ENUM -- Telephone Number Mapping M. Haberler
Working Group IPA
Internet-Draft R. Stastny
Intended status: Informational Oefeg
Expires: June 18, 2007 December 15, 2006
Combined User and Infrastructure ENUM in the e164.arpa tree
draft-ietf-enum-combined-03
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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. Introducing a branch into the e164.arpa tree . . . . . . . . . 4
5. Defining the Infrastructure ENUM branch location . . . . . . . 4
6. Locating the ENUM branch location record . . . . . . . . . . . 5
7. Example for the location of the EBL . . . . . . . . . . . . . 6
8. Recommended resolver behaviour . . . . . . . . . . . . . . . . 7
9. Security considerations . . . . . . . . . . . . . . . . . . . 8
10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Interoperability considerations . . . . . . . . . . . . . . . 9
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
13.1. Normative References . . . . . . . . . . . . . . . . . . 9
13.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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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 an Uniform
Resource Identifier (URI) [5]. This URI maps to a specific point of
interconnection to the service provider's network that could enable
the originating party to establish communication with the associated
terminating party. This URI 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 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
may take some time. It was therefore decided to develop an Interim
Solution that can be used by individual countries to implement an
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interoperable Infrastructure ENUM tree immediately. The Interim
Solution will be deprecated upon approval (loosely timed) of the
long-term solution. Is 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. Introducing a branch into the e164.arpa tree
A convention is needed how, given a fully qualified E.164 number [3],
a resolver can determine the location of the Infrastructure ENUM
subdomain for this country. 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. Defining the Infrastructure ENUM branch location
[7] specifies an extension to the ENUM DDDS application which adds an
extra mapping step using a DNS resource record (ENUM Branch Location
- EBL) to the E.164 to domain-name translation algorithm.
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The decision where to place the Infrastructure ENUM tree is a
national or group-of-countries decision. The EBL affecting the
translation of any E.164 number thus needs to reside under the
e164.arpa tree for the country code of that number.
[7] specifies a DNS resource record (ENUM Branch Location - EBL) and
an algorithm to branch the ENUM DNS tree for specific use-cases with
the following parameters:
1. the name of EBL use-case,
2. a SEPARATOR,
3. a POSITION,
4. an APEX.
These parameters can be used to describe the tree shape for the
Interim Solution of Infrastructure ENUM as follows:
o The national or group-of-countries decision about the location of
the Infrastructure ENUM branch is documented in the e164.arpa tree
by inserting an EBL resource record into a subdomain at the
country code level.
o The EBL subdomain label for the Infrastructure ENUM use-case MUST
be "infrastructure". This EBL carries the above mentioned three
values for maximum flexibility:
1. the branching label (SEPARATOR) to be inserted into the ENUM
domain to branch off to the Infrastructure ENUM sub-tree.
This MAY be an empty (zero-length) string which means no label
will be inserted.
2. an insertion POSITION, indicating after which digit this label
(SEPARATOR) should be inserted. A value of 0 means to the
right of all digits.
3. an APEX indicating what domain MUST replace "e164.arpa" for
this application. "e164.arpa" MAY also be replaced by itself.
o A resolver looking for an Infrastructure ENUM domain needs to
retrieve this EBL once during first resolution within a country
code. This is described in Section 6.
o The construction of the FQDN is described in [7] the recommended
resolver behavior in Section 8.
6. Locating the ENUM branch location record
This section specifies the EBL location for the use-case
"Infrastructure ENUM". The EBL records for Infrastructure ENUM
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SHOULD be positioned at the level of individual country codes as
assigned by ITU-T, and MUST use the subdomain label "infrastructure".
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.
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.
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.
If a branch location record is not found according to this table (for
instance, in the unlikely case the ITU-T 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. Example for the location of the EBL
This example shows the location of the EBL records for the use-case
"Infrastructure ENUM" defined in this document. It defines that the
EBL resides at "infrastructure".<reverse-country-code>.e164.arpa.
Thus for example:
infrastructure.3.4.e164.arpa. IN EBL 2 "i" e164.arpa.
infrastructure.1.e164.arpa. IN EBL 4 "i" example.com.
infrastructure.9.4.e164.arpa. IN EBL 0 "" ie164.arpa.
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These records indicate how the transformation from E.164 number to
ENUM domains for the application "Infrastructure ENUM" should be done
for numbers in country code +43, +1, and +49. A detailed example for
the intermediate steps necessary is given in [7].
This leads to the following mappings:
+43 15056416 6.1.4.6.5.0.5.1.i.3.4.e164.arpa
+1 5551234567 7.6.5.4.3.2.1.i.5.5.5.1.example.com
+49 891234567 7.6.5.4.3.2.1.9.8.9.4.ie164.arpa
The last example for CC +49 also shows how the migration to the long
term solution can be accomplished. This assumes that the apex for
the long-term solution is "ie164.arpa".
8. 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:
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 (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
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.
* 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.
+ Optionally 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
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retrieve the EBL record in the 'infrastructure' subdomain
each time, again storing the country code and associated EBL
values if successful.
+ If both attempts fail, use the triple ("", 0, "e164.arpa")
as default. This corresponds to the RFC 3671 "golden tree"
* Construct a domain name according to the algorithm given in
Section 5.
* 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
EBL records is thus recommended.
9. 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
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
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authenticity of the conveyed value.
10. IANA considerations
None.
11. 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 EBL records for this country would then be
changed to the values "position=0", "seperator="" and
"apex=example.com" (whatever is defined). When finally all countries
have migrated, the EBL records may be removed.
12. 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.
13. References
13.1. 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.
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[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-02 (work in progress),
December 2006.
13.2. Informative References
[8] Lind, S. and P. Pfautz, "Infrastrucure ENUM Requirements",
draft-ietf-enum-infrastructure-enum-reqs-03 (work in progress),
August 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-03 (work in
progress), November 2006.
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/
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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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