ENUM -- Telephone Number Mapping O. Lendl
Working Group enum.at
Internet-Draft June 12, 2007
Intended status: Standards Track
Expires: December 14, 2007
The ENUM Branch Location Record
draft-ietf-enum-branch-location-record-03
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 14, 2007.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This documents defines an extension to the E.164 Number Mapping
(ENUM) algorithm by adding a mapping step which indicates where the
ENUM tree for a specific ENUM application is located. A new DNS
record (IEBL, the Infrastructure ENUM Branch Location record) is
defined which provides an interim solution for the Infrastructure
ENUM tree location.
Lendl Expires December 14, 2007 [Page 1]
Internet-Draft ENUM Branch Location Record June 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The generalized ENUM Application . . . . . . . . . . . . . . . 4
4. The EBL Resource Record . . . . . . . . . . . . . . . . . . . 5
4.1. The EBL RDATA Format . . . . . . . . . . . . . . . . . . . 5
4.2. The EBL Presentation Format . . . . . . . . . . . . . . . 6
4.3. The IEBL Record . . . . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Combined Infrastructure ENUM . . . . . . . . . . . . . . . 6
5.2. Tree Aggregation . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
Lendl Expires December 14, 2007 [Page 2]
Internet-Draft ENUM Branch Location Record June 2007
1. Introduction
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 [4].
ENUM (E.164 Number Mapping) as defined in RFC 3761 [1] (User-ENUM) is
based on the concept of a single "golden" tree (e164.arpa) which
stores telephone number to URI mappings.
Experience has shown that this single tree is not suitable for all
applications and usage scenarios. The rules regarding administrative
control of domains, opt-in requirements, and delegation hierarchy can
vary between applications. See e.g. Infrastructure ENUM [6].
While non-terminal NAPTRs (see [3]) can redirect the ENUM resolution
algorithm to another DNS tree, their semantics are not powerful
enough to support an integration of Infrastructure ENUM into User
ENUM at the number level.
A more generic application-specific redirection mechanism is thus
needed.
An ENUM Branch Location Record as defined by this document contains
information to drive a generalized algorithm which transforms a
telephone number into a domain name. This extends the original
algorithm as defined in section 2.4 of RFC 3761 [1] for specific use-
cases.
This document defines the layout of a generic ENUM Branch Location
(EBL) DNS Resource Record type and allocates a specific RRTYPE code
for the Infrastructure ENUM use-case. In order for multiple such
application to co-exist, each will need to allocate its own RRTYPE
code.
2. Context
RFC 3761 defines ENUM as a Dynamic Delegation Discovery System (DDDS)
application according to RFC 3401 ff [2]. 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 generalized ENUM application extends only the definition of the
Lendl Expires December 14, 2007 [Page 3]
Internet-Draft ENUM Branch Location Record June 2007
"Valid Databases" part of the DDDS algorithm. All other aspects of
ENUM (e.g. further processing, valid enum-service types) are not
affected.
The terminology can be confusing: ENUM is a DDDS Application. This
draft generalizes ENUM to allow specific applications (e.g.
Infrastructure ENUM) to use EBL records to tailor the ENUM algorithm
to their individual needs. To distinguish these two layers of
"applications", this document uses the term "use-case" for specific
applications of the EBL-enabled ENUM algorithm.
This document does not define where EBL records are located in the
DNS, that is left to documents which describe an actual use-case of
the generalized ENUM application. These use-cases need to include a
clear specification on where to look for EBLs, as well as allocate a
RRTYPE code for this use-case.
3. The generalized ENUM Application
To recap, RFC 3761 (section 2.4) uses the following four steps as the
"Valid Databases" part of the DDDS Algorithm:
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.
This small algorithm translates the "Application Unique String" (AUS,
the E.164 telephone number) to a fully qualified domain name (FQDN)
which is then used to query for NAPTR (Naming Authority Pointer, [3])
records containing rewriting rules.
Any use-case which uses EBL records to generalize the basic ENUM
algorithm MUST clearly define where EBLs for this use-case are
located in the DNS and MUST define the client behavior for the case
if the EBL is not found at that location in the DNS tree. The EBL
itself contains three parameters which affect the translation
algorithm: SEPERATOR, POSITION, and APEX.
The generalized algorithm to derive the initial FQDN for the NAPTR
lookup (thus replacing steps 1-4 from above) is defined as:
1. Apply the use-case specific algorithm to translate the AUS (the
E.164 telephone number) to the location of the EBL record in the
DNS. This needs to yield a fully qualified domain name (FQDN).
Lendl Expires December 14, 2007 [Page 4]
Internet-Draft ENUM Branch Location Record June 2007
2. Query the DNS for an EBL record at the location of this FQDN, and
retrieve the triple (SEPERATOR, POSITION, APEX) from this record.
If multiple records are present, take any one and ignore the
others.
If no EBL record was found, the client MUST proceed according to
the definition of the use-case, which could either be falling
back to a default (e.g. use the triple ("", 0, "e164.arpa") to
indicate the RFC 3671 "golden tree") or returning an error.
3. Build an ordered list of single-digit strings from all digits
appearing in the AUS. All non-digit characters are ignored.
4. If SEPERATOR is not the empty string, then insert a string
consisting of SEPERATOR after POSITION strings into this list.
If the list of strings was shorter than POSITION elements, then
report an error.
5. Reverse the order of the list.
6. Append a string containing APEX to the end of the list.
7. Create a single domain-name by joining the list together with
dots (".") between each string.
Further processing is done according to RFC 3271: This domain-name is
used to request NAPTR records which may contain the end result or, if
the flags field is blank, produce new keys in the form of domain-
names from the DNS.
Section 5 contains examples.
4. The EBL Resource Record
Multiple use-cases of this algorithm can look for EBL records at the
same location in the DNS. To distinguish EBL records from different
use-cases, each use-case MUST allocate its own RRTYPE code for the
EBL records associated with it. This document describes the generic
RDATA format for all these EBL records, but allocates only the RRTYPE
code for the Infrastructure ENUM use-case.
4.1. The EBL RDATA Format
The RDATA for an EBL RR consists of a position number, separator
string and an apex domain:
Lendl Expires December 14, 2007 [Page 5]
Internet-Draft ENUM Branch Location Record June 2007
0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+
| POSITION |
+--+--+--+--+--+--+--+--+
/ SEPARATOR /
+--+--+--+--+--+--+--+--+
/ APEX /
+--+--+--+--+--+--+--+--+
where POSITION is a single byte, SEPARATOR is a <character-string>
and APEX is a <domain-name>.
<character-string> and <domain-name> are defined in RFC 1035 [5].
The APEX field MUST NOT be empty; name-compression MUST NOT be used.
4.2. The EBL Presentation Format
The master file format follows the standard rules in RFC 1035.
POSITION is represented as decimal integer. SEPARATOR is a quoted
string, APEX is a domain name and thus does not require quoting.
4.3. The IEBL Record
The EBL record for the Combined Infrastructure ENUM use-case [7] is
using the mnemonic "IEBL".
The RR type code for the IEBL RR is /IANA-ACTION/.
IEBL records are stored in the User-ENUM tree (e164.arpa) at the
country-code (or group-of-countries) level, e.g. 1.e164.arpa,
3.4.e164.arpa, or 3.5.3.e164.arpa. A simple algorithm to determine
the country-code length is given in draft-ietf-enum-combined-01 [7].
For up-to-date information regarding currently assigned country-code
the see E.164 [8] and the ITU website under "ITU-T / Service
Publications".
If no IEBL record is found at the country-code level then the ENUM
client MUST report an error.
5. Examples
5.1. Combined Infrastructure ENUM
This example shows the use of IEBL records for the combined
Infrastructure ENUM use-case.
Lendl Expires December 14, 2007 [Page 6]
Internet-Draft ENUM Branch Location Record June 2007
This use-case defines that the IEBL resides at <reverse-country-
code>.e164.arpa. Thus for example:
1.e164.arpa. IN IEBL 4 "i" e164.arpa.
4.4.e164.arpa. IN IEBL 2 "i" e164.arpa.
These records indicate how the transformation from E.164 number to
ENUM domains for the use-case "Infrastructure ENUM" should be done
for numbers in country-codes +44 and +1. This leads to the following
mappings:
+1 21255501234 4.3.2.1.0.5.5.5.i.2.1.2.1.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 as defined in Section 3 operates on "+44
2079460123":
1. According to the combined I-ENUM specification, retrieve the
country-code from the number and build a FQDN using the reversed,
dot-separated country-code and "e164.arpa", yielding
"4.4.e164.arpa".
2. The IEBL lookup for this domain sets SEPERATOR to "i", POSITION
to "2" and APEX to "e164.arpa".
3. The list of strings is
("4","4","2","0","7","9","4","6","0","1","2","3").
4. The SEPERATOR is "i", 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")
5. Reversing the list gives:
("3","2","1","0","6","4","9","7","0","2","i","4","4")
6. Appending APEX yields:
("3","2","1","0","6","4","9","7","0","2","i","4","4","e164.arpa")
7. Concatenation with dots: "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 "ienum.example.net" as the new apex for I-ENUM, the
administrators of +44 can implement a smooth transition by changing
its IEBL record in the following way:
4.4.e164.arpa. IN IEBL 0 "" ienum.example.net.
Lendl Expires December 14, 2007 [Page 7]
Internet-Draft ENUM Branch Location Record June 2007
This way, clients using the interim I-ENUM solution end up querying
the same tree as clients implementing the long-term solution.
5.2. Tree Aggregation
EBL records can also be helpful in private ENUM settings. Consider a
Voice over IP (VoIP) operator called "example.com" which participates
in various country-specific VoIP peering services that all use their
own private ENUM tree.
In order to avoid hardcoding country-specific ENUM lookups in its
soft-switch, "example.com" can establish its own private ENUM tree
which is populated with EBL records pointing to each fabric's ENUM
tree. For example:
$ORIGIN enum.example.com.
1 IN IEBL 0 "" nanp-exchange.example.org.
4.4 IN IEBL 0 "" uk-peering.example.net.
3.5.3 IN IEBL 0 "" ie-link.example.net.
2.5.3 IN IEBL 0 "" enum.benelux.example.net.
1.3 IN IEBL 0 "" enum.benelux.example.net.
2.3 IN IEBL 0 "" enum.benelux.example.net.
6. Security Considerations
EBLs are used to direct ENUM resolvers to other places in the DNS.
The security of DNS in both the location of the EBLs and wherever
they point to needs to be maintained.
Use-case specifications need to be careful when designing their EBL
location: Information concerning which numbers have been dialed could
be leaked to the nameserver hosting the EBL records.
7. IANA Considerations
This documents allocates the Resource Records Type field for the IEBL
record according to the definition in Section 4.
8. Acknowledgements
The author would like to thank Alexander Mayrhofer, Michael Haberler,
Richard Stastny, Ed Lewis, and Olafur Gudmundsson for their
contributions.
Lendl Expires December 14, 2007 [Page 8]
Internet-Draft ENUM Branch Location Record June 2007
9. References
9.1. Normative References
[1] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
[2] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
[3] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403,
October 2002.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
9.2. Informative References
[6] Lind, S. and P. Pfautz, "Infrastrucure ENUM Requirements",
draft-ietf-enum-infrastructure-enum-reqs-02 (work in progress),
April 2006.
[7] Haberler, M. and R. Stastny, "Combined User and Infrastructure
ENUM in the e164.arpa tree", draft-ietf-enum-combined-01 (work
in progress), October 2006.
[8] International Telecommunications Union, "The International
Public Telecommunication Numbering Plan", ITU-T Recommendation
E.164, 1991.
Author's Address
Otmar Lendl
enum.at GmbH
Karlsplatz 1/9
Wien A-1010
Austria
Phone: +43 1 5056416 33
Email: otmar.lendl@enum.at
URI: http://www.enum.at/
Lendl Expires December 14, 2007 [Page 9]
Internet-Draft ENUM Branch Location Record June 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Lendl Expires December 14, 2007 [Page 10]
