Network Working Group                                    J. Klensin, Ed.
Request for Comments: 3245                                           IAB
Category: Informational                                       March 2002

       The History and Context of Telephone Number Mapping (ENUM)
       Operational Decisions: Informational Documents Contributed
                      to ITU-T Study Group 2 (SG2)

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   RFC 2916 assigned responsibility for a number of administrative and
   operational details of Telephone Number Mapping (ENUM) to the IAB.
   It also anticipated that ITU would take responsibility for
   determining the legitimacy and appropriateness of applicants for
   delegation of "country code"-level subdomains of the top-level ENUM
   domain.  Recently, three memos have been prepared for the ITU-T Study
   Group 2 (SG2) to explain the background of, and reasoning for, the
   relevant decisions.  The IAB has also supplied a set of procedural
   instructions to the RIPE NCC for implementation of their part of the
   model.  The content of the three memos is provided in this document
   for the information of the IETF community.

Table of Contents

   1. Introduction: ENUM Background Information .....................  2
   2. Why one and only one domain is used in ENUM ...................  2
   3. Why .ARPA was selected as the top level domain for ENUM .......  4
   4. The selection of an operator for E164.ARPA ....................  7
   5. Procedures to be followed by RIPE NCC .........................  8
   6. References ....................................................  8
   6.1. Normative references ........................................  8
   6.2. Informative and explanatory references ......................  8
   7. Security Considerations .......................................  9
   8. IANA Considerations ...........................................  9
   9. Authors' Addresses ............................................  9
   10. Full Copyright Statement ..................................... 10

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RFC 3245   History and Context of ENUM Operational Decisions  March 2002

1. Introduction: ENUM Background Information

   In January 2002, in response to questions from the ITU-T Study Group
   2 (referred to just as "SG2", below), specifically its group working
   on "Questions 1 and 2", and members of the IETF and
   telecommunications communities, Scott Bradner, as Area Director
   responsible for the ENUM work and ISOC Vice President for Standards,
   initiated an effort to produce explanations of the decisions made by
   the IETF about ENUM administration.  The effort to produce and refine
   those documents eventually involved him, Patrik Faltstrom (author of
   RFC 2916), and several members of the IAB.

   The documents have now been contributed to ITU-T, and are being
   published as internal SG2 documents.  This document provides the IETF
   community a copy of the information provided to SG2.  Section 2 below
   contains the same content as COM 2-11-E, section 3 contains the same
   content as COM 2-12-E, and section 4 contains the same content as SG2
   document COM 2-10-E.  The documents being published within SG2 show
   their source as "THE INTERNET SOCIETY ON BEHALF OF THE IETF", which
   is a formality deriving from the fact that ISOC holds an ITU sector
   membership on behalf of the IETF.

2. Why one and only one domain is used in ENUM

2.1. Introduction

   This contribution is one of a series provided by the IETF to ITU-T
   SG2 to provide background information about the IETF's ENUM Working
   Group deliberations and decisions.  This particular contribution
   addresses the IETF's decision that only a single domain could be
   supported in ENUM.

2.2. The need for a single root in the DNS

   In the Domain Name System (DNS), each domain name is globally unique.
   This is a fundamental fact in the DNS system and follows
   mathematically from the structure of that system as well as the
   resource identification requirements of the Internet.  Which DNS
   server is authoritative for a specific domain is defined by
   delegations from the parent domain, and this is repeated recursively
   until the so-called root zone, which is handled by a well-known set
   of DNS servers.  Note that words like "authoritative" and
   "delegation" and their variations are used here in their specific,
   technical, DNS sense and may not have the same meanings they normally
   would in an ITU context.

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   Given that one starts with the well-known root zone, every party
   querying the DNS system will end up at the same set of servers for
   the same domain, regardless of who is sending the query, when the
   query is sent and where in the network the query is initiated.  In
   May 2000 the IAB published a document on the need for a single root
   in the DNS.  This document explores the issues in greater detail.
   See RFC 2826 (

2.3. Storing E.164 numbers in the DNS

   An E.164 number is also globally unique, and because of that it has
   most of the same properties as a domain name.  This was the reason
   why storing E.164 numbers in the DNS system is technically a simple
   mapping.  ENUM is just that, a way to store E.164 numbers in the DNS.
   Multiple ENUM trees in the DNS hierarchy would have the telephony
   equivalent of permitting every carrier to assign a different meaning
   to an E.164 country code, with each one potentially mapping a given
   number to a different circuit or rejecting it entirely.  For the
   Internet, if there were multiple trees, there would be no way to
   determine which domains might contain ENUM records.  Thus, each
   application that uses ENUM facilities would have to be manually
   configured with a list of domains to be searched.  This would incur
   the same problems of scaling and updates that led to the development
   of the DNS.

   The goal with ENUM is that one party should be able to look up
   information in DNS, which another party has stored in DNS.  This must
   be possible with only the E.164 number as input to the algorithm.

   If the party storing information in DNS has two (or more) places to
   choose from, and chooses one of them, how is a second party looking
   up things to know what place was selected?  An analogy would be if
   one knew only www.whitehouse, and not the TLD, and ask people to go
   to that website.  Is the correct domain name, or  It should be noted that exists and is a pornography site.

   Thus, the only way of knowing where to look up E.164/ENUM numbers in
   DNS is to use one and only one domain, and have everyone agree on
   what that domain is.  Note that ENUM is a system for use with E.164
   numbers in their general, global, context.  Nothing technical can, or
   should, try to prevent parties that wish to use ENUM-like mechanisms,
   or other systems that have the same general structure as telephone
   numbers, from working out private, out of band, agreements to support
   those applications.  However, such applications are neither E.164 nor
   ENUM, any more than internal extension numbers in a PBX are normally
   considered to be part of either.

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RFC 3245   History and Context of ENUM Operational Decisions  March 2002

3. Why .ARPA was selected as the top level domain for ENUM

3.1. Introduction

   This memo is one of a series provided by the IETF to SG2 to provide
   background information about the IETF's ENUM Working Group
   deliberations and decisions.  This particular memo addresses the
   IETF's decision that the ENUM DNS tree would use the .ARPA top level

3.2. IAB Statement on Infrastructure Domain and Subdomains

   (Taken from, May

   Over the last several months, the IAB has been reviewing, and
   discussing with ICANN and other parties, the handling of various
   Internet Protocol-related infrastructure components that the
   community has concluded should be placed into the DNS.

   Historically, the most visible infrastructure domain has been the
   IPv4 address reverse-mapping domain.  This domain was placed in "in-" as part of the initial ARPANET transition strategy from
   host table naming (see RFC 881- rfc0881.txt).
   Other than the IPv4 reverse-mapping subdomain, it became the only
   active subdomain of that domain as the <host-table-name>.ARPA names
   that were also part of the transition were gradually removed.  Other
   infrastructure domains were, in the past, placed under the "INT" TLD
   and various organizational names.

   It is in the interest of general Internet stability, to pay adequate
   attention to the placement of secondary DNS servers, and
   administrative cleanliness, to start rationalizing this situation by
   locating new infrastructure subdomains in a single domain and
   migrating existing ones to it as appropriate.  It appears that our
   original infrastructure domain "ARPA", redesignated from an
   abbreviation for "ARPANET" to an acronym for "Address and Routing
   Parameters Area" is best suited for this purpose.

3.3. Infrastructure subdomains

   Operationally, it is easier to ensure good stability for DNS in
   general if we have as few DNS zones as possible that are used for
   parameters for infrastructure purposes.  Today, new infrastructure
   domains are put in ARPA and old assignments which were made in other
   domains are being migrated to ARPA.  Currently, ARPA is used for in- (for reverse mapping of IPv4 addresses),, (for

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   reverse mapping of IPv6 addresses), and, (the subject of
   this memo).  In the future, URI schemes, URN namespaces and other new
   address families will be stored in ARPA.

   Theoretically, each set of infrastructure parameters could be stored
   in a separate domain as a TLD.  (For example, .URI, .UNI, .IPV6, new
   TLD, which only can be created via the ICANN process (which might
   take a year or more) and would unnecessarily and undesirably flatten
   the DNS tree.  It is much easier to have one TLD with easily created
   new subdomains (2nd level domains), one for each parameter.  Thus it
   was logical to store E.164 numbers in ARPA.

3.4. The ARPA domain (derived from RFC 3172, September 2001)

   The "arpa" domain was originally established as part of the initial
   deployment of the DNS, to provide a transition mechanism from the
   Host Tables that were previously standard in the ARPANET.  It was
   also used to provide a permanent home for IPv4 address to name
   mappings ("reverse mappings") which were previously also handled
   using the Host Table mechanism.  The Internet Architecture Board
   (IAB), in cooperation with the Internet Corporation for Assigned
   Names and Numbers (ICANN), is currently responsible for managing the
   Top Level Domain (TLD) name "arpa".  This arrangement is documented
   in Appendix A of RFC 3172.  This domain name provides the root of the
   name hierarchy of the reverse mapping of IP addresses to domain
   names.  More generally, this domain name undertakes a role as a
   limited use domain for Internet infrastructure applications, by
   providing a name root for the mapping of particular protocol values
   to names of service entities.  This domain name provides a name root
   for the mapping of protocol values into lookup keys to retrieve
   operationally critical protocol infrastructure data records or
   objects for the Internet.

   The IAB may add other infrastructure uses to the "arpa" domain in the
   future.  Any such additions or changes will be in accordance with the
   procedures documented in Section 2.1 and Section 3 of this document.
   [referring to RFC 3172] This domain is termed an "infrastructure
   domain", as its role is to support the operating infrastructure of
   the Internet.  In particular, the "arpa" domain is not to be used in
   the same manner (e.g., for naming hosts) as other generic Top Level
   Domains are commonly used.

   The operational administration of this domain, in accordance with the
   provisions described in this document, shall be performed by the IANA
   under the terms of the MoU between the IAB and ICANN concerning the
   IANA [RFC 2860].

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3.5. Assignment of the .ARPA top level domain

   As documented in appendix A of RFC 3172, on April 28, 2000 the US
   Department of Commerce, acting under the authority of its purchase
   order with ICANN, directed ICANN to operate the .ARPA TLD under the
   guidance of the IAB, as a limited use domain for internet
   infrastructure applications.

3.6. Name Server Requirements for .ARPA (from RFC 3172)

   As this domain is part of the operationally critical infrastructure
   of the Internet, the stability, integrity and efficiency of the
   operation of this domain is a matter of importance for all Internet

   The "arpa" domain is positioned as a top level domain in order to
   avoid potential operational instabilities caused by multiple DNS
   lookups spanning several operational domains that would be required
   to locate the servers of each of the parent names of a more deeply
   nested infrastructure name.  The maximal lookup set for ARPA is a
   lookup of the name servers for the "arpa" domain from a root server,
   and the query agent is then provided with a list of authoritative
   "arpa" name servers.

   The efficient and correct operation of the "arpa" domain is
   considered to be sufficiently critical that the operational
   requirements for the root servers apply to the operational
   requirements of the "arpa" servers.  All operational requirements
   noted in RFC 2870, as they apply to the operational requirements of
   the root servers, shall apply to the operation of the "arpa" servers.
   Any revision to RFC 2870 in relation to the operation of the root
   servers shall also apply to the operation of the "arpa" servers.

   Many of the servers that are authoritative for the root zone (or the
   "." zone) also currently serve as authoritative for the "arpa" zone.
   As noted in RFC 2870, this arrangement is likely to change in the

3.7. Summary: ENUM use of .ARPA

   The ARPA domain is the preferred TLD for infrastructure and parameter
   use.  The ENUM structure should be placed in a single domain subtree
   (see separate contribution, COM 2-11), and is expected to evolve into
   important Internet infrastructure, and hence should be placed there.
   This decision is facilitated by the MOU between ICANN and IETF and
   the instructions from the US Government to ICANN, which provide for
   IAB supervision of that domain.  Despite some confusion with the name
   of a US Department of Defense agency, DARPA, these uses are

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   consistent with all of the historical uses of the ARPA domain, which
   have been for infrastructure purposes (initially when the
   hierarchical DNS was created to replace the old flat namespace of
   ARPANET): the domain was never used for any internal or specific
   DARPA purpose.  Recognizing the potential difficulties with multiple
   infrastructure domains, the Internet Architecture Board concluded in
   May 2000 that all new infrastructure information was to be stored in
   the ARPA domain and existing infrastructure subtrees migrated there
   as feasible.
   provides additional context for these decisions.

   The ENUM Working Group decided to follow that recommendation.

4. The selection of an operator for E164.ARPA

4.1. Introduction

   This contribution is one of a series provided by the IETF to SG2 to
   provide background information about the IETF's ENUM Working Group
   deliberations and decisions.  This particular contribution addresses
   the IETF's selection of an operator for the E164.ARPA domain.

4.2. Name server operator requirements

   RFC 2870 ( describes the
   requirements for operating DNS root servers.  Important DNS-based
   infrastructure services require that their servers be operated with
   the same level of attention to reliability and security that the root
   servers require.  In addition, for an infrastructure service such as
   E164.ARPA some additional requirements were felt by the IAB to be
   important.  Organizations that operate core services such as IN-
   ADDR.ARPA and E164.ARPA must have a history of reliable operation of
   DNS servers and be highly respected and known for both their relevant
   technical skills and their fairness and impartiality.  In addition,
   the IAB felt that the organization that operates such infrastructure
   domains must be a non-profit and public-service-oriented one to
   remove any incentive for exploitative behavior based on profit
   motives that depend on, e.g., the number of records in the database
   even if some reasonable registration fee is charged to recover costs.
   The IAB also felt that they wanted an organization with good (and
   extensive) experience working with governments when necessary and one
   with experience working with the IAB and the IETF more generally.

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4.3. Evaluating possible operators

   The IAB researched various options for operators and came to the
   conclusion that the regional IP address registries (RIRs) met all of
   the criteria.  They all had extensive experience providing and
   supporting infrastructure services reliably and securely and all
   three of them had a long history of working with the IETF.

4.4. Selecting a particular operator

   Given that all of the RIRs would have met the criteria, the selection
   of a particular RIR required looking at other factors.  The IAB
   concluded that RIPE NCC would be the best operator for E164.ARPA,
   based largely on their somewhat greater experience in running DNS
   servers and on their location in a neutral legal jurisdiction.

4.5. Country administration of cc subdomains

   Of course, once a subdomain associated with a country code is
   assigned for registration and operations to an appropriately-
   designated entity for the associated country or numbering plan,
   administration of that subdomain is entirely a National Matter, with
   no involvement anticipated from the IAB/IETF, the E164.ARPA registry,
   or from the ITU.

5. Procedures to be followed by RIPE NCC

   The IAB and the RIPE NCC have agreed on procedures for the latter to
   follow in making ENUM registrations at the country code level.  Those
   instructions are expected to evolve as experience is accumulated.
   Current versions will be posted on the IAB and/or RIPE NCC web sites.

6. References

6.1. Normative references

   None.  This document is intended to be self-contained and purely

6.2.  Informative and explanatory references.

   [RFC 2860] Carpenter, B., Baker, F. and M.  Roberts, "Memorandum of
              Understanding Concerning the Technical Work of the
              Internet Assigned Numbers Authority", RFC 2860, June 2000.

   [RFC 2870] Bush, R., Karrenberg, D., Kosters, M. and R. Plzak, "Root
              Name Server Operational Requirements", BCP 40, RFC 2870,
              June 2000.

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RFC 3245   History and Context of ENUM Operational Decisions  March 2002

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

   [RFC 3172] Huston, G., Ed., "Management Guidelines & Operational
              Requirements for the Address and Routing Parameter Area
              Domain ('arpa')", BCP 52, RFC 3172, September 2001.

7. Security Considerations

   This document provides information only and raises no new security
   issues.  The security issues associated with the underlying protocols
   are described in RFC 2916.

8. IANA Considerations

   There are no IANA considerations regarding this document.  Sections 3
   and 4 contain a record of actions already performed by IANA and
   partial explanations for them.

9.  Authors' Addresses

   Internet Architecture Board EMail:

      Membership at time this document was completed:

      Harald Alvestrand
      Ran Atkinson
      Rob Austein
      Fred Baker
      Steve Bellovin
      Brian Carpenter
      Jon Crowcroft
      Leslie Daigle
      Steve Deering
      Sally Floyd
      Geoff Huston
      John Klensin
      Henning Schulzrinne

   Scott Bradner

   Patrik Faltstrom

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10. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


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

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