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Network Working Group                                       J. LivingoodRequest for Comments: 4769                  Comcast Cable CommunicationsCategory: Standards Track                                     R. Shockey                                                                 NeuStar                                                           November 2006IANA Registration for an Enumservice ContainingPublic Switched Telephone Network (PSTN) Signaling InformationStatus 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 2006Table of Contents1. Introduction ....................................................32. Distribution of Data ............................................43. ENUM Service Registration for PSTN ..............................53.1. ENUM Service Registration for PSTN with Subtype "tel" ......53.2. ENUM Service Registration for PSTN with Subtype "sip" ......54. Examples ........................................................64.1. Example of a Ported Number, Using a 'tel' URI Scheme .......64.2. Example of a Ported Number, Using a 'sip' URI Scheme .......64.3. Example of a Non-Ported Number, Using a 'tel' URI Scheme ...74.4. Example of a Non-Ported Number, Using a 'sip' URI Scheme ...74.5. Example Using a Regular Expression .........................75. Implementation Recommendations ..................................75.1. Call Processing When Multiple Records Are Returned .........75.2. NAPTR Configuration issues .................................86. Examples of E2U+pstn in Call Processing .........................86.1. Dialed Number Not Available On-Net .........................86.2. Dialed Number Available On-Net and on the PSTN .............97. Security Considerations .........................................98. IANA Considerations ............................................109. Acknowledgements ...............................................1010. References ....................................................1010.1. Normative References .....................................1010.2. Informative References ...................................11Livingood & Shockey        Standards Track                      [Page 2]

RFC 4769                    PSTN Enumservice               November 20061.  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 inRFC 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 inRFC 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 ProtocolLivingood & 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 inRFC 4694 [10], and implementations must be prepared to   accept them.   The service parameters defined inRFC 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 20063.  ENUM Service Registration for PSTN3.1.  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 inRFC 4694 [10].   Security Considerations: SeeSection 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 inSection 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: SeeSection 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 inSection 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 inRFC 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 inRFC 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 inRFC 3261, Section 19.1.6 [11], as well as   inRFC 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 inRFC 3261, Section 19.1.6 [11], as well as inRFC 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 Recommendations5.1.  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 whichLivingood & 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 inSection 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 20066.2.  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 elementsLivingood & 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 inRFC 3761   [1].  A thorough analysis of threats to the DNS itself is covered inRFC 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 inRFC 3761.  Details of this registration are   provided inSection 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. References10.1.  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",RFC4035, 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",RFC3482, February 2003.Livingood & Shockey        Standards Track                     [Page 11]

RFC 4769                    PSTN Enumservice               November 2006Authors' 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.bizLivingood & Shockey        Standards Track                     [Page 12]

RFC 4769                    PSTN Enumservice               November 2006Full Copyright Statement   Copyright (C) The IETF Trust (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,   AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT   THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY   IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR   PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Livingood & Shockey        Standards Track                     [Page 13]