Telephone Number Mapping A. Brown Internet Draft Nortel Networks Document: Greg Vaudreuil Lucent Technologies February 23, 2001 ENUM Service Reference Model 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. Brown, Vaudreuil Expires August 2001 1 ENUM Reference Model February 23, 2001 Table of Contents 1. Abstract........................................................1 2. Introduction....................................................2 3. Scope...........................................................2 4. Overview........................................................4 4.1 Relationship with Dynamic Services.............................4 4.2 Number Portability.............................................5 5. The ENUM Service................................................5 5.1 First Tier: Determining the Service Registrar..................5 5.2 Second Tier: Retrieving Resource records.......................6 5.3 Third Tier: Service-Specific Queries...........................6 6. Interesting Numbering Topologies...............................8 6.1 Sub-addressing.................................................8 6.2 Default and Range-based Service Records........................9 6.3 Permissive dialing for dialing plan transitions...............9 7 Illustrative System Examples....................................10 7.1 Example: Hypothetical Reachme Service.........................10 7.2 Example: SIP Call Setup Service Request.......................11 8. Security Considerations........................................12 9. References.....................................................13 10. Acknowledgments...............................................13 11. Author's Addresses............................................13 12. Full Copyright Statement......................................14 Appendix:.........................................................15 Changes from draft-ietf-enum-operations-00.txt....................15 Changes from draft-ietf-enum-operations-01.txt....................15 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 RFC 2916. Please send comments on this document to the ENUM working group. 3. Scope This document defines the reference architecture behind the ENUM protocols 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, store and forward or real-time Internet Fax, VPIM voice messaging, Internet paging, geographic phone location, and many others. Each service is to be separately defined and identified using a unique, registered service identifier. Brown, Vaudreuil Expires August 2001 2 ENUM Reference Model February 23, 2001 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, fax, pager, or email messages are routed. Brown, Vaudreuil Expires August 2001 3 ENUM Reference Model February 23, 2001 4. Overview This telephone number-based directory system implements a three-tier information model; the first two constituting the ENUM service. This abstract model is based on analysis of pre-existing administrative structures, generalized service requirements, and the capabilities of candidate protocols. This model does not itself specify an administrative model, but provides a reference to guide implementers or conforming clients and servers. The mechanics of the ENUM service are specified in [ENUM]. The first tier is the mapping of the telephone number delegation tree to the service registrar. Conceptually, this delegated authority knows nothing about service-specific information associated with the telephone number but provides a reference to the service registrar that does know the specific information. 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 second tier is the set of service records themselves. The service records indicate which of several services may be available for a given telephone number. Multiple records indicating redundant or competitive service providers may be provided. The set of records may be provided or modified by any number of service providers. The ENUM service defines these records to be NAPTR records yielding a valid URL for a potentially useful service. It is up to the client initiating the service request to sort through the set of NAPTR records to determine which services are appropriate for the intended action. The service registrar is conceptually responsible for maintaining the set of service records for a given telephone number. 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. If necessary, an additional service-specific tier of information can be provided by the service provider itself. This tier 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 by the provider of that specific service. 4.1 Relationship with Dynamic Services 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, misrouting 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 and service record update intervals. Use of the ENUM system to implement time-of-day and other highly dynamic services is discouraged. Where such a service is desired, Brown, Vaudreuil Expires August 2001 4 ENUM Reference Model February 23, 2001 it is recommended that itself be implemented as part of a service indicated by the service records. 4.2 Number Portability The concept of number portability generally refers to the ability of a subscriber to change service providers, service types, or locations without changing their telephone number. For a full discussion of number portability, see [PORT]. In support of number portability, the ENUM service provides mechanism at the three conceptual tiers of the ENUM service. 1. If the telephone number has been re-delegated to another authority, and that authority also provides the Tier-1 ENUM service, the telephone number can be re-delegated in the ENUM service by changing the name server "NS" records to point to the new authority. This may be the case where numbers are re- delegated from the incumbent service provider to another or to a portability authority. The immediately higher delegated authority coordinates the transfer. 2. The service registrar may be reassigned. This may be the case where an individual or corporation changes telephony service providers and wishes that telephony service provider to also provide service registrar functions. The new service registrar would recreate the appropriate service specific NAPTR records and the delegated authority would coordinate the transfer from one registrar to the other. 3. If a specific service for a given telephone number was changed from one provider to another, such as switching telephone answering / voice messaging providers, the NATPR record indicating the specific service would change. The service registrar would coordinate the deletion of the record for the previous service provider and the insertion of a record for the new service provider. It is anticipated that in the early stages of an ENUM deployment, the delegated authority and the service registrar may be the same entity. 5. The ENUM Service 5.1 First Tier: Determining the Service Registrar The first tier is the mapping of an E.164-formatted international telecommunication number into the identity of the service registrar for that number. This may or may not involve more than one referral in DNS. The delegation of telephone numbers from the root authority (the ITU) down to individuals is a well-established system. While there are differing Tier-1 administrative models, to a client they each aim to represent in a single tree the trusted relationship between the delegated carriers and subsidiary registrars; a necessary precondition to ensure protection against various attacks. The delegated authority, sub-delegated authority, or individual may arrange to have a third-party (e.g., a service provider) list their Brown, Vaudreuil Expires August 2001 5 ENUM Reference Model February 23, 2001 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-tier 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 "additional address information " within numbering plans. 5.2 Second Tier: Retrieving Resource records. The second tier is the request for NAPTR RRs to discover the URL of the appropriate service-specific directory such as an LDAP directory server, H.323 gatekeeper, or specific endpoint addresses. The 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. 5.3 Third Tier: Service-Specific Queries An additional tier of query may be used to a service-specific directory for service-specific information. As indicated in the Brown, Vaudreuil Expires August 2001 6 ENUM Reference Model February 23, 2001 URI, such a query may include a SIP query to a designated gatekeeper or an LDAP query to a designated directory server. This tier 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. Brown, Vaudreuil Expires August 2001 7 ENUM Reference Model February 23, 2001 6. Interesting Numbering Topologies The following numbering uses require special consideration in the provision and use of ENUM services. 6.1 Sub-addressing The E.164 standard provides for sub-addressing through "additional information" within the 16 digits of an E.164 number. This information is passed through many telecommunications networks to be used by terminal equipment to select between alternate services or terminal devices. The sub-address digits are not processed by the switching system and are not used by intermediate processes to select services or route calls. In many cases, the network- numbering infrastructure may be unaware of the existence or use of sub-addressing by a given endpoint. Within ENUM, sub-addressing may be supported in two ways. The service registrar may explicitly provision NAPTR records for each sub-address, or the service registrar may provision default records for a range of sub- addresses. Using common DNS server implementations, the registrar may provision default records for a block of sub-addresses. A combination of explicit entries and default entries may be provided in common DNS server implementations using a longest-match algorithm. It is important to note that if a NAPTR or any other RR is provisioned for a sub-address, then all NAPTR records that are useful for that sub- address must also be provisioned. It is also important to note that numbers with optional sub- addresses may be queried without the sub-address component. For example, it may be useful to dial an address when placing a PSTN telephone call. The telephone number may terminate on an automated attendant application that can prompt for the appropriate internal extension. However, when placing a SIP call using IP telephony, the address plus the sub-address may be queried. The following set of records for company.com illustrate one configuration where a PSTN caller will be directed to the automated attendant application whether they dial the number or the number plus a sub-address, and whether the sub-address is explicitly provisioned or not. Calling using SIP to the explicitly provisioned sub-address will result in a direct call to the intended recipient. Example: 1.2.3.4.5.6.7.8.9.e164.arpa IN NAPTR 102 10 "u" "tel+E2U" "!^.*$!tel:+987654321!" . IN NAPTR 10 10 "u" "sip+E2U" "!+(.*)!sip:AA@company.com!" . *.1.2.3.4.5.6.7.8.9.e164.arpa IN NAPTR 102 10 "u" "tel+E2U" "!^.*$!tel:+987654321!" . IN NAPTR 10 10 "u" "sip+E2U" "!+(.*)!sip:AA@company.com!" . 1.0.1.1.2.3.4.5.6.7.8.9.e164.arpa IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:joe@company.com!" . IN NAPTR 102 10 "u" "tel+E2U" "!^.*$!tel:+987654321!" . Brown, Vaudreuil Expires August 2001 8 ENUM Reference Model February 23, 2001 6.2 Default and Range-based Service Records It is envisioned that a corporation or service provider not subject to number portability may wish to maintain a set of default NAPTR records for all E.164 telephone numbers within a delegation block. Similar to sub-addressing, a service registrar may provision a set of NAPTR records for a set of E.164 numbers with similar service requirements. Example: *.3.4.5.6.7.8.9.164.arpa IN NAPTR 102 10 "u" "tel+E2U" "!+(.*)!Tel:+\1" . IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:AA@company.com!" . IN NAPTR 10 10 "U" "mailto+E2U" \ "!+(.*)!mailto:+\1@company.com!" . 1.0.3.4.5.6.7.8.9.164.arpa IN NAPTR 102 10 "u" "tel+E2U" "!^.*$!tel:+987654310!" . IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:AA@company.com!" . 2.2.3.4.5.6.7.8.9.164.arpa IN NAPTR 102 10 "u" "tel+E2U" "!^.*$!tel:+987654322!" . IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:joe@company.com!" . IN NAPTR 10 10 "U" "mailto+E2U" \ "!^.*$!tel:+987654322@company.com!" . In this example, mail sent to the phone number +987654311 using 1.1.3.4.5.6.7.8.9.164.arpa will be sent to +987654311@company.com. Mail is explicitly not accepted at the automated attendant number as indicted by the lack of a mailto service record. Because extension 22 has an explicit NAPTR record for inbound calls via the tel record, it must also have an explicit mailto: URL in a NAPTR record. 6.3 Permissive dialing for dialing plan transitions In the real-world environment, the telephone number hierarchy is modified as necessary to prevent number exhaustion and to facilitate new services. These re-numberings either insert additional digits at arbitrary parts of the previous telephone number or result in the re-assignment of a sub-tree of numbers to a new prefix. To avoid the operational and social disruption involved with a _flash cut_, a practice of _permissive dialing_ has been created. Permissive dialing enables and end-user to use either the previous or new telephone number for a period of time. During this time, there may be two different telephone numbers pointing to the same set of service records, or a duplicate set of service records for the new and previous number. Brown, Vaudreuil Expires August 2001 9 ENUM Reference Model February 23, 2001 7 Illustrative System Examples 7.1 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. 2.1.2.1.5.5.5.3.1.6.1.e164.arpa Sample configuration file for the top tier delegations from ITU: 1.e164.arpa. IN NS ns.NANP.phone.net. ;for NANP 3.3.e164.arpa. IN NS ns.FR.phone.net. ; for France 2.7.9.e164.arpa. 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.arpa. IN NS ns.Zcorporation.com. ;for +1 613 555 XXXX Zcorporation is the designated service registrar for the block of 100 numbers +1 613 555 12XX. Zcorporation provides the following service specific record for all telephone numbers within it's 100 number block: *.2.1.5.5.5.3.1.6.1.e164.arpa. IN NAPTR 100 10 "u" "ldap+E2U"\ "$!ldap://ldap1.Zcorporation.com/cn=\1!" . Assuming the resolver is using non-extended DNS, the query using telephone number +1 613 555 1212 for the_reachme service is as follows: QueryType: NAPTR QueryName: _ 2.1.2.1.5.5.5.3.1.6.1.e164.arpa. Response: IN NAPTR 10 10 "u" "Reachme+E2U" \ "!LDAP:\\ldap1.zcorporation.com\cn=\1!" . The client can then apply the regular expression to yield an LDAP URI of LDAP:\\ldap1.zcorporation.com\cn=16135551212 and then use LDAP with the reachme schema to determine the set of communications technologies available for +1 613 555 1212. Brown, Vaudreuil Expires August 2001 10 ENUM Reference Model February 23, 2001 7.2 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 (Sipphonecall) 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 tier delegations from ITU: 1.e164.arpa. IN NS ns.NANP.phone.net. ;for NANP 3.3.e164.arpa. IN NS ns.FR.phone.net. ; for France 2.7.9.e164.arpa. 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.arpa. 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.arpa. IN NS ns.ServiceProviderB.net. ;for 972 555 1313 ServiceProviderB provides service registrar functions directly for the telephone number. The following configuration entry is provided for +1 972 555 1313. 3.1.3.1.5.5.2.7.9.1.ServiceProviderB.net. IN NAPTR 10 10 "u" "sip+E2U"\ "!^.*$!sip:19725551313@ServiceProviderB.net!" . The DNS Query and response using telephone number +1 972 555 1313: QueryType: NAPTR QueryName: 3.1.3.1.5.5.5.2.7.9.1.e164.arpa Result: IN NAPTR 10 10 "u" "sip+E2U" \ "!^.*$!sip:19725551313@ServiceProviderB.net!" . The client can now use the SIP protocols to contact the SIP gateway to initiate a phone call. Brown, Vaudreuil Expires August 2001 11 ENUM Reference Model February 23, 2001 8. Security Considerations The following are known security issues taken into consideration in the definition of this directory service. 1. Service provider customer information is very sensitive, especially in this time of local phone competition. Service providers require the maximum flexibility to protect this data. 2. Registration of a domain name for the telephone numbers delegated to another carrier may result in messages being misdirected to the wrong carrier. As subdelegations are implemented, the risk that phone numbers delegated to one enterprise may be incorrectly pointed at another will increase. 3. Service providers operate in a regulated environment where certain 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. Brown, Vaudreuil Expires August 2001 12 ENUM Reference Model February 23, 2001 9. 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. [REQ] Brown, Anne, "ENUM Requirements", work-in-progress, November 1999 [ENUM] Faltstrom, Patrick, "E.164 number and DNS", RFC 2916, September 2000. [NAPTR] M. Mealling, R. Daniel _The Naming Authority Pointer (NAPTR) DNS Resource Record_, RFC 2915, September 2000. [PORT] M. Foster, T. McGarry, j. Yu, _Number Portability in the GSTN: An Overview_, Work-in-Progress, July 2000. 10. Acknowledgments 11. 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 Brown, Vaudreuil Expires August 2001 13 ENUM Reference Model February 23, 2001 12. Full Copyright Statement "Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into Brown, Vaudreuil Expires August 2001 14 ENUM Reference Model February 23, 2001 Appendix: Changes from draft-ietf-enum-operations-00.txt o Discussion of interesting numbering topologies was added o Retrieval of NAPTR records are now described in a separate step from the determination of a service registrar. o A new example was created to illustrate ENUM using sub-addressing. Changes from draft-ietf-enum-operations-01.txt O Changed the title to more clearly reflect the intent of the document. o Collapsed Level 1 and 2 into a single tier. Aligned the document to use the _tier_ terminology. o Added missing text for dynamic updates. o Reworked the examples to eliminate all references to the controversial DNAME and CNAME redirection. o Generalized descriptions of the administrative model to avoid exclusion of any particular tier-1 approach. o Corrected various errors in the examples. Brown, Vaudreuil Expires August 2001 15