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The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application for Infrastructure ENUM
RFC 5526

Document Type RFC - Informational (April 2009)
Authors Richard Stastny , Penn Pfautz , Jason Livingood
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD Jon Peterson
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RFC 5526
Network Working Group                                       J. Livingood
Request for Comments: 5526                  Comcast Cable Communications
Category: Informational                                        P. Pfautz
                                                              R. Stastny
                                                              April 2009

            The E.164 to Uniform Resource Identifiers (URI)
        Dynamic Delegation Discovery System (DDDS) Application
                        for Infrastructure ENUM

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

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   document authors.  All rights reserved.

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RFC 5526                  Infrastructure ENUM                 April 2009


   This document defines the use case for Infrastructure ENUM and
   proposes its implementation as a parallel namespace to "",
   as defined in RFC 3761, as the long-term solution to the problem of
   allowing carriers to provision DNS records for telephone numbers
   independently of those provisioned by end users (number assignees).

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Zone Apex for Infrastructure ENUM ...............................3
   4. IANA Considerations .............................................3
   5. Security and Privacy Considerations .............................4
   6. Acknowledgements ................................................4
   7. Normative References ............................................4

1.  Introduction

   ENUM (E.164 Number Mapping [1]) is a system that transforms E.164
   numbers [2] into domain names and then uses the DNS (Domain Name
   Service) [3] to discover NAPTR records that specify what services are
   available for a specific domain name.

   ENUM as originally defined was based on the end-user opt-in
   principle.  While this has great potential to foster new services and
   end-user choices in the long term, the current requirements for
   IP-based interconnection of Voice over IP (VoIP) domains require the
   provisioning of large numbers of allocated or served (hosted) numbers
   of a participating service provider, without the need for individual
   users to opt-in.  This way, service providers can provision their own
   ENUM information that is separate, distinct, and likely to be
   different from what an end-user may provision.  This is particularly
   important if Infrastructure ENUM is used for number-portability
   applications, for example, which an end-user would be unlikely
   interested in provisioning but which a service provider would likely
   find essential.

   In addition, while it is possible that service providers could
   mandate that their users opt-into through end-user contract
   terms and conditions, there are substantial downsides to such an
   approach.  Thus, for all these reasons and many others, ENUM for
   end-user provisioning is ill-suited for use by service providers for
   the interconnection of VoIP domains.

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RFC 5526                  Infrastructure ENUM                 April 2009

   As VoIP evolves and becomes pervasive, E.164-addressed telephone
   calls need not necessarily traverse the Public Switched Telephone
   Network (PSTN).  Therefore, VoIP service providers have an interest
   in using ENUM on a so-called "Infrastructure" basis in order to keep
   VoIP traffic on IP networks on an end-to-end basis, both within and
   between service provider domains.  This requires a means of
   identifying a VoIP point of interconnection to which calls addressed
   to a given E.164 number may be delivered; Infrastructure ENUM
   provides this means.  Calls that can originate and terminate on IP
   networks, and do not have to traverse the PSTN, will require fewer or
   no points of transcoding, and can also involve additional IP network
   services that are not possible on the PSTN, among other benefits.

   Requirements for Infrastructure ENUM are provided in [4].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119 [5].

3.  Zone Apex for Infrastructure ENUM

   This document proposes that Infrastructure ENUM be implemented by
   means of a parallel namespace to dedicated to
   Infrastructure ENUM, in a domain that is yet to be determined.  Use
   of a parallel namespace allows carriers and end-users to control
   their ENUM registrations independently, without forcing one to work
   through the other.

   Infrastructure ENUM Tier 2 resource records in the Infrastructure
   ENUM tree will be controlled by the service provider that is
   providing services to a given E.164 number, generally referred to in
   various countries as the "carrier-of-record" (see [4]).  The
   definition of a carrier-of-record for a given E.164 number is a
   national matter or is defined by the entity controlling the numbering

   See also Section 3, "Requirements for Infrastructure ENUM", of [4].

4.  IANA Considerations

   IANA has created a registry for Enumservices as originally specified
   in RFC 2916 and revised in RFC 3761.  Enumservices registered with
   IANA are valid for Infrastructure ENUM as well as end-user ENUM.

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RFC 5526                  Infrastructure ENUM                 April 2009

5.  Security and Privacy Considerations

   This document proposes a new zone apex for ENUM to meet the
   requirements of Infrastructure ENUM.  The over-the-network protocol
   of ENUM is unchanged by the addition of an apex and, as such, the
   Security Considerations of RFC 3761 [1] still apply.  Specific
   considerations related to the security of an Infrastructure ENUM apex
   are given in more detail in Section 4, "Security Considerations", of

   Infrastructure ENUM registrations proposed by this document should
   resolve to service provider points-of-interconnection rather than to
   end-user equipment.  Service providers need to take appropriate
   measures to protect their end-user customers from unwanted
   communications as with other types of interconnections.

6.  Acknowledgements

   The authors wish to thank Lawrence Conroy, Patrik Faltstrom, Michael
   Haberler, Otmar Lendl, Steve Lind, Alexander Mayrhofer, Jim Reid, and
   Richard Shockey for their helpful discussions of this document and
   the concept of Infrastructure ENUM.

7.  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] Lind, S. and P. Pfautz, "Infrastructure ENUM Requirements", RFC
       5067, November 2007.

   [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

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RFC 5526                  Infrastructure ENUM                 April 2009

Authors' Addresses

   Jason Livingood
   Comcast Cable Communications
   1500 Market Street
   Philadelphia, PA 19102

   Phone: +1-215-981-7813

   Penn Pfautz
   200 S. Laurel Ave
   Middletown, NJ  07748

   Phone: +1-732-420-4962

   Richard Stastny
   Anzbachgasse 43
   1140 Vienna

   Phone: +43-664-420-4100

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