Telephone Number Mapping A. Brown Internet Draft Nortel Networks Document: Greg Vaudreuil Lucent Technologies April 27, 2000 ENUM Service Specific Provisioning: Principles of Operation Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. 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. 1. Abstract This document outlines the principles for the operation of a telephone number directory service. This service provides for the resolution of telephone numbers into Internet domain name addresses and service specific directory discovery. Telephone Number Based Directory Services April 2000 Table of Contents 1. Abstract........................................................1 2. Introduction....................................................2 3. Scope...........................................................2 4. Overview........................................................3 4.1 First Level: Determining the Delegated Authority...............3 4.2 Second Level: Determining the service registrar and retrieving Resource records...................................................5 4.3 Third Level: Service-Specific..................................5 5 Illustrative System Examples.....................................6 5.1 Example: Hypothetical Reachme Service..........................6 5.25.3 Example: SIP Call Setup Service Request.....................8 6. Security Considerations.........................................9 7. References.....................................................10 9. Acknowledgments................................................10 10. Author's Addresses............................................12 2. Introduction This document outlines the principles for the operation of a telephone number directory service. This service provides for the resolution of telephone numbers into the address of a service specific directory or where applicable for a given service, directly into a service-specific endpoint addresses. This directory service uses the algorithms and methods described in draft-faltstrom-e164-06.txt. Please send comments on this document to the ENUM working group. 3. Scope This document defines the architecture and mechanics necessary to implement a telephone number based Internet directory system. This solution enables an extensible set of services to be provided for a given telephone number. Example services may include IP telephony, Internet Fax, VPIM voice messaging, Internet paging, geographic phone location, and others. Each service is to be separately defined and identified using a unique, registered service identifier. This document does not specify the particulars of any telephone number based service. In particular, it does not describe how phone calls are placed, routed, or terminated or how voice messages are routed. Telephone Number Based Directory Services April 2000 4. Overview This phone number based directory system implements a three level information model, the first two constitute the ENUM service itself. This model is based on analysis of pre-existing administrative structures, generalized service requirements, and the capabilities of candidate protocols. The first level is the mapping of the telephone number delegation tree to the authority to which the number has been delegated. Conceptually this delegated authority knows nothing about service- specific information associated with the telephone number but can provide a reference to the appropriate entity that does know. The second level is the provision of the requested DNS resource records from a service registrar. Because there may be multiple service providers for a given telephone number, conceptually this registrar of services assumes a role of managing service registrations and arbitrating conflicts between service providers. Where this services registrar is different from the delegated authority, a query redirection from the delegated authority to the name server of the service registrar for a given telephone number is necessary The third level is the provision of service specific data from the service provider itself. Where necessary for a given service, this level provides specific attributes including any necessary attributes to place a call, route a message, validate capabilities, or other data necessary for that service that are known only to the provider of that specific service. 4.1 First Level: Determining the Delegated Authority The first level is the mapping of an E.164 formatted international telecommunication number into an Internet domain name identifier. This may or may not involve more than one referral in DNS. From the client's perspective, this level is transparent, bundled within the query for the service-specific resource records stored at the second level. The delegation of telephone numbers from the root authority (the ITU) down to individuals is a well-established system that can be utilized. These telephone number registrars have a trusted relationship with their delegated carriers or subsidiary registrars; a valuable asset to ensure protection against various attacks. Note that in this model, the delegation of telephone number blocks or individual numbers to a corporation or to an individual can be administratively and technically modeled as a sub-delegation to another carrier. With that additional information publicly registered, the mapping between telephone number and these domain names can be provided by any neutral entity. The delegated authority, sub-delegated authority, or individual may arrange to have a third-party (e.g., a service provider) list their Telephone Number Based Directory Services April 2000 information. In this case the service provider's domain would be returned in the ENUM query. The Internet Domain Name System provides an ideal technology for the first-level directory due to its hierarchical structure, fast connectionless queries, and distributed administrative model. Earlier experimentation with the TPC.INT remote printing experiment has shown how the hierarchical assignment of telephone numbers can be mapped directly to the hierarchy of domains within the DNS. The ENUM directory uses that approach to map any arbitrary telephone number into a single domain name. ITU standard E.164 defines the structure of the public telephone number as follows: country code, followed by nationally significant part, followed by sub-address. The country code may be from one to three digits, and the total length may be up to 15 digits. The nationally significant portion may be arbitrarily divided on any number boundary. In many countries numbering plans, the divisions are not uniform, that is, the "Area codes" or "City codes" may be of varying lengths within a single country and the total number of digits may be variable. Where supported by the relevant service, an optional sub-address of up to four digits may be utilized to designate an extension telephone number. Note that while sub- addressing is not well supported in GSTN calling, it is more widely supported for voice messaging. It is important to note that the national long-distance access or international dialing prefix sequence is not part of the canonical E.164 number. Within this delegation flexibility, it is always the case that the delegation of authority is always done left-to-right. With this assumption, a numbering tree can be built on a digit-by-digit basis that can represent any arbitrary hierarchical structure. DNS permits the delegation of authority on arbitrary boundaries such that a delegation to country code "1", "44", and "972" can all coexist under a single numbering plan root. The same applies for "service selectors", "area codes", "city codes", "Line number", or "sub-address extension" within numbering plans. The telephone number delegation information changes infrequently. However, when a change to this data is made, the information must be rapidly propagated through the directory system. Inconsistencies between the authoritative data and cached data may result in loss of service, mis-routing of communications, and/or service loops. An effective ENUM service requires that DNS time-to-live fields be set to an appropriate value consistent with the telephone number reassignment policies if the delegating authority. For example, ported telephone numbers may be reassigned from one carrier to another, a change that must be made quickly to avoid routing loops. Records supporting that service require a TTL of Zero and secondary DNS servers that use rapid replication techniques to ensure the data is consistent. The mechanics of the ENUM service are specified in [faltstrom] Telephone Number Based Directory Services April 2000 4.2 Second Level: Determining the service registrar and retrieving Resource records. The second level uses MX or SRV RRs to discover the identity of the appropriate service-specific directory such as an LDAP directory server, H.323 gatekeeper, or NAPTR RRs to discover the URL for other service-specific endpoint addresses. In this proposal, the second-level service registrar is responsible for ensuring that multiple services may be provided on behalf of a single telephone number, potentially by different service providers. This function includes an arbiter function to ensure that there is a deterministic instance of any given service assigned to a single telephone number. The service-specific directory locator function is a new service modeled upon existing telco-service provisioning models. Long-distance carrier selection within the United States is one well-known example of a service-specific registration requiring an arbiter function within the current network. The rate of change for the second level, service location function may range from static through dynamic under the control of the entity to which the phone number has been delegated. A registrar in the public network may wish to make this information relatively static to take advantage of DNS query caching, while a corporation may be willing to accept a higher query load to provide a dynamic service such as time-of-day inbound service routing. It is important that the costs of providing a dynamic service at this level are limited to the second-level service registrar provider and do not impose a burden on clients or the first level telephone number registrar. The DNAME and CNAME records provide the redirection from the designated authority to the service registrar. The DNAME provides a means for reforming and re-issuing a query for a "non-terminal" domain name. As is standard for compliant DNS resolver libraries, clients must support the CNAME record type. Servers that provide for substitution MAY support the DNAME record to provide redirection for an entire telephone number range as a DNS sub-tree. These servers MUST provide synthesized CNAME records for the proper operation of older resolver libraries that have not been extended to understand DNAME. Servers that redirect queries on a per-telephone number basis MUST support CNAMES. 4.3 Third Level: Service-Specific The third level is the query of the service-specific directory for service-specific information. This third level is specific to the service and is to be described in service specific documents. The service specific directory is expected to be dynamic. It is important that as little coordination as possible be required between the directories of innovative and potentially competing service-specific providers. Telephone Number Based Directory Services April 2000 5 Illustrative System Examples 5.1 Example: Hypothetical Reachme Service The following hypothetical service enables an end-user to discover the various means by which she can reach a recipient represented by their corporate telephone number +1 613-555-1212 using the hypothetical "reachme" service. This service is hosted by directly by the recipient's corporation. The telephone number is transformed into a domain name form to be used in a DNS query. 2.1.2.1.5.5.5.6.1.3.1.e164.foo Sample configuration file for the top level delegations from ITU: 1.e164.int. IN NS ns.NANP.phone.net. ;for NANP 3.3.e164.int. IN NS ns.FR.phone.net. ; for France 2.7.9.e164.int. IN NS ns.il.phone.net. ; for Israel Sample configuration file for numbers delegated from the NANP node in the DNS tree: 5.5.5.3.1.6.1.e164.int. IN NS ns.ServiceProviderA.net. ;for +1 613 555 XXXX In this example, ServiceProviderA.net is the authority to which the telephone number has been delegated. ServiceProviderA.net provides a non-terminal redirection pointer to Zcorporation, the designated service registrar for the block of 100 numbers +1 613 555 12XX. The configuration for this block of numbers is: 2.1.5.5.5.3.1.6.1.e164.int. DNAME 2.1.5.5.3.1.6.1.Zcorporation.com. Zcorporation provides the following service specific record for all telephone numbers within it's 100 number block: *.2.1.5.5.3.1.6.1.Zcorporation.com. SRV ldap1.Zcorporation.com. weight=10, preference =10, port = 389 Assuming the resolver is using non-extended DNS, the query using telephone number +1 613 555 1212 for the _reachme service is as follows: Query: _reachme._tcp.2.1.2.1.5.5.5.3.1.6.1.e164.foo. Response: Name=__reachme_tcp.2.1.2.1.5.5.5.3.1.6.1.e164.foo CNAME=2.1.2.1.5.5.5.3.1.6.1.Zcorporation.com SRV=ldap1.Zcorporation.com weight=10 preference=10 port=389 Telephone Number Based Directory Services April 2000 The client can then use LDAP with the reachme schema to determine the set of communications technologies available for +1 613 555 1212. Telephone Number Based Directory Services April 2000 5.25.3 Example: SIP Call Setup Service Request This example provides resolution of a telephone number to the identifier for the SIP gatekeeper designated to support real-time calling (Sip) to 972 555 1313. The telephone number is part of a block of ported telephone numbers that have been ported out of the donor carriers block to another carrier. The telephone number is transformed into a domain name form to be used in a DNS query. Sample configuration file for the top level delegations from ITU: 1.e164.int. IN NS ns.NANP.phone.net. ;for NANP 3.3.e164.int. IN NS ns.FR.phone.net. ; for France 2.7.9.e164.int. IN NS ns.il.phone.net. ; for Israel Sample DNS configuration file for the ported number block serviced by the 972 555 number portability authority delegated from the NANP node in the DNS tree: 5.5.5.2.7.9.1.e164.int. IN NS ns.972555Port.NANP.phone.net. ;for 972 555 The number portability authority manages the delegation on a per- telephone number basis. Logically, the ns.972555Port.NANP.phone.net has the following record for the telephone number. 3.1.3.1.5.5.5.2.7.9.1.e164.int. IN NS ns.ServiceProviderB.net. ;for 972 555 1313 ServiceProviderB records the designated service registrar for the telephone number as itself and hosts the service records directly without using a DNAME record. 3.1.3.1.5.5.3.1.6.1.ServiceProviderB.net. SRV sipgw1.ServiceProviderB.net. weight=10, preference =10, port = 389 ;for 972 555 1313 The DNS Query and response using telephone number +1 613 555 1313: Query: _sip._tcp.3.1.3.1.5.5.5.2.7.9.1.e164.int Result: name=__sip._tcp.3.1.3.1.5.5.5.2.7.9.1.e164.int SRV=sipgw1.ServiceProviderB.net weight=1 preference=10 port=100 The client can now use the SIP protocols to contact the SIP gateway to initiate a phone call. Telephone Number Based Directory Services April 2000 6. Security Considerations The following are known security issues taken into consideration in the definition of this directory service. 1. especially in this time of local phone competition. Service providers require the maximum flexibility to protect this data. 2. to another carrier may result in messages being misdirected to the wrong carrier. As sub-delegations are implemented, the risk that phone numbers delegated to one enterprise may be incorrectly pointed at another will increase. 3. information about subscribers must not be disclosed. Telephony services and Voice Messaging are subject to caller-ID blocking restrictions, restrictions normally enforced in the telephony network. No such protection is available on the Internet. The protection of this data is essential, but is up to the individual service providers to not disclose this information outside of their control. Telephone Number Based Directory Services April 2000 7. References [DNS1] Mockapetris, P., "Domain names - implementation and specification", RFC1035, Nov 1987. [DNS2] Mockapetris, P., "Domain names - concepts and facilities", RFC 1034, Nov 1987. [SRV] Arnt Gulbrandsen, Paul Vixie, Levon Esibov, "A DNS RR for specifying the location of services (DNS SRV)", Work in Progress [E164] ITU, "CCITT Recommendation E.164 (1991), Telephone Network and ISDN Operation, Numbering, Routing and Mobile Service - Numbering Plan for the ISDN Era." [TPC1] Malamud, Carl, Rose, Marshall, "Principles of Operation for the TPC.INT Subdomain: Remote Printing -- Technical Procedures", RFC 1530, October 1993. [VPIM2] Vaudreuil, Greg, Parsons, Glen, "Voice Profile for Internet Mail, Version 2", RFC 2421, September 1998. [SRV] Gulbrandsen, A., Vixie, P., "A DNS RR for specifying the location of services (DNS SRV)", RFC 2052, October 1996. [Brown] Brown, Anne, "ENUM Requirements", work-in-progress, November 1999 [Faltstrom] Faltstrom, Patrick, " E.164 number and DNS", work in progress, January 2000 [DNAME] 9. Acknowledgments This experimental directory builds upon the earlier work of: Carl Malamud and Marshall Rose in their TPC.INT remote printing experiment. Bernhard Elliot working with the TMIA has provided much of the organizational impetus to get this project moving, a substantial task given the sometimes slow and bureaucratic nature in the telephony business and regulatory environment. Dave Dudley and the Messaging Alliance (TMA) for their early work in pioneering a shared directory service for voice messaging and their continuing efforts to apply those learnings to this effort. Jeff Sherer and Mike Dimitroff of Lucent Technologies provided invaluable insight and review based on a prototype implementation of this service using actual data for the North American Numbering Plan numbering region. Telephone Number Based Directory Services April 2000 Telephone Number Based Directory Services April 2000 10. Author's Addresses Anne R. Brown Nortel Networks P.O. Box 3511, Station C Ottawa, ON K1Y 4H7 Canada Phone: +1-613-765-5274 Fax: +1-613-763-2697 Email: ARBrown@NortelNetworks.com Gregory M. Vaudreuil Lucent Technologies, Communications Application Group 17080 Dallas Parkway Dallas, TX 75248-1905 United States Phone/Fax: +1-972-733-2722 Email: GregV@IEEE.org Telephone Number Based Directory Services April 2000 11. Full Copyright Statement "Copyright (C) The Internet Society (2000). All Rights Reserved. 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