Technical description of ENUM

In accordance with IETF's RFC 3761, ENUM means that an E.164 number is converted into a domain name that complies with the Internet DNS. In addition, this domain name can be used to identify various communications services connected to the E.164 number, such as Internet calls and e-mail. Even though ENUM supports many services, such as instant messaging and e-mail, it is primarily used for routing VoIP calls.

Numbers are converted into domain names by reversing the order of the digits that comprise an E.164 telephone number and by removing other additional characters. Each single number is separated by a dot from the other numbers of the domain name. Finally, the domain (e164.arpa) is appended to the end. In accordance with this description, the Finnish telephone number

+358 9 6966 634

is converted into the domain name

4.3.6.6.6.9.6.9.8.5.3.e164.arpa

The resulted domain name can be used to perform an ENUM query for specifying the Universal Resource Identifiers (URIs) that correspond to the domain name. If the ENUM domain has been entered into the ENUM system, the response to the query shall include data on the Tier 2 name servers configured for the ENUM domain. The response to a query sent to these Tier 2 name servers, on the other hand, includes one or more Naming Authority Pointer (NAPTR) records used for describing the URIs. The response contains a list of the URIs (e.g. SIP URI or tel URI) that correspond to the name arranged on the basis of set priorities. Below is an example of NAPTR records at Tier 2 level:

$ORIGIN 4.3.6.6.6.9.6.9.8.5.3.e164.arpa.
@ IN NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:kn@iptel.org!" .
@ IN NAPTR 10 101 "u" "E2U+sip" "!^.*$!sip:klaus.nieminen@vivi.fi!" .
@ IN NAPTR 10 102 "u" "E2U+email:mailto" "!^.*$!mailto:klaus.nieminen@vivi.fi!" .

The information stored in Tier 2 name servers is publicly available on the Internet. One should therefore be careful not to enter such information that is not meant to be available to everyone on the Internet (such as secret telephone numbers). Entering e-mail addresses into name servers may also cause problems, since the addresses may be used, for example, to send spam.

It may be more convenient to give the information in the following form:

IN NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:+35896966634@vivi.fi!" .

Alternatively, it is possible to enter only the address of the operator's server instead of the customer's SIP address, for example

IN NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:+35896966634@sbc.oper2.fi!" .

The web pages of IETF's ENUM working group are at:
http://www.ietf.org/html.charters/enum-charter.html

Implementers should pay special attention to certain issues, such as partial ENUM support of some ENUM capable products, the old test delegations that are in use at least for now, delegations that comply with RFC 2916, and possible errors and NAPTR records that have not been updated or removed.

A list of some products and applications supporting ENUM can be found from:
http://www.e164.org/wiki/StuffThatSupportsENUM

In addition to the user ENUM, there are several ENUM services provided within organisations or interconnection coalitions in other countries. IETF is presently working to define the infrastructure ENUM that shall be reserved for operators, and the planning of a national operator ENUM has started in Finland as well.