Combined User and Infrastructure ENUM in the e164.arpa tree
draft-ietf-enum-combined-08

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 April 24, 2008.

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. This interim solution will be deprecated after approval and implementation of the long-term solution.

Table of Contents

1.  Introduction

2.  Terminology

3.  Interim Solution

4.  The Algorithm

5.  Determing the Position of the Branch

6.  Transition to the long-term Solution

7.  Examples

8.  Security considerations

9.  IANA considerations

10.  Acknowledgments

11.  References
    11.1.  Normative References
    11.2.  Informative References

§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

ENUM (E.164 Number Mapping, RFC 3761 (Faltstrom, P. and M. Mealling, “The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM),” April 2004.) [RFC3761]) is a system that transforms E.164 numbers (ITU-T, “The International Public Telecommunication Number Plan,” February 2005.) [refs.E164] into domain names and then queries the DNS (Domain Name Service) (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) [RFC1034] for NAPTR (Naming Authority Pointer) records (Mealling, M., “Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS,” October 2002.) [RFC3401] 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 (Faltstrom, P. and M. Mealling, “The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM),” April 2004.) [RFC3761] by the carrier-of-record (Lind, S. and P. Pfautz, “Infrastructure ENUM Requirements,” May 2007.) [I‑D.ietf‑enum‑infrastructure‑enum‑reqs] (voice service provider) for a specific E.164 number (ITU-T, “The International Public Telecommunication Number Plan,” February 2005.) [refs.E164] to publish a mapping of this telephone number to one or more Uniform Resource Identifiers (URIs) (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) [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 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 [I‑D.ietf‑enum‑infrastructure‑enum‑reqs] (Lind, S. and P. Pfautz, “Infrastructure ENUM Requirements,” May 2007.).

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. This process of defining the Dynamic Delegation Discovery System (DDDS) (Mealling, M., “Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS,” October 2002.) [RFC3401] application for Infrastructure ENUM is work in progress [I‑D.ietf‑enum‑infrastructure] (Livingood, J., “The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application for Infrastructure ENUM,” December 2007.). This is called the long term solution.

This document presents an interim solution for Infrastructure ENUM and a mechanism for transitioning to a 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 Algorithm). 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 (Determing the Position of the Branch). Finally, Section 6 (Transition to the long-term Solution) provides a way that implementations following the procedures of Section 4 (The Algorithm) 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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [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 becomes available. 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 (Faltstrom, P. and M. Mealling, “The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM),” April 2004.) [RFC3761] continue to work without any modification.

Because of various reasons (e.g. potentially different delegation points, different reliability requirements, use of DNS wildcards), 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.

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.

This allows countries using a dedicated country code to introduce the Interim Solution as a national matter by the concerned National Regulation Authority (NRA). The governing body of a shared country code and the owner of a global network code can also chose to implement this solution within their area of responsibility.

Under this approach, ITU-T (International Telecommunication Union / Telecommunication Standardization Sector) and IETF (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 (Mealling, M., “Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS,” October 2002.) [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 a E.164 telephone number into a domain name. Section 2.4 of RFC 3761 uses the following four 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 (Determing the Position of the Branch).



2. Build an ordered list of single-digit strings from all digits appearing in the telephone number. All non-digit characters are ignored.



3. Insert a string consisting of "i" after POSITION strings into this list. 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 solution differs from straight RFC 3761 ENUM. All other parts of User-ENUM, including the enumservices registrations, apply to I-ENUM as well.

5.  Determing the Position of the Branch

In order to allow for the deployment of this Interim Solution independently 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) managed currently 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 ([refs.E164] (ITU-T, “The International Public Telecommunication Number Plan,” February 2005.) 6.2.1) and for international E.164-numbers for global services ([refs.E164] (ITU-T, “The International Public Telecommunication Number Plan,” February 2005.) 6.2.2) the most sensible choice for POSITION is 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 ([refs.E164] (ITU-T, “The International Public Telecommunication Number Plan,” February 2005.) 6.2.3) 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 ([refs.E164] (ITU-T, “The International Public Telecommunication Number Plan,” February 2005.) 6.2.4) the value for POSITION is 4.

The authoritative source for up-to-date country code and network Identity Code allocations is a published by ITU-T as a complement to the recommendation E.164 [refs.E164] (ITU-T, “The International Public Telecommunication Number Plan,” February 2005.). The current version of this complement is available from 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. The POSITION parameter for the NANP is thus 1.

As of 2007, the POSITION value for a specific E.164 number can be determined with the following algorithm:

• If the number starts with 1 or 7 then POSITION is 1
• If the number is in one of the following 2-digit country codes: 20, 27, 30-34, 36, 39, 40, 41, 43-49, 51-58, 60-66, 81, 82, 84, 86, 90-95, or 98, then POSITION is 2.
• If the number starts with 388 or 881, then POSITION is 4
• If the number starts with 878 or 882, then POSITION is 5
• If the number starts with 883 and the next digit is < 5, then POSITION is 6
• If the number starts with 883 and the next digit is >= 5, then POSITION is 7
• 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 a country split as happened recently e.g. to Yugoslavia.

Regarding network codes, the ITU-T has up to 2007 only allocated one and two digit ICs. Assignments of three and four digit ICs started in May 2007 in the +883 country code. A 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 [I‑D.ietf‑enum‑infrastructure] (Livingood, J., “The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application for Infrastructure ENUM,” December 2007.) 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 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 which have already switched to the long-term solution. This SHOULD be done by placing DNAME (Crawford, M., “Non-Terminal DNS Name Redirection,” August 1999.) [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.

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 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 as defined in Section 4 (The Algorithm) 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")



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: "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 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 .

9.  IANA considerations

None.

10.  Acknowledgments

We gratefully acknowledge suggestions and improvements by Jason Livingood and Tom Creighton of Comcast, Penn Pfautz of ATT, Lawrence Conroy of Roke Manor Research, Jim Reid, and Alexander Mayrhofer of enum.at.

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses

Full Copyright Statement

Copyright © 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.