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.
AbstractThis 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] records in DNS  , 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. RFC2119 .
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: 18.104.22.168.22.214.171.124.3.4 5. Reverse the order of the digits. Example: 126.96.36.199.188.8.131.52.6.4 6. Append the string ".e164.arpa" to the end. Example: 184.108.40.206.220.127.116.11.6.4.e164.arpa
It is the string which is the result of step 2 in section 2 above which is input to the NAPTR algorithm. RFC2396  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.
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 ) 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.  Mealling, M. and R. Daniel, "The Naming Authority Pointer (NAPTR) DNS Resource Record", RFC 2915, September 2000.  Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.  Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Berners-Lee, T., Fielding, R.T. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998.  Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, March 1999.  Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, April 2000.
 Howes, T. and M. Smith, "An LDAP URL Format", RFC 1959, June 1996.  Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999.  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
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 18.104.22.168.22.214.171.124.6.4.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: $ORIGIN 126.96.36.199.188.8.131.52.6.4.e164.arpa. IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:email@example.com" . IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!mailto:firstname.lastname@example.org" . IN NAPTR 10 10 "u" "http+E2U" "!^.*$!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@example.com" is applied to the original string, "+4689761234". The output is "sip:firstname.lastname@example.org" which is used according to SIP resolution.
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