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Network Working Group                                        P. FaltstromRequest for Comments: 2916                             Cisco Systems Inc.Category: Standards Track                                  September 2000E.164 number and DNSStatus 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 inRFC2119 [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 DNSFaltstrom                   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:      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.arpa2.1 Special note about the '+'   The '+' is kept in stage 2 insection 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.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 insection 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 inRFC2396 [5]   An E.164 number, without any characters but leading '+' and digits,   (result of step 2 insection 2 above) is the input to the NAPTR   algorithm.   The service supported for a call is E2U.Faltstrom                   Standards Track                     [Page 3]

RFC 2916                  E.164 number and DNS            September 20003.1.2 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 insection 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 insection 2 of this document.3.2 Examples3.2.1 Example 1$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.   IN NAPTR 100 10 "u" "sip+E2U"    "!^.*$!sip:info@tele2.se!"     .   IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!mailto:info@tele2.se!"  .   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.Faltstrom                   Standards Track                     [Page 4]

RFC 2916                  E.164 number and DNS            September 20003.2.2 Example 2$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.  IN NAPTR  10 10 "u" "sip+E2U"     "!^.*$!sip:paf@swip.net!"    .  IN NAPTR 102 10 "u" "mailto+E2U"  "!^.*$!mailto:paf@swip.net!" .  IN NAPTR 102 10 "u" "tel+E2U"     "!^.*$!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 inRFC 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.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.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.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        1996.   [9]  Eastlake, D., "Domain Name System Security Extensions",RFC2535, 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: paf@cisco.com   URI:http://www.cisco.comFaltstrom                   Standards Track                     [Page 7]

RFC 2916                  E.164 number and DNS            September 2000Appendix 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:sven@sips.se".  The ISP with the name   ISP A runs email and webpages for Sven, under the email address   sven@ispa.se, and URIhttp://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:sven@sips.se!"        .    IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!mailto:sven@ispa.se!"     .    IN NAPTR 10 10 "u" "http+E2U"   "!^.*$!http://svensson.ispa.se!" .    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   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:sven@sips.se"        .    IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!mailto:sven@ispa.se"     .    IN NAPTR 10 10 "u" "http+E2U"   "!^.*$!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:sven@sips.se" is applied to   the original string, "+4689761234". The output is "sip:sven@sips.se"   which is used according to SIP resolution.Faltstrom                   Standards Track                     [Page 9]

RFC 2916                  E.164 number and DNS            September 2000Full 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   English.   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   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Faltstrom                   Standards Track                    [Page 10]