ENUM -- Telephone Number Mapping M. Haberler Working Group IPA Internet-Draft R. Stastny Expires: January 9, 2006 Oefeg July 8, 2005 Combined User and Carrier ENUM in the e164.arpa tree draft-haberler-carrier-enum-00 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 January 9, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract ENUM as defined now in RFC3761 is not well suited for the purpose of interconnection by carriers, as can be seen by the use of various private tree arrangements based on ENUM mechanisms. A combined end- user and carrier ENUM tree solution would leverage the ENUM infrastructure in e164.arpa, increase resolution rates, and decrease the cost per registered telephone number. This document describes a minimally invasive scheme to provide both end-user and carrier data Haberler & Stastny Expires January 9, 2006 [Page 1] Internet-Draft Combined User and Carrier ENUM July 2005 in ENUM. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Introducing a branch into the e164.arpa tree . . . . . . . . 4 4. Resolver behavior options and the Carrier ENUM boundary . . 5 5. Recommended resolver behavior . . . . . . . . . . . . . . . 7 6. Security considerations . . . . . . . . . . . . . . . . . . 7 7. IANA considerations . . . . . . . . . . . . . . . . . . . . 8 8. Interoperability considerations . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1 Normative References . . . . . . . . . . . . . . . . . . 8 10.2 Informative References . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . 10 Haberler & Stastny Expires January 9, 2006 [Page 2] Internet-Draft Combined User and Carrier ENUM July 2005 1. Introduction ENUM as defined in RFC3761 is based on the end-user opt-in principle. While this has great potential to foster new services and end-user choice in the long-term, the current requirements for IP-based interconnection of carriers and IP Telephony Service Providers require the provisioning of all allocated or served (hosted) numbers of a participating carrier of record. Also, an interconnection scenario through Carrier ENUM typically implies underlying closed user arrangements where URIs are used in authenticated context, an assumption which cannot reasonably be imposed on User ENUM entries. While in principle solutions like compulsory opt-in through terms and conditions for end users are conceivable, there are substantial downsides to such an approach. ENUM for end-user provisioning remains an ill-suited solution for the PoI (point-of-interconnect) information discovery problem. Both from an OPEX (Operational Expenditure) perspective as well as overall resolution rates achievable through a given approach, a combined ENUM tree both for end-users and carrier of record ENUM stands to be superior over a forest of disparate private trees now as well as long-term. Also, as a common infrastructure easily supports both usage scenarios, a combined approach will support the end-user ENUM vision by driving down the average cost per number. Lastly, any later convergence between ENUM for end-users and carriers of record will be significantly easier and cheaper, thus benefitting users as well as carriers. For the rest of the document the terms User ENUM and Carrier ENUM will be used to distinguish between the two approaches. 2. Requirements A solution for combined User and Carrier ENUM within the e164.arpa tree should meet the following requirements: o A single DNS lookup should suffice to resolve any given number in the public DNS in both scenarios. o It should leave User ENUM resolution semantics and tree shape intact, i.e. requiring no wholesale changes to existing User ENUM resolvers or tree layout. o Additional functionality should only be imposed on carrier resolvers. o It should work with both closed and open number plans without resorting to wildcard records in the non-user controlled part of the DNS, both to avoid associated semantic problems as well as keeping the route to DNSSEC deployment open. Haberler & Stastny Expires January 9, 2006 [Page 3] Internet-Draft Combined User and Carrier ENUM July 2005 o It should not require the introduction of new constructs within existing standards, such as new types or changed semantics of NAPTR records. o It should be possible to introduce the scheme in a timely manner, supporting current carrier needs. Consequently, it is desirable to deploy the scheme without re-opening already settled questions of roles, responsibilites and international coordination, and in particular the country code delegation process. o It should meet all reasonable privacy concerns about visibility of information an end user has no control over, for example discovery of unlisted numbers, or inadvertent disclosure of user identity. o It should keep the option open for other types of closed-user- group type applications, which might not naturally fit into the - predominantly voice oriented - Carrier ENUM scenario. Note in particular that we assume all entries to properly resolve in the public DNS, both user and carrier. Usage restrictions on Carrier ENUM records are to be handled at the URI level, and not by restriction on the visibility of entries in the public DNS. 3. Introducing a branch into the e164.arpa tree The method most easily fulfilling the abovementioned requirements is to branch off the e164.arpa tree into a subdomain at a given point, and deploy a Carrier ENUM subtree underneath without touching User ENUM semantics at all. For readability, we will use the 'carrier' subdomain from now on, while in practice a single character subdomain like 'c' will suffice. For interoperability it is desirable to have that branch sit in a commonly agreed, or easily discoverable place. Several options for this branch location exist, among them are: o above the country code delegation level, e.g. '4.9.7.1.carrier.e164.arpa', alternatively: o somewhere below the country code delegation level, e.g. '4.9.7.carrier.1.e164.arpa'. In the first case, heavy involvement of ITU-T, RIPE as well as the applicable NRAs (National Regulatory Authorities) is needed. Also, reopening the discussion of the interim procedures already agreed is a tedious process, as is the adaptation of the current delegation mechanism. However, no changes to resolver semantics are required as this approach amounts to just a different apex definition for the resolver. Therefore the remainder of this paper addresses only the second scenario. This approach, putting aside significant process and timing concerns, appears to be an easier to manage long-term approach to tree naming. Haberler & Stastny Expires January 9, 2006 [Page 4] Internet-Draft Combined User and Carrier ENUM July 2005 In the second case issues could be resolved as a national matter, or as a regional opt-in within in a given Numbering Plan Area such as the North American NPA. However, a convention is needed how, given a fully qualified E.164 [2] number, a resolver can determine the location of the carrier subdomain. The involvement of the NRA is needed in all cases since the definition of the proper carrier of record follows national telecommunications law. In the second case however, ITU-T and IETF (IAB) involvement is only lightweight, e.g. to recommend the proper algorithm defined here to enable international interoperability. 4. Resolver behavior options and the Carrier ENUM boundary Whatever choice is taken, a Carrier ENUM resolver needs to determine the place applicable in a given number to search for the 'carrier' subdomain for international interoperability. To determine the subdomain location, the following schemes could be used: o a convention whereas the subdomain is always located right below the country code delegation level, e.g. '4.9.7.carrier.1.e164.arpa', o a per-country or per-NPA decision about the subdomain location, for instance 'carrier.4.9.7.1.e164.arpa'. The first option would enable a fixed table-based mapping in the resolver from country codes to subdomain location. Given the fact that the ITU recently allocated only 3-digit country codes, there is no more spare 1- and 2-digit country codes and existing 1- and 2-digit country codes are extremely unlikely to be be recovered, a 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 happend recently e.g. to Yugoslavia. The current scheme to determine country code length is as follows: 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. While easily implemented, this method precludes some national options, for instance to move their Carrier ENUM tree under carrier.e164.arpa, or have the subdomain location deeper in the tree, for instance under a numbering plan spanning several countries, right under the number plan area (NPA) level, e.g. 'carrier.4.9.7.1.e164.arpa' . In practice, current regional co-operative number plan areas such as North America partition the number plan at the 4th digit (thus the "number plan area code" +1444 is discrete from the "number plan area Haberler & Stastny Expires January 9, 2006 [Page 5] Internet-Draft Combined User and Carrier ENUM July 2005 code" +1443). This partitioning may be reflected in routing choices within the PSTN, so a PSTN routing choice can be made by inspection of the first 4 digits of the E.164 number. For flexibility, some PSTN routing choices may be made by examining a further digit, so that, for example, +87810 could be treated differently from +87811, and might be routed to a different interconnection point. This would reflect a difference between the administration controlling these two number spaces. It seems unlikely that further inspection would be required even in the medium to long term, so that an algorithm that inspects the first 5 digits will suffice to detect the boundary for Carrier ENUM space within the global ENUM space for any realistic numbering administrative partitioning. Therefore, we suggest a more flexible approach which subsumes all of the above scenerios. It is based on a table of well-known subdomain locations for those countries or NPAs which have opted in to this scheme. This Carrier ENUM subdomain location table would look as follows: +--------------+---------+ | country code | location| +--------------+---------+ | 43 | 2 | | 1 | 4 | +--------------+---------+ Figure 1 The interpretation would be as follows: o Country codes 43 and 1 have opted to use a combined User and Carrier ENUM tree under e164.arpa. o Country code 43 (Austria) has opted to locate the carrier subtree right under the country code, therefore to be found two digits into the number. Example: '5.6.4.3.1.2.4.4.6.6.carrier.3.4.e164.arpa'. o The +1 NPA has decided to locate the subdomain after country code plus NPA, i.e. 4 digits into the number. Example: '1.5.6.5.4.3.4.carrier.1.7.5.1.e164.arpa' . This table would be found at a fixed location registered with IANA and retrieved by Carrier ENUM resolvers from time to time. We could imagine other methods to distribute this - fairly static - information, for instance eventually in the DNS itself, and encourage suggestions how this could be done. Haberler & Stastny Expires January 9, 2006 [Page 6] Internet-Draft Combined User and Carrier ENUM July 2005 5. Recommended resolver behavior A User ENUM resolver as per RFC3761 need not be aware of any Carrier ENUM conventions at all. A combined User and Carrier ENUM resolver shall behave as follows: The input to the resolver routine shall be: 1. the called number in fully qualified E.164 (international) format, 2. a 'subtree' parameter indicating wether the search should proceed in the User ENUM tree, or in the subtree indicated by the parameter (example: 'c' standing for carrier, or a null value for defaulting to User ENUM resolution), 3. access to the subdomain location table, 4. any other paramters 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: 1. if the subtree parameter indicates a User ENUM search, proceed as per RFC3761. 2. If the subtree parameter indicates a Carrier ENUM query: 1. determine whether or not the country code is contained in the subdomain table. If not, indicate failure. 2. If it is, retrieve the subdomain location parameter for the given country code and insert the subdomain accordingly while creating the inverted dotted domain name. 3. search the DNS for any NAPTR records for the given number. 6. Security considerations Privacy issues have been raised regarding unwarranted disclosure of user information by publishing Carrier 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 Carrier 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, wether serviced or not, or listed or unlisted, will prevent mining attempts for such number attributes. Haberler & Stastny Expires January 9, 2006 [Page 7] Internet-Draft Combined User and Carrier ENUM July 2005 The result would be that the information in the public DNS would mirror number range allocation information, but not more. Carrier ENUM will not tell you more than you can get by just dialing numbers. 7. IANA considerations The following parameters need to be registered with IANA: 1. The name of the Carrier ENUM subdomain, for example 'c'. In the future other labels could be registered for different purposes. 2. According to RFC 3761, the IETF requested IANA to delegate the E164.ARPA domain following instructions provided by the IAB. Names within this zone are to be delegated to parties according to the ITU-T Recommendation E.164. If the first option outlined in this proposal is accepted, there will be no changes requested of IANA with respect to the E164.ARPA domain. However, if the second option outlined in this document is accepted, this would require IETF to request IANA to create a new sub-domain C.E164.ARPA. 3. the URI and format of the subdomain location table. 8. Interoperability considerations A resolver needs to indicate which information is requested - User or Carrier ENUM, or both. A user-ENUM-only resolver need not be aware of the carrier subtree and no changes with respect to RFC3761 semantics are required. A resolver desiring to retrieve Carrier ENUM or both types of records needs to be aware of the conventions laid out in this draft. 9. Acknowledgements We gratefully acknowledge suggestions and improvements by Jason Livingood and Tom Creighton of Comcast, Penn Pfautz of ATT, and Lawrence Conroy of Roke Manor Research. 10. References 10.1 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. 10.2 Informative References [2] ITU-T, "The International Public Telecommunication Number Plan", Recommendation E.164, May 1997. Haberler & Stastny Expires January 9, 2006 [Page 8] Internet-Draft Combined User and Carrier ENUM July 2005 Authors' Addresses Michael Haberler Internet Foundation Austria Waehringerstrasse 3/19 Wien A-1090 Austria Phone: +43 664 4213465 Email: mah@eunet.at URI: http://www.nic.at/ipa/ Richard Stastny Oefeg Postbox 147 Vienna A-1030 Austria Phone: +43 664 420 4100 Email: richard.stastny@oefeg.at URI: http://www.oefeg.at Haberler & Stastny Expires January 9, 2006 [Page 9] Internet-Draft Combined User and Carrier ENUM July 2005 Intellectual Property Statement 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. 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