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

E.164 number and

Status of this

This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited

Copyright

Copyright (C) The Internet Society (2000). All Rights Reserved

This document discusses the use of the Domain Name System (DNS)
storage of E.164 numbers. More specifically, how DNS can be used
identifying available services connected to one E.164 number
Routing of the actual connection using the service selected
these methods is not discussed

1.

Through transformation of E.164 numbers into DNS names and the use
existing DNS services like delegation through NS records, and use
NAPTR [1] records in DNS [2] [3], one can look up what services
available for a specific domain name in a decentralized way
distributed management of the different levels in the lookup process

1.1

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

The domain "e164.arpa" is being populated in order to provide
infrastructure in DNS for storage of E.164 numbers. In order
facilitate distributed operations, this domain is divided
subdomains. Holders of E.164 numbers which want to be listed in

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RFC 2916 E.164 number and DNS September 2000

should contact the appropriate zone administrator in order to
listed, by examining the SOA resource record associated with
zone, just like in normal DNS operations

Of course, as with other domains, policies for such listings will
controlled on a subdomain basis and may differ in different parts
the world

To find the DNS names for a specific E.164 number, the
procedure is to be followed

1. See that the E.164 number is written in its full form,
the countrycode IDDD. Example: +46-8-9761234

2. Remove all non-digit characters with the exception of the
'+'. 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.

2.1 Special note about the '+'

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

3. Fetching URIs given an E.164

For a record in DNS, the NAPTR record is used for
available ways of contacting a specific node identified by that name
Specifically, it can be used for knowing what services exists for
specific domain name, including phone numbers by the use of
e164.arpa domain as described above

The identification is using the NAPTR resource record defined for
in the URN resolution process, but it can be generalized in a
that suits the needs specified in this document

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RFC 2916 E.164 number and DNS September 2000

It is the string which is the result of step 2 in section 2
which is input to the NAPTR algorithm

3.1 The NAPTR

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
processed when multiple NAPTR records are returned in response
a single query

o The preference field specifies the order in which records
be processed when multiple NAPTR records have the same value
"order".

o The service field specifies the resolution protocol and
service(s) that will be available if the rewrite specified by
regexp or replacement fields is applied

o The flags field contains modifiers that affect what happens in
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
rewrite rule

Note that the client applies all the substitutions and performs
lookups, they are not performed in the DNS servers. Note that
are stored in the regexp field

3.1.1 Specification for use of NAPTR Resource

The input is an E.164 encoded telephone number. The output is
Uniform Resource Identifier in its absolute form according to
'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
algorithm

The service supported for a call is E2U

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RFC 2916 E.164 number and DNS September 2000

3.1.2 Specification of Service E2U (E.164 to URI

* Name: E.164 to
* Mnemonic: E2
* 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
form as specified in step 2 in section 2 in this document
* Algorithm:
* Output: One or more
* Error Conditions
o E.164 number not in the numbering
o E.164 number in the numbering plan, but no URIs exist
that
o Service

* Security Considerations
o Malicious
One of the fundamental dangers related to any service
as this is that a malicious entry in a resolver's
will cause clients to resolve the E.164 into the wrong URI
The possible intent may be to cause the client to
a resource containing fraudulent or damaging material
o Denial of
By removing the URI to which the E.164 maps, a
intruder may remove the client's ability to access
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
non-digits apart from the leading '+' from the E.164 number
described in step 1 and 2 in section 2 of this document

3.2

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: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
preferably contacted by SIP, and secondly by SMTP

In both cases, the next step in the resolution process is to use
resolution mechanism for each of the protocols, (SIP and SMTP)
know what node to contact for each

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RFC 2916 E.164 number and DNS September 2000

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: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
result of the rewrite of the NAPTR record is a URI (the "u" flag
the NAPTR record). In the case of the protocol SIP, the URI might
a SIP URI, which is resolved as described in RFC 2543 [6]. In
case of the "tel" URI scheme [7], the procedure is restarted
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
the 46 countrycode (for Sweden) exists in an LDAP server, and
search to do is specified by the LDAP URI [8].

4. IANA

This memo requests that the IANA delegate the E164.ARPA
following instructions to be provided by the IAB. Names within
zone are to be delegated to parties according to the
recommendation E.164. The names allocated should be hierarchic
accordance with ITU Recommendation E.164, and the codes
assigned in accordance with that Recommendation

Delegations in the zone e164.arpa (not delegations in
domains of e164.arpa) should be done after Expert Review, and
IESG will appoint a designated expert

5. Security

As this system is built on top of DNS, one can not be sure that
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
is to be recommended

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RFC 2916 E.164 number and DNS September 2000

The caching in DNS can make the propagation time for a change
the same amount of time as the time to live for the NAPTR records
the zone that is changed. The use of this in an environment
IP-addresses are for hire (for example, when using DHCP [11])
therefore be done very carefully

There are a number of countries (and other numbering environments)
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
information may provide incentives for changes that are
unauthorized (and, in some cases, for denial of legitimate
requests). Such environments should be designed with
mechanisms for identification and authentication of those
changes and for authorization of those changes

6.

Support and ideas have come from people at Ericsson, Bjorn
and the group which implemented this scheme in their lab to see
it worked. Input has also come from ITU-T SG2, Working Party 1/2
(Numbering, Routing, Global Mobility and Service Definition),
ENUM working group in the IETF, John Klensin and Leif Sunnegardh

[1] Mealling, M. and R. Daniel, "The Naming Authority
(NAPTR) DNS Resource Record", RFC 2915, September 2000.

[2] Mockapetris, P., "Domain names - concepts and facilities",
13, RFC 1034, November 1987.

[3] Mockapetris, P., "Domain names - implementation
specification", STD 13, RFC 1035, November 1987.

[4] Bradner, S., "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997.

[5] Berners-Lee, T., Fielding, R.T. and L. Masinter, "
Resource Identifiers (URI): Generic Syntax", RFC 2396,
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,
2000.

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RFC 2916 E.164 number and DNS September 2000

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

[9] Eastlake, D., "Domain Name System Security Extensions",
2535, March 1999.

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

[11] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.

Author's

Patrik
Cisco Systems
170 W Tasman Drive SJ-13/2
San Jose CA 95134

EMail: paf@cisco.
URI: http://www.cisco.

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RFC 2916 E.164 number and DNS September 2000

Appendix A.

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
telco with the name Telco-A. The regulator because of that has
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
+46-8-9761234. The user gets the service of running DNS from
company Redirection Service. Sven Svensson has asked Telco A
point out Redirection Service as the authoritative source
information about the number +46-8-9761234. Telco A because of
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
SIP service from the company Sip Service which provides Sven
the SIP URI "sip:sven@sips.se". The ISP with the
ISP A runs email and webpages for Sven, under the email
sven@ispa.se, and URI http://svensson.ispa.se

The DNS for the redirection service because of this contains
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!" .

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RFC 2916 E.164 number and DNS September 2000

A user, John Smith, want to contact Sven Svensson, he to start
only has the E.164 number of Sven, i.e. +46-8-9761234. He takes
number, and enters the number in his communication client,
happen to know how to handle the SIP protocol. The client
the dashes, and ends up with the E.164 number +4689761234. That
what is used in the algorithm for NAPTR records, which is
follows

The client converts the E.164 number into the domain
4.3.2.1.6.7.9.8.6.4.e164.arpa., and queries for NAPTR records
this domainname. Using DNS mechanisms which includes following
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
the original string, "+4689761234". The output is "sip:sven@sips.se
which is used according to SIP resolution

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RFC 2916 E.164 number and DNS September 2000

Full Copyright

Copyright (C) The Internet Society (2000). All Rights Reserved

This document and translations of it may be copied and furnished
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or assist in its implementation may be prepared, copied,
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Faltstrom Standards Track [Page 10]

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