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rfc:rfc4769

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 (C) 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

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 contained in BCP 78, and except as set forth therein, the authors
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 Internet Society.

Livingood & Shockey Standards Track [Page 13]

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