ENUM -- Telephone Number Mapping M. Haberler
Working Group IPA
Internet-Draft R. Stastny
Intended status: Informational Oefeg
Expires: December 15, 2007 June 13, 2007
Combined User and Infrastructure ENUM in the e164.arpa tree
draft-ietf-enum-combined-05
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Abstract
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 approval of
the long-term solution.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Interim Solution . . . . . . . . . . . . . . . . . . . . . . . 3
4. Leveraging the e164.arpa infrastructure . . . . . . . . . . . 4
5. Locating the Infrastructure ENUM Branch . . . . . . . . . . . 4
6. Position of the IEBL Record . . . . . . . . . . . . . . . . . 5
7. Recommended resolver behaviour . . . . . . . . . . . . . . . . 6
8. Security considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Interoperability considerations . . . . . . . . . . . . . . . 8
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
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. 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 Name Server (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 RFC 3761 (User-ENUM) is not well suited for the
purpose of interconnection by carriers and voice service providers,
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 RFC
3761 [2] by the carrier-of-record [8] (Voice service provider) for a
specific E.164 number [3] to map a telephone number into one or more
Uniform Resource Identifiers (URIs) [5].
These URIs will be used to derive specific points of interconnection
into the service provider's network that could enable the originating
party to establish communication with the associated terminating
party. These URIs are 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 techniques 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 is work in
progress [9]. This is called the long term solution.
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 BCP 14, RFC 2119 [1].
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
long-term solution. It is therefore also required that the Interim
Solution includes a smooth migration path to the long-term solution.
It is also required that existing ENUM clients querying User ENUM as
defined in RFC 3761 [2] continue to work without any modification.
Because of various reasons, sharing a single domain name between the
user itself and the respective carrier for a number is not possible.
Hence, a different domain name must be used to store infrastructure
ENUM information.
The method most easily fulfilling this 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.
4. Leveraging the e164.arpa infrastructure
A convention is needed how, given a fully qualified E.164 number [3],
a resolver can determine the location of the Infrastructure ENUM
domain for this number. In order to avoid the delays associated with
the long term solution, the existing delegations and agreements
around e164.arpa need to be leveraged for the discovery algorithm.
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 National Regulation Authority (NRA) or as a regional
opt-in within in a given Numbering Plan Area (NPA) 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.
5. Locating the Infrastructure ENUM Branch
[7] specifies an extension to the ENUM DDDS application which adds an
extra mapping step using a DNS resource record (Infrastructure ENUM
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Branch Location - IEBL) to the E.164 to domain-name translation
algorithm.
The decision where to place the Infrastructure ENUM tree is a
national or group-of-countries decision. The IEBL affecting the
translation of any E.164 number thus needs to reside under the
e164.arpa tree for the country code of that number.
The IEBL contains the following parameters:
1. a SEPARATOR,
2. a POSITION,
3. an APEX.
Together, these three parameters describe the tree shape for a
country's Infrastructure ENUM tree according to the Interim Solution.
These parameters provide enough flexibility to describe setups
ranging from branches under e164.arpa at NPA level, branches at
country-code level, independent trees per country, and also the long-
term solution.
o Existence of the IEBL Record: The national or group-of-country's
decision to implement the Interim Solution is documented in the
e164.arpa tree by inserting an IEBL resource record at the country
code level.
o SEPARATOR: This branching label will be inserted into the ENUM
domain to branch off from the User-ENUM tree into the
Infrastructure ENUM sub-tree. This MAY be an empty (zero-length)
string which means no label will be inserted.
o POSITION: A number indicating after which digit this label
(SEPARATOR) should be inserted. A value of 0 means to the right
of all digits.
o APEX: A domain name indicating what domain replaces "e164.arpa"
for this application. "e164.arpa" MAY also be replaced by itself.
o The IEBL record is extremely well suited for caching: The layout
of a country's Infrastructure ENUM setup is very static
information, allowing large TTLs on the IEBL records. Overall,
the number of possible IEBL records in the DNS is bounded by the
number of countries, which in combination means very high cache
hit rates.
6. Position of the IEBL Record
The EBL record for Infrastructure ENUM (IEBL), as defined in [7]), is
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stored in the DNS at the country code level within the e164.arpa ENUM
tree.
If a country or group-of-countries decides to implement the interim
solution for Infrastructure ENUM, then it SHALL put an IEBL record at
the level of individual country codes as assigned by ITU-T.
The same algorithm applies for non-geographic country codes (global
services, e.g. +800, +878, +808 or networks, e.g. +882): If I-ENUM is
introduced for these numbers, the IEBL record SHALL be stored at
X.Y.Z.e164.arpa, even if the Tier-1 delegation is not at that level
in the ENUM tree.
The only remaining a-priori knowledge an Infrastructure ENUM resolver
needs to have is the current list of country codes, or an equivalent
method to determine where the country code in the number ends.
The authoritative source for up-to-date country code allocations is
published by ITU-T as complement to the recommendation E.164 [3].
The current version of this complement is available from ITU website
under "ITU-T / Service Publications".
As of 2007, the country code length can be determined with the
following simple algorithm:
o 3 digits is the default length of a country code.
o country codes 1 and 7 are a single digit.
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-T 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.
Examples can be found in [7]
7. Recommended resolver behaviour
An 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:
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The input to the resolver routine shall be:
1. the E.164 number in fully qualified (international) format,
2. a mode parameter indicating whether resolution should follow User
ENUM or Infrastructure ENUM rules,
3. optionally a table or algorithm to calculate country code lengths
(Section 6),
4. any other parameters used to drive the search, for instance an
enumservice type. These parameters are outside the scope of this
draft.
The resolver shall proceed as follows:
o If the mode parameter indicates a User ENUM search, proceed as per
RFC 3761.
o If the mode parameter indicates an Infrastructure ENUM query:
* Determine country code length, and thus the location of the
IEBL record.
* Check for a cached IEBL lookup result.
* If no cached result is present:
Retrieve the IEBL record from the country code zone, and store
the result in a cache. For positive result, normal DNS caching
semantics apply. For negative results, it is RECOMMENDED that
the ENUM client sets the caching timeout to 24 hours.
* If no IEBL is present at the calculated position in the DNS,
return an error.
* If an IEBL was found, construct a domain name according to the
algorithm given in [7].
* Search the DNS for any ENUM NAPTR records for the resulting
domain name.
It is assumed that the location of the Infrastructure ENUM tree for
each country will be rather static. Extensive caching of discovered
IEBL records (and their absence) is thus recommended.
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
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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
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 either anonymized
UserIDs or the E.164 number itself in the user-part of the URI, such
as in sip:+441632960084@example.com .
The 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
None.
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 RFC 3761 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 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 IEBL records for this country would then be
changed to the values "position=0", "separator="" and
"apex=example.com" (whatever is defined). When finally all countries
have migrated, the IEBL records may be removed.
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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
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, February 2005.
[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 Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[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-03 (work in progress),
June 2007.
12.2. Informative References
[8] Lind, S. and P. Pfautz, "Infrastructure ENUM Requirements",
draft-ietf-enum-infrastructure-enum-reqs-04 (work in progress),
May 2007.
[9] Livingood, J., "The E.164 to Uniform Resource Identifiers (URI)
Dynamic Delegation Discovery System (DDDS) Application for
Infrastructure ENUM", draft-ietf-enum-infrastructure-05 (work in
progress), January 2007.
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Authors' Addresses
Michael Haberler
Internet Foundation Austria
Waehringerstrasse 3/19
Wien A-1090
Austria
Phone: +43 664 4213465
Email: mah@inode.at
URI: http://www.nic.at/ipa/
Richard Stastny
Oefeg
Postbox 147
Vienna A-1030
Austria
Phone: +43 664 420 4100
Email: richard.stastny@oefeg.at
URI: http://www.oefeg.at
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