The principles and conditions of implementation of the ENUM protocol in France

Public consultation

23 May 2001 - 12 June 2001

Version française
to download the public consultation


I- Introduction of ENUM

II- Applications of the ENUM protocol and expected benefits for consumers

III-Questions regarding numbering and naming


The context

The integration of telecommunications services and Internet services is accelerating in a move towards the convergence and generalisation of IP technologies in networks. We are currently seeing an increased use of IP technologies in telecommunications networks, which have traditionally been based on circuit switching. This movement is picking up even more speed by the increase in IP traffic linked to the penetration of Internet services, but also to the technological innovations allowing IP networks to support real-time applications. These changes are seen in new services integrating voice and video and by the appearance of new players providing these services.

The coexistence of the two types of network with different architectures and functions may require the implementation of solutions which guarantee the interoperability of the networks and services. In particular, numbering, naming and addressing aspects seem to play a determining role in the ability to implement truly convergent projects.

Thus, projects associating E.164 numbering resources and Internet domain names are currently being developed. In particular, the ENUM protocol project, defined by RFC 2916 of the IETF, involves creating Internet domain names from E.164 telephone numbers, and linking them to communication services.

ENUM is an important project in the area of convergence and in the development of telecommunications networks. However, its implementation raises a number of problems both internationally and nationally. In particular, the use of telephone numbers and their integration in Internet domain names raises many questions regarding the coherence of these two systems.

This is why the Secretary of State to Industry and Autorité de régulation des télécommunications (ART), which is responsible for managing the national numbering plan, wish to consult players on the implications and the means of implementing the ENUM project1. The goal of this consultation is to raise awareness among players about the ENUM project and to gather their opinions on the possible conditions of implementation. It goes beyond the French context. Indeed, the conditions of implementation in France (Metropolitan France and DOM) will depend to a great extent on the options chosen at the international level. Therefore, in order to enlighten it in preparing the French position, the Secretary of State to Industry and ART also wish to gather the opinions of players on the conditions of international implementation which have not yet been finalised.

This document is divided into three parts. First, we present the ENUM protocol as its principle is defined by IETF2 specifications (part I). We then discuss the questions regarding the applications of the ENUM protocol and the expected benefits for consumers (part II). Finally, we discuss the questions regarding numbering and naming (Part III).

How the consultation is held

Those persons or organisations wishing to contribute to the discussion in this public consultation should send their comments, in French, to Autorité de régulation des télécommunications by 12 June 2001 at 12:00 noon.

In particular, contributors are invited to comment on the questions posed in this document, but may also share with ART their thoughts on any subject related to the implications of the convergence of IP and non-IP network services.

Comments in writing must be identified as being a "réponse à la consultation publique sur les conditions de mise en œuvre du protocole ENUM en France" and sent to:
Autorité de régulation des télécommunications
Service Interconnexion et Nouvelles Technologies
7, square Max Hymans
75730 Paris Cedex 15

Comments may also be submitted by e-mail to the following address:

Contributors are reminded that unsecured e-mails sent via Internet are not guaranteed confidential.

Contributors may add a link between ART's site ( and their response, which they may publish on their Web site. Interested persons are invited to contact Ingrid Violet, on +33 (0)1 40 47 70 35.

ART reserves the right to publish all or part of the responses it receives, unless otherwise stipulated by the author. In this case, the author is invited to place confidential elements in an appendix.

For more information, contact Stanislas Bourgain (Tel: +33 (0)1 40 47 71 06 Fax: +33 (0)1 40 47 71 88) or Olivier Esper (Tel: +33 (0)1 40 47 71 57  Fax: +33 (0)1 40 47 7188)

I- Introduction of ENUM

This first part briefly reviews the principles of E.164 numbering and of the Internet domain name system. It explains how the ENUM protocol project works.

I.1 Definition of ENUM

ENUM is a protocol born of the work of the IETF, whose principles are explained in specification RFC 29163. It defines the conversion of a E.164 telephone number into a domain name, which can be used for various communication services (telephone service, e-mail, fax, location, messaging, etc.)

I.2 E.164 telephone numbering

The organisation of traditional telephone numbering was defined at the international level by the ITU in its E.164 recommendation which describes the structure of the numbers used in the international telecommunication system and defines the general framework and criteria for assigning country codes. The ITU coordinates the international numbering plan and assigns geographic country codes to member states.

At the national level, each member state has sovereignty in assigning and using its numbering resources on its own territory. It must guarantee the integrity and coherence of the national numbering plan. In France, Autorité de régulation des télécommunications (ART) plays this role. It manages the resources corresponding to the following E.164 codes: 33 (Metropolitan France), 262 (Reunion), 508 (St-Pierre-et-Miquelon), 590 (Guadeloupe), 594 (Guyana) and 596 (Martinique).

I.3 How DNS works

The domain naming system (DNS) is composed of a system of distributed and hierarchical servers, supervised by a system of root servers4.

Its purpose is to link to the numeric addresses of the machines connected to Internet (IP addresses) to "names" which better identify the entity sought and which are easier to remember: e.g. is easier to read than

Domain names

A suffix indicates the top level domain (TLD) the names are recorded in. In practice, there are three types of top level domains:

  • 243 national domains (ccTLD for country code Top Level Domain) defined by the two-letter codes of standard ISO-3166: .fr, .uk, .be, etc.
  • 1 international domain (.int) reserved for international organisations
  • 6 generic domains (gTLD for generic Top Level domain) which correspond to the type or main activity of the organisation registered

- 3 gTld are open to organisations worldwide wishing to register their domain:

.com., .org, .net.

- 3 other gTLD, .edu (education), .gov (government) and .mil (military), are reserved to American organisations


  • 2 new gTLD (.biz and .info) have recently been accredited by ICANN5 and five others will be soon (.aero, .coop, .museum, .name, .pro)
  • the TLD .arpa, is managed jointly by the IANA and IAB, under the authority of the American government

Each top level domain name manager (AFNIC defines the rules for assigning names under this top level.

IP addresses

IP addresses are managed by three regional organisations, as delegated by ICANN:
ARIN for North America
RIPE for Europe and Africa6
APNIC for Asia-Pacific

These non-profit organisations draw up the rules which determine how they redistribute the IP addresses to their members (service providers).

ICANN coordinates the DNS system: it delegates the management of top level domains and ensures the global database is coherent. The Internet root is a file, whose maintenance is delegated by the Department of Commerce of the American government and ICANN to a technical service provider7. This file is then replicated on the other root servers.

In practice, to determine the IP address of the site, a local DNS server sends a query to the root servers which return the address of the server hosting the zone ".fr" to which it then resubmits its query. The server of the domain ".fr" gives the address of the server hosting "", which provides the IP address for the site

The management rules of E.164 numbering and Internet naming resources are drafted by different bodies according to different mechanisms. For E.164 numbers, the management principles are defined by an international governmental organisation, the ITU, and the specific allocation rules by the member states. Originally held by the American government, the responsibility for the Internet domain name system has been progressively transferred to ICANN since 1998. This California private non-profit organisation is under contract with the US Department of Commerce. The mechanism put in place in 1998 aims to have market players coordinate and define the rules ("auto-regulation").

However, the changes in the issues and problems posed by Internet raise the question of the role of the states in coordinating and managing Internet. In a recently published report8, the European Commission suggests that European governments participate more actively in Internet management in order to strengthen their role and to guarantee the eventual independence of ICANN.

I.4 How ENUM works

ENUM defines an Internet domain name using an E.164 telephone number, and links it to communication services (telephone service, fax, email, pager, etc.).

Creating an Internet domain name from an E.164 number

In order to respect the hierarchical structure of domain names, the conversion involves adding the number's country code and inverting the telephone number. Plus, a decimal point (.) in the character string indicates a position where database query is possible. ENUM has chosen to separate each individual digit with a decimal. Thus, each digit determines a domain whose administration and technical management can be delegated.

For example, the telephone number 01 40 47 70 00 would be converted as "" (add France's country code +33 and invert the number), where XXX is the domain in which the ENUM domain names would be registered.

II- Applications of the ENUM protocol and expected benefits for consumers

II.1- Services

According to the ENUM protocol, a list of communication services is linked to each ENUM domain name which are used to reach the user linked to the ENUM domain name.

For example, the database queried on the domain name "" (i.e. the number 01 40 47 70 00) would return a list indicating that the number 01 40 47 70 00 can be reached:

It would also indicate the order of preference, among the communication services, of the person linked to the number 01 40 47 70 00.

The ENUM domain name would thus be converted in a SIP URL9 for IP telephony, an e-mail address, a URL, etc.

The diagram below shows how a service would work using the ENUM protocol:

1. A dials 01 40 47 70 00 and initiates the call
2. The telephone network routes the call to a gateway with ENUM functions
3. The gateway converts the telephone number into an Internet address...
4. The gateway queries the DNS server
5. The DNS server returns the address of the domain name... at which the person can be reached, e.g. an SIP address such as
6. The DNS returns the IP address of the SIP server linked to the URL
7. The SIP server routes the call to B

The applications created by the implementation of the ENUM project make concrete some of the first services of the convergence of Internet and the telecommunications world. They might for example be of interest to users who wish to be reached via different methods of communication with a single number.

These applications, intended for a broad public, are nevertheless still poorly known and their success will depend to a great extent on their benefit to consumers and on the uses that consumers will make of them.

However, it appears that the penetration of these convergence services in the general public will be facilitated through the adoption of simple mechanisms. In this respect, the reuse of telephone numbers which have been generally well accepted seems to be an important factor in their development. The use of E.164 telephone numbers in Internet naming offers the advantage of making the service easily accessible and understandable by the consumer. The E.164 "naming" system of the "telecommunications world" indeed offers the advantage of being stable, used worldwide and accepted and understood by users.

Therefore, ENUM might facilitate the penetration of Internet-based communication services.

Moreover, the question of whether trials are needed can be raised. These trials might help us better identify ENUM services and system constraints. Such trials are already planned in several countries of the European Union.


The Secretary of State to Industry and ART wish to receive the comments and opinions of players concerned by the principle of a system like ENUM, the services which might be offered to the public using a system like ENUM, its underlying interests and issues. In particular:

Q1. To what types of services might the implementation of the ENUM protocol lead? Would these be new services? What existing services would be influenced by ENUM?

Q2.What client base would be targeted by these services? What penetration can be expected, and according to what timescale?

Q3. How do you see users accepting and adopting these services? Is the use of traditional telephone numbers for access to convergence services a real advantage to acceptance and adoption?

Q4. Is it necessary to conduct trials in France? Who should take the initiative for testing? What would be the goals of trials?

II.2 Accessibility of ENUM servers

The proper functioning of ENUM services will depend on the ability of the DNS infrastructure to support the frequency and the number of queries generated by ENUM services with an optimal availability rate. The position of domain servers used for ENUM and available national and international connectivity to and from these servers will be key elements. An ill-adapted or insufficient infrastructure could cause bottlenecks which would reduce the interest of the service for consumers.


Q5. What bottlenecks to the proper functioning of ENUM, can already be identified?

II.3 The legal obligations to which ENUM communication services are subject

The right to provide public of telecommunications services is subject to the Post and Telecommunications Code and in particular to the respect of essential requirements and prescriptions regarding defence and public safety. In particular, obligations such as routing emergency calls, legal interception of calls, contribution to the financing of universal service, etc. apply to the provision to the public of telephone service.

In that ENUM domains might be linked to communication services and in particular to the provision of a public telephone service, it is important that the legal obligations related to these services be taken into account in implementing the ENUM protocol.


Q6. Does ENUM and the communication services which might be linked to it raise problems with respect to the legal obligations currently applied to telecommunications services? In particular to telephone service?

III-Questions regarding numbering and naming

III.1 The insertion of E.164 numbers in the DNS

Primarily, ENUM raises the question of the coherence between E.164 telephone numbers and domain names. If the general public is to use services made possible by ENUM, it appears that perfect coherence will have to be guaranteed in order to protect the main advantage of E.164 numbering, which is the use of a system already widely used and accepted by the public.

Special attention should be paid to this question, for the following reasons:

  • Management of ENUM domain names which is not coordinated with that of E.164 numbers could result in the creation of ENUM subdomains which do not correspond to the country codes assigned by the ITU.
  • Similarly, management of ENUM domain names which is not coordinated with that of E.164 numbers could result in the assignment of ENUM domain names which do not respect the numbering plan matching a given country code.
  • Finally, poor correspondence between the assigned ENUM domain names and E.164 numbers could cause incoherence between the recipient of a telephone number and the recipient of the corresponding ENUM domain name.

These inconsistencies might, in the long run, threaten the principle of the protocol and its acceptance and adoption by users. Indeed, the system would lose its interest and understandability since the automatic correspondence between domain names and telephone numbers—its principal advantage—would no longer be assured.

As a result, rules must be defined for inserting E.164 numbers in the ENUM system, in order to protect coherence between the two systems. Since ENUM reuses telephone numbers and transforms them, the subordination of the management of the ENUM system to that of the E.164 numbering seems the simplest and most logical solution to guarantee coherence.

These elements lead us to suggest that ITU play a coordinating role in the implementation of ENUM.

On the one hand, ITU might oversee the coherence, of the ENUM system with the E.164 numbering plan at the highest level10, by monitoring the subdomain delegation database. In particular, this would make it possible to guarantee that a State is made responsible for managing the domain corresponding to its own country code, for example, "" for France. However, ITU would be able to delegate technical management of the base to a third party.

On the other hand, it might define certain essential rules at the supra-national level regarding the insertion of E.164 numbers in the DNS.

Such a measure should make it possible to guarantee coherence between the ENUM DNS and E. 164 numbering.

Moreover, the management of the ENUM base by ITU would guarantee that ENUM resources are managed independently of market players. This independence would guarantee equal and simple access for users to the various networks and telecommunications services and would prevent the creation of dominant positions. As for E.164 numbering resources, such independence constitutes an essential element in the development of competition.

The question of coordination of ENUM by the ITU is not unrelated to the choice of the top level domain (".xxx") which will be used to implement ENUM (Cf. II-2 Choosing the top level domain). These two questions could, however, be dealt with separately, at least initially.


The Secretary of State to Industry and ART wish to receive the comments and opinions of players concerned by its analysis of the question of the insertion of E.164 numbers in the Internet naming system.

In particular:

Q7. Must the domain name management used with the ENUM protocol be subordinate to the management of E.164 numbers? What special rules would be needed to ensure the perfect correspondence between E.164 telephone numbers and ENUM domain names? Who should define these rules?

Q8. Do you feel it would be necessary that the ITU provide coordination in order to guarantee the coherence of ENUM with the numbering plan? What might be the alternatives? In your opinion, what might be the relation between the ITU, which is responsible for the international telephone numbering and ICANN, the domain name management and Internet address supervisory body?

Q9. How can we protect the principles of free competition, non-discrimination and transparency between the players in implementing ENUM? What roles should be played by ITU and the bodies responsible for national numbering plans?

III.2 Choosing the top level domain

IETF's ENUM specifications stipulate that domain names corresponding to the E.164 telephone numbers will be located below the top level domain ".arpa". For example, if this suffix is chosen, the domain name for the number 01 40 47 70 00 would be "".

This suffix is currently used for the Internet infrastructure and is administered by IANA11. IAB12, which is responsible for the technical development of Internet, seems to have delegated the technical management of the second domain "" to RIPE NCC13 for the registration of ENUM domain names. As for the ITU, it would be responsible for administration. In this hypothesis, a member state would inform ITU of its desire to be delegated the domain corresponding to its country code in the "" domain. For metropolitan France, for example, this would be the domain "". ITU would then notify RIPE NCC of its authorisation to delegate the subdomain to the entity designated by the member state.

However, these choices (which are not confirmed14) are not yet final and remain controversial. For example, certain countries seem reticent to insert their numbering plan in a domain controlled by the American government (Department of Commerce), which through its contract with ICANN, has a right of inspection of the management of ".arpa". They suggest using another suffix.

Moreover, some prefer to let parallel root systems develop, like the ".gprs" naming system for mobile telephony, letting the market arbitrate between the various systems. On the other hand, others consider it important that ENUM be implemented in the DNS currently used in Internet and coordinated by ICANN.

In this context, it is possible that several systems like ENUM might be developed. Second-level domains similar to parallel ENUM systems have already been created by companies already having a strong presence in domain name management: such as "" for the company Vérisign, registry for the domains ".com", ".net" and ".org", "" registered by NetNumber or "" by the company Neustar which manages the numbering plan for North America and which was delegated the new top level domain ".biz" by ICANN. Moreover, the company Vérisign has initiated a test phase for ENUM services in a real-life environment in order to study the development of ENUM services and to solve any functioning and interoperability problems which might appear. For the time being, these systems still seem to be in the experimental phase. However their parallel development leads us to believe that several systems could be implemented. The existence of several second-level domains might cause confusion for the end user.

Alongside these initiatives, ITU is studying the feasibility of a public and universal system like ENUM. The domain used would be under the full responsibility of ITU, which could delegate technical management to a specialised service provider. Domains like "", or "", have been mentioned. Another solution mentioned would be that the management of "" be delegated.

A system developed by ITU would not only guarantee coherence with E.164 numbering plans (cf. III-1 Inserting E.164 numbers in the DNS), but also neutrality, reliability and global coverage.


Q10. Does it seem essential that a top level domain be chosen in implementing ENUM? What impact will this choice have on the potential applications of ENUM? On the development of competition?

Q11. What do you think of the IAB's proposal to use the domain in implementing ENUM? What do you think of IAB's "decision" to delegate the technical management of the domain to RIPE NCC?

Q12. What domain name do you consider the best suited to implementing ENUM? Who should manage it?

Q13. What would be the implications of implementing the ENUM protocol under several different domains? (, .enum,,,, etc.)? Could the choice of the domain have an impact on the acceptance and adoption of services by users? For example, do you think that parallel ENUM services developed under a domain such as "" would hold back the emergence of a public and universal ENUM service recognised by the ITU?

III.3 Implementation at the national level

The conditions of implementation of the ENUM protocol are currently under discussion in various international bodies. They are therefore not made official.

The models for implementation identify three levels of players:

- Tier 0: the registry of the domain under which ENUM is implemented (see III.2). It maintains an Internet domain name server which recognises the ENUM subdomain servers for the various country codes ("Level 1" below). For example, a query sent to the Tier 0 server on the name "" is rerouted to the server responsible for subdomain "". Questions related to the insertion of E.164 numbers in the DNS lead ART to recommend that the ITU be designated at Tier 0 (cf. III-1 and III-2).

- Tier 1: the governments are responsible for the domain corresponding to their country code(s), and designate an entity responsible for managing the subdomain(s). A Tier 1 entity is the equivalent of the registry of a "ccTLD"15, i.e. AFNIC for ".fr".

- Tier 2: a Tier-2 body is responsible for registering the numbers of the end users in the domain "". For Internet domain names, this tier corresponds to Internet Registrars, those bodies authorised to register names. In addition to registering numbers as domain names, the Tier 2 maintains a database linking each ENUM domain name with the communication services providing access to the end user (cf. II.4). An ENUM domain name must be stored by a single Tier 2 service provider16.

- Tier 3: communication service providers. The database of a Tier 2 service provider routes to the service provider servers (gateway to IP telephony, messaging server, etc.).

The diagram below shows the tiers explained above:

Role of Tier 1

If the role of Tier 1 is not provided by the manager of the E.164 numbering plan, it seems indispensable that there be coordination between this manager and the Tier 1. In particular, registrations of new ENUM domain names in the database maintained by the Tier 1 should be validated by a check against the E.164 numbering plan. The definition of delegation rules for subdomains corresponding to the ranges of assigned numbers and the purely technical management of the domain might be distributed between these bodies.

Role of Tier 2

Concerning the activity of Tier 2, this role might be held either by the operator holding the telephone number or by a service provider freely chosen by the user. The first solution would easily guarantee the identity of the recipient of the registered domain name and of the telephone number associated with it. However, the operator might render it difficult to use services provided by another service provider on this domain name. The second solution would prevent this and would offer users the choice of their Tier 2 service provider regardless of their telephone operator. On the other hand, it would make it more difficult to guarantee that the person requesting registration of the domain name is the actual recipient of the corresponding telephone number. This might increase the risk of "cyberpirating" of ENUM domains. A verification procedure involving the operator could be put in place.

The implementation of telephone number portability must also be taken into account in creating a model. In the case of the first model (Tier 2 service provider = operator holding the telephone number), when a number is ported to another operator, the ENUM domain name should logically also be ported to this operator, and the Tier 1 entity must be informed of this change so that it can modify its database accordingly. In the case of the second model (service provider chosen freely by the user), when a telephone number is ported from one operator to another, the Tier 2 service provider responsible for this number should be informed of this change so that it can modify its database accordingly. In both cases, a mechanism must be put in place to inform the Tier 1 entity or Tier 2 service provider that the number of an E.164 block has been transferred to another operator.

On these aspects, the ITU is preparing an additional document on ENUM, which will guide member states in implementing ENUM in their countries. This document is expected to be completed in September 2001.


Q14. What is the foreseeable impact on the development of ENUM services of the way responsibilities will be shared?

Q15. What are the risks of a dominant position which you can foresee in dividing responsibilities in implementing ENUM?

Q16. Should the Tier-2 activity be reserved to operators or be exercised by any service provider or other body? Must competition between Tier-2s be introduced in a way similar to that for domain name registers (registrars)?

Q17. How should domain names corresponding to telephone numbers be registered? How can we prevent pirating or cyberpirating of domain names matching a telephone number?

Q18. For portability, what procedures must be put in place to update the domain name database, to add or remove a subscriber?

1- Telephony Number Mapping; the ENUM project is described in the first part of the document.

2- IETF = Internet Engineering Task Force, standardisation body.


4- There is one root server primaire "a.root-server" and 12 secondary root servers.

5- ICANN = Internet Corporation for Assigned Names and Numbers, coordination body for Internet technical resources (domain names, IP addresses, protocols) at the global level (California private non-profit company).

6- An organisation for the Africa region (AFRINIC) is currently being created.

7- Currently it is Verisign, a company located in the United States.

8- "The organisation and Management of the Internet - International and European Policy Issues 1998-2000" (COM(2000) 202 - C-0263/2000-2000/2140 (COS))

9- Session Initiation Protocol Uniform Resource Locators

10- Coherence between the E.164 country codes or networks (for example 33 for France) with the ENUM subdomains.

11- Internet Assigned Numbers Authority

12- Internet Architecture Board

13- RIPE NCC = European IP networks Network Coordination Centre: agency responsible for managing IP addressing space for Europe.

14- In particular, if registration data for the domain "" indicate that the RIPE NCC is the technical contact for this domain, ITU is not designated as administrative contact. No agreement seems to have been signed between IAB and ITU.

15- Country Code Top Level Domain.

16- A domain name can be registered with one service provider only. If it is registered with more than one, there will be an incoherence when the DNS query is resolved, or the domain name belongs to a different tree structure in the DNS system used for Internet today.

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