Internet Draft A. Gallant Document: draft-gallant-e164-tier-defs-00.txt NeuStar, Inc. Expires: January 10, 2002 July 11, 2001 Numbering Tiers Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 document is an individual submission and is not the product of any working group. Comments should be sent to the author. Abstract Describing numbering tiers has been informal to date. This document proposes a simple recognition principle for tiers in terms of the protocol mapping of an E.164 numbering resource to a DNS domain name, as described for ENUM in RFC 2916. Tier value is a formal attribute of the digit string for an arbitrary E.164 resource, and it applies also to the resulting domain name. No assumptions about assignment, existence, or delegation are involved. The lower boundaries for Tier 1 and Tier 2 are a complete E.164 country code and a complete E.164 number, respectively. The Tier 1 attribute would apply when an E.164 resource contains a complete E.164 country code but does not contain a complete E.164 number. This document suggests terms for sub-tiers in Tier 1. It also looks at some confusion and ambiguity that arise when talking about tiers as attributes in a DNS context, e.g., for domains or zones, or as attributes in ENUM administration, such as Tier 1 entities (T1Es). Gallant Expires January 10, 2002 1 Numbering Tiers July 11, 2001 Table of Contents 1. Introduction...................................................2 2. Terminology and Assumptions....................................3 3. ENUM Mapping from E.164 Resources to DNS Domain Names..........3 4. Defined Levels of E.164 Resources..............................4 5. Recognition Principle..........................................5 6. Suggestions on Usage...........................................6 7. Flavors Beyond Vanilla.........................................9 8. Security Considerations........................................9 9. Acknowledgement...............................................10 10. References....................................................10 11. Author's Address..............................................10 1. Introduction A mapping between telephone numbers and domain names is defined for ENUM and is described in RFC 2916 [1]. At the heart of ENUM is the domain name representation of an E.164 number [2]. For example: - the E.164 number "+1 202 533 2812" is mapped to - the domain name "2.1.8.2.3.3.5.2.0.2.1.e164.arpa." In E.164 numbering, E.164 resources are administered at certain distinct levels, such as E.164 numbers, E.164 country codes, or the E.164 numbering plan itself. Informally, the terms Tier 2, Tier 1, and Tier 0, respectively, have been used to characterize entities, zones, functions, etc., at those levels. This practice was useful when discussing concepts. However, some significant problems arose from the informal usage of "tier" terms in ENUM. One problem is that "tier" terms are not well defined, and the resolution of technical, administrative, and coordination matters is impaired as a result. Another problem is the use of "tier" terms with other specific terms (such as a "Tier 1 zone" or a "Tier 2 entity") that appears to conflict with existing usage. Terms already defined and used for DNS have also been used beyond a DNS-specific context, especially when considering how ENUM would be implemented and administered to reflect how E.164 resources are administered. However useful or well-intentioned the broader use of terminology was, it increased the discussion of meta-issues such as what context a term is used in, or which organization is authoritative for a definition. This document seeks to remedy some of those problems. Gallant Expires January 10, 2002 2 Numbering Tiers July 11, 2001 This document contains a simple recognition principle for numbering tiers in ENUM, and it takes into account the mapping from E.164 resources to DNS domain names for ENUM. It also discusses how this approach could be useful in certain ENUM-related aspects of coordinating administrative functions for ENUM. 2. Terminology and Assumptions For this document, some conventions are based on those in RFC 2916, such as the use of "e164.arpa" for writing "ENUM names" for E.164 numbers. In addition, terminology about E.164 and about DNS is used in accordance with the respective context of each: "country code" is an E.164 term, "zone" is a DNS term, etc. The mapping from numbers to names is an abstract function from a "numbering space" to a name space. This mapping is one-to-one: two different digit strings map to two different names, never to the same name. Therefore, an attribute of a number may also, in theory, be used as an attribute of the name that the number maps to. There is a difference between a set and its contents. An E.164 country code is not the same as the set of all numbers that begin with that country code. Likewise, an attribute of a domain name is not the same as an attribute of members of a subset of that domain. This is important to remember when talking about tiers. It is important to note that no assumptions are made about whether or not an E.164 number is assigned, whether or not a domain or its related DNS Resource Records exist, or delegation. Note also that this document does not discuss "registry/registrar" terms. 3. ENUM Mapping from E.164 Resources to DNS Domain Names RFC 2916 describes a method for mapping an E.164 number to a fully qualified domain name (an "ENUM name"). That ENUM name (for the E.164 number) is used to identify DNS Resource Records associated with that domain name. For ENUM, the RRs include NAPTR records, each containing a service- specific URI associated with the given E.164 number. The following, based on RFC 2916, Section 3.2.1 Example 1, illustrates this: $ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa. @ IN NAPTR 100 10 "u" "sip+E2U" "!^.*$!sip:info@tele2.se!" . IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!mailto:info@tele2.se!" . The mapping used in ENUM can be applied to any single E.164 resource. In particular, any leading digit string (LDS) of an E.164 number can be mapped into a domain name. Note that this mapping is completely unrelated to whether or not the domain actually exists. Gallant Expires January 10, 2002 3 Numbering Tiers July 11, 2001 That domain contains the domain name that was mapped from the whole number. This is illustrated by the following example, based on the RFC 2916 Appendix A Scenario: - The E.164 number is: +46 8 976 1234. - A leading digit string of the number is: +46 8 976. - The string maps to: 6.7.9.8.6.4.e164.arpa. - The whole number maps to: 4.3.2.1.6.7.9.8.6.4.e164.arpa. It is clear that the mapped image of the number is a sub-domain of the mapped image of the leading string of the number, at least in theory. As mentioned above, the number-to-name mapping is an abstract function, and no assumptions are made about actual number assignments, existence of a domain name's RRs, or delegation. 4. Defined Levels of E.164 Resources Roughly speaking, there are three distinct levels of E.164 resources. The highest level is the E.164 numbering plan, as described in ITU-T Rec. E.164 and related recommendations. The next level is the Country Code level. Below that is the level of the E.164 number itself. Assignment of E.164 country codes, as well as management of the overall E.164 numbering plan, are international matters. The use of E.164 resources within a country code, and the assignment of E.164 numbers, are national matters. 4.1 Tiers in ENUM Informally, "tier" terms were used as attributes (e.g., "Tier 2 Entity") to describe the level of an E.164 numbering resource: - Tier 0 referred to the E.164 numbering plan; - Tier 1 referred to an E.164 country code; and - Tier 2 referred to an E.164 number. When some models for ENUM administration were described, "tier" terms were very useful for looking at different levels of functions, entities, and sets of domain names. For example, Tier 0 was used to discuss issues about the ENUM "root" (e.g., a convention from RFC 2916 is to write "e164.arpa"). Tier 1 has been used to refer to different options for structuring ENUM administration for national numbering resources (see Section 4.2 below). Then, Tier 2 was used to refer to "where the NAPTRs live," depending on how a particular model might be nationally implemented. It was convenient to assume that an actual delegated zone for a number would be created to hold its NAPTR RRs, but this does not have to be the case. Gallant Expires January 10, 2002 4 Numbering Tiers July 11, 2001 Note that "Tier 3" was sometimes used in an ENUM-related sense to refer to specific services (e.g., SIP, LDAP) whose URIs are in the "Tier 2" NAPTR records for an E.164 number. However, "Tier 3" is not formally a part of ENUM. Instead, client software for ENUM would end after receiving a definitive response to an ENUM query, which when successful provides all of the NAPTR records (from "Tier 2") for a number. After that, other client software would likely take over, based in part on analysis of the records returned. 4.2 Problems with Using "Tier" Terms One complication of the use of "tier" terms comes from the different varieties of national (or integrated) numbering plans. For example, some plans use an NDC (national destination code) field. The North American Numbering Plan (for E.164 Country Code 1) uses NPA (Numbering Plan Area) codes. A further complication is the possible options for structuring ENUM administration within a country code. The possibilities range from a single monolithic structure to a mix of multiple structures (see for example the RFC 2916 Appendix A Scenario, where a single national level lies above several intermediate levels, one of which lies above the E.164 number used in the example). 5. Recognition Principle One specific way to identify ENUM tiers as attributes is suggested by considering the level of the E.164 resource that is mapped to an ENUM domain name. The recognition principle extends this attribute to apply to any digit string that could be an E.164 number. Tier Recognition Principle: - Input: A (possibly empty) decimal digit string preceded by "+". - Case 0: If the digit string is empty or does not begin with a complete E.164 country code, then it is a Tier 0 string. - Case 1: If the digit string begins with a complete E.164 country code but is not a complete E.164 number, then it is a Tier 1 string. - Case 2: If the digit string is a complete E.164 number, then it is a Tier 2 string. Notes: 1. The input is to be interpreted as a (possibly empty) leading digit string of an E.164 number. The empty string is included because it maps to "e164.arpa" (as per the RFC 2916 convention). Gallant Expires January 10, 2002 5 Numbering Tiers July 11, 2001 2. Equivalently, this recognition principle could be written in terms of the domain names to which digit strings would be mapped. It is unambiguous because the mapping is one-to-one. 3. For the domain-name-based formulation, the input is equivalent to a (possibly empty) string of labels, where each label is a single decimal digit, the labels are separated by "."s, and the string ends with "e164.arpa.". Also recall that the "ENUM name" of a complete E.164 number would be a sub-domain of the "ENUM name" of any leading digit string of that number, as in the Section 3 example above. For digit strings, the Tier cases are illustrated by the following table, based on the RFC 2916 example E.164 number of +46 8 976 1234: Leading Digit String ENUM Tier Note ==================== ========= ===================== (+) Tier 0 (See Note 1 above.) (+)4 Tier 0 (+)46 Tier 1 A complete E.164 Country Code. (+)46 8 Tier 1 (+)46 8 9 Tier 1 (+)46 8 97 Tier 1 (+)46 8 976 Tier 1 (See Sections 6.2 and 7 below.) (+)46 8 976 1 Tier 1 (+)46 8 976 12 Tier 1 (+)46 8 976 123 Tier 1 (+)46 8 976 1234 Tier 2 A complete E.164 Number. The (+) is used here for digit strings in the E.164 numbering space. 6. Suggestions on Usage The proposed recognition principle for ENUM tiers provides an exact and unambiguous characterization of the specific ENUM tier of an E.164 resource. Then, "tier" terms may be used as attributes whenever an E.164 resource is mentioned. One benefit of the recognition principle is that a tier value can be assigned even if an E.164 resource is not a complete number or a complete country code. The tier value that this gets is consistent with that of the complete resource (e.g., country code) above it. 6.1 Tier 0 Tier 0 is the attribute for "above the country code level". For example, Tier 0 could refer to "e164.arpa" and related aspects involving the E.164 numbering plan as a whole. Gallant Expires January 10, 2002 6 Numbering Tiers July 11, 2001 Note that while the E.164 numbering plan deals with country codes, an E.164 country code is a Tier 1 resource. This is an example of the need to be specific about is being described by a tier value. 6.2 Tier 1 Tier 1 is the attribute for national numbering resources that E.164 country codes or that are within a country code but are not complete E.164 numbers. For example, in the RFC 2916 Appendix A Scenario, both the "regulator" (for +46) and "Telco-A" (for +46 8 976) would be "Tier 1 entities". Note again that while an E.l64 country code is a Tier 1 resource, and an E.164 number begins with a country code, a complete E.164 number is a Tier 2 resource. 6.3 Tier 2 Tier 2 is the attribute for an E.164 number. For example, in the RFC 2916 Appendix A Scenario, the "Redirection Service company" (for the complete number +46 8 976 1234) would be a "Tier 2 entity". 6.4 DNS Zones With the understanding that "zone" is a DNS-specific term, and as long as ENUM "tiers" are understand in terms of attributes of mapped E.164 resources, then "tier" terms could be used for zones in ENUM, but only if care is taken to avoid confusion and ambiguity. For example, in one hypothetical model for ENUM administration, in the spirit of the canonical example (in Appendix A of RFC 2916): - e164.arpa is the Tier 0 zone, - 6.4.e164.arpa is the Tier 1 zone for the country code +46, and - 4.3.2.1.6.7.9.8.6.4.e164.arpa is the Tier 2 zone for the number. This particular hypothetical model depends on the assumption that DNS delegations would be made to those zones. However, this is not a necessary assumption. Consider, for a different hypothetical example, that the NAPTR records associated with an E.164 number could in fact be part of the zone file for a country code's zone, without the need to create a zone specifically for that number. DNS queries would still work. Therefore, issues about zones for ENUM would have aspects of both numbering administration and ENUM administration, in addition to issues that would be specific to DNS administration. Gallant Expires January 10, 2002 7 Numbering Tiers July 11, 2001 Coordination of these multi-administration aspects is a necessary part of defining and managing the specific zone structure of a particular implementation of ENUM. A restatement of the previous example illustrates the need for clarity during coordination: - a complete E.164 number is a Tier 2 resource, - a NAPTR record for a complete number is a Tier 2 RR, but - the DNS zone containing such a NAPTR record could be a Tier 2 zone, a Tier 1 zone, or possibly even the Tier 0 zone. 6.5 Sets or Contents Discussing relationships between resources and possible zone delegations identifies special cases of a more general problem. Talking about a set is different than talking about its contents. An E.164 country code is not the same as the set of all numbers that begin with that country code. Likewise, an attribute of a domain name (such as its tier value) is not necessarily the same as an attribute of some or all of the members that domain. There are times, such as delegation, when this is an important distinction. In particular, experience suggests that great care be taken when discussing zones. The convention used in this document is that a zone should be identified by its name (i.e., origin). It is dangerous and confusing to refer to a zone by its contents. For example, "What kind of zone is a telephone number in?" is ambiguous at best. In a hypothetical model where a zone is delegated for a number, it is a Tier 2 zone, and its origin is the domain name that is the mapped image of the number. There would then be an appropriate SOA RR defining that Tier 2 zone. The NS records delegating "to the number" would live in Tier 1 (or above). Consider the simple hypothetical model where there is one Tier 1 zone for an E.164 country code, and where that zone contains the delegations to the Tier 2 zones for each number. Then, roughly speaking, there are Tier 2 RRs in Tier 2 zones, and there are Tier 2 RRs in the Tier 1 zone! That's why it is confusing, even with clear terminology, to talk about tiers and zones at the same time. Discussing tiers becomes even more problematic for zones that might contain RRs for a mix of leading digit strings and complete numbers. These topics are inherently complex, and more precise use of terms is needed to deal with these complexities. Gallant Expires January 10, 2002 8 Numbering Tiers July 11, 2001 7. Flavors Beyond Vanilla With suitable care, it is possible to discuss different flavors of attributes, especially when options for Tier 1 are discussed. Here, "options for Tier 1" refers to the many different ways that ENUM administration of Tier 1 resources could be structured. It is important to note that Tier 1 options are national matters. Very briefly, when describing certain sets of E.164 resources, the attribute "Tier 1" could be flavored by the following modifiers: - "upper" Tier 1: E.164 resources including the country code, - "lower" Tier 1: E.164 resources directly above numbers, - "middle" Tier 1: neither upper nor lower, and - "complete" Tier 1: both upper and lower. For example, in the RFC 2916 Appendix A Scenario, the "regulator" (for +46) would be an "upper Tier 1 entity", and "Telco-A" (for +46 8 976) would be a "lower Tier 1 entity". Again, "tier" terms are not to be considered as defining anything about DNS. Instead, "tier" terms describe aspects of how ENUM would be administered to take into account how numbering resource are administered, and in particular, how those various aspects are then handled when E.164 resources are mapped into DNS using ENUM. The development of criteria and procedures for managing the non-DNS aspects of ENUM administration is not discussed in this document. 8. Security Considerations RFC 2916 discussed security considerations related to ENUM. The discussion of terminology in this document indicates the importance of effective coordination of numbering resource administration, ENUM administration, and DNS administration functions in general. More specifically, looking at "tiers" has highlighted some of the complexity involved with the delegation of zones for ENUM. It would be desirable for ENUM-related delegations to be carefully planned well in advance of implementation. There is a range of possible options for structuring ENUM administration functions, and there are many different factors involved. Delegation of zones for ENUM is likely to be an area of some concern with respect to Tier 1 resource administration, which is a national matter, and effective use of terminology should help make addressing such matters more manageable. Gallant Expires January 10, 2002 9 Numbering Tiers July 11, 2001 9. Acknowledgement Inputs on some of the intricacies of DNS terminology and administration were provided by Patrik Faltstrom. 10. References 1 RFC 2916, "E.164 number and DNS." P. Faltstrom. September 2000. 2 ITU-T Recommendation E.164 (05/97), "The international public telecommunication numbering plan." 11. Author's Address Andrew Gallant NeuStar, Inc. 1120 Vermont Avenue NW, 4th Floor Washington, DC 20005 USA Tel: +1 202 533 2812 Fax: +1 202 533 2987 Email: andrew.gallant@neustar.com Gallant Expires January 10, 2002 10