ENUM Lookup

Telephone Number Mapping (ENUM from TElephone NUmber Mapping) is a suite of protocols used to unify the telephone system with the Internet by using E.164 addresses with the Domain Name System (DNS). With ENUM, an E.164 number can be expressed as a Fully Qualified Domain Name (FQDN) in a specific Internet infrastructure domain defined for this purpose (e164.arpa). E.164 numbers are globally unique, language independent identifiers for resources on Public Switched Telecommunication Networks (PSTNs). ITU-T recommendation E.164 is the international public telecommunication telephony numbering plan.

How ENUM Works

ENUM uses DNS-based architecture and protocols for mapping a complete international telephone number (for example, [phone]) to a series of Uniform Resource Identifiers (URIs).

The protocol itself is defined in the document E.164 number and DNS (RFC 3761) that provides facilities to resolve E.164 telephone numbers into other resources or services on the Internet. The syntax of Uniform Resource Identifiers (URIs) is defined in RFC 2396. ENUM uses Naming Authority Pointer (NAPTR) records defined in RFC 2915 in order to identify available ways or services for contacting a specific node identified through the E.164 number.

Translating the Telephone Number

A telephone number is translated into an Internet address using the following steps:

1. The number is first stored in the following format, [phone]. 1 is the country code for the United States, Canada, and the seventeen other countries that make up the North American Numbering Plan (NANP). The + indicates that the number is a complete, international E.164 telephone number.
2. All characters are removed except for the digits. For example, 12025551234.
3. The order of the digits is reversed. For example, 43215552021. The telephone number is reversed because DNS reads addresses from right to left, from the most significant to the least significant character. Dots are placed between each digit. Example: 4.3.2.1.5.5.5.2.0.2.1. In DNS terms, each digit becomes a zone. Authority can be delegated to any point within the number.
4. A domain (for example, e164.arpa) is appended to the end of the numbers in order to create a FQDN. For example,4.3.2.1.5.5.5.2.0.2.1.e164.arpa.
5. The domain name is queried for the resource records that define URIs necessary to access SIP-based VoIP.

   Once the authoritative name server for that domain name is found, ENUM retrieves relevant records and uses that data to complete the call or service. For example, the number 12025551234 returns sip:[e-mail].

About NAPTR Records

ENUM uses NAPTR records for URI resource records. NAPTR records are used to translate E.164 addresses to SIP addresses. An example of a NAPTR record is:

$ORIGIN 4.3.2.1.5.5.5.2.0.2.1.e164.arpa.
IN NAPTR 100 10 "u" "sip+E2U"  "!^.*$!sip:[e-mail]!"

This example specifies that if you want to use the "sip+E2U" service, you should use sip:[e-mail] as the address.

The regular expression can be used by a telephone company to easily assign addresses to all of its clients. For example, if your number is [phone], your SIP address is sip:[e-mail]; if your number is [phone], your SIP address is sip:[e-mail].

About the Oracle® Enterprise Session Border Controller ENUM Functionality

The ENUM functionality lets the Oracle® Enterprise Session Border Controller make an ENUM query for a SIP request. The ENUM lookup capability lets the Oracle® Enterprise Session Border Controller transform E.164 numbers to URIs during the process of routing (or redirecting) a call. During the routing of a SIP call, the Oracle® Enterprise Session Border Controller uses a local policy attribute to determine if an ENUM query is required and if so which ENUM server(s) need to be queried. A successful ENUM query results in a URI that is used to continue routing or redirecting the call.

Configurable Lookup Length

You can configure a lookup length in the ENUM configuration that provides for more efficient caching of URI lookup results; in it, you can specify the length of the string for the DNS request starting from the most significant digit. This provides more flexibility for length matching, which is useful given the amount of wild card matching available in ENUM services. Specific ENUM groups might only be intended to provide NPANXX or wild card results.

UDP Datagram Support for DNS NAPTR Responses

The Oracle® Enterprise Session Border Controller’s default behavior is to conform to the DNS standard defined in RFC 1035 Domain Names: Implementation and Specification, which sets a maximum size for UDP responses of 512 bytes. This limitation means that responses larger than 512 bytes are truncated (set with the TC, or truncation, bit). In addition, this limitation protects network and system resources because using TCP consumes an undesirable amount of both.

However, you can configure support ENUM queries that manage larger UDP DNS responses as set out in RFC 2671, Extension Mechanisms for DNS (EDNS0), enabling your Oracle® Enterprise Session Border Controller to manage responses beyond 512 bytes. According to RFC 2671, senders can advertise their capabilities using a new resource record (OPT pseudo-RR), which contains the UDP payload size the sender can receive. When you specify a maximum response size over 512 bytes, then the Oracle® Enterprise Session Border Controller add the OPT pseudo-RR to the ENUM query—without which the ENUM server will truncate the response.

Custom ENUM Service Type Support

You can configure the ENUM service type that you want to use for an ENUM group. The Oracle® Enterprise Session Border Controller has always supported E2U+sip and sip+E2U by default, and still does. With Release S-C6.1.0, however, you are also able to configure the service type to those supported in RFCs 2916 and 3721.

For example, you can now set the service type in the ENUM configuration to support E2U+sip and E2U+voicemsg:sip. When you configure customer ENUM service types on your system, however, you should note the following:

• New entries in the service-type parameter overwrite pre-existing values, including the default values.
• Because of the overwriting noted above, you must include the defaults (if you want them configured) when you are adding additional ENUM service type support. That is, you have to also type in E2U+sip and sip+E2U if you want them to be used in addition to the customized types you are setting.