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Telephone Number Mapping A. Brown
Internet Draft Nortel Networks
Document: <draft-ietf-enum-operation-02.txt> 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.
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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.
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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.
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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,
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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
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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
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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.
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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!" .
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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.
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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.
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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.
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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.
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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
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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
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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.
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