Network Working Group                                        P Faltstrom
Internet-Draft                                         Cisco Systems Inc
Expires: January 31, 2001                                 August 2, 2000

                          E.164 number and DNS

Status of this Memo

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

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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on January 31, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.


   This document discusses the use of DNS for storage of E.164 numbers.
   More specifically, how DNS can be used for identifying available
   services connected to one E.164 number. Routing of the actual
   connection using the service selected using these methods is not

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1. Introduction

   Through transformation of E.164 numbers into DNS names and the use
   of existing DNS services like delegation through NS records, and use
   of NAPTR[1] records in DNS[2][3], one can look up what services are
   available for a specific domainname in a decentralized way with
   distributed management of the different levels in the lookup

1.1 Terminology

   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
   in this document are to be interpreted as described in RFC2119[4]

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2. E.164 numbers and DNS

   The domain "" 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.

   To find the DNS names for a specific E.164 number, the following
   procedure is to be followed:

   1.  See that the E.164 number is written in its full form, including
       the countrycode IDDD. Example: +46-8-9761234

   2.  Remove all non-digit characters with the exception of the
       leading '+'. Example: +4689761234

   3.  Remove all characters with the exception of the digits. Example:

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

   5.  Reverse the order of the digits. Example:

   6.  Append the string "" to the end. Example:

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3. Fetching URIs given an E.164 number

   For a record in DNS, the NAPTR record is used for identifying
   available ways of contacting a specific node identified by that
   name. Specifically it can be used for knowing what services exists
   for a specific domainname, including phone numbers by the use of the domain as described above.

   The identification is using the NAPTR resource record defined for
   use in the URN resolution process, but it can be generalized in a
   way that suits the needs specified in this document.

   It is the string which is the result of step 2 in section 2 above
   which is input to the NAPTR algorithm.

3.1 The NAPTR record

   The key fields in the NAPTR RR are order, preference, service,
   flags, regexp, and replacement. For a detailed description, see:

   o  The order field specifies the order in which records MUST be
      processed when multiple NAPTR records are returned in response to
      a single query.

   o  The preference field specifies the order in which records SHOULD
      be processed when multiple NAPTR records have the same value of

   o  The service field specifies the resolution protocol and
      resolution service(s) that will be available if the rewrite
      specified by the regexp or replacement fields is applied.

   o  The flags field contains modifiers that affect what happens in
      the next DNS lookup, typically for optimizing the process.

   o  The regexp field is one of two fields used for the rewrite rules,
      and is the core concept of the NAPTR record.

   o  The replacement field is the other field that may be used for the
      rewrite rule.

   Note that the client applies all the substitutions and performs all
   lookups, they are not performed in the DNS servers. Note that URIs
   are stored in the regexp field.

3.1.1 Specification for use of NAPTR Resource Records

   The input is an E.164 encoded telephone number. The output is a
   Uniform Resource Identifier in its absolute form according to the

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   'absoluteURI' production in the Collected ABNF found in RFC2396[5]

   An E.164 number, without any characters but leading '+' and digits,
   (result of step 2 in section 2 above) is the input to the NAPTR

   The service supported for a call is E2U.

3.1.2 Specification of Service E2U (E.164 to URI)

   * Name: E.164 to URI
   * Mnemonic: E2U
   * Number of Operands: 1
   * Type of Each Operand: First operand is an E.164 number.
   * Format of Each Operand: First operand is the E.164 number in the
     form as specified in step 2 in section 2 in this document.
   * Algorithm: Opaque
   * Output: One or more URLs
   * Error Conditions:
      o E.164 number not in the numbering plan
      o E.164 number in the numbering plan, but no URLs exist for that number
      o Service unavailable

   * Security Considerations:
      o Malicious Redirection
        One of the fundamental dangers related to any service such
        as this is that a malicious entry in a resolver's database
        will cause clients to resolve the E.164 into the wrong URL.
        The possible intent may be to cause the client to retrieve
        a resource containing fraudulent or damaging material.
      o Denial of Service
        By removing the URL to which the E.164 maps, a malicious
        intruder may remove the client's ability to access the

   This operation is used to map a one E.164 number to a list of URIs.
   The first well-known step in the resolution process is to remove all
   non-digits apart from the leading '+' from the E.164 number as
   described in step 1 and 2 in section 2 of this document.

3.2 Examples

3.2.1 Example 1

    IN NAPTR 100 10 "u" "sip+E2U"  "!^.*$!!"       .
    IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!!"  .

   This describes that the domain is

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   preferably contacted by SIP, and secondly by SMTP.

   In both cases, the next step in the resolution process is to use the
   resolution mechanism for each of the protocols, (SIP and SMTP) to
   know what node to contact for each.

3.2.2 Example 2

    IN NAPTR  10 10 "u" "sip+E2U"   "!^.*$!!"      .
    IN NAPTR 102 10 "u" "mailto+E2U"  "!^.*$!!" .
    IN NAPTR 102 10 "u" "tel+E2U"   "!^.*$!tel:+4689761234!"       .

   Note that the preferred method is to use the SIP protocol, but the
   result of the rewrite of the NAPTR record is a URI (the "u" flag in
   the NAPTR record). In the case of the protocol SIP, the URI might be
   a SIP URI, which is resolved as described in RFC 2543[6]. In the
   case of the "tel" URI scheme[7], the procedure is restarted with
   this new E.164 number. The client is responsible for loop detection.

   The rest of the resolution of the routing is done as described

3.2.3 Example 3

   * IN NAPTR 100 10 "u" "ldap+E2U" "!^+46(.*)$!ldap://\1!" .

   We see in this example that information about all E.164 numbers in
   the 46 countrycode (for Sweden) exists in an LDAP server, and the
   search to do is specified by the LDAP URI[8].

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4. 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.

   Delegations should be done after Expert Review, and the IESG will
   appoint a designated expert.

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5. Security Considerations

   As this system is built on top of DNS, one can not be sure that the
   information one get back from DNS is more secure than any DNS query.
   To solve that, the use of DNSSEC[9] 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 and
   SRV[10] records in the zone that is changed. The TTL should because
   of that be kept to a minimum. The use of this in an environment
   where IP-addresses are for hire (for example when using DHCP[11])
   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.

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6. Acknowledgement

   Support and ideas has 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 come from ITU-T SG2, Working Party 1/2
   (Numbering, Routing, Global Mobility and Service Definition), the
   ENUM working group in the IETF, John Klensin and Leif Sunnegardh.

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   [1]  Mealling, M and R Daniel, "The Naming Authority Pointer (NAPTR)
        DNS Resource Record", draft-ietf-urn-naptr-rr-03.txt (work in
        progress), June 1998.

   [2]  Mockapetris, P.V., "Domain names - concepts and facilities",
        RFC 1034, STD 13, Nov 1987.

   [3]  Mockapetris, P.V., "Domain names - implementation and
        specification", RFC 1035, STD 13, Nov 1987.

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

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

   [6]  Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
        "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [7]  Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, April

   [8]  Howes, T. and M. Smith, "An LDAP URL Format", RFC 1959, June

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

   [10]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
         specifying the location of services (DNS SRV)", RFC 2782,
         February 2000.

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

Author's Address

   Patrik Faltstrom
   Cisco Systems Inc
   170 W Tasman Drive SJ-13/2
   San Jose CA 95134


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Appendix A. Scenario

   Say that the content of the zone is the following:

   6.4 IN NS

   The regulator has in turn given a series of 10000 numbers to the
   telco with the name Telco-A. The regulator because of that has in
   his DNS.


   A user named Sven Svensson has from Telco A got the phone number
   +46-8-9761234. The user gets the service of running DNS from the
   company Redirection Service. Sven Svensson has asked Telco A to
   point out Redirection Service as the authoritative source for
   information about the number +46-8-9761234. Telco A because of this
   puts in his DNS the following.


   Sven Svensson has already plain telephony from Telco A, but also a
   SIP service from the company Sip Service which provides Sven with
   the SIP URI "". The ISP with the name
   ISP A runs email and webpages for Sven, under the emailaddress, and URL

   The DNS for the redirection service because of this contains the

    IN NAPTR 10 10 "u" "sip+E2U"  "!^.*$!!"  .
    IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!!"   .
    IN NAPTR 10 10 "u" "http+E2U" "!^.*$!!" .
    IN NAPTR 10 10 "u" "tel+E2U"  "!^.*$!tel:+46-8-9761234!"               .

   A user, John Smith, want to contact Sven Svensson, he to start with
   only has the E.164 number of Sven, i.e. +46-8-9761234. He takes the
   number, and enters the number in his communication client, which
   happen to know how to handle the SIP protocol. The client removes
   the dashes, and ends up with the E.164 number +4689761234. That is
   what is used in the algorithm for NAPTR records, which is as

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   The client converts the E.164 number into the domainname, and queries for NAPTR records for
   this domainname. Using DNS mechanisms which includes following the
   NS record referals, the following records are returned:

    IN NAPTR 10 10 "u" "sip+E2U"  "!^.*$!"  .
    IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!"   .
    IN NAPTR 10 10 "u" "http+E2U" "!^.*$!" .
    IN NAPTR 10 10 "u" "tel+E2U"  "!^.*$!tel:+46-8-9761234"               .

   Because this client know sip, the first record above is selected,
   and the SIP URI is extracted, and used according to SIP resolution.

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