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ENUM Technical issues / DNS. Patrik Fältström Area Director, Applications Area, IETF [email protected] Agenda. Background Problem we want to solve Technical solution Conclusion. Background. ENUM. 3. The Domain Name System. It is a distributed database It is a protocol

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Enum technical issues dns l.jpg

ENUM Technical issues / DNS

Patrik FältströmArea Director, Applications Area, [email protected]

Agenda l.jpg

  • Background

  • Problem we want to solve

  • Technical solution

  • Conclusion

Background l.jpg




The domain name system l.jpg
The Domain Name System

  • It is a distributed database

  • It is a protocol

  • Often the two get mixed up

Dns as distributed database l.jpg
DNS as distributed database

Central server,

“root server”

Server for


Server for





Server for


Recursion l.jpg

Ask “se”!





Ask “paf.se”!













Domains and zones l.jpg

zone "a.se."

domain "a.se."

domain/zone "b.a.se."

Domains and zones









Delegations l.jpg

Root server

se. IN NS ns.nic-se.se

ns.nic-se.se. IN A


ns.se. IN NS ns.nic-se.se.

a.se. IN NS ns.a.se.

c.se. IN NS ns.c.se.

ns.nic-se.se. IN A

ns.a.se. IN A

ns.c.se. IN A


a.se. IN NS ns.c.se.

b.a.se. IN NS ns.b.se.

ns.a.se. IN A

ns.b.se. IN A


c.se. IN NS ns.c.se.

ns.c.se. IN A


b.a.se. IN N S ns.b.a.se.

ns.b.a.se. IN A

E 164 numbers l.jpg

Allocation at CC level by the ITU, SG2/WP1/2 (from an IETF perspective)

E.164 provides the number structure and functionality of numbers used for international public

telecommunicationsHierarchically allocated

E.164 numbers

Structure of e 164 l.jpg

Structure to use for geographic areas perspective)

Structure of E.164




1-3 digits

N digits

Max (15-N) digits

National (significant) number

International public telecommunication

Number for geographical areas

Problem we want to solve l.jpg

Problem we want to solve perspective)



Problem statements 1 2 l.jpg
Problem statements 1(2) perspective)

  • How do network elements find services on the Internet if you only have a telephone (E.164) number?

  • How can subscribers (as opposed to carriers) define their preferences for incoming communications?

  • How can we (IETF/ITU) enable new services while enabling competition, consumer choice and consumer protection?

Problem statements 2 2 l.jpg
Problem statements 2(2) perspective)

  • How can consumers, carriers and new players have equal access to these new opportunities?

  • How do we (IETF/ITU) make this simple and easy to use without reinventing the wheel?

Technical solution l.jpg

Technical solution perspective)



Solution in short l.jpg
Solution in short perspective)

  • Put telephone numbers* in the global domain name system, DNS

  • ENUM Working Group created to solve the problem of using the DNS for:

    • Domainname in

    • [Numbers reformatted as domain names]

    • URI out

    • [mailto, sip, tel, http or other URI scheme]

  • Solution was to use NAPTR records

* Maybe not in the original E.164 format

Example l.jpg
Example perspective)

  • $ORIGIN NAPTR 10 10 "U""sip+E2U""!^.*$!sip:[email protected]!"IN NAPTR 20 10 "U""ldap+E2U""!^.*$!mailto:[email protected]!"

Why dns l.jpg
Why DNS? perspective)

  • It’s there

    • It works… It’s global… It scales… It’s fast… It’s open…

  • Grandfather existing DNS delegation mechanisms, which looks like delegation in E.164 world

  • ENUM enabled DNS provides a low cost, shared SCP-like infrastructure for IP infrastructure

  • ENUM facilitates PSTN/IP convergence

Step 1 l.jpg

The input to the perspective)

NAPTR algorithm

Step 1

  • Take an E.164 number and create a fully qualified domain name in a single highly defined and structured domain

  • +46-8-971234

  • +468971234


Step 1 explanation l.jpg
Step 1 Explanation perspective)

  • Each digit becomes a definable and distributed “zone” in DNS terms

  • Delegation can (doesn’t have to) happen at every digit, including at last digit

  • Zones such as country codes, area codes or primary delegated blocks of numbers can be delegated as well as individual numbers

  • DNS defines authoritative nameservers for NAPTR/service resource records (RR’s)

Step 2 l.jpg
Step 2 perspective)

  • Lookup NAPTR RR’s in DNS, and apply NAPTR/ENUM algorithm


  • !^.*$!mailto:[email protected]!!^+46(.*)$!ldap://ldap.telco.se/cn=0\1!

  • Use rewrite rules using regular expressions which operate on the E.164 number (+468971234)

Regular expressions l.jpg
Regular expressions perspective)

  • For ENUM, the NAPTR regexp field may yield an (unchanged) URL

  • !<regexp>!<string>!

    • “Match <regexp> on original E.164, and apply rewrite rule <string>”

^ - Match beginning

$ - Match end

. - Match any character

.* - Match any number of any character

() - Grouping, \n in <string> is replaced with group number ‘n’ in <regexp>

Step 2 in detail l.jpg
Step 2 in detail perspective)


  • IN NAPTR 10 10 “U” “mailto+E2U” “!^.*$!mailto:[email protected]!”

  • IN NAPTR 20 10 “U” “ldap+E2U” “!^+46(.*)$!ldap://ldap.telco.se/cn=0\1”

  • Note that no line break should be in the records

Enum dns hierarchy l.jpg

e164.arpa perspective)


ENUM DNS Hierarchy

Alternative 1

Alternative 2

46 8 976123 l.jpg
+46-8-976123 perspective)


e164.arpa. IN NS ns.ripe.net.

6.4.e164.arpa. IN NS ns.e164.se.

ns.ns.e164.se. IN A

Root server

e164.arpa. IN NS. ns.ripe.net

ns.ripe.net. IN A


6.4.e164.arpa IN NS ns.e164.se. IN NS e164.telco.se.

e164.telia.se. IN A

e164.telco.se. IN NS e164.telco.se. IN NS ns.eservice.net.

ns.eservice.net. IN NS eservice.net. IN NAPTR …….

Dns hierarchy l.jpg
DNS Hierarchy perspective)

  • Strictly delegated

  • One authoritative server for each name, and only one

    • Have led to the registry/registrar model

  • One responsible registry, but many registrars which talk with customers

Registry registrar l.jpg

Registrar for .com perspective)

interQ Incorporated

Registrar for .com

Active ISP

Registrar for .se


Registrar for .com

Network Solutions

Registrars for .se

Today 227

Registrars for .com

Today 77


Root server

Server for .com

Verisign Global Registry

Server for .se


Conclusion l.jpg

Conclusion perspective)



Enum example flow usage l.jpg

DNS-Server perspective)

Sip proxy

Sip proxy

ENUM example flow/usage



sip:[email protected]

“Call setup”




sip:[email protected]

Enum and voip l.jpg
ENUM and VoIP perspective)

  • ENUM allows VoIP proxies and servers to find each other

    • Intra and inter domain call setup

    • Only for declaring ability to accept certain applications

  • ENUM is an opt-in system (also on country code level)

Enum in universal messaging l.jpg
ENUM in Universal Messaging perspective)

  • IETF Voice Profile for Internet Mail

  • ENUM enables carrier and enterprise voice mail systems to find each other, interoperate and exchange messages

  • Linkage to directory gives access to “spoken name” as well as authenticated access to sensitive information (privacy)

Enum in internet fax l.jpg
ENUM in Internet Fax perspective)

  • IETF RFC 2305 / ITU-T Rec. T.37

  • ENUM enables internet aware fax machines to find each other, interoperate and exchange messages

Detailed example of fax l.jpg
Detailed example of Fax perspective)

  • Person X wants to send a fax to Person Y; she knows Y's E.164 number.

  • Person Y has many IP applications tied to her own (Y's) E.164 number.

  • Device C became responsible for delivering the fax. Device C is Internet Aware, is ENUM-enhanced, can do SMTP and SIP, but can't do H.323 yet.

  • Device C does a DNS query on the E.164 number. Device C discovers from the response that Person Y prefers real-time to store-and-forward fax delivery, likes SIP and H.323 equally, and has SMTP, too.

  • Device C tries SIP first. If SIP fails, Device C does not try H.323. Instead, Device C then tries SMTP.

  • Note: Device C uses ENUM in its application selection logic. ENUM is an enabler and not the same sort of thing as SMTP, SIP, H.323, voice/fax/data, etc.

From rfc 2916 l.jpg
From RFC 2916 perspective)

  • 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.

Status as of jan 10 2001 done l.jpg
Status as of Jan 10, 2001 (done) perspective)

  • Base spec published as RFC 2916

  • e164.arpa delegated to RIPE-NCC

  • ITU SG2/WP1/2 liaison statement:

    • Member state can inform ITU on entry of numbers in DNS, provision of E.164 information

Status as of jan 10 2001 not done l.jpg
Status as of Jan 10, 2001 (not done) perspective)

  • Delegation of domains below e164.arpa to managing entities that are appointed by member states

  • National issues

Resources l.jpg
Resources perspective)

  • NAPTR Resource Records (RFC 2915)

    • http://www.ietf.org/rfc/rfc2915.txt

  • ENUM Specification (RFC 2916)

    • http://www.ietf.org/rfc/rfc2916.txt

  • Liaison Statement (RFC 3026)

    • http://www.ietf.org/rfc/rfc3026.txt

  • IAB on need for unique root (RFC 2826)

    • http://www.ietf.org/rfc/rfc2826.txt

  • IAB Statement on ARPA

    • http://www.iab.org/iab/DOCUMENTS/statement-on-infrastructure-domains.txt

  • My email address: [email protected]