Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


Network Working Group P. Faltstrom Request for Comments: 2916 Cisco Systems Inc. Category: Standards Track September 2000

                        E.164 number and DNS

Status 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 Internet Society (2000).  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.
 Routing of the actual connection using the service selected using
 these methods is not discussed.

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 domain name in a decentralized way with
 distributed management of the different levels in the lookup process.

1.1 Terminology

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

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

Faltstrom Standards Track [Page 1] RFC 2916 E.164 number and DNS September 2000

 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:

2.1 Special note about the '+'

 The '+' is kept in stage 2 in section 2 to flag that the number which
 the regular expression is operating on is a E.164 number.  Future
 work will be needed to determine how other numbering plans (such as
 closed ones) might be identified.  It is possible, but not definite,
 that they would use a similar mechanism as the one described in this

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 domain name, 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.

Faltstrom Standards Track [Page 2] RFC 2916 E.164 number and DNS September 2000

 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
 '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.

Faltstrom Standards Track [Page 3] RFC 2916 E.164 number and DNS September 2000

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 URIs
  • Error Conditions:

o E.164 number not in the numbering plan

    o E.164 number in the numbering plan, but no URIs 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 URI.
      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 URI 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
 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.

Faltstrom Standards Track [Page 4] RFC 2916 E.164 number and DNS September 2000

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 above.

3.2.3 Example 3

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

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

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.

Faltstrom Standards Track [Page 5] RFC 2916 E.164 number and DNS September 2000

 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 [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.

6. Acknowledgements

 Support and ideas 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 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.


 [1]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
      (NAPTR) DNS Resource Record", RFC 2915, September 2000.
 [2]  Mockapetris, P., "Domain names - concepts and facilities", STD
      13, RFC 1034, November 1987.
 [3]  Mockapetris, P., "Domain names - implementation and
      specification", STD 13, RFC 1035, November 1987.
 [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, 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

Faltstrom Standards Track [Page 6] RFC 2916 E.164 number and DNS September 2000

 [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

Faltstrom Standards Track [Page 7] RFC 2916 E.164 number and DNS September 2000

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 email address, and URI
 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!"       .

Faltstrom Standards Track [Page 8] RFC 2916 E.164 number and DNS September 2000

 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
 The client converts the E.164 number into the domain name, and queries for NAPTR records for
 this domainname.  Using DNS mechanisms which includes following the
 NS record referrals, 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 the client knows sip, the first record above is selected,
 and the regular expression "!^.*$!" is applied to
 the original string, "+4689761234". The output is ""
 which is used according to SIP resolution.

Faltstrom Standards Track [Page 9] RFC 2916 E.164 number and DNS September 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Faltstrom Standards Track [Page 10]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2916.txt · Last modified: 2000/09/25 16:15 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki