ENUM                                                        P. Faltstrom
Internet-Draft                                         Cisco Systems Inc
Obsoletes: 2916 (if approved)                                M. Mealling
Expires: July 23, 2003                                          VeriSign
                                                        January 22, 2003

                The E.164 to URI DDDS Application (ENUM)

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on July 23, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This document discusses the use of the Domain Name System (DNS) for
   storage of E.164 numbers.  More specifically, how DNS can be used for
   identifying available services connected to one E.164 number.  It
   specifically obsoletes RFC 2916 to bring it in line with the Dynamic
   Delegation Discovery System (DDDS) Application specification found in
   the document series specified in RFC 3401.  It is very important to
   note that it is impossible to read and understand this document
   without reading the documents discussed in RFC 3401.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2   Use for these mechanisms for private dialing plans . . . . .  3
   1.3   Application of local policy  . . . . . . . . . . . . . . . .  3
   2.    The ENUM Application Specifications  . . . . . . . . . . . .  4
   2.1   Application Unique String  . . . . . . . . . . . . . . . . .  4
   2.2   First Well Known Rule  . . . . . . . . . . . . . . . . . . .  4
   2.3   Expected Output  . . . . . . . . . . . . . . . . . . . . . .  4
   2.4   Valid Databases  . . . . . . . . . . . . . . . . . . . . . .  4
   2.4.1 Flags  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.4.2 Services Parameters  . . . . . . . . . . . . . . . . . . . .  6
   3.    Registration mechanism for Enumservices  . . . . . . . . . .  7
   3.1   Registration Requirements  . . . . . . . . . . . . . . . . .  7
   3.1.1 Functionality Requirement  . . . . . . . . . . . . . . . . .  7
   3.1.2 Naming requirement . . . . . . . . . . . . . . . . . . . . .  7
   3.1.3 Security requirement . . . . . . . . . . . . . . . . . . . .  7
   3.1.4 Publication Requirements . . . . . . . . . . . . . . . . . .  8
   3.2   Registration procedure . . . . . . . . . . . . . . . . . . .  8
   3.2.1 IANA Registration  . . . . . . . . . . . . . . . . . . . . .  8
   3.2.2 Registration Template  . . . . . . . . . . . . . . . . . . .  9
   4.    Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1   Example  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 11
   6.    Security Considerations  . . . . . . . . . . . . . . . . . . 12
   7.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 13
   8.    Changes since RFC 2916 . . . . . . . . . . . . . . . . . . . 14
         Normative References . . . . . . . . . . . . . . . . . . . . 15
         Non-normative references . . . . . . . . . . . . . . . . . . 16
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
         Intellectual Property and Copyright Statements . . . . . . . 17

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1. Introduction

   Through transformation of E.164 [4] numbers into DNS names and the
   use of existing DNS services like delegation through NS records and
   NAPTR records, one can look up what services are available for a
   specific domain name in a decentralized way with distributed
   management of the different levels in the lookup process.

   The domain "e164.arpa" is being populated in order to provide the
   infrastructure in DNS for storage of E.164 numbers.  In order to
   facilitate distributed operations, this domain is divided into
   subdomains.  Holders of E.164 numbers which want to be listed in DNS
   should contact the appropriate zone administrator in order to be
   listed, by examining the SOA resource record associated with the
   zone, just like in normal DNS operations.

   Of course, as with other domains, policies for such listings will be
   controlled on a subdomain basis and may differ in different parts of
   the world.

1.1 Terminology

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

   All capitalized terms are taken from the vocabulary found in the DDDS
   algorithm specification found in RFC 3403 [1].

1.2 Use for these mechanisms for private dialing plans

   This document specifies how "ENUM" works, that is how to handle
   numbers allocated according to the ITU-T standard E.164.  But, a
   similar mechanism can be used also for other numbers, such as private
   dialing plans.  To implement that (a) the suffix MUST be selected,
   MUST NOT be e164.arpa, MUST be known for all parties using the same
   dialing plan (b) the application unique string SHOULD be the full
   number as specified but without the leading '+'.

1.3 Application of local policy

   The preference field in the NAPTR is a request from the holder of the
   E.164 in what order the records are to be used.  It is to be noted
   that the party looking up the records MAY apply a local policy for in
   what order the records are to be used based on a combination of the
   service fields and URI schemes.  This overrides the MUST requirement
   in the DDDS algorithm.

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2. The ENUM Application Specifications

   This template defines the ENUM DDDS Application according to the
   rules and requirements found in [1].  The DDDS database used by this
   Application is found in [2] which is the document that defines the
   NAPTR DNS Resource Record type.

2.1 Application Unique String

   The Application Unique String is a fully qualified E.164 number minus
   any non-digit characters except for the '+' character which appears
   at the beginning of the number.  The "+" is kept to provide a well
   understood anchor for the AUS in order to distinguish it from other
   telephone numbers that are not part of the E.164 namespace.

   For example, the E.164 number could start out as "+1-770-923-9595".
   To ensure that no syntactic sugar is allowed into the AUS, all
   non-digits except for "+" are removed, yielding "+17709239595".

2.2 First Well Known Rule

   The First Well Known Rule for this Application is the identity rule.
   The output of this rule is the same as the input.  This is because
   the E.164 namespace and this Applications databases are organized in
   such a way that it is possible to go directly from the name to the
   smallest granularity of the namespace directly from the name itself.

   Take the previous example, the AUS is "+17709239595".  Applying the
   First Well Known Rule produces the exact same string, "+17709239595".

2.3 Expected Output

   The output of the last DDDS loop is a Uniform Resource Identifier in
   its absolute form according to the 'absoluteURI' production in the
   Collected ABNF found in RFC2396 [3].

2.4 Valid Databases

   At present only one DDDS Database is specified for this Application.
   "Dynamic Delegation Discovery System (DDDS) Part Three:  The DNS
   Database" (RFC 3403) [2] specifies a DDDS Database that uses the
   NAPTR DNS resource record to contain the rewrite rules.  The Keys for
   this database are encoded as domain-names.

   The output of the First Well Known Rule for the ENUM Application is
   the E.164 number minus all non-digit characters except for the +.  In
   order to convert this to a unique key in this Database the string is
   converted into a domain-name according to this algorithm:

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   1.  Remove all characters with the exception of the digits.  For
       example, the First Well Known Rule produced the Key
       "+4689761234".  This step would simply remove the leading "+",
       producing "4689761234".

   2.  Put dots (".") between each digit.  Example:

   3.  Reverse the order of the digits.  Example:

   4.  Append the string ".e164.arpa" to the end.  Example:

   This domain-name is used to request NAPTR records which may contain
   the end result or, if the flags field is blank, produces new keys in
   the form of domain-names from the DNS.

   DNS servers MAY interpret Flag values and use that information to
   include appropriate resource records in the Additional Information
   portion of the DNS packet.  Clients are encouraged to check for
   additional information but are not required to do so.  See the
   Additional Information Processing section of RFC 3404 for more
   information on NAPTR records and the Additional Information section
   of a DNS response packet.

   The character set used to encode the substitution expression is
   UTF-8.  The allowed input characters are all those characters that
   are allowed anywhere in an E.164 number.  The characters allowed to
   be in a Key are those that are currently defined for DNS

2.4.1 Flags

   This Database contains a field that contains flags that signal when
   the DDDS algorithm has finished.  At this time only one flag, "U", is
   defined.  This means that this Rule is the last one and that the
   output of the Rule is a URI [3].  See RFC 3404 [2].

   If a client encounters a record with an unknown flag, it MUST ignore
   it and move to the next Rule.  This test takes precedence over any
   ordering since flags can control the interpretation placed on fields.
   A novel flag might change the interpretation of the regexp and/or
   replacement fields such that it is impossible to determine if a
   record matched a given target.

   If this flag is not present then this rule is non-terminal.  If a
   Rule is non-terminal then clients MUST use the Key produced by this
   Rewrite Rule as the new Key in the DDDS loop (i.e.  causing the
   client to query for new NAPTR records at the domain-name that is the

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   result of this Rule).

2.4.2 Services Parameters

   Service Parameters for this Application take the following form and
   are found in the Service field of the NAPTR record.

                    service_field = "E2U" 1*(servicespec)
                    servicespec   = "+" enumservice
                    enumservice   = type 0*(subtypespec)
                    subtypespec   = ":" subtype
                    type          = 1*32(ALPHA / DIGIT)
                    subtype       = 1*32(ALPHA / DIGIT)

   In other words, a non-optional "E2U" (used to denote ENUM only
   Rewrite Rules in order to mitigate record collisions) followed by 1
   or more or more Enumservices which indicate what class of
   functionality a given end point offers.  Each Enumservice is
   indicated by an initial '+' character. ENUM Services

   Enumservice specifications contain the functional specification (i.e.
   what it can be used for), the valid protocols, and the URI schemes
   that may be returned.  Note that there is no implicit mapping between
   the textual string "type" or "subtype" in the grammar for the
   Enumservice and URI schemes or protocols.  The mapping, if any, have
   to be made explicit in the specification for the Enumservice itself.
   A registration of a specific Type also have to specify the Subtypes

   The only exception to the registration rule is for Types and Subtypes
   used for experimental purposes, and those are to start with the facet
   "X-".  These elements are unregistered, experimental, and should be
   used only with the active agreement of the parties exchanging them.

   The registration mechanism is specified in Section 3.

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3. Registration mechanism for Enumservices

   Registration of Enumservices requires approval by the IESG and
   publication of the Enumservice registration as an RFC on Standards
   Track or BCP.

3.1 Registration Requirements

   Service registration proposals are all expected to conform to various
   requirements laid out in the following sections.

3.1.1 Functionality Requirement

   An Enumservice registered must be able to function as a selection
   mechanism when choosing one NAPTR resource record from another.  That
   means that the registration MUST specify what is expected when using
   that very NAPTR record, and the URI which is the outcome of the use
   of it.

   Specifically, a registered Enumservice MUST specify the URI scheme(s)
   that may be used for the Enumservice, and, when needed, other
   information which will have to be transfered into the URI resolution
   process itself (LDAP DNs, transferring of the AUS into the resulting
   URI, etc).

3.1.2 Naming requirement

   The Enumservice MUST be unique, conform to the ABNF specified in
   Section 2.4.2, and MUST NOT start with the facet "X-" which is
   reserved for experimental, private use.

   The type MUST be unique, conform to the ABNF specified in Section
   2.4.2, and MUST NOT start with the facet "X-" which is reserved for
   experimental, private use.

   The subtype MUST be named so that the Enumservice is unique, conform
   to the ABNF specified in Section 2.4.2, and MUST NOT start with the
   facet "X-" which is reserved for experimental, private use.  The
   subtype for one type MAY be the same as a subtype for a different
   registered type.

3.1.3 Security requirement

   An analysis of security issues is required for all Enumservices
   registered.  (This is in accordance with the basic requirements for
   all IETF protocols.)

   All descriptions of security issues must be as accurate as possible

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   regardless of registration tree.  In particular, a statement that
   there are "no security issues associated with this Enumservice" must
   not be confused with "the security issues associates with this
   Enumservice have not been assessed".

   There is no requirement that Enumservices registered must be secure
   or completely free from risks.  Nevertheless, all known security
   risks must be identified in the registration of a Enumservice.

   The security considerations section of all registrations is subject
   to continuing evaluation and modification.

   Some of the issues that should be looked at in a security analysis of
   a Enumservice are:

   1.  Complex Enumservices may include provisions for directives that
       institute actions on a user's resources.  In many cases provision
       can be made to specify arbitrary actions in an unrestricted
       fashion which may then have devastating results.  Especially if
       there is a risk for a new ENUM lookup, and because of that an
       infinite loop in the overall resolution process of the E.164.

   2.  Complex Enumservices may include provisions for directives that
       institute actions which, while not directly harmful, may result
       in disclosure of information that either facilitates a subsequent
       attack or else violates the users privacy in some way.

   3.  An Enumservice might be targeted for applications that require
       some sort of security assurance but do not provide the necessary
       security mechanisms themselves.  For example, a Enumservice could
       be defined for storage of confidential medical information which
       in turn requires an external confidentiality service.

3.1.4 Publication Requirements

   Proposals for Enumservices registered must be published as RFCs on
   Standards track or as BCP.  IANA will retain copies of all
   Enumservice registration proposals and "publish" them as part of the
   ENUM Enumservice Registration tree itself.

3.2 Registration procedure

3.2.1 IANA Registration

   Provided that the Enumservice has obtained approval that is
   necessary, and the RFC is published, IANA will register the
   Enumservice and make the Enumservice registration available to the

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   community in addition to the RFC publication itself. Location of ENUM Enumservice Registrations

   Enumservice registrations will be published in the IANA repository
   and made available via anonymous FTP at the following URI: "ftp://
   ftp.isi.edu/in-notes/iana/assignments/enum-services/". Change Control

   Change control of Enumservices stay with the IETF via the RFC
   publication process.  Especially, Enumservice registrations may not
   be deleted; Enumservices which are no longer believed appropriate for
   use can be declared OBSOLETE by publication of a new RFC and a change
   to their "intended use" field; such Enumservice will be clearly
   marked in the lists published by IANA.

3.2.2 Registration Template

   Enumservice Name:



   URI Scheme(s):

   Functional Specification:

   Security considerations:

   Intended usage: (One of COMMON, LIMITED USE or OBSOLETE)


   Any other information that the author deems interesting:

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4. Examples

   The examples below use theoretical services which uses Enumservices
   which might not make sense, but they are still used for educational
   purposes.  For example, the protocol used is in some cases exactly
   the the same string as the URI scheme.  That was the specification in
   RFC 2916, but this default specification of a Enumservice is no
   longer allowed.  All Enumservices need to be registered explicitly by
   the procedure specified in section Section 3N.

4.1 Example

     IN NAPTR 10 100 "u" "E2U+sip"        "!^.*$!sip:info@example.com!"     .
     IN NAPTR 10 101 "u" "E2U+h323:voice" "!^.*$!h323:info@example.com!"    .
     IN NAPTR 10 102 "u" "E2U+msg:mailto" "!^.*$!mailto:info@example.com!"  .

   This describes that the domain is
   preferably contacted by SIP, secondly via H.323 for voice, and
   thirdly by SMTP for messaging.  Note that the tokens "sip", "msg",
   "h323", "voice" and "mailto" are Types and Subtypes registered with
   IANA, and they have no implicit connection with the protocols or URI
   schemes with the same names.

   In all cases, the next step in the resolution process is to use the
   resolution mechanism for each of the protocols, (specified by the URI
   schemes sip, h323 and mailto) to know what node to contact for each.

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5. IANA Considerations

   This memo requests that the IANA delegate the E164.ARPA domain
   following instructions to be provided by the IAB.  Names within this
   zone are to be delegated to parties according to the ITU
   recommendation E.164.  The names allocated should be hierarchic in
   accordance with ITU Recommendation E.164, and the codes should
   assigned in accordance with that Recommendation.

   IAB is to coordinate with ITU-T TSB if the technical contact for the
   domain e164.arpa is to change, as ITU-T TSB has an operational
   working relationship with this technical contact which needs to be

   Delegations in the zone e164.arpa (not delegations in delegated
   domains of e164.arpa) should be done after Expert Review, and the
   IESG will appoint a designated expert.

   IANA is to create a registry for ENUM Enumservices as specified in
   Section 3.  Whenever a new ENUM Enumservice is registered by the RFC
   process in the IETF, IANA is at the time of publication of the RFC to
   register the Enumservice and add a pointer to the RFC itself.

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6. Security Considerations

   As this system is built on top of DNS, one can not be sure that the
   information one gets back from DNS is more secure than any DNS query.
   To solve that, the use of DNSSEC [7] for securing and verifying zones
   is to be recommended.

   The caching in DNS can make the propagation time for a change take
   the same amount of time as the time to live for the NAPTR records in
   the zone that is changed.  The use of this in an environment where
   IP-addresses are for hire (for example, when using DHCP [8]) must
   therefore be done very carefully.

   There are a number of countries (and other numbering environments) in
   which there are multiple providers of call routing and number/name-
   translation services.  In these areas, any system that permits users,
   or putative agents for users, to change routing or supplier
   information may provide incentives for changes that are actually
   unauthorized (and, in some cases, for denial of legitimate change
   requests).  Such environments should be designed with adequate
   mechanisms for identification and authentication of those requesting
   changes and for authorization of those changes.

   A large amount of Security Issues have to do with the resolution
   process itself, and use of the URIs produced by the DDDS mechanism.
   Those have to be specified in the registration of the ENUM
   Enumservice used, as specified in Section 3.1.3.

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7. Acknowledgments

   Support and ideas leading to RFC 2916 have come from people at
   Ericsson, Bjorn Larsson and the group which implemented this scheme
   in their lab to see that it worked.  Input has also arrived from
   ITU-T SG2, Working Party 1/2 (Numbering, Routing, Global Mobility and
   Enumservice Definition), the ENUM working group in the IETF, John
   Klensin and Leif Sunnegardh.

   This update of RFC 2916 is created with specific input from: Randy
   Bush, David Conrad, Richard Hill, John Petersen, Jim Reid, Joakim
   Stralmark, Robert Walter and James Yu.

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8. Changes since RFC 2916

   Part from clarifications in the text in this document, the major
   changes are two:

   The document uses an explicit DDDS algorithm, and not only NAPTR
   resource records in an "ad-hoc" mode.  In reality this doesn't imply
   any changes in deployed base of applications, as the algorithm used
   for ENUM resolution is exactly the same.

   The format of the service field has changed.  The old format was of
   the form "example+E2U", while the new format is "E2U+example".
   Reason for this change have to with the added subtypes in the
   enumservice, the ability to support more than one enumservice per
   NAPTR RR, and a general agreement in the IETF that the main selector
   between different NAPTR with the same owner (E2U in this case) should
   be first.

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Normative References

   [1]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Three: The DNS Database", RFC 3403, February 2002.

   [2]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Four: The URI Resolution Application", RFC 3404, February 2002.

   [3]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

   [4]  ITU-T, "The International Public Telecommunication Number Plan",
        Recommendation E.164, May 1997.

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Non-normative references

   [5]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        One: The Comprehensive DDDS Standard", RFC 3401, February 2002.

   [6]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Two: The Algorithm", RFC 3402, February 2002.

   [7]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [8]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

Authors' Addresses

   Patrik Faltstrom
   Cisco Systems Inc
   273 71 Lovestad

   EMail: paf@cisco.com
   URI:   http://www.cisco.com

   Michael Mealling
   21345 Ridgetop Circle
   Sterling, VA  20166

   EMail: michael@neonym.net
   URI:   http://www.verisignlabs.com

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