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What is ENUM?

ENUM was developed as a solution to the question of how network elements can find services on the Internet using only a telephone number, and how telephones, which have an input mechanism limited to twelve keys on a keypad, can be used to access Internet services. ENUM at its most basic is the convergence of PSTN and IP networks; it is the mapping of a telephone number from the public switched telephone network to Internet functionalities.

"ENUM" has a number of meanings. It is the name of a protocol that resolves fully qualified telephone numbers to fully qualified domain name addresses using a DNS-based architecture. It is the name of a chartered working group of the Internet Engineering Task Force (IETF) chartered to develop protocols that map telephone numbers to resources found on the Internet using the Domain Name System. It is also the title of RFC 2916, the approved protocol document that discusses the use of DNS for the storage of E.164 numbers and the available services connected to an E.164 number.

ENUM does not change the Numbering Plan and does not change telephony numbering or its administration in any way. ENUM will not drain already scarce numbering resources because it uses existing numbers.

What is E.164? Why use E.164 for ENUM?

E.164 is the name of the international telephone numbering plan administered by the International Telecommunication Union (ITU), which specifies the format, structure, and administrative hierarchy of telephone numbers. "E.164" refers to the ITU document that describes the structure of telephone numbers. The ITU issues country codes to sovereign nations, but administration of telephone numbers within each country is governed by that country’s telecommunications regulatory agency. A fully qualified E.164 number is designated by a country code, an area or city code, and a phone number. For example, a fully qualified E.164 number for the phone number 555-1234 in Washington, DC (area code 202) in the United States (country code 1) would be +1-202-555-1234.

E.164 numbers are appropriate for use in ENUM because they are an existing system for global traceability. Further, because country codes and assignments change only in the event of the collapse or complete restructuring of governments, these numbers tend to be static.

What are the main benefits that ENUM gives to subscribers?

ENUM enables Internet-based users to make a selection from a range of services available for communicating with another person when the caller knows only a telephone number or has access only to a telephone keypad. ENUM allows users to access Internet-based services and resources from Internet-aware telephones, ordinary telephones connected to Internet gateways or proxy services, and other Internet-connected devices where input is limited to numeric digits.

ENUM enables users to specify their preferences for receiving incoming communications, and gives greater user control over communications. For example, a user can specify a preference for voice mail messages over live calls during certain times of day, or may indicate a destination for call forwarding. Some of ENUM's potential applications are discussed later in this document.

What kinds of customers could use ENUM?

Although ENUM can be used by individual residential telephone customers, users could also be corporations, individuals, government agencies, military, and hosts of other non-individual users.

How is ENUM going to work for the average telephone subscriber? What does this system look like to that subscriber?

If an average telephone customer were to make a telephone call using, for instance, an Internet-enabled phone to another Internet-enabled phone, all of the steps in between are invisible to the user. To the call initiator and call receiver, this phone call would appear the same as a call made over the PSTN. Subscribers will, of course, have to identify the ENUM services that they wish to use.

What kinds of applications could use ENUM?

ENUM links a telephone number to a host or resources on the Internet that can connect the call, either end-to-end over IP networks or through a designated gateway to the PSTN. This is useful for connecting SIP- or H.323-compatible endpoints that exist across domain boundaries.

The potential applications of ENUM are far-reaching; however, the principal applications for ENUM have centered on two areas. One is Voice over IP (VoIP) and the other centers on Voice Protocol for Internet Mail (VPIM). The long-stated goal of the VoIP industry has been to make a phone call over the Internet as easy to make and as high-quality as a regular PSTN phone call. The goal of the VPIM industry has been to develop a comprehensive mechanism by which voice mail systems could exchange messages over IP networks. ENUM enables carrier and enterprise voice mail systems to find each other, interoperate, and exchange messages.

Although VoIP and VPIM are the most widely discussed applications, other application possibilities for ENUM, including but not limited to Internet Fax and Instant Messaging, exist.

How does ENUM work?

Once a telephone number is entered, it is translated into an Internet address using the following steps:

1. The phone number is translated into a fully qualified E.164 number by adding the city (or area) and country code. Example: 555-1234 dialed in Washington, DC becomes +1-202-555-1234, where the "1" represents the North American country code. The "+" indicates that the number is a fully qualified E.164 number.
2. All characters are removed except for the digits. Example: 12025551234
3. The order of the digits is reversed. Example: 43215552021
4. Dots are placed between each digit. Example: 4.3.2.1.5.5.5.2.0.2.1
5. The domain "e164.arpa" is appended to the end. Example: 4.3.2.1.5.5.5.2.0.2.1.e164.arpa

ENUM then issues a DNS query on this domain. Once the authoritative name server is found, ENUM retrieves relevant NAPTR Resource records and will perform according to the user's registered services for that number.

What is .arpa, and why is it the top level domain for ENUM? Why not create a new top-level domain specifically for ENUM?

E164.arpa has been proposed as the DNS domain for use with ENUM. This designation may change as a result of ongoing discussions between the ITU, the IETF, and other international organizations involved with ENUM. In the event that the international community chooses a different ENUM domain, the structures discussed here and in IETF RFC 2916 will apply to that new designated domain. The .arpa domain has been designated for Internet infrastructure purposes. It is managed by the Internet Assigned Numbers Authority (IANA) in cooperation with the Internet technical community under the guidance of the Internet Architecture Board (IAB). A new top-level domain (e.g., .e164) was not created because ENUM is an infrastructure application appropriate for designation within the previously established .arpa domain. ENUM is consid-ered appropriate as an infrastructure application because it provides a set of DNS-based resource directories, referenced by phone number, for use by various ENUM-enabled application clients (such as telephones, SIP servers, and voice messaging systems).

Why is the number reversed?

DNS names are structured from right to left. In the example used above (4.3.2.1.5.5.5.2.0.2.1.e164.arpa), DNS would first search for the top-level domain arpa, then search second-level domains for e164, then search the next level for the country code 1, etc. More information on DNS structuring can be found in the RFC for Non-Terminal DNS Name Redirection, at http://www.rfc-editor.org/rfc/rfc2672.txt.

Why are there dots between the numbers?

Each dot separates the number into administrative domains, or zones. This allows for delegation of authority at various points along the name and eliminates the requirement for clients to know individual delegation schemes to know where to put the dots.

Will a user have to type in the dots and reversed numbers?

No, this will be done by the application (e.g., a web browser) or device (e.g., an Internet-enabled telephone) that supports ENUM. The user simply dials a telephone number in the traditional manner.

Will ENUM telephone routing confuse the PSTN routing system?

ENUM facilitates the discovery of resources associated with a telephone number. Therefore, it facilitates how various applications will identify appropriate peer servers associated with an intended end-user. It does not, however, impact how those applications will operate once the location of an end-user associated application server has been established. Consequently, ENUM does not affect application-level functions such as call routing and signaling, regardless of the underlying technology employed. It should be noted, however, that a telephone company call-routing mechanism could use ENUM as well.

It is a core principle that in providing a unified resource directory service, ENUM will not change the existing right-to-use rules and principles for telephone numbers. ENUM is not intended to change how telephone numbers are administered, but instead facilitate a wide range of applications using phone numbers as subscriber names. ENUM also will not interfere with existing PSTN functions and technology, such as circuit switching, SS7 (ISUP or TCAP), or Intelligent Networking, where similar resource discovery activities are performed through the PSTN legacy technologies.

How will the e164.arpa domain be organized?

One convenient way of doing this would be to delegate according to the 243 country codes designated by the ITU. It is important to understand, however, that delegation in DNS can occur at any digit, or zone domain in DNS terms.

For example, within the root e164.arpa, there would be:

• An NS listing for .1.e164.arpa - representing the country code (1) of the North American Numbering Plan (US, Canada, and several Caribbean countries),
• An NS listing for .4.4.e164.arpa - representing the country code (44) of the UK,
• An NS listing for .6.4.e164.arpa - representing the country code (46) of Sweden,
• An NS listing for .1.8.e164.arpa - representing the country code (81) of Japan, and
• An NS listing for .8.5.3.e164.arpa - representing the country code (358) of Finland.

At the national TN/NS level, further NS delegation [DNAME, CNAME, PTR] can occur to enterprises, TN/NS application service providers, carriers, and even individuals who have DNS servers in their homes.

Why use DNS?

DNS has been selected for use with ENUM because it's there. It already works, it's global, it scales, it's fast, and it's open. ENUM-enabled DNS provides a low-cost, shared infrastructure for IP services similar to SCP functionality.

What is the effect of e164.arpa deployment on the global DNS system?

This is going to require research, such as the effect of "wrong dials" on the root of e164. That is, caller specification of a wrong number can result in many additional queries to the e164.arpa root. Additional work will be necessary in advising ENUM applications such things as the level of data caching necessary in order to relieve stress, suppress escalating of poorly formed queries, mis-dials, or cache misses on the root structure. For telephony applications, performance and load engineering is critical, as query volumes from small to medium sized cities alone can easily reach many thousands per second. Response times, as well as transaction loads, must be carefully considered. Conventional DNS caching is of significantly reduced value in ENUM due to the huge size of the name space and relatively even distribution of queries into the space over arbitrary time intervals. Unlike conventional DNS queries, call volumes are not highly concentrated into a popular small subset of the number space.

What are SRV and NAPTR Records?

These are DNS Resource Records that contain information about what resources, services, and applications are associated with a specific phone number. These services are determined by the subscriber.

What happens if a user dials a number that cannot be resolved by DNS?

In a similar manner to a 404-Not Found message in a web browser, an error message will be returned to the device or software initiating the call. In the case of a web browser, a 404-Not Found message will be displayed. In the case of a SIP telephone, it will open a gateway to the PSTN and connect in the traditional way.

What happens if a user dials an emergency number (e.g., 911 in the United States and 112 in Europe)?

Emergency numbers are generally considered "access codes" and are outside of E.164 and ENUM services. If the user dials an emergency number from a SIP phone, the phone will recognize that it cannot make a SIP connection and will open a gateway to the PSTN.

What protocol does ENUM use for Internet Telephony?

ENUM itself is "protocol agnostic" because it is application agnostic. It does not specify what applications a particular number is associated with, but instead provides a unified way of discovering resources associated with it. It can, for example, work with either H.323 or SIP.

What can be said about articles in the technical press that say that Voice over IP just doesn't work?

VoIP is an evolving technology that is in an early stage of development but that is rapidly improving. It is only a question of when, not if, Internet Telephony will become a reality, fully integrated with the existing, global telephone service. However, ENUM is not intended solely to facilitate VoIP, but to work with a range of applications where a telephone number is used as a subscriber name.

What is SIP? How does ENUM relate to SIP?

SIP, or Session Initiation Protocol, performs the initiation of interactive communications sessions between users, as well as termination of those communications and modifications to sessions. SIP is one protocol that may be used by ENUM to send out initiation attempts to multiple locations in order to find the user who is receiving a call.

What happens to a phone number when a subscriber ports from one service provider to another?

When a number ports, the service provider of record changes. That is, the industry recognizes a different service provider as the holder for that particular number. This is important for routing and billing purposes.

Subscribers who port from one service to another should still be able to continue using their ENUM-enabled services, assuming their new service providers support them. The actual location of server resources identified by ENUM will likely change as the subscriber changes any of the underlying service providers.

When the user disconnects services, the number goes back to the original service provider's inventory, not to the new service provider's inventory.

What happens to the ENUM services when a subscriber cancels telephone service?

As we now know, the number returns to the communications service provider's inventory for the purposes of reassignment. The subscriber who cancels telephone services must realize that the carrier that issued that phone number will have to cancel the associations that number has with all ENUM services, even those provided by other service providers. If this is not done, the new user of that telephone number could have a conflict with the old user's services. However, where number portability is available, a user has the option of porting the telephone number over to a new service provider instead of canceling the existing service and losing that current number.

Could ENUM be used to provide telephone number portability?

In those countries that do not yet have a centralized database administration service, having a shared directory service like ENUM might be of interest. However, ENUM is not intended to serve this function, as there are very significant technical, regulatory, security, and operational limitations in using ENUM for this purpose. ENUM is a shared resource discovery service, not an industry provisioning service. In most countries where number portability is deployed, telephone service providers are generally required to comply with regulatory/industry processes, procedures, and systems, regardless of the underlying technology they employ for telephony service delivery (SIP, H.323, circuit-switched, etc.).

How ENUM is administered in those countries will also likely require mirroring of provisioning rules (e.g., anti- slamming) employed for number portability and number administration so as to ensure that service providers using ENUM-enabled services do not violate applicable regulatory rules or industry guidelines. ENUM is another downstream use of numbering provisioning and administration activities, and will need to be deployed consistent with applicable national requirements. It does not create an alternate numbering system with its own set of rules and policies.

How is the user of a number authenticated?

Users could be corporations, individuals, government agencies, military organizations, and hosts of other non-individual users. Service providers typically assign large blocks of numbers to these entities. The telecom manager within these entities then assigns numbers to users, so even the service providers cannot identify the users for a large portion of the allocated numbers. This is an unresolved issue but one that must be resolved prior to deploying a robust and secure ENUM service. It is likely that the service provider that allocated the number(s) to the user will be involved in the process of authentication.

What about private numbering plans within a company?

The ENUM protocol can be used in private numbering plans the same way it can be used in the public E.164 numbering plan. The Internet Telephony gateway or proxy needs some intelligence to "decode" a particular dialing string and then decide how to look up resources for that particular number. Instead of looking for resources in e164.arpa, the gateway or proxy would look for SRV or NAPTR records for private numbers under some other structure, such as e164.bigcompany.com.

Are users going to have to pay to have their telephone numbers ENUM-provisioned?

Probably yes, but most likely the costs will be indirectly recovered through the underlying prices for ENUM-enabled services that subscribers pay. This is a DNS-based system, and someone must pay to have a domain name registered in DNS. Listing telephone numbers will be no different. Whether the cost will be charged directly to the subscriber or will be an indirect charge as part of some larger services will depend on those offering the services.

It is important to remember that a user does not have to have ENUM list his phone number. ENUM would be a subscriber-controlled "opt-in" system to "announce," over the Internet, the availability of a particular telephone number to accept service sessions and how to manage those sessions as a result of having subscribed to an ENUM-enabled service. If a customer does not have an Internet telephony device or service, the associated phone number will likely not be listed. On the other hand, subscribers may not necessarily be aware that they have subscribed to such a service, and have had ENUM provisioned for that service by their service provider on their behalf.

Are users going to have control over how this system is used with their phone numbers?

Ultimately yes. To reiterate, the first principle in the creation and operation of a global ENUM service is that phone number subscribers or their designated representatives are the ultimate decision makers on how a DNS record for a phone number is to be provisioned.

How will the rights of telephone number subscribers be protected?

This is an essential question that must be resolved, but a clear statement of policy protecting subscribers should be part of any ENUM system charter. A simple answer is by respecting existing regulatory and business rules regarding number administration, slamming, non-reliance, etc. Only by replicating or re-implementing ENUM analogs to the existing rules of the road will we avoid a wide range of serious administrative, operational, and political conflicts.

How are you going to prevent "slamming" or "hijacking"?

Slamming, or the involuntary transfer of service provider, must be avoided in any ENUM system. However, it is a serious problem in the PSTN and we must be careful not to expect more from Internet services than we are able to guarantee elsewhere. Note that anti-slamming fundamentally requires a neutral third party solution. The US industry is grappling with this issue on long distance right now. It was solved on number portability from the outset. Authenticated subscriber access is not a total solution, because if subscribers disconnect their telephony service, they lose rights to the phone number. Consequently, some combination of originator authentication as well as telephone number rights validation, using new and existing validation sources, can be used to solve the problem, depending on the level of standard required.

Are there any examples of global namespace delegation that should be considered as models?

The closest, technical equivalent is in-addr.arpa. That domain provides a reverse mapping, from IP address to domain name. It is used as part of the Internet infrastructure operation, to help authenticate an IP address and identify the operator associated with an IP address. It is not seen directly by users. The same is true for e164.arpa. It will be for operational infrastructure, rather than for direct access by end users.

As with e164.arpa, in-addr.arpa, allocations are hierarchical, according to the infrastructure administrative structure. For in-addr.arpa, the hierarchy uses the "CIDR" address allocation hierarchy. For e164.arpa, the hierarchy will be based on the ITU E.164 Recommendation.

What will be the effort to administer the root of the e164.arpa namespace?

Any solution ought to require little or no work on the part of the e164.arpa root administrator. Optimally the root of e164.arpa should contain a small listing of all of the national ENUM top-level country code name servers.

Who can administer the ENUM registry in the near-term?

ENUM is approaching the stage where the industry will want to start interoperability testing, and they will want to test using the e164.arpa domain. The interoperability test would have the same principles that current ones do; that is, no charge, sharing of information, etc. One method of enabling the registry would be to develop an RFC that defines the interim delegation principles for IANA as well as principles for the transition to the permanent registry.

What can be done in the long term?

There will need to be a formal effort to define and establish the structure for this activity. An example of the charter for that effort would be:

1. Define the global ENUM Service.
2. Perform the task of certifying organizations to IANA that wish to operate national TN/NS once they have been nominated by their respective nation states. A simple letter could be sent to appropriate national authorities asking them how they wish to proceed or if they even want to participate.
3. Coordinate technical standards for the operation of ENUM service in cooperation with the IETF.
4. Establish guidelines and policies, which national TN/NS administrators operate.
5. Promote public policy on how ENUM resources should be used.

Oversight for this activity should comprise several constituencies, such as:

1. The potential ENUM user community,
2. The potential ENUM provider community,
3. National governments, at least as an advisory, and
4. IAB-IESG representatives.

Who will administer the national telephone number name servers?

There are many competent companies or organizations that can operate these servers. A number of companies have already come forward to express their interest in running these servers, initially free of charge and on an experimental basis, until such time as consensus can be reached on how this system is to ultimately organize.

There are a number of regulatory constraints in various countries that might apply to the ENUM administrator, name service operators, and delegation policies below the national level. For example, where local telephony service competition and number portability are being deployed in a country, it is not unusual that a neutral third party is required to provide master database administration services, and a requirement for anti-slamming and non-reliance on competing carriers for routing or resolution functions.