rfc2916

Network Working Group P. Faltstrom

Request for Comments: 2916 Cisco Systems Inc.

Category: Standards Track September 2000

E.1 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.1 numbers. More specifically, how DNS can be used for

identifying available services connected to one E.1 number.

Routing of the actual connection using the service selected using

these methods is not discussed.

1. Introduction

Through transformation of E.1 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

he digits. Example: 4.3.2.1.6.7.9.8.6.4

6. Append the string ".e1.arpa" to the end. Example:

4.3.2.1.6.7.9.8.6.4.e1.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.1 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.1 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

e1.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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RFC 2916 E.1 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

"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.1 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.1 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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RFC 2916 E.1 number and DNS September 2000

3.1.2 Specification of Service E2U (E.1 to URI)

* Name: E.1 to URI

* Mnemonic: E2U

* Number of Operands: 1

* Type of Each Operand: First operand is an E.1 number.

* Format of Each Operand: First operand is the E.1 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.1 number not in the numbering plan

o E.1 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.1 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.1 maps, a malicious

intruder may remove the client's ability to access the

resource.

This operation is used to map a one E.1 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.1 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.e1.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.e1.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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RFC 2916 E.1 number and DNS September 2000

3.2.2 Example 2

$ORIGIN 4.3.2.1.6.7.9.8.6.4.e1.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:+46761234!" .

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.1 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.e1.arpa.

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

We see in this example that information about all E.1 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 E1.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.1. The names allocated should be hierarchic in

accordance with ITU Recommendation E.1, and the codes should

assigned in accordance with that Recommendation.

Delegations in the zone e1.arpa (not delegations in delegated

domains of e1.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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RFC 2916 E.1 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

is, P., "Domain names - concepts and facilities

.6.4.e1.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.1 number and DNS September 2000

A user, John Smith, want to contact Sven Svensson, he to start with

only has the E.1 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.1 number +46761234. That is

what is used in the algorithm for NAPTR records, which is as

follows.

The client converts the E.1 number into the domain name

4.3.2.1.6.7.9.8.6.4.e1.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.e1.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, "+46761234". The output is "sip:sven@sips.se"

which is used according to SIP resolution.

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RFC 2916 E.1 number and DNS September 2000

Full Copyright Statement

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

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d 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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