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IETF RFC 4769
IANA Registration for an Enumservice Containing Public Switched Telephone Network (PSTN) Signaling Information
Last modified on Thursday, November 30th, 2006
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Network Working Group J. Livingood
Request for Comments: 4769 Comcast Cable Communications
Category: Standards Track R. Shockey
NeuStar
November 2006
IANA Registration for an Enumservice Containing
Public Switched Telephone Network (PSTN) Signaling Information
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The IETF Trust (2006).
Abstract
This document registers the Enumservice type "pstn" and subtype "tel"
using the URI scheme 'tel', as well as the subtype "sip" using the
URI scheme 'sip' as per the IANA registration process defined in the
ENUM specification, RFC 3761. This Enumservice is used to facilitate
the routing of telephone calls in those countries where number
portability exists.
Livingood & Shockey Standards Track PAGE 1 
RFC 4769 PSTN Enumservice November 2006
Table of Contents
1. Introduction ....................................................3
2. Distribution of Data ............................................4
3. ENUM Service Registration for PSTN ..............................5
3.1. ENUM Service Registration for PSTN with Subtype "tel" ......5
3.2. ENUM Service Registration for PSTN with Subtype "sip" ......5
4. Examples ........................................................6
4.1. Example of a Ported Number, Using a 'tel' URI Scheme .......6
4.2. Example of a Ported Number, Using a 'sip' URI Scheme .......6
4.3. Example of a Non-Ported Number, Using a 'tel' URI Scheme ...7
4.4. Example of a Non-Ported Number, Using a 'sip' URI Scheme ...7
4.5. Example Using a Regular Expression .........................7
5. Implementation Recommendations ..................................7
5.1. Call Processing When Multiple Records Are Returned .........7
5.2. NAPTR Configuration issues .................................8
6. Examples of E2U+pstn in Call Processing .........................8
6.1. Dialed Number Not Available On-Net .........................8
6.2. Dialed Number Available On-Net and on the PSTN .............9
7. Security Considerations .........................................9
8. IANA Considerations ............................................10
9. Acknowledgements ...............................................10
10. References ....................................................10
10.1. Normative References .....................................10
10.2. Informative References ...................................11
Livingood & Shockey Standards Track PAGE 2
RFC 4769 PSTN Enumservice November 2006
1. Introduction
ENUM (E.164 Number Mapping, RFC 3761 [1]) is a technology that
transforms E.164 numbers (The International Public Telecommunication
Numbering Plan, ITU-T Recommendation E.164 [2]) into domain names and
then uses DNS (Domain Name System, RFC 1034 [3]) delegation through
NS records and NAPTR records (Dynamic Delegation Discovery System
(DDDS) Part Three: The Domain Name System (DNS) Database, RFC 3403
[4]) to look up what services are available for a specific domain
name.
This document registers Enumservices according to the guidelines
given in RFC 3761 [1] to be used for provisioning in the services
field of a NAPTR [4] resource record to indicate the types of
functionality associated with an end point and/or telephone number.
The registration is defined within the DDDS (Dynamic Delegation
Discovery System [4][5][6][7][8]) hierarchy, for use with the "E2U"
DDDS Application defined in RFC 3761.
Number Portability allows telephone subscribers to keep their
telephone numbers when they change service providers, move to a new
location, or change the subscribed services [14]. In many countries,
such as the United States and Canada, the functions of naming and
addressing on the Public Switched Telephone Network (PSTN) have been
abstracted. In the case of a ported number, the dialed number is not
directly routable on the PSTN and must be translated into a routing
number for call completion. Other numbers, which are not ported, and
which can be routed directly on the PSTN based on the dialed number,
are typically assigned to carriers and other entities in large blocks
or pools. Number Portability and other numbering information are
distributed in a variety of methods and formats around the world.
The Enumservices described here could enable service providers to
place ported numbers, pooled numbers, and blocks of numbers and their
associated PSTN contact information, into externally available or
highly locally cached ENUM databases. This, in turn, could enable
such parties to consolidate all telephone number lookups in their
networks into a single ENUM lookup, thereby simplifying call routing
and network operations, which would then result in either an on-net
(IP-based) response or an off-net (PSTN-based) response.
The following Enumservice is registered with this document: "pstn" to
indicate PSTN routing data, including number portability data, non-
ported telephone number data (individually or in number blocks), and
other PSTN-oriented data that is associated with E.164 telephone
numbers. The purpose of this Enumservice is to provide routing
information for telephone numbers that do not designate an endpoint
resident on the public Internet or a private/peered Internet Protocol
Livingood & Shockey Standards Track PAGE 3
RFC 4769 PSTN Enumservice November 2006
(IP) network. Thus, these are numbers that are only routable via the
traditional PSTN, even if the call originates from an IP network.
The URIs returned in this service may use the TEL URI parameters
defined in RFC 4694 [10], and implementations must be prepared to
accept them.
The service parameters defined in RFC 3761 indicate that a "type" and
a "subtype" may be specified. Within this set of specifications, the
convention is assumed that the "type" (being the more generic term)
defines the service and the "subtype" defines the URI scheme.
When only one URI scheme is associated with a given service, it
should be assumed that an additional URI scheme to be used with this
service may be added at a later time. Thus, the subtype is needed to
identify the specific Enumservice intended.
2. Distribution of Data
The distribution of number portability data is often highly
restricted, either by contract or regulation of a National Regulatory
Authority (NRA); therefore, NAPTR records specified herein may or may
not be part of the e164.arpa DNS tree.
The authors believe that it is more likely that these records will be
distributed on a purely private basis. Distribution of this NAPTR
data could be either (a) on a private basis (within a service
provider's internal network, or on a private basis between one or
more parties using a variety of security mechanisms to prohibit
general public access), (b) openly available or, (c) distributed by
the relevant number portability organization or other industry
organization, but possibly on a national basis and subject to or in
accordance with national regulatory policy.
If such data were distributed nationally, the national telephone
numbering authority, or some other regulatory body or numbering
organization, may have jurisdiction. Such a body may choose to
restrict distribution of the data in such a way that it may not pass
over that country's national borders.
Livingood & Shockey Standards Track PAGE 4
RFC 4769 PSTN Enumservice November 2006
3. ENUM Service Registration for PSTN
3.1. ENUM Service Registration for PSTN with Subtype "tel"
Enumservice Name: "pstn"
Enumservice Type: "pstn"
Enumservice Subtype: "tel"
URI Scheme: 'tel:'
Functional Specification:
These Enumservices indicate that the remote resource identified can
be addressed by the associated URI scheme in order to initiate a
telecommunication session, which may include two-way voice or other
communications, to the PSTN. These URIs may contain number
portability data as specified in RFC 4694 [10].
Security Considerations: See Section 7.
Intended Usage: COMMON
Authors:
Jason Livingood (jason_livingood@cable.comcast.com)
Richard Shockey (richard.shockey@neustar.biz)
Any other information the author deems interesting:
A Number Portability Dip Indicator (npdi) should be used in practice
(see examples below in Section 4).
3.2. ENUM Service Registration for PSTN with Subtype "sip"
Enumservice Name: "pstn"
Enumservice Type: "pstn"
Enumservice Subtype: "sip"
URI Scheme: 'sip:'
Livingood & Shockey Standards Track PAGE 5
RFC 4769 PSTN Enumservice November 2006
Functional Specification:
These Enumservices indicate that the remote resource identified can
be addressed by the associated URI scheme in order to initiate a
telecommunication session, which may include two-way voice or other
communications, to the PSTN.
Security Considerations: See Section 7.
Intended Usage: COMMON
Authors:
Jason Livingood (jason_livingood@cable.comcast.com)
Richard Shockey (richard.shockey@neustar.biz)
Any other information the author deems interesting:
A Number Portability Dip Indicator (npdi) should be used in practice
(see examples below in Section 4).
4. Examples
The following sub-sections document several examples for illustrative
purposes. These examples shall in no way limit the various forms
that this Enumservice may take.
4.1. Example of a Ported Number, Using a 'tel' URI Scheme
$ORIGIN 3.2.1.0.5.5.5.5.1.2.1.e164.arpa.
NAPTR 10 100 "u" "E2U+pstn:tel"
"!^.*$!tel:+1-215-555-0123;npdi;rn=+1-215-555-0199!".
In this example, a Routing Number (rn) and a Number Portability Dip
Indicator (npdi) are used as shown in RFC 4694 [10]. The 'npdi'
field is included in order to prevent subsequent lookups in legacy-
style PSTN databases.
4.2. Example of a Ported Number, Using a 'sip' URI Scheme
$ORIGIN 3.2.1.0.5.5.5.5.1.2.1.e164.arpa.
NAPTR 10 100 "u" "E2U+pstn:sip"
"!^.*$!sip:+1-215-555-0123;npdi;rn=+1-215-555-0199
@gw.example.com;user=phone!".
In this example, a Routing Number (rn) and a Number Portability Dip
Indicator (npdi) are used as shown in RFC 4694 [10]. The 'npdi'
field is included in order to prevent subsequent lookups in legacy-
Livingood & Shockey Standards Track PAGE 6
RFC 4769 PSTN Enumservice November 2006
style PSTN databases. The method of conversion from a tel to a SIP
URI is as demonstrated in RFC 3261, Section 19.1.6 [11], as well as
in RFC 4694, Section 6 [10].
4.3. Example of a Non-Ported Number, Using a 'tel' URI Scheme
$ORIGIN 3.2.1.0.5.5.5.5.1.2.1.e164.arpa.
NAPTR 10 100 "u" "E2U+pstn:tel"
"!^.*$!tel:+1-215-555-0123;npdi!".
In this example, a Number Portability Dip Indicator (npdi) is used
[10]. The 'npdi' field is included in order to prevent subsequent
lookups in legacy-style PSTN databases.
4.4. Example of a Non-Ported Number, Using a 'sip' URI Scheme
$ORIGIN 3.2.1.0.5.5.5.5.1.2.1.e164.arpa.
NAPTR 10 100 "u" "E2U+pstn:sip"
"!^.*$!sip:+1-215-555-0123;npdi@gw.example.com;user=phone!".
In this example, a Number Portability Dip Indicator (npdi) is used
[10]. The 'npdi' field is included in order to prevent subsequent
lookups in legacy-style PSTN databases. The method of conversion
from a tel to a SIP URI is as demonstrated in RFC 3261, Section
19.1.6 [11], as well as in RFC 4694, Section 6 [10].
4.5. Example Using a Regular Expression
$ORIGIN 3.2.1.0.5.5.5.5.1.2.1.e164.arpa.
NAPTR 10 100 "u" "E2U+pstn:tel"
"!(^.*)$!tel:\1;npdi!".
In this example, a regular expression replacement function is used to
reduce the size of the NAPTR record. The tel URI uses "\1", which
would dynamically replace the expression with the TN plus the leading
"+" -- in this case, +1-215-555-0123.
5. Implementation Recommendations
5.1. Call Processing When Multiple Records Are Returned
It is likely that both E2U+sip and E2U+pstn Enumservice type records
will be returned for a given query. In this case, this could result
in what is essentially an on-net and off-net pstn record. Thus, one
record gives the associated address on an IP network, while the other
gives the associated address on the PSTN. As with multiple records
resulting from a typical ENUM query of the e164.arpa tree, it is up
to the application using an ENUM resolver to determine which
Livingood & Shockey Standards Track PAGE 7
RFC 4769 PSTN Enumservice November 2006
record(s) to use and which record(s) to ignore. Implementers should
take this into consideration and build logic into their applications
that can select appropriately from multiple records based on
business, network, or other rules. For example, such a resolver
could be configured to grant preference to the on-net record, or
execute other logic, as required by the application.
5.2. NAPTR Configuration issues
It has been suggested that tel URIs may be easier and more efficient
to use in practice than SIP URIs. In addition, the use of tel URIs
may result in somewhat smaller NAPTR records, which, when considering
adding hundreds of millions of these records to the DNS, could have a
substantial impact on the processing and storage requirements for
service providers or other entities making use of this Enumservice
type.
Implementers may wish to consider using regular expressions in order
to reduce the size of individual NAPTRs. This will have a
significant effect on the overall size of the database involved.
Using the example in Section 4.5, above, this is 11 bytes per record.
6. Examples of E2U+pstn in Call Processing
These are examples of how a switch, proxy, or other calling
application may make use of this Enumservice type during the call
initiation process.
6.1. Dialed Number Not Available On-Net
When the dialed number is not available on-net, the call processing
is as follows.
a) A user, which is connected to a calling application, dials an
E.164 telephone number: +1-215-555-0123.
b) The calling application uses the dialed number to form a NAPTR
record: 3.2.1.0.5.5.5.5.1.2.1.e164.arpa.
c) The DNS finds an E2U+pstn:tel record and returns a tel URI for
processing by the calling application: tel:+1-215-555-0123;npdi.
d) The calling application uses routing logic to determine which
media gateway is the closest to this number and routes the call
appropriately.
Livingood & Shockey Standards Track PAGE 8
RFC 4769 PSTN Enumservice November 2006
6.2. Dialed Number Available On-Net and on the PSTN
When the dialed number is available on-net and on the PSTN, the call
processing is as follows.
a) A user, which is connected to a calling application, dials an
E.164 telephone number: 1-215-555-0123.
b) The calling application uses the dialed number to form a NAPTR
record: 3.2.1.0.5.5.5.5.1.2.1.e164.arpa.
c) The DNS finds both an E2U+pstn record, as well as an E2U+sip
record, since this number happens to be on the IP network of a
connected network.
d) The calling application prioritizes the on-net record first:
sip:+1-215-555-0123;npdi@gw.example.com;user=phone.
e) The calling application sets up the SIP call to gw.example.com.
f) Should the IP call route fail for whatever reason, the calling
application may be able to utilize the E2U+pstn record to invoke a
fallback route to a media gateway that is connected to the PSTN.
7. Security Considerations
DNS, as used by ENUM, is a global, distributed database. Should
implementers of this specification use e164.arpa or any other
publicly available domain as the tree for maintaining PSTN
Enumservice data, this information would be visible to anyone
anonymously. While this is not qualitatively different from
publication in a telephone directory, it does open or ease access to
such data without any indication that such data has been accessed or
by whom it has been accessed.
Such data harvesting by third parties is often used to generate lists
of targets for unsolicited information. Thus, a third party could
use this to generate a list that they can use to make unsolicited
"telemarketing" phone calls. Many countries have do-not-call
registries or other legal or regulatory mechanisms in place to deal
with such abuses.
As noted earlier, carriers, service providers, and other users may
simply choose not to publish such information in the public e164.arpa
tree. They may instead simply publish this in their internal ENUM
routing database that is only able to be queried by trusted elements
Livingood & Shockey Standards Track PAGE 9
RFC 4769 PSTN Enumservice November 2006
of their network, such as softswitches and SIP proxy servers. They
may also choose to publish such information in a carrier-only branch
of the E164.ARPA tree, should one be created.
Although an E.164 telephone number does not appear to reveal as much
identity information about a user as a name in the format
sip:username@hostname or email:username@hostname, the information is
still publicly available; thus, there is still the risk of unwanted
communication.
An analysis of threats specific to the dependence of ENUM on the DNS
and the applicability of DNSSEC [12] to this is provided in RFC 3761
[1]. A thorough analysis of threats to the DNS itself is covered in
RFC 3833 [13].
8. IANA Considerations
This document registers the 'pstn' Enumservice type and the subtype
"tel" and "sip" under the Enumservice registry described in the IANA
considerations in RFC 3761. Details of this registration are
provided in Section 3 of this document.
9. Acknowledgements
The authors wish to thank Lawrence Conroy, Tom Creighton, Jason
Gaedtke, Jaime Jimenez, Chris Kennedy, Alexander Mayrhofer, Doug
Ranalli, Jonathan Rosenberg, Bob Walter, and James Yu for their
helpful discussions of this topic, and detailed reviews of this
document. The authors also wish to thank the IETF's ENUM Working
Group for helpful feedback in refining and developing this document.
10. References
10.1. Normative References
[1] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
[2] ITU-T, "The International Public Telecommunication Number Plan",
Recommendation E.164, February 2005.
[3] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[4] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403, October
2002.
Livingood & Shockey Standards Track PAGE 10
RFC 4769 PSTN Enumservice November 2006
[5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
[6] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Two: The Algorithm", RFC 3402, October 2002.
[7] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The Uniform Resource Identifiers (URI)", RFC 3404, October
2002.
[8] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Five: URI.ARPA Assignment Procedures", BCP 65, RFC 3405, October
2002.
[9] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004.
[10] Yu, J., "Number Portability Parameters for the "tel" Uniform
Resource Identifier", RFC 4694, October 2006.
[11] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
10.2. Informative References
[12] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions", RFC
4035, March 2005.
[13] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name
System (DNS)", RFC 3833, August 2004.
[14] Foster, M., McGarry, T., and J. Yu, "Number Portability in the
Global Switched Telephone Network (GSTN): An Overview", RFC
3482, February 2003.
Livingood & Shockey Standards Track PAGE 11
RFC 4769 PSTN Enumservice November 2006
Authors' Addresses
Jason Livingood
Comcast Cable Communications
1500 Market Street
Philadelphia, PA 19102
USA
Phone: +1-215-981-7813
EMail: jason_livingood@cable.comcast.com
Richard Shockey
NeuStar
46000 Center Oak Plaza
Sterling, VA 20166
USA
Phone: +1-571-434-5651
EMail: richard.shockey@neustar.biz
Livingood & Shockey Standards Track PAGE 12
RFC 4769 PSTN Enumservice November 2006
Full Copyright Statement
Copyright © The IETF Trust (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Livingood & Shockey Standards Track PAGE 13
IANA Registration for an Enumservice Containing Public Switched Telephone Network (PSTN) Signaling Information
RFC TOTAL SIZE: 24945 bytes
PUBLICATION DATE: Thursday, November 30th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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