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.

Abstract

  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 "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





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  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:
     4689761234

 4.  Put dots (".") between each digit.  Example: 4.6.8.9.7.6.1.2.3.4

 5.  Reverse the order of the digits.  Example: 4.3.2.1.6.7.9.8.6.4

 6.  Append the string ".e164.arpa" to the end.  Example:
     4.3.2.1.6.7.9.8.6.4.e164.arpa

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
  document.

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
  e164.arpa 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.




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  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
     "order".

  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
  algorithm.

  The service supported for a call is E2U.







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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
       resource.

  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

$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
  IN NAPTR 100 10 "u" "sip+E2U"    "!^.*$!sip:[email protected]!"     .
  IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!mailto:[email protected]!"  .

  This describes that the domain 4.3.2.1.6.7.9.8.6.4.e164.arpa 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.




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3.2.2 Example 2

$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
 IN NAPTR  10 10 "u" "sip+E2U"     "!^.*$!sip:[email protected]!"    .
 IN NAPTR 102 10 "u" "mailto+E2U"  "!^.*$!mailto:[email protected]!" .
 IN NAPTR 102 10 "u" "tel+E2U"     "!^.*$!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

  $ORIGIN 6.4.e164.arpa.
  * IN NAPTR 100 10 "u" "ldap+E2U" "!^+46(.*)$!ldap://ldap.se/cn=01!" .

  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 e164.arpa (not delegations in delegated
  domains of e164.arpa) 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.







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  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.

References

  [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
       1998.

  [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
       2000.




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  [8]  Howes, T. and M. Smith, "An LDAP URL Format", RFC 1959, June
       1996.

  [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
  USA

  EMail: [email protected]
  URI:   http://www.cisco.com




























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Appendix A. Scenario

  Say that the content of the e164.arpa zone is the following:

  $ORIGIN e164.arpa.
  6.4 IN NS ns.regulator-e164.example.se.

  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.

  $ORIGIN 6.4.e164.arpa.
  6.7.9.8 IN NS ns.telco-a.example.se.

  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.

  $ORIGIN 6.7.9.8.6.4.e164.arpa.
  4.3.2.1 IN NS ns.redirection-service.example.se.

  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 "sip:[email protected]".  The ISP with the name
  ISP A runs email and webpages for Sven, under the email address
  [email protected], and URI http://svensson.ispa.se.

  The DNS for the redirection service because of this contains the
  following.

  $ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
   IN NAPTR 10 10 "u" "sip+E2U"    "!^.*$!sip:[email protected]!"        .
   IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!mailto:[email protected]!"     .
   IN NAPTR 10 10 "u" "http+E2U"   "!^.*$!http://svensson.ispa.se!" .
   IN NAPTR 10 10 "u" "tel+E2U"    "!^.*$!tel:+46-8-9761234!"       .













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  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
  follows.

  The client converts the E.164 number into the domain name
  4.3.2.1.6.7.9.8.6.4.e164.arpa., and queries for NAPTR records for
  this domainname.  Using DNS mechanisms which includes following the
  NS record referrals, the following records are returned:

  $ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
   IN NAPTR 10 10 "u" "sip+E2U"    "!^.*$!sip:[email protected]"        .
   IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!mailto:[email protected]"     .
   IN NAPTR 10 10 "u" "http+E2U"   "!^.*$!http://svensson.ispa.se" .
   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 "!^.*$!sip:[email protected]" is applied to
  the original string, "+4689761234". The output is "sip:[email protected]"
  which is used according to SIP resolution.




























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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
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  The limited permissions granted above are perpetual and will not be
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Acknowledgement

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



















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