ENUM -- Telephone Number Mapping O. Lendl
Working Group enum.at
Internet-Draft December 12, 2006
Intended status: Standards Track
Expires: June 15, 2007
The ENUM Branch Location Record
draft-ietf-enum-branch-location-record-02
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Copyright (C) The IETF Trust (2006).
Abstract
This documents defines the ENUM Branch Location record (EBL) which is
used to indicate where the ENUM tree for special ENUM application is
located. The primary application for the EBL record is to provide a
temporary solution for the Infrastructure ENUM tree location.
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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 . . . . . . . . . . . . . . . . 5
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
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 [5].
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.
The ENUM Branch Location (EBL) 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.
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 as based on EBL records extends only
the definition of the "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.
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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.
3. The generalized ENUM Application
To recap, RFC 3761 (section 2.4) uses the following steps for 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 needs to define where EBLs for this use-case are located in
the DNS. 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 yields a fully qualified domain name (FQDN).
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, use the triple ("", 0, "e164.arpa")
as default. This corresponds to the RFC 3671 "golden tree".
3. Build an ordered list of single-digit strings from all digits
appearing in the AUS. All non-digit characters will be 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.
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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
The RR type code for the EBL RR is /IANA-ACTION/.
4.1. The EBL RDATA Format
The RDATA for a EBL RR consists of a position number, separator
string and an apex domain. <character-string> and <domain-name> refer
to the definitions of RFC 1035 [4].
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> and must not be empty. Name-compression
is not to be used for the APEX field.
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.
5. Examples
This example shows the use of EBL records as defined by the interim
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solution to Infrastructure ENUM as defined by
draft-ietf-enum-combined-01 [6].
This application 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.
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. 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
Here is the list of the intermediate steps for the first example to
visualize how the algorithm as defined in Section 3 operates on "+43
15056416":
1. According to the interim, combined I-ENUM specification, retrieve
the country-code from the number and build a FQDN using
"infrastructure", the reversed, dot-separated country-code and
"e164.arpa", yielding "infrastructure.3.4.e164.arpa".
2. The EBL lookup for this domain sets SEPERATOR to "i", POSITION to
"2" and APEX to "e164.arpa".
3. The list of strings is ("4","3","1","5","0","5","6","4","1","6").
4. The SEPERATOR is "i", POSITION is 2, thus "i" is inserted between
the second and the third string, yielding:
("4","3","i","1","5","0","5","6","4","1","6")
5. Reversing the list gives:
("6","3","4","6","5","0","5","1","i","3","4")
6. Appending APEX yields:
("6","3","4","6","5","0","5","1","i","3","4","e164.arpa")
7. Concatenation with dots: "6.3.4.6.5.0.5.1.i.3.4.e164.arpa"
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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 EBL
record according to the definition in Section 4.
8. Acknowledgements
The author would like to thank Alexander Mayrhofer, Michael Haberler,
Richard Stastny, Klaus Nieminen, Richard Shockey, and Karsten
Fleischhauer for their contributions.
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] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
9.2. Informative References
[5] Lind, S. and P. Pfautz, "Infrastrucure ENUM Requirements",
draft-ietf-enum-infrastructure-enum-reqs-02 (work in progress),
April 2006.
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[6] 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.
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/
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