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Alcatel-Lucent | ENUM Use and Management for the Successful Deployment of ENUM-Enabled Services
Overview of ENUM and Its Operation
As desribed in the Introduction, ENUM is defined in IETF RFC 3761. In this RFC, the phrase E.164 number
frequently appears, and it refers to Recommendation E.164, which was written by the International
Telecommunication Union (ITU) and specifies the international public telecommunication numbering
plan. In brief, ENUM specifies a method for storing information in DNS servers to map an E.164 number
(a.k.a., telephone number) to Uniform Resource Identifiers (URIs) for associated communication services
(e.g., VoIP service). Each URI (for VoIP phone, cell phone, e-mail, web page, SIP Proxy Server, Session
Border Controller, etc.) is stored in a DNS Naming Authority Pointer (NAPTR) record, which is in an
E.164 domain, such as
4.4.4.4.3.3.3.2.2.2.1.e164.arpa. Note that the digits in the E.164 domain name
consist of a phone number in reverse order. As defined in section 4 in RFC 3403, there are six resource
data fields in a NAPTR record, and they are as follows:
Order
Preference
Flags
Services
Regular Expression
Replacement
Figure 1: Typical ENUM Operation
An example NAPTR record is as follows:
4.4.4.4.3.3.3.2.2.2.1.e164.arpa. IN NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:[email protected]!" .
In this example record, the following values were used (and Replacement was not used).
Order = 100
Preference = 10
Flags = u
Regular Expression =
!^.*$!sip:[email protected]!
Services = E2U+sip
The figure below provides a brief overview of the typical operation of ENUM.
The main steps that occur in this figure
are as follows:
1. An ENUM client starts with a called phone
number, e.g.
1-222-333-4444.
2. The client turns the number into a domain
name, e.g.
4.4.4.4.3.3.3.2.2.2.1.e164.arpa.
3. The client, using a resolver, queries a DNS
server with the domain name.
4. The DNS server returns the NAPTR records
in the domain to the client.
5. If multiple NAPTR records are returned, the
client picks one to use based on the Order,
Preference, and Services field values in
the records.
If the URI in the selected NAPTR record contains a host name (e.g., ims.acme. com), then the
client will do a second non-ENUM DNS query to determine the host’s IP address.
