Network Working Group                                        P Faltstrom
Internet-Draft                                                     Tele2
Expires: October 24, 2000                                 April 25, 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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    This Internet-Draft will expire on October 24, 2000.

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[4], 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[3]

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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.

    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: +46-8-56264000

    2.  Remove all non-digit characters part from the leading '+'.
        Example: +4689761234

    3.  Use this string as input to the NAPTR algorithm.

    4.  Remove all characters part from the digits. Example: 4689761234

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

    6.  Change the order of the digits. Example:

    7.  Append the domain "" to the end. Example:

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3. Identifying available services

    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.

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 also that
    it is the belief that regexps should rarely be used. The replacement
    field seems adequate for the vast majority of situations.

3.1.1 Specific handling of non-terminal NAPTR records

    A E.164 number, without any characters but leading '+', is the input
    to the NAPTR algorithm, and because of this non-terminal NAPTR
    records (i.e. rewrite rules) operate on this E.164 number.

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    The service supported for a call is N2R.

3.1.2 Example of use
    ;;        ord pr fl  service            re replacement
     IN NAPTR 100 10 "a" "sip+N2R"      "" ""
     IN NAPTR 102 10 "a" "smtp+N2R"     "" ""

    This describes that the domain is preferrable contacted via
    the SIP protocol, secondly via SMTP (for VPIM voicemail over SMTP
    for example).

    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.

3.1.3 When the virtual address is a phone number

    When the target address is a phone number, it is first translated
    into a RR name in the domain according to the method
    described above.

     IN NAPTR  10 10 "a" "sip+N2R"      "" ""
     IN NAPTR 102 10 "a" "smtp+N2R"     "" ""
     IN NAPTR 102 10 "a" "tel+N2R"      "" "tel:+4689761234"

    Note that the prefered method is to use the SIP protocol, but the
    result of the rewrite of the NAPTR record is a URI (the "a" 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[5]. In the
    case of the "tel" URI scheme, 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

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4. 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 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
    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 (i.e. DHCP) must therefore be done very

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5. Acknowledgement

    Support and ideas has come from people at Ericsson, especially Bjorn
    Larsson, especially 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, and Leif Sunnegardh
    at Tele2 for information about how SS7 really works.

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

    [3]  Bradner, S, "Key words for use in RFCs to Indicate Requirement
         Levels", RFC 2119, Mars 1997.

    [4]  Mockapetris, P, "Domain names - Implementation and
         Specification", RFC 1035, November 1987.

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

Author's Address

    Patrik Faltstrom
    Borgarfjordsgatan 16
    127 61 Kista


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Appendix A. Example 1

    ITU has delegated the country code 46, and the zone to
    the regulator of telephony in Sweden. Because of that, the content
    of the zone is.

    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 "a" "sip+N2R"      "" ""
     IN NAPTR 10 10 "a" "smtp+N2R"     "" ""
     IN NAPTR 10 10 "a" "http+N2R"     "" ""
     IN NAPTR 10 10 "a" "tel+N2R"      "" "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

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    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

    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 "a" "sip+N2R"      "" ""
     IN NAPTR 10 10 "a" "smtp+N2R"     "" ""
     IN NAPTR 10 10 "a" "http+N2R"     "" ""
     IN NAPTR 10 10 "a" "tel+N2R"      "" "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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