Network Working Group M. Haberler
Request for Comments: 5527 IPA
Category: Informational O. Lendl
enum.at
R. Stastny
Unaffiliated
May 2009
Combined User and Infrastructure ENUM in the e164.arpa Tree
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.
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Abstract
This memo defines an interim solution for Infrastructure ENUM in
order to allow a combined User and Infrastructure ENUM implementation
in e164.arpa as a national choice. This interim solution will be
deprecated after implementation of the long-term solution.
Haberler, et al. Informational [Page 1]RFC 5527 Combined User and Infrastructure ENUM May 2009
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Interim Solution ................................................3
4. The Algorithm ...................................................4
5. Determining the Position of the Branch ..........................5
6. Transition to the Long-Term Solution ............................6
7. Examples ........................................................7
8. Security Considerations .........................................8
9. Acknowledgments .................................................9
10. References .....................................................9
10.1. Normative References ......................................9
10.2. Informative References ....................................9
1. Introduction
ENUM (E.164 Number Mapping, [RFC3761]) is a system that transforms
E.164 numbers [E164] into domain names and then queries the DNS
(Domain Name Service) [RFC1034] for NAPTR (Naming Authority Pointer)
records [RFC3401] in order 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 [RFC3761] by the carrier-of-record (voice service provider)
[RFC5067] for a specific E.164 number [E164] in order to publish a
mapping of this telephone number to one or more Uniform Resource
Identifiers (URIs) [RFC3986].
Other voice service providers can query the DNS for this mapping and
use the resulting URIs as input into their call-routing algorithm.
These URIs are separate from any URIs that the end-user who registers
an 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 [RFC5067].
Using the same E.164 number to domain mapping techniques for other
applications under a different, internationally agreed-upon apex
(instead of e164.arpa) is straightforward on the technical side.
This process of defining the Dynamic Delegation Discovery System
(DDDS) [RFC3401] application for Infrastructure ENUM is defined in
[RFC5526]. This is the long-term solution.
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This document presents an interim solution for Infrastructure ENUM
and a mechanism for transitioning to the long-term solution. The
interim solution is based on establishing a branch in the e164.arpa
tree, which resolvers may locate by following the algorithm described
in Section 4. The location of the branch is dependent upon country-
code length, and thus resolvers must determine the position of the
branch based on the method described in Section 5. Finally,
Section 6 provides a way that implementations following the
procedures of Sections 4 and 5 may be seamlessly redirected to the
long-term solution, when it becomes available.
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
[RFC2119].
3. Interim Solution
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 interoperable
Infrastructure ENUM tree immediately. The interim solution will be
deprecated when the long-term solution [RFC5526] is deployed. 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 [RFC3761] continue to work without any
modification.
Because of various reasons (e.g., potentially different delegation
points, different reliability requirements, and use of DNS
wildcards), sharing a single domain name between the user itself and
the respective carrier for a given number is not possible. Hence, a
different domain name must be used to store infrastructure ENUM
information.
In order to avoid the delays associated with the long-term solution,
the existing delegations and agreements around e164.arpa need to be
leveraged.
The method most easily fulfilling the requirements is to branch off
the e164.arpa tree into a subdomain at the country-code delegation
level below e164.arpa and deploy an Infrastructure ENUM subtree
underneath, without touching User ENUM semantics at all.
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RFC 5527 Combined User and Infrastructure ENUM May 2009
This allows countries using a dedicated country code to introduce the
interim solution as a national matter to the concerned National
Regulation Authority (NRA). The governing body of a shared country
code and the owner of a global network code can also choose to
implement this solution within their area of responsibility.
Under this approach, ITU-T (International Telecommunication Union /
Telecommunication Standardization Sector), IETF, and IAB involvement
is only lightweight, e.g., to recommend the proper algorithm defined
here to enable international interoperability.
4. The Algorithm
RFC 3761 defines ENUM as a Dynamic Delegation Discovery System (DDDS)
application according to RFC 3401 [RFC3401]. As such, ENUM defines
the following components of the DDDS algorithm:
1. Application Unique String
2. First Well-Known Rule
3. Expected Output
4. Valid Databases
The "Valid Databases" part contains the transformation of an E.164
telephone number into a domain name. Section 2.4 of RFC 3761 uses
the following 4-step algorithm for this:
1. Remove all characters with the exception of the digits.
2. Put dots (".") between each digit.
3. Reverse the order of the digits.
4. Append the string ".e164.arpa" to the end.
The interim solution for Infrastructure ENUM uses a modified version
of this algorithm:
1. Determine the proper POSITION parameter for this E.164 number
according to the algorithm in Section 5 of this document.
2. Build an ordered list of single-digit strings from all digits
appearing in the telephone number. All non-digit characters are
ignored.
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3. Insert a string consisting of "i" into this list, after POSITION
strings. If the list of strings was shorter than POSITION
elements, then report an error.
4. Reverse the order of the list.
5. Append the string "e164.arpa" to the end of the list.
6. Create a single domain name by joining the list together with
dots (".") between each string.
This is the only point where the interim Infrastructure ENUM (I-ENUM)
solution differs from straight RFC 3761 ENUM. All other parts of
User ENUM, including the enumservices registrations, apply to I-ENUM
as well.
5. Determining the Position of the Branch
In order to allow for the deployment of this interim solution
independent of IAB/ITU-T/RIPE-NCC negotiations, the branching label
"i" cannot be inserted in the Tier-0 zone (i.e., the e164.arpa zone
itself) currently managed by RIPE NCC. This condition acts as a
lower bound on the choice of the POSITION parameter.
For international E.164-numbers for geographic areas (Section 6.2.1
of [E164]) and for international E.164-numbers for global services
(Section 6.2.2 of [E164]), the most sensible choice for POSITION is
the number of digits in the country code of the number in question.
This places the branch directly under the country-code level within
the e164.arpa ENUM tree.
For international E.164-number for networks (Section 6.2.3 of
[E164]), the appropriate choice for POSITION is the combined length
of the CC (Country Code) and IC (Identification Code) fields.
For international E.164-number for groups of countries (Section 6.2.4
of [E164]), the value for POSITION is 4.
The authoritative source for up-to-date country code and network
Identification Code allocations is published by the ITU-T as a
complement to the recommendation E.164 [E164]. The current version
of this complement is available from the ITU website under "ITU-T /
Service Publications".
Please note that country code 1 of the North American Numbering Plan
(NANP) does not fall under the ITU classification of "groups of
countries", but is a "shared country code" for a geographic area.
Thus, the POSITION parameter for the NANP is 1.
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As of 2007, the POSITION value for a specific E.164 number can be
determined with the following algorithm:
o If the number starts with 1 or 7, then POSITION is 1.
o If the number is in one of the following 2-digit country codes,
then POSITION is 2: 20, 27, 30-34, 36, 39, 40, 41, 43-49, 51-58,
60-66, 81, 82, 84, 86, 90-95, or 98.
o If the number starts with 388 or 881, then POSITION is 4.
o If the number starts with 878 or 882, then POSITION is 5.
o If the number starts with 883 and the next digit is < 5, then
POSITION is 6.
o If the number starts with 883 and the next digit is >= 5, then
POSITION is 7.
o In all other cases, POSITION is 3.
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 list of existing 1- and 2-digit country codes
can be considered very stable. The only problem may be for a country
that has split, as happened recently, for example, to Yugoslavia.
Regarding network codes, up to 2007, the ITU-T has only allocated 1-
and 2-digit ICs. Assignments of 3- and 4-digit ICs started in May
2007 in the +883 country code. Any further change in the ITU-T
policy in this respect will need to be reflected in the above
algorithm.
6. Transition to the Long-Term Solution
The proposed long-term solution for Infrastructure ENUM [RFC5526] is
the establishment of a new zone apex for that tree. This apex will
play the same role as "e164.arpa" does for User ENUM.
It is unrealistic to assume that all countries and all ENUM clients
will manage to migrate from the interim solution to the long-term
solution at a single point in time. It is thus necessary to plan for
an incremental transition.
In order to achieve this, clients using the interim solution need to
be redirected to the long-term I-ENUM tree for all country codes that
have already switched to the long-term solution. This SHOULD be done
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by placing DNAME [RFC2672] records at the branch (the "i") label
pointing to the appropriate domain name in the long-term I-ENUM tree.
All descendants at that branch label location where the DNAME record
is inserted MUST be removed, as required by Section 3 of RFC 2672.
Therefore, ALL entities involved in making or answering DNS queries
for I-ENUM MUST fully support the DNAME record type and its
semantics. In particular, entities involved in I-ENUM lookups MUST
correctly handle responses containing synthesized CNAMEs that may be
generated as a consequence of DNAME processing by any other element
in resolution, typically an iterative mode resolving name server.
These entities MUST also apply adequate measures to detect loops and
prevent non-terminating resolutions because of improperly configured
DNAME records or combinations of DNAME and CNAME records.
Note: Some caching name server implementations are known to handle
DNAMEs incorrectly. In the worst case, such bugs could stay
undetected until a country transitions to the long-term solution.
Therefore, ensuring full DNAME support from the start (and carefully
testing that it actually works) is important.
The domain name for the branch location and its DNAME record SHOULD
be removed once the transition to the long-term solution is completed
and all entities involved in I-ENUM have migrated to the new zone
apex for I-ENUM.
7. Examples
These are two examples of how E.164 numbers translate to
Infrastructure ENUM domains according to the interim solution.
+1 21255501234 4.3.2.1.0.5.5.5.2.1.2.i.1.e164.arpa
+44 2079460123 3.2.1.0.6.4.9.7.0.2.i.4.4.e164.arpa
Here is the list of the intermediate steps for the second example to
visualize how the algorithm defined in Section 4 operates on "+44
2079460123":
1. "+44 2079460123" is within a 2-digit country code; thus, POSITION
is 2.
2. The list of strings is
("4","4","2","0","7","9","4","6","0","1","2","3")
3. POSITION is 2; thus, "i" is inserted between the second and the
third string, yielding:
("4","4","i","2","0","7","9","4","6","0","1","2","3")
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4. Reversing the list gives:
("3","2","1","0","6","4","9","7","0","2","i","4","4")
5. Appending "e164.arpa" yields:
("3","2","1","0","6","4","9","7","0","2","i","4","4","e164.arpa")
6. Concatenation with dots yields:
"3.2.1.0.6.4.9.7.0.2.i.4.4.e164.arpa"
After the introduction of the long-term Infrastructure ENUM solution,
using, for example, "ienum.example.net" as the new apex for I-ENUM,
the administrators of +44 can implement a smooth transition by
putting the following DNAME record in their zone:
i.4.4.e164.arpa. IN DNAME 4.4.ienum.example.net.
This way, clients using the interim I-ENUM solution end up querying
the same tree as clients implementing the long-term solution.
8. Security Considerations
Privacy issues have been raised regarding the unwarranted disclosure
of user information that would result from publishing Infrastructure
ENUM information in the public DNS. For instance, such disclosure
could be used for harvesting numbers in service or obtaining 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 or not a number is served 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 whether listed or unlisted, will prevent mining attempts
for such number attributes.
The result will be that the information in the public DNS will mirror
number-range allocation information, but no 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 the 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.
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9. Acknowledgments
We gratefully acknowledge suggestions and improvements by Jason
Livingood and Tom Creighton of Comcast, Penn Pfautz of AT&T, Lawrence
Conroy of Roke Manor Research, Jim Reid, and Alexander Mayrhofer of
enum.at.
10. References
10.1. Normative References
[E164] ITU-T, "The International Public Telecommunication Number
Plan", Recommendation E.164, February 2005.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
RFC 2672, August 1999.
[RFC3401] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part One: The Comprehensive DDDS", RFC 3401, October 2002.
[RFC3761] Faltstrom, P. and M. Mealling, "The E.164 to Uniform
Resource Identifiers (URI) Dynamic Delegation Discovery
System (DDDS) Application (ENUM)", RFC 3761, April 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
10.2. Informative References
[RFC5067] Lind, S. and P. Pfautz, "Infrastructure ENUM
Requirements", RFC 5067, November 2007.
[RFC5526] Livingood, J., Pfautz, P., and R. Stastny, "The E.164 to
Uniform Resource Identifiers (URI) Dynamic Delegation
Discovery System (DDDS) Application for Infrastructure
ENUM", RFC 5526, April 2007.
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Authors' Addresses
Michael Haberler
Internet Foundation Austria
Karlsplatz 1/2/9
Wien 1010
Austria
Phone: +43 664 4213465
EMail: ietf@mah.priv.at
URI: http://www.nic.at/ipa/
Otmar Lendl
enum.at GmbH
Karlsplatz 1/2/9
Wien A-1010
Austria
Phone: +43 1 5056416 33
EMail: otmar.lendl@enum.at
URI: http://www.enum.at/
Richard Stastny
Unaffiliated
Anzbachgasse 43
1140 Vienna
Austria
Phone: +43 664 420 4100
EMail: richardstastny@gmail.com
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