl sub-field contents.

   Thus, the case-sensitivity flag is inappropriate for ENUM, and SHOULD
   NOT be provisioned into E2U NAPTRs.

2.3.  Regexp Field Delimiter

   It is not possible to select a delimiter character that cannot appear
   in one of the sub-fields.  The '!' character is used as a delimiter
   in all of the examples in [RFC3403] and in [RFC3761].  It is the only
   character seen in existing zones, and a number of different client
   implementations are still "hardwired" to expect this character as a
   delimiter.

   The '!' character will not normally 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).  If it is present in the Regexp field, then
   that instance MUST be escaped using the standard technique proposed
   in Section 3.2 of [RFC3402]: a backslash character (U+005C) should be
   inserted before it in the string.  Otherwise, a client may attempt to
   process this as a standard delimiter and interpret the Regexp field
   contents differently from the system that provisioned it.

2.4.  Regexp Meta-Character Issue

   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 (to produce the sub-string U+005C U+002B), as '+'
   is a meta-character in POSIX Extended Regular Expression syntax.

   Not escaping the '+' character produces an invalid ERE, but is a
   common mistake.  Even standards have given incorrect examples; the
   obsolete [RFC2916] (Section 3.4.3, example 3) has this problem.

   For example, the following NAPTR example is incorrect:
   * IN NAPTR 100 10 "u" "E2U+sip" "!^+4655(.*)$!sip:\\1@example.net!" .

   A correct way to write this example is:
   * IN NAPTR 100 10 "u"
       "E2U+sip" "!^\\+4655(.*)$!sip:\\1@example.net!" .

   Note that when a NAPTR resource record is shown in DNS master file
   syntax (as in this example above), the backslash itself must be
   escaped using a second backslash.  The DNS on-the-wire packet will
   have only a single backslash.

3.  Unsupported NAPTRs

   An ENUM client MAY discard a NAPTR received in response to an ENUM
   query because:

   o  the NAPTR is syntactically or semantically incorrect,

   o  the NAPTR has a different (non-empty) DDDS Application identifier
      from the 'E2U' used in ENUM,

   o  the NAPTR's ERE does not match the Application Unique String for
      this ENUM query,

   o  the ENUM client does not recognise any Enumservice held in this
      NAPTR, or

   o  this NAPTR (only) contains an Enumservice that is unsupported.

   These conditions SHOULD NOT cause the whole ENUM query to terminate,
   and processing SHOULD continue with the next NAPTR in the returned
   Resource Record Set (RRSet).

   When an ENUM client encounters a compound NAPTR (i.e., one containing
   more than one Enumservice -- see also Section 4.4.1) and cannot
   process or cannot recognise one of the Enumservices within it, that
   ENUM client SHOULD ignore this Enumservice 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.  These
   conditions SHOULD NOT be considered errors.

   ENUM uses regular-expression processing when generating URIs from the
   Regexp field of "terminal" NAPTRs.  Just as with all uses of regular
   expressions, there is a potential for buffer overrun when generating
   this output.  There may be repeated back-reference patterns in a
   NAPTR's Repl sub-field, and the output these generate may consume a
   considerable amount of buffer space.

   Even if an ENUM client would normally encounter only NAPTRs with
   short URIs, it may also receive NAPTRs with repeated back-reference
   patterns in their Repl sub-fields that could generate strings longer
   than the client's buffer.  Such NAPTRs may have been misconfigured
   accidentally or by design.  The client MUST NOT fail in this case.
   It SHOULD NOT discard the entire ENUM query, but instead just discard
   the NAPTR that would otherwise have caused this overrun.

   If a problem is detected when processing an ENUM query across
   multiple domains (by following non-terminal NAPTR references), then
   the ENUM query SHOULD NOT be abandoned, but instead processing SHOULD
   continue at the next NAPTR after the non-terminal NAPTR that referred
   to the domain in which the problem would have occurred.  See
   Section 5.2.2 for more details.

3.1.  Non-Compliant Client Behaviour

   Through monitoring current ENUM clients, a number of non-compliant
   behaviours have been detected.  These behaviours are incorrect, but
   may be encountered in still-operational client implementations.

   ENUM clients have been known to discard NAPTRs in which the Services
   field holds more than one Enumservice.

   ENUM clients have also been known to discard NAPTRs with a "non-
   greedy" ERE sub-field expression (i.e., EREs that are dissimilar to
   "^.*$").

   ENUM clients have been known to discard NAPTRs that do not use '!' as
   their Regexp delimiter character.

   ENUM clients have been known to discard NAPTRs in which the delimiter
   is NOT the last character in the Regexp field.

   ENUM clients have been known to discard NAPTRs with an empty Flags
   field (i.e., "non-terminal" NAPTRs).

   ENUM clients have been known to ignore the ORDER field value
   entirely, sorting the NAPTRs in an RRSet based solely on the
   PREFERENCE/PRIORITY field values.

   Finally, many ENUM clients have been known to discard a NAPTR where
   they have local knowledge that the URI that would be generated by
   processing the NAPTR is unusable.  This behaviour is, strictly
   speaking, non-compliant, but might be considered reasonable (see
   Section 4.1).

4.  ENUM NAPTR Processing

   ENUM is a DDDS Application, and the way in which NAPTRs in an RRSet
   are processed reflects this.  The details are described in Section
   3.3 of [RFC3402].  The client is expected to sort the records it
   receives into a sequence and then process those records in that
   sequence.  The sequence reflects the ORDER and PREFERENCE/PRIORITY
   field values in each of the NAPTRs.

   The ORDER field value is the major, or most significant, sort term
   and the PREFERENCE/PRIORITY field value is the minor, or least
   significant, sort term.  The combination of ORDER and PREFERENCE/
   PRIORITY field values indicates the sequence chosen by the publisher
   of this data, and NAPTRs will be considered in this sequence.

   Once the NAPTRs are sorted into sequence, further processing is done
   to determine if each of the NAPTRs is appropriate for this ENUM
   evaluation.  This involves looking at the Flags field.  If the Flags
   field is empty, this is a "non-terminal" NAPTR and is processed as
   described in Section 5.

   If the "u" Flag is present (and so the NAPTR is a "terminal" rule
   that generates a URI), the Services field is checked to ensure that
   this NAPTR is intended for ENUM (i.e., that this NAPTR includes the
   "E2U" DDDS Application identifier in the Services field).  The ERE in
   the Regexp field is checked and must match the Application Unique
   String (AUS) for this ENUM evaluation (the queried telephone number).
   Unless each of these checks succeeds, the NAPTR is discarded and the
   next in sequence is processed.

   During this processing, clients will also consider the Enumservices
   within the Services field.  Enumservices indicate the kind of
   interaction that can be achieved through use of the URI this NAPTR
   generates.  If there is local knowledge that a NAPTR includes only an
   Enumservice that is either not supported or not recognised, then this

   NAPTR can be discarded and the next in sequence will be processed.
   Thus, for a system that has support only for SIP interactions, if it
   receives an RRSet in which the "best" NAPTR indicates the H323
   Enumservice, then that client could reasonably discard that NAPTR and
   go on to the next in sequence.

4.1.  Common Non-Compliant ENUM Processing

   The processing of ORDER and PREFERENCE/PRIORITY fields has been a
   significant source of confusion, and many ENUM clients do not
   implement the processing exactly as specified.

   In particular, many ENUM clients use local prior knowledge about URIs
   during ENUM processing.  If a client has prior knowledge that a
   particular URI will not result in an acceptable outcome, it might
   discard that NAPTR and consider the next one in the sequence.
   Examples of such local prior knowledge include: the URI does not
   resolve, authentication has been recently rejected, or user policies
   mark a particular URI as unacceptable (the URI could be a "premium
   rate" telephone number that would be charged at an unacceptable
   rate).

   Strictly speaking, this behaviour is non-compliant if the next NAPTR
   record has a different ORDER value.  The ENUM algorithm (Section 3.3
   of [RFC3402] and Section 4.1 of [RFC3403]) states that once a match
   has been found for the Application Unique String (AUS), and the
   service description satisfies the client's requirements, NAPTR
   records with larger ORDER values must not be considered (but other
   NAPTR records with the same ORDER value can still be considered).

   However, embedding local knowledge about the URI within the ENUM
   evaluation process is almost universal in systems employing ENUM.
   Also, since the difference between ORDER and PRIORITY/PREFERENCE has
   been unclear, NAPTR records have been provisioned in ways that would
   make strictly compliant systems unusable in practice.  Given that
   such systems are intended to provide communications, this non-
   compliant, "embedded decision" behaviour is understandable.

   It is proposed that when the ENUM specification is updated,
   processing of ORDER and PRIORITY/PREFERENCE should be updated based
   on implementation and deployment experiences described in this
   document.

4.1.1.  Example

   The example in this section is intended to further understanding
   about the difference between what [RFC3402] and [RFC3403] specify and
   what existing ENUM clients do.

   WARNING: The NAPTR records shown in this section are intended to
   illustrate somewhat unclear corner cases, and are not intended as
   good examples of how to do ENUM provisioning.

   Consider the following RRset, which maps numbers in the UK drama
   range to one server, and all other numbers to a second server:
   * 3600 IN NAPTR 1 1 "u" "e2u+sip"
       "!^(\\+441632960.*)$!sips:\\1@atlanta.example.com!" .
   * 3600 IN NAPTR 2 1 "u" "e2u+sip"
       "!^(.*)$!sip:\\1@biloxi.example.com!" .

   According to the processing specified in [RFC3402] and [RFC3403], the
   ENUM client is never intended to consider the second rule for e.g.,
   AUS "+441632960123", even if it does not support "sips" URIs, or the
   atlanta.example.com server cannot be reached, or the user indicates
   he or she doesn't wish to contact atlanta.example.com.  However,
   existing ENUM implementations are known to do this, and as described
   above, it can be useful if the alternative is failing to communicate
   at all.

   To prevent a client from considering the second rule for the UK drama
   range, the example could be rewritten to have more predictable
   behaviour as follows:
   * 3600 IN NAPTR 1 1 "u" "e2u+sip"
       "!^(\\+441632960.*)$!sips:\\1@atlanta.example.com!" .
   * 3600 IN NAPTR 2 1 "u" "e2u+sip"
       "!^(\\+[^4].*|\\+4[^4].*|\\+44[^1].*|\\+441[^6].*|\\+4416[^3].*|
       \\+44163[^2].*|\\+441632[^9].*|\\+4416329[^6].*|
       \\+44163296[^0].*)$!sip:\\1@biloxi.example.com!" .

4.2.  Order/Priority Values - Processing Sequence

   [RFC3761] and [RFC3403] 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 worse
   ORDER values must only be dealt with once the current ones with a
   better ORDER value have been processed.

   However, as described in the introduction to this section, this
   stated behaviour is a simplification.  Once sorted into a sequence
   reflecting ORDER and PREFERENCE/PRIORITY values, other fields are
   also considered during evaluation of retrieved NAPTRs; local
   knowledge may play a factor in the decision process, once a NAPTR has
   reached that point in the sequence at which it is considered.

   ENUM clients may also include the end user "in the decision loop",
   offering the end user the choice from a list of possible NAPTRs.
   Conceptually this choice is embedded within step 4 of the DDDS
   algorithm (as described in Section 3.3 of [RFC3402]).  Given that the
   ORDER field value is the major sort term, one would expect a
   conforming ENUM client to present only those NAPTRs with the
   currently "best" ORDER field value as choices.  When/if all the
   presented options 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
   at once, in the sequence defined by the ORDER and PREFERENCE/PRIORITY
   fields.

   In summary, ENUM clients will take into account the Services field
   value, the Flags field, and the Regexp ERE sub-field, along with the
   ORDER and PREFERENCE/PRIORITY field values, and may consider local
   policies or available local knowledge.

   The Registrant and the ENUM zone provisioning system he or she uses
   must be aware of this and SHOULD NOT rely on ENUM clients solely
   taking account of the value of the ORDER and the PREFERENCE/PRIORITY
   fields alone.

   Specifically, it is unsafe to assume that an ENUM client will not
   consider another NAPTR if there is one with a better ORDER value.
   The instructions in Section 4.1 and Section 8 of [RFC3403] may or may
   not be followed strictly by different ENUM clients for perfectly
   justifiable reasons.

   Where the ENUM client presents a list of possible URLs to the end
   user for his or her choice, it MUST do so in the sequence defined by
   the ORDER and PREFERENCE/PRIORITY values specified by the Registrant.

   However, a Registrant SHOULD place into his or her zone only contacts
   that he or she is willing to support; even those with the worst ORDER
   and PREFERENCE/PRIORITY values MAY be selected by an end user.

4.3.  Use of Order and Preference Fields

   NAPTRs in ENUM zones that hold incorrect ORDER values can cause major
   problems.  [RFC3403] highlights that having both ORDER and
   PREFERENCE/PRIORITY fields is a historical artifact of the NAPTR
   resource record type.  It is reasonable to have a common default
   value for the ORDER field, relying on the PREFERENCE/PRIORITY field
   to indicate the preferred sort.

   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, or
   simply a difference of interpretation of the standards.

   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; not the
   other way around.

   To avoid these common interoperability issues, it is recommended that
   ENUM NAPTRs SHOULD hold a default value in their ORDER field.

4.4.  NAPTRs with Identical ORDER/PRIORITY Values

   From experience, it has been learned that 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 normally occur.

   Such a condition indicates that these NAPTRs are truly identical in
   priority and that there is no preference between the services these
   NAPTRs offer.  Implementers SHOULD NOT assume that the DNS will
   deliver NAPTRs within an RRSet in a particular sequence.

   Multiple NAPTRs with identical ORDER and identical PREFERENCE/
   PRIORITY field values SHOULD NOT be provisioned into an RRSet unless
   the intent is that these NAPTRs are truly identical in priority and
   there is no preference between them.

   Some ENUM client implementations have considered this case to be an
   error and have rejected such duplicates entirely.  Others have
   attempted to further randomise the order in which such duplicates are
   processed.  Thus, use of such duplicate NAPTRs is unwise, as client
   implementations exist that will behave in different ways.

4.4.1.  Compound NAPTRs and Implicit ORDER/REFERENCE Values

   With [RFC3761], it is possible to have more than one Enumservice
   associated with a single NAPTR.  These Enumservices share the same
   Regexp field and so generate the same URI.  Such a "compound" NAPTR
   could well be used to indicate a mobile phone that supports both
   "voice:tel" and "sms:tel" Enumservices.  The Services field in that
   case would be "E2U+voice:tel+sms:tel".

   A compound NAPTR can be treated as a set of NAPTRs that each hold a
   single Enumservice.  These reconstructed NAPTRs share the same ORDER
   and PREFERENCE/PRIORITY field values but should be treated as if each
   had a logically different priority.  In this case, the reconstructed
   NAPTR holding the leftmost Enumservice within the compound NAPTR has
   the best priority, and the reconstructed NAPTR holding the rightmost
   Enumservice has the worst priority in this set.

   To avoid indeterminate behaviour, it is recommended that ENUM clients
   SHOULD process the Enumservices within a compound NAPTR in a left-to-
   right sequence.  ENUM provisioning systems SHOULD assume that such a
   processing order will be used and provision the Enumservices within a
   compound NAPTR accordingly.

4.5.  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.  If
   they are supported, then the way that ORDER and PREFERENCE/PRIORITY
   field values are processed is affected.

   Two main questions remain from the specifications of DDDS and
   [RFC3761]:

   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?

   Whilst one interpretation of [RFC3761] 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.

   In keeping with the interpretation made so far, ENUM implementations
   MUST consider the ORDER and PREFERENCE/PRIORITY values only within
   the context of the domain currently being processed in an ENUM query.
   These values MUST be discarded when processing other RRSets in the
   query.

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 holds, as its target, 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
   referenced 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 form 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" Private
   Branch Exchange (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.

   To avoid interoperability problems, some kind of acceptable advice is
   needed on non-terminal NAPTRs.  As current support is limited, non-
   terminal NAPTRs SHOULD NOT be used in ENUM unless it is clear that
   all of the ENUM clients this environment supports can process these.

5.2.  Non-Terminal NAPTRs - Considerations

   The following specific issues need to be considered if non-terminal
   NAPTRs are to be supported in a particular environment.  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 4.5
   for details.

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, a "non-terminal" or referential loop
   is implied.  An implementation MAY treat a chain of more than 5
   domains traversed during a single ENUM query as an indication that a
   self-referential loop has been entered.

   There are many techniques that can be used to detect such a loop, but
   the simple approach of counting the number of domains queried in the
   current ENUM query suffices.

   Where a loop has been detected, processing SHOULD continue at the
   next NAPTR in the referring domain (i.e., after the non-terminal
   NAPTR that included the reference that triggered the loop detection).

5.2.3.  Field Content in Non-Terminal NAPTRs

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

5.2.3.1.  Flags Field Content with Non-Terminal NAPTRs

   Section 2.4.1 of [RFC3761] states that the only flag character valid
   for use with the "E2U" DDDS Application is 'u'.  The flag 'u' is
   defined (in Section 4.3 of [RFC3404]) thus: 'The "u" flag means that
   the output of the Rule is a URI'.

   Section 2.4.1 of [RFC3761] 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 [RFC3404].  One
   could well argue that this is a feature potentially common to all
   DDDS Applications, and so might have been specified in [RFC3402] or
   [RFC3403].

   The Flags field will be empty in non-terminal NAPTRs encountered in
   ENUM processing.  ENUM does not have any other way to indicate a non-
   terminal NAPTR.

5.2.3.2.  Services Field Content with Non-Terminal NAPTRs

   Furthermore, [RFC3761] 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 a non-terminal NAPTR cannot hold a valid
   (non-empty) Services field when used in ENUM.  Section 2.4.2 of
   [RFC3761] 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 cannot be a non-terminal
   Enumservice (and so no such Registered Enumservice identifier), this
   syntax cannot be met with a non-terminal NAPTR; there are no non-
   terminal Enumservices to put into this field.

   A reasonable interpretation of the specifications is that for a non-
   terminal NAPTR, 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.

   It is expected that future revisions of the ENUM standard will
   clarify this text, making this interpretation plain.  This was the
   intent of the current standard, and the intent will be made explicit
   in its revision.

   In keeping with existing implementations, in a non-terminal NAPTR
   encountered in an ENUM query, the Services field SHOULD be empty, and
   clients SHOULD ignore any content it contains.

   Of course, such non-terminal NAPTRs with an empty Services field are
   not specific to any DDDS Application.  Thus, other means must be used
   to ensure a non-terminal NAPTR that is intended only for a particular
   DDDS Application cannot be encountered during a lookup for another
   DDDS Application (for example, by ensuring that the same domain is
   not used to host NAPTRs for more than one such DDDS Application).

5.2.3.3.  Regular Expression and Replacement Field Content with Non-
          Terminal NAPTRs

   The descriptive text in Section 4.1 of [RFC3403] 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.

   [RFC3403] 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,
   [RFC3403] does not specify which is used with terminal and non-
   terminal rules.

   The descriptive text of Section 4.1 of [RFC3403] 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
   should 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 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 includes specifying 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 what is the expected
   output).  ENUM (as specified in Section 2.4.1 of [RFC3761]) uses 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 implementations.  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.  Generating an ENUM domain name from the Regexp field is
   difficult at best and impossible for the general case of a variable-
   length telephone number, or one that has more than 9 digits.  Thus,
   it is proposed that when the ENUM specification is updated, this
   option is deprecated, and using the Regexp field for non-terminal
   ENUM NAPTRs is prohibited.

   In keeping with current implementations, the target domain of a non-
   terminal ENUM NAPTR MUST be placed in the (non-empty) Replacement
   field.  This field MUST be interpreted as holding the domain name
   that forms the next key output from this non-terminal rule.
   Conversely, the Regexp field MUST be empty in a non-terminal NAPTR
   encountered in ENUM processing, and ENUM clients MUST ignore its
   content.

6.  Backwards Compatibility

6.1.  Services Field Syntax

   [RFC3761] is the current standard for the syntax for NAPTRs
   supporting the ENUM DDDS Application.  This obsoletes the original
   specification that was given in [RFC2916].  RFC 3761 made 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 [RFC3403], there is now a single identifier that
   indicates the DDDS Application.  In the obsolete specification
   [RFC2915], there were zero or more "Resolution Service" identifiers
   (the equivalent of the DDDS Application).  The same identifier string
   for the DDDS identifier or the Resolution Service is defined in both
   the [RFC3761] and [RFC2916] specifications: "E2U".

   Also, [RFC3761] 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.  [RFC3761] 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.  [RFC2916] specified that the protocol sub-
   field was the leftmost, followed by the '+' delimiter, in turn
   followed by the "E2U" resolution service tag.

   [RFC2915] and [RFC2916] have been obsoleted by [RFC3401] - [RFC3404]
   and by [RFC3761].  However, [RFC3824] 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.  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.

   To facilitate this method, IANA should reject any request to register
   an Enumservice with the label "E2U".

   To summarise, ENUM clients MUST support ENUM NAPTRs according to
   [RFC3761] syntax.  ENUM clients SHOULD also support ENUM NAPTRs
   according to the obsolete syntax of [RFC2916]; there are still zones
   that hold "old" syntax NAPTRs.  ENUM zones MUST NOT be provisioned
   with NAPTRs according to the obsolete form, and MUST be provisioned
   with NAPTRs in which the Services field is according to [RFC3761].

7.  Collected Implications for ENUM Provisioning

   ENUM NAPTRs SHOULD NOT include characters outside the printable US-
   ASCII equivalent range (U+0020 to U+007E) unless it is clear that all
   ENUM clients they are designed to support will be able to process
   such characters correctly.  If ENUM zone provisioning systems require
   non-ASCII characters, these systems SHOULD encode the non-ASCII data
   to emit only US-ASCII characters by applying the appropriate

   mechanism ([RFC3492], [RFC3987]).  Non-printable characters SHOULD
   NOT be used, as ENUM clients may need to present NAPTR content in a
   human-readable form.

   The case-sensitivity flag ('i') is inappropriate for ENUM, and SHOULD
   NOT be provisioned into the Regexp field of E2U NAPTRs.

   ENUM zone provisioning systems SHOULD use '!'  (U+0021) as their
   Regexp delimiter character.

   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 [RFC3402]
   (i.e., "\!", U+005C U+0021).  Note that when a NAPTR resource record
   is entered in DNS master file syntax, the backslash itself must be
   escaped using a second backslash.

   If present in the ERE sub-field of an ENUM NAPTR, the literal
   character '+' MUST be escaped as "\+" (i.e.  U+005C U+002B).  Note
   that, as always, when a NAPTR resource record is entered in DNS
   master file syntax, the backslash itself must be escaped using a
   second backslash.

   The Registrant and the ENUM zone provisioning system he or she uses
   SHOULD NOT rely on ENUM clients solely taking account of the value of
   the ORDER and the PREFERENCE/PRIORITY fields in ENUM NAPTRs.  Thus, a
   Registrant SHOULD place into his or her zone only contacts that he or
   she is willing to support; even those with the worst ORDER and
   PREFERENCE/PRIORITY values MAY be selected by an end user.

   Many apparent mistakes in ORDER and PREFERENCE/PRIORITY values have
   been detected in provisioned ENUM zones.  To avoid these common
   interoperability issues, provisioning systems SHOULD NOT use
   different ORDER field values for NAPTRs in a Resource Record Set
   (RRSet).  To generalise, all ENUM NAPTRs SHOULD hold a default value
   in their ORDER field.  A value of "100" is recommended, as it seems
   to be used in most provisioned domains.

   Multiple NAPTRs with identical ORDER and identical PREFERENCE/
   PRIORITY field values SHOULD NOT be provisioned into an RRSet unless
   the intent is that these NAPTRs are truly identical and there is no
   preference between them.  Implementers SHOULD NOT assume that the DNS
   will deliver NAPTRs within an RRSet in a particular sequence.

   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.

   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).

   Whilst this client behaviour is non-compliant, ENUM provisioning
   systems and their users should be aware that some ENUM clients have
   been detected with poor (or no) support for non-trivial ERE sub-field
   expressions.

   ENUM provisioning systems SHOULD be cautious in the use of multiple
   back-reference patterns in the Repl sub-field of NAPTRs they
   provision.  Some clients have limited buffer space for character
   expansion when generating URIs (see also Section 3).  These
   provisioning systems SHOULD check the back-reference replacement
   patterns they use, ensuring that regular expression processing will
   not produce excessive-length URIs.

   As current support is limited, non-terminal NAPTRs SHOULD NOT be
   provisioned in ENUM zones unless it is clear that all ENUM clients
   that this environment supports can process these.

   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.

   In a non-terminal NAPTR encountered in an ENUM query (i.e., one with
   an empty Flags field), the Services field SHOULD be empty.

   A non-terminal NAPTR MUST include its target domain in the (non-
   empty) Replacement field.  This field MUST be interpreted as holding
   the domain name that forms the next key output from this non-terminal
   rule.  The Regexp field MUST be empty in a non-terminal NAPTR
   intended to be encountered during an ENUM query.

   ENUM zones MUST NOT be provisioned with NAPTRs according to the
   obsolete form, and MUST be provisioned with NAPTRs in which the
   Services field is according to [RFC3761].

8.  Collected Implications for ENUM Clients

   ENUM clients SHOULD NOT discard NAPTRs in which they detect
   characters outside the US-ASCII printable range (0x20 to 0x7E
   hexadecimal).

   ENUM clients MAY discard NAPTRs that have octets in the Flags,
   Services, or Regexp fields that have byte values outside the US-ASCII
   equivalent range (i.e., byte values above 0x7F).  Clients MUST be
   ready to encounter NAPTRs with such values without failure.

   ENUM clients SHOULD NOT assume that the delimiter is the last
   character of the Regexp field.

      Unless they are sure that in their environment this is the case,
      in general an ENUM client may still encounter NAPTRs that have
      been provisioned with a following 'i' (case-insensitive) flag,
      even though that flag has no effect at all in an ENUM scenario.

   ENUM clients SHOULD discard NAPTRs that have more or less than 3
   unescaped instances of the delimiter character within the Regexp
   field.

      In the spirit of being liberal with what it will accept, if the
      ENUM client is sure how the Regexp field should be interpreted,
      then it may choose to process the NAPTR even in the face of an
      incorrect number of unescaped delimiter characters.  If it is not
      clear how the Regexp field should be interpreted, then the client
      must discard the NAPTR.

   Where the ENUM client presents a list of possible URLs to the end
   user for his or her choice, it MAY present all NAPTRs -- not just the
   ones with the highest currently unprocessed ORDER field value.  The
   client SHOULD keep to the ORDER and PREFERENCE/PRIORITY values
   specified by the Registrant.

   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 might have done this already).

   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.

   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.

   ENUM clients MUST be ready to process NAPTRs that use a different
   character from '!' as their Regexp Delimiter without failure.

   ENUM clients MUST be ready to process NAPTRs that have non-trivial
   patterns in their ERE sub-field values without failure.

   ENUM clients MUST be ready to process NAPTRs with a DDDS Application
   identifier other than 'E2U' without failure.

   ENUM clients MUST be ready to process NAPTRs with many copies of
   back-reference patterns within the Repl sub-field without failure
   (see also Section 3).

   If a NAPTR is discarded, this SHOULD NOT cause the whole ENUM query
   to terminate and processing SHOULD continue with the next NAPTR in
   the returned Resource Record Set (RRSet).

   When an ENUM client encounters a compound NAPTR (i.e., one containing
   more than one Enumservice) and cannot process or cannot recognise one
   of the Enumservices within it, that ENUM client SHOULD ignore this
   Enumservice 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.  These conditions SHOULD NOT be
   considered errors.

   ENUM clients MUST support ENUM NAPTRs according to [RFC3761] syntax.
   ENUM clients SHOULD also support ENUM NAPTRs according to the
   obsolete syntax of [RFC2916]; there are still zones that hold "old"
   syntax NAPTRs.

8.1.  Non-Terminal NAPTR Processing

   ENUM clients MUST be ready to process NAPTRs with an empty Flags
   field ("non-terminal" NAPTRs) without failure.  More generally, non-
   terminal NAPTR processing SHOULD be implemented, but ENUM clients MAY
   discard non-terminal NAPTRs they encounter.

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

   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.  If
   present in a non-terminal NAPTR, a non-empty Regexp field MUST be
   ignored by ENUM clients.

   If a problem is detected when processing an ENUM query across
   multiple domains (by following non-terminal NAPTR references), then
   the ENUM query SHOULD NOT be abandoned, but instead processing SHOULD
   continue at the next NAPTR after the non-terminal NAPTR that referred
   to the domain in which the problem would have occurred.

   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 the non-terminal NAPTR that caused the
   traversal).

   ENUM clients MAY consider a chain of more than 5 "non-terminal"
   NAPTRs traversed in a single ENUM query as an indication that a
   referential loop has been entered.

   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
   referential 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.

9.  Security Considerations

   In addition to the security implications of recommendations in this
   document, 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 2, 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.

   As mentioned in Section 3, ENUM uses regular expressions to generate
   URIs.  Though it is a standard feature of DDDS, use of "non-greedy"
   regular expressions with multiple back-reference patterns in the Repl
   sub-field does create the potential for buffer-overrun attacks.
   Provisioning system designers SHOULD be aware of this and SHOULD
   limit the repeated use of back-reference replacement patterns.
   Conversely, ENUM client implementers SHOULD avoid using fixed
   character buffers when generating URIs from Repl sub-fields that
   include Back-reference patterns, and MUST avoid failure in the case
   of buffer exhaustion.

10.  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 [ETSI-TS102172] 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 rigorous framework on which to build.
   This document reflects how complex a system it is: without the
   intricacy of [RFC3401] - [RFC3404] and the work that went into them,
   it could have been very difficult to ensure interoperability.

11.  References

11.1.  Normative References

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

   [IEEE.1003-2.1992]
              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.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3402]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Two: The Algorithm", RFC 3402, October 2002.

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

   [RFC3404]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Four: The Uniform Resource Identifiers (URI)",
              RFC 3404, October 2002.

   [RFC3405]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Five: URI.ARPA Assignment Procedures", BCP 65,
              RFC 3405, October 2002.

   [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
              (IDNA)", RFC 3492, March 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
              Resource Identifiers (URI) Dynamic Delegation Discovery
              System (DDDS) Application (ENUM)", RFC 3761, April 2004.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

11.2.  Informative References

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

   [ETSI-TS102172]
              ETSI, "Minimum Requirements for Interoperability of
              European ENUM Implementations", ETSI TS 102 172,
              October 2004.

   [RFC2915]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
              (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [RFC2916]  Faltstrom, P., "E.164 number and DNS", RFC 2916,
              September 2000.

   [RFC3401]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part One: The Comprehensive DDDS", RFC 3401, October 2002.

   [RFC3824]  Peterson, J., Liu, H., Yu, J., and B. Campbell, "Using
              E.164 numbers with the Session Initiation Protocol (SIP)",
              RFC 3824, June 2004.

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 Services 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.co.jp/en/

 

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