ENUM -- Telephone Number Mapping                             M. Haberler
Working Group                                                        IPA
Internet-Draft                                                R. Stastny
Expires: February 8, 2007                                          Oefeg
                                                          August 7, 2006

      Combined User and Infrastructure ENUM in the e164.arpa tree

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This memo defines an interim solution for Infrastructure ENUM to
   allow a combined User and Infrastructure ENUM implementation in
   e164.arpa as a national choice until the long-term solution is
   approved.  This interim solution will be deprecated after deployment
   of the long-term solution.

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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   3.  Interim Solution . . . . . . . . . . . . . . . . . . . . . . .  3

   4.  Introducing a branch into the 164.arpa tree  . . . . . . . . .  4

   5.  Defining the Infrastructure ENUM branch location . . . . . . .  4

   6.  Finding the ENUM branch location record  . . . . . . . . . . .  5

   7.  Recommended resolver behaviour . . . . . . . . . . . . . . . .  6

   8.  Security considerations  . . . . . . . . . . . . . . . . . . .  7

   9.  IANA considerations  . . . . . . . . . . . . . . . . . . . . .  8

   10. Interoperability considerations  . . . . . . . . . . . . . . .  8

   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8

   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     12.1.  Normative References  . . . . . . . . . . . . . . . . . .  9
     12.2.  Informative References  . . . . . . . . . . . . . . . . .  9

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC2119 [1].

   Note: The ENUM WG decided at IETF#64 to prefer the term
   Infrastructure ENUM.  Therefore, this document uses the term
   Infrastructure ENUM as synonymous to Carrier ENUM.

2.  Introduction

   ENUM (E.164 Number Mapping, RFC 3761 [2]) is a system that transforms
   E.164 numbers [3] into domain names and then uses DNS (Domain Name
   Service) [6] services like delegation through NS records and NAPTR
   (Naming Authority Pointer) records [4] to look up which services are
   available for a specific domain name.

   ENUM as defined in RFC3761 (User-ENUM) is not well suited for the
   purpose of interconnection by carriers, as can be seen by the use of
   various private tree arrangements based on ENUM mechanisms.

   Infrastructure ENUM is defined as the use of the technology in
   RFC3761 [2] by the carrier-of-record [8] (Voice service provider) for
   a specific E.164 [3] number to map a telephone number into an URI [5]
   that identifies a specific point of interconnection to that service
   provider's network that could enable the originating party to
   establish communication with the associated terminating party.  It is
   separate from any URIs that the end-user who registers his E.164
   number in ENUM may wish to associate with that E.164 number.

   The requirements, terms and definitions for Infrastructure ENUM are
   defined in [8].

   Using the same E.164 number to domain mapping technique for other
   applications under a different, internationally agreed apex (instead
   of e164.arpa) is straightforward on the technical side.  Establishing
   the international agreements necessary to delegate the country-code
   level subdomains under the new apex is non-trivial and time-
   consuming.  This process of defining the Dynamic Delegation Discovery
   System DDDS [4] application for Infrastructure ENUM in "ie164.arpa"
   is under way [9].  This is called the "proper" long term solution.

3.  Interim Solution

   As stated above, the agreements to establish the long-term solution

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   may take some time.  It was therefore decided to develop an interim
   solution that can be used by individual countries to implement an
   interoperable Infrastructure ENUM tree immediately.  The Interim
   solution will be deprecated upon approval (loosely timed) of the
   "proper" long-term solution.

   Is is therefore also required that the Interim solution is compatible
   with the "right" long-term solution to allow for easy migration.

4.  Introducing a branch into the 164.arpa tree

   A convention is needed how, given a fully qualified E.164 [3] number,
   a resolver can determine the location of the Infrastructure ENUM
   subdomain for this country.  Under this approach, ITU-T and IETF
   (IAB) involvement is only lightweight, e.g. to recommend the proper
   algorithm defined here to enable international interoperability.

   This allows to introduce the Interim solution as a national matter by
   the concerned NRA or as a regional opt-in within in a given Numbering
   Plan Area such as the North American NPA.

   Beyond the setup phase, an NRA need not be involved operationally -
   it is sufficient to establish a convention linking the national
   definition of a carrier of record to the credentials for write access
   to the Infrastructure ENUM tree.

   The method most easily fulfilling the above mentioned requirements is
   to branch off the e164.arpa tree into a subdomain at or somewhere
   below the country code delegation level below e164.arpa, and deploy
   an Infrastructure ENUM subtree underneath without touching User ENUM
   semantics at all.

5.  Defining the Infrastructure ENUM branch location

   The decision where to place the Infrastructure ENUM tree below
   e164.arpa is a national or group-of-countries decision.  To branch
   off the e164.arpa tree for a given country code, a DNS label is
   inserted at a specific position into the ENUM fully qualified domain
   name (FQDN).

   For international interoperability, an Infrastructure ENUM resolver
   needs to determine for a given country code

   1.  the name of the label to be inserted

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   2.  the position where to insert the label in an Infrastructure ENUM
       domain name for a given country code
   3.  a convention how to discover these parameters.

   We propose a mechanism to discover these parameters dynamically for
   any given tree shape as follows:

   o  the national or group-of-countries decision about subdomain
      location is documented in the e164.arpa tree proper by inserting a
      special DNS resource record at the country code level, called ENUM
      Branch Location Record (EBL) [7], into a subdomain in the country
      code zone.  In case of the Infrastructure ENUM application, this
      subdomain name will be "infrastructure".  This ENUM Branch
      Location Record carries three values for maximum flexibility:
      1.  the branching label to be inserted into the ENUM domain to
          branch off to the application-specific tree.  This may be an
          empty (zero-length) string.
      2.  an insertion position, indicating after which digit this label
          should be inserted into the ENUM domain to branch off to the
          application-specific tree.  A value of 0 means "after all
      3.  an apex: indicating what domain should replace "e164.arpa" for
          this application.
   o  a resolver looking for an Infrastructure ENUM domain needs to
      retrieve this EBL once during first resolution within a country
   o  while constructing the FQDN, the branching label as retrieved from
      the EBL resource record is inserted at the insertion position
      (also as per EBL) and finally the apex is appended.  Labels,
      digits and apex are separated by dots as usual.  A zero-length
      branching label is not inserted at all.

6.  Finding the ENUM branch location record

   The only remaining a-priori knowledge a Infrastructure ENUM resolver
   should have is the current list of country codes, or an equivalent
   method to determine where the country code in the number ends.

   To prime the country code extraction algorithm, the current scheme to
   determine country code length as follows could be employed:

   o  3 digits is the default length of a country code.
   o  country codes 1 and 7 are a single digit.

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   o  the following country codes are two digits: 20, 27, 30-34, 36, 39,
      40, 41, 43-49, 51-58, 60-66, 81, 82, 84, 86, 90-95, 98.

   Figure 1

   Given the fact that the ITU recently allocated only 3-digit country
   codes, there are no more spare 1- and 2-digit country codes and
   existing 1- and 2-digit country codes are extremely unlikely to be
   recovered, the above table consisting of the existing 1- and 2-digit
   country codes can be considered very stable.  The only problem may be
   a country split as happened recently e.g. to Yugoslavia.

   If a branch location record is not found according to this table (for
   instance, in the unlikely case the ITU allocates a country code not
   according to these rules), it is still possible to determine the
   branch location record by "iterating down" the tree digit-by-digit.
   Such a fallback strategy would rely on the assumption that there is
   never a branch location record inserted above the country code zone,
   for which there would be no use in the first place.

   It seems unlikely that inspection of more than the first five digits
   will be required to locate the branch location record under any
   realistic numbering administrative partitioning.

7.  Recommended resolver behaviour

   A User ENUM resolver as per RFC 3761 need not be aware of any
   Infrastructure ENUM conventions at all.  A combined User and
   Infrastructure ENUM resolver shall behave as follows:

   The input to the resolver routine shall be:
   1.  the called number in fully qualified E.164 (international)
   2.  a mode parameter indicating wether resolution should follow User
       ENUM or Infrastructure ENUM rules (for instance, a null value for
       defaulting to User ENUM, or 'infrastructure' for Infrastructure
       ENUM semantics).
   3.  optionally a table or algorithm to easily detect country codes
       (Section 6),
   4.  any other parameters used to drive the search, for instance an
       enumservice type.  These parameters are outside the scope of this

   The resolver shall proceed as follows:
   o  if the mode parameter indicates a User ENUM search, proceed as per

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   o  If the mode parameter indicates an Infrastructure ENUM query:
      *  determine country code length.
      *  consult table if an EBL record for this country code was
         already retrieved since resolver boot time.
      *  if not:
            retrieve the EBL record from the 'infrastructure' subdomain
            of the country code zone, and store the country code and
            associated EBL values in an EBL table.
            optional fallback for irregular country code not covered by
            the CC extraction algorithm: (Figure 1) if the last step
            fails, iterate over the number up to five digits and try to
            retrieve the EBL record in the 'infrastructure' subdomain
            each time, again storing the country code and associated EBL
            values in the cache if successful.
            if both attempts fail, return NXDOMAIN.
      *  valid EBL record found: if the branching label is non-zero
         length, insert it at the insertion position in the FQDN and add
         a trailing dot, add the remaining digits and dots, and append
         the apex.
      *  search the DNS for any ENUM NAPTR records for the resulting
         domain name.

   It is assumed that already discovered EBL values are stored in a
   cache table of country code and already discovered EBL parameters.

8.  Security considerations

   Privacy issues have been raised regarding unwarranted disclosure of
   user information by publishing Infrastructure ENUM information in the
   public DNS, for instance the use for harvesting of numbers in
   service, or unlisted numbers.

   Given that number range allocation is public information, we believe
   the easiest way to cope with such concerns is to fully unroll
   allocated number ranges in the Infrastructure ENUM subtree, wherever
   such privacy concerns exist.  Whether a number is served or not would
   be exposed by the carrier of record when an attempt is made to
   contact the corresponding URI.  We assume this to be an authenticated
   operation, which would not leak information to unauthorized parties.

   Entering all numbers in an allocated number range, whether serviced
   or not, or listed or unlisted, will prevent mining attempts for such
   number attributes.

   The result would be that the information in the public DNS would
   mirror number range allocation information, but not more.
   Infrastructure ENUM will not tell you more than you can get by just

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   dialing numbers.

   The URI pointing to the destination network of the Carrier of Record
   should also not disclose any privacy information about the identity
   of end-user, it is therefore recommended to use in the user-part of
   the SIP URI either anonymized UserIDs or the E.164 number itself,
   such as sip:441632960084@example.com .

   The definition of a new resource record (RR) type or a new
   enumservice does not introduce security problems into the DNS.  Usage
   of the Branch Location record conveys only static setup information
   under a country code subtree of e164.arpa.  The intended use of DNS
   Security Extensions (DNSSEC) within ENUM will prove authenticity of
   the conveyed value.

9.  IANA considerations


10.  Interoperability considerations

   An application using the combined resolver needs to indicate which
   information is requested - User or Infrastructure ENUM, or both.  A
   user-ENUM-only resolver need not be aware of the Infrastructure ENUM
   subtree and no changes with respect to RFC3761 semantics are
   required.  A resolver desiring to retrieve Infrastructure ENUM or
   both types of records needs to be aware of the conventions laid out
   in this draft.

   When the "proper" long-term solution is adopted, each country using
   the interim solution may decide on its own when to migrate to the
   long-term solution.  The EBL records for this country would then be
   changed to the values "insertion position=0", "branching label=''"
   and "apex=ie164.arpa".  When finally all countries have migrated, the
   EBL records may be removed.

11.  Acknowledgements

   We gratefully acknowledge suggestions and improvements by Jason
   Livingood and Tom Creighton of Comcast, Penn Pfautz of ATT, Lawrence
   Conroy of Roke Manor Research, and Alexander Mayrhofer and Otmar
   Lendl of enum.at.

12.  References

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12.1.  Normative References

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

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

   [3]  ITU-T, "The International Public Telecommunication Number Plan",
        Recommendation E.164, May 1997.

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

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

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

   [7]  Lendl, O., "The ENUM Branch Location Record",
        draft-ietf-enum-branch-location-record-00 (work in progress),
        August 2006.

12.2.  Informative References

   [8]  Lind, S. and P. Pfautz, "Infrastrucure ENUM Requirements",
        draft-ietf-enum-infrastructure-enum-reqs-02 (work in progress),
        April 2006.

   [9]  Livingood, J., "The E.164 to Uniform Resource Identifiers (URI)
        Dynamic Delegation Discovery  System (DDDS) Application for
        Infrastructure ENUM", draft-ietf-enum-infrastructure-00 (work in
        progress), April 2006.

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Authors' Addresses

   Michael Haberler
   Internet Foundation Austria
   Waehringerstrasse 3/19
   Wien  A-1090

   Phone: +43 664 4213465
   Email: mah@inode.at
   URI:   http://www.nic.at/ipa/

   Richard Stastny
   Postbox 147
   Vienna  A-1030

   Phone: +43 664 420 4100
   Email: richard.stastny@oefeg.at
   URI:   http://www.oefeg.at

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