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 (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 ..................... 22. Why one and only one domain is used in ENUM ................... 23. Why .ARPA was selected as the top level domain for ENUM ....... 44. The selection of an operator for E164.ARPA .................... 75. Procedures to be followed by RIPE NCC ......................... 86. References .................................................... 86.1. Normative references ........................................ 86.2. Informative and explanatory references ...................... 87. Security Considerations ....................................... 98. IANA Considerations ........................................... 99. Authors' Addresses ............................................ 910. Full Copyright Statement ..................................... 10
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
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.
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 (http://www.ietf.org/rfc/rfc2826.txt).
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 www.whitehouse.gov,
www.whitehouse.com or www.whitehouse.se? It should be noted that
www.whitehouse.com 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.
3. Why .ARPA was selected as the top level domain for ENUM
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 http://www.iab.org/iab/DOCUMENTS/iab-arpa-stmt.txt, 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-
addr.arpa" as part of the initial ARPANET transition strategy from
host table naming (see RFC 881-http://www.ietf.org/rfc/ 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-
addr.arpa (for reverse mapping of IPv4 addresses), ip6.arpa, (for
reverse mapping of IPv6 addresses), and e164.arpa, (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].
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
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
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. http://www.iab.org/iab/DOCUMENTS/iab-arpa-stmt.txt
provides additional context for these decisions.
The ENUM Working Group decided to follow that recommendation.
4. The selection of an operator for E164.ARPA
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 (http://www.ietf.org/rfc/rfc2780.txt) 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.
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.
6.1. 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,
[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: firstname.lastname@example.org
Membership at time this document was completed:
10. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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