Network Working Group A. Mayrhofer

Network Working Group A. Mayrhofer
Request for Comments: 4725 enum.at
Category: Informational B. Hoeneisen
Switch
November 2006

ENUM Validation Architecture

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2006).

Abstract

An ENUM domain name is tightly coupled with the underlying E.164
number. The process of verifying whether or not the Registrant of an
ENUM domain name is identical to the Assignee of the corresponding
E.164 number is commonly called "validation". This document
describes validation requirements and a high-level architecture for
an ENUM validation infrastructure.

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RFC 4725 ENUM Validation Architecture November 2006

Table of Contents

1. Introduction ....................................................3
2. Requirements ....................................................3
3. ENUM Provisioning Model and Roles ...............................4
3.1. Number Assignment Entity (NAE) .............................5
3.2. Assignee ...................................................7
3.3. Registrant .................................................7
3.4. Validation Entity (VE) .....................................7
3.5. Registry ...................................................8
3.6. Registrar ..................................................8
3.7. Domain Name System Service Provider (DNS-SP) ...............8
3.8. Application Service Provider (ASP) .........................8
4. Validation Process Assumptions ..................................9
4.1. Workflow ...................................................9
4.2. Trust Relations ...........................................10
4.3. Data Flow and Format ......................................11
5. Example Scenarios ..............................................11
5.1. E.164 Number Assignment along with ENUM Registration ......11
5.2. Fully Disjoint Roles ......................................13
6. Security Considerations ........................................14
6.1. Fraud Prevention ..........................................14
6.2. Assignee Data .............................................14
7. Acknowledgements ...............................................15
8. References .....................................................15
8.1. Normative References ......................................15
8.2. Informative References ....................................15

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1. Introduction

E.164 Number Mapping (ENUM) [1] uses the Domain Name System (DNS) [4]
to refer from E.164 numbers [2] to Uniform Resource Identifiers
(URIs) [3]. E.164 numbers are mapped to domain names through means
described further in RFC 3761 [1].

"Ordinary" domain names are usually allocated on a first-come-first-
served basis, where the associated registration data is the complete
source of ownership. However, ENUM domain names are linked to E.164
numbers, and thus intrinsically tied to the status and the "Assignee"
(defined in Section 3.2) of the corresponding E.164 number.

2. Requirements

Preserving integrity between ENUM and E.164 is one of the main
concerns in ENUM implementations, and often one of the reasons why
"trials" precede commercial implementations.

To maintain this relationship between E.164 numbers and ENUM domain
names, registration processes must ensure that the following
requirements are fulfilled during the entire lifetime of an ENUM
delegation:

o The ENUM domain name corresponds either to an assigned E.164
number or to a respective E.164 number that is assigned during the
registration process itself.

o The corresponding E.164 number is within a number range approved
to be used with ENUM.

o The registration of the ENUM domain name is authorized by the
Assignee of the corresponding E.164 number; i.e., the entity
requesting the registration of an ENUM domain name is either the
Assignee of the corresponding E.164 number itself or an entity
authorized to request registration on behalf of said Assignee.

o The "Registrant" (see Section 3.3) of the ENUM domain is identical
to the Assignee of the corresponding E.164 number.

The process of verifying the above requirements during registration
is commonly called "initial validation". In addition to this one-
time validation process, provisions must be made that ENUM domain
name delegations are revoked when the above requirements are no
longer met. In other words, it must be ensured that the state of the
ENUM domain name tracks any change in state and ownership of the

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corresponding E.164 number. The regular process of checking that the
above requirements are still satisfied is commonly called "recurring
validation" or "revalidation".

The above requirements are usually part of the local registration
policy issued by the authorities in charge of ENUM administration.

3. ENUM Provisioning Model and Roles

The above requirements lead to the introduction of a new role in the
provisioning model, an entity performing validation related tasks:
The Validation Entity (VE). A typical ENUM provisioning model, on
which this document is based, is depicted in Figure 1:

+----------+
.| Registry |- -- -- -- -- -- --
. +----------+ |
. |
. | | Trust
DNS Delegation | Relation
. | Registration |
. |
. | |
+--------+ +-----------+ +----+
| DNS-SP |-- -- -- -- --| Registrar |----------------| VE |
+--------+ Nameservers +-----------+ Validation +----+
: | / |
: | E.164 Number
: | ENUM Assignment
: NAPTR | Management _ Verification
: | / |
: | _
: | / |
+-----+ ENUM enabled +------------+ E.164 Number +-----+
| ASP |- -- -- -- -- --| Assignee = |-- -- -- -- --| NAE |
+-----+ Service | Registrant | Assignment +-----+
+------------+

Legend:

ASP: Application Service Provider
DNS-SP: Domain Name System Service Provider
NAE: Number Assignment Entity
VE: Validation Entity

Figure 1: ENUM Model

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These different roles are described further below. Note that an
entity can act in more than one of these roles simultaneously; for
example, the Registrar, the DNS-SP, and the ASP roles could be
performed by a single company.

3.1. Number Assignment Entity (NAE)

A Number Assignment Entity (NAE) assigns E.164 numbers to end-users.
Often, but not always, the Communication Service Provider (CSP) of
the end-user (Assignee) acts as NAE. There are two main variants for
E.164 number assignments:

1. Indirect assignment:

The National Number Plan Administrator (NNPA) assigns ranges of
E.164 numbers to CSPs. Out of these ranges, the CSPs assign
numbers (or number blocks) to their customers (end-users,
Assignees). In this variant, the CSPs perform the role of the
NAE.

2. Direct assignment:

In certain cases, an NNPA assigns E.164 numbers directly to
Assignees (end-users), and therefore the NNPA acts as NAE in this
variant. Typically, this concerns the assignment of special
purpose numbers (e.g., premium rate).

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These two variants of E.164 number assignment are depicted in
Figure 2:

+--------------------------------------------+
| International Telecommunication Union (ITU)|
+--------------------------------------------+
|
Country codes (e.g., +44)
|
v
+-------------------------------------------+
| National Number Plan Administrator (NNPA) |------------+
+-------------------------------------------+ |
| |
Number Ranges |
(e.g., +44 20 7946 xxxx) |
| |
v |
+--------------------------------------+ |
| Communication Service Provider (CSP) | |
+--------------------------------------+ |
| |
| Single Numbers
Either Single Numbers (e.g., +44 909 8790879)
or Number Blocks (Variant 2)
(e.g., +44 20 7946 0999, +44 20 7946 07xx) |
(Variant 1) |
| |
v |
+----------+ |
| Assignee |<------------------------------+
+----------+

Figure 2: E.164 Number Assignment

(Note: Numbers above are "drama" numbers and are shown for
illustrative purpose only. Assignment polices for similar "real"
numbers in country code +44 may differ.)

As the Assignee (subscriber) data associated with an E.164 number is
the primary source of number assignment information, the NAE usually
holds the authoritative information required to confirm the
assignment.

A CSP that acts as NAE (indirect assignment) may therefore easily
assert the E.164 number assignment for its subscribers. In some
cases, such CSPs operate database(s) containing service information
on their subscribers' numbers. Typically, authorized entities such

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as other CSPs are allowed to access these databases, in real-time,
under contract for the limited purposes of billing and validation (no
marketing, data mining, or otherwise). These databases could be re-
used for ENUM validation purposes.

Number portability transactions may lead to situations where the CSP
that originally acted as NAE no longer has authoritative assignment
information about ported numbers. Whether the old and/or the new CSP
act(s) as NAE for ported numbers depends on local policy.

However, it is unlikely that all CSPs acting as NAEs will participate
in ENUM validation.

3.2. Assignee

The person or organization to whom a NAE assigns an E.164 number is
called Assignee of this number. For the scope of this document, the
terms Assignee, subscriber, and number-holder are used equivalently.

The Assignee has the "right to use" on the assigned E.164 number.

3.3. Registrant

The ENUM Registrant is the end-user, the person or organization who
is the "holder" of the ENUM domain name.

The Registrant usually has control over his ENUM domain name(s) and
its DNS zone content.

3.4. Validation Entity (VE)

The Validation Entity (VE) verifies whether or not the Registrant of
an ENUM domain name is identical to the Assignee of the corresponding
E.164 number.

Often it also verifies that the entity requesting the registration of
an ENUM domain name is either the Assignee of the corresponding E.164
number itself or an entity authorized to request registration on
behalf of said Assignee.

This role may be performed by several parties and is not necessarily
limited to a single entity.

The actual validation methods applied may vary depending on, e.g.,
the particular party, available data sources, Assignee's choice, and
regulatory requirements. Validation methods are out of scope of this
document.

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3.5. Registry

The ENUM Registry operates the master database of ENUM domain
delegations and runs the authoritative nameservers for the relevant
zone under e164.arpa. There must always be a single authoritative
ENUM Registry for a specific zone.

3.6. Registrar

An ENUM Registrar performs ENUM domain delegations on behalf of a
Registrant by interacting with the Registry, typically through a
protocol like Extensible Provisioning Protocol (EPP) [5]. This role
is similar to the one that Registrars fulfill in the "ordinary"
domain name registration world.

The Registrar may well not be the same entity as the CSP of the
Registrant. Therefore, a Registrar may lack authoritative number-
assignment information. If the Registrar and the CSP are the same
entity (or has a source of authoritative data), the Registrar could
perform the role of the VE itself.

In any case, a Registrar has to ensure a proper validation through a
VE prior to the registration of an ENUM domain name.

3.7. Domain Name System Service Provider (DNS-SP)

The Domain Name System Service Provider (DNS-SP) operates the
nameservers for the ENUM DNS zones, which contain the ENUM Naming
Authority Pointer (NAPTR) Resource Record (RR) entries [1].

In most cases, the Registry delegates the ENUM DNS zones to the
nameservers at the DNS-SP.

The DNS-SP is usually not involved in the validation process.

3.8. Application Service Provider (ASP)

The Application Service Provider (ASP) operates a service for the
Registrant. This service could be an IP telephony service, whereby
the service provider populates the ENUM zone for its customers so
that others can discover that customer's URI.

Usually, the ASP is not involved in the validation process.

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4. Validation Process Assumptions

4.1. Workflow

The prototypical initial validation workflow using the above roles
and definitions consists of the following steps:

1. A potential Registrant approaches a Registrar, and orders an ENUM
domain name.

2. The Registrar chooses a cooperating Validation Entity, and
requests an initial validation for the ENUM domain name ordered.

3. The Validation Entity performs the actual validation, which could
require interaction with the Assignee/Registrant.

4. The Validation Entity indicates the result of the initial
validation to the Registrar.

5. If the validation process was successful, the Registrar
provisions the ENUM domain name with the Registry. Depending on
the local Registry policy, validation-related information may be
provided to the Registry along with this registration.

In most cases, local policy mandates expiration dates to be imposed
on successful validations. If the ENUM delegation is to be kept
beyond this expiration date, recurring validation has to be
performed. A typical revalidation workflow involves the following
steps:

1. In good time before the current validation expires, the Registrar
requests the Validation Entity to revalidate the domain name in
question.

2. The Validation Entity verifies if the delegation requirements are
still met. It may use information acquired during the initial
validation or associated to the registration data.

3. The Validation Entity indicates the result of the recurring
validation to the Registrar.

4. In case the revalidation has been successful, the domain
delegation may persist. Local Registry policy may require
updating domain name registration data, especially in case the
Registry keeps validation-related expiry information.

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5. In case the revalidation has failed, the ENUM domain delegation
must be suspended, either by explicit interaction with the
Registry or -- if the Registry keeps validation-related
information -- automatically when the current validation expires.
Local policy may grant a grace period on the expiration date.

This workflow ensures the integrity between the E.164 and ENUM
namespaces. ENUM domain delegations that fail to meet the validation
requirements are suspended from the DNS.

4.2. Trust Relations

The above validation workflow implies the following trust relations:

o The Registry trusts the Validation Entities to enforce the local
validation policy.

o The Registrars trust the Validation Entities to properly perform
validation based on the Registrar's request.

o Depending on the amount of validation data provided to the
Registry additional trust relations may be necessary. Three cases
can be differentiated:

* The Registry receives no validation-related data: The Registry
needs to trust the Registrar that validation has been
performed, and the result was positive. In addition, the
Registry needs to trust the Registrar that it will properly
remove delegations for which revalidation fails.

* The Registry receives validation-related data including expiry
date, but there are no means of checking its authenticity: The
Registry needs to trust the Registrar that the validation data
provided is authentic.

* The Registry receives validation-related data including expiry
date and means to verify its authenticity (e.g., a
cryptographic signature issued by the VE): No additional trust
relations are necessary.

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4.3. Data Flow and Format

The validation process requires the following regular data flows
(Note: data flows not directly related to validation are out of scope
of this document):

o Registrars communicate with Validation Entities to initiate,
modify, or cancel validation requests. Validation Entities act
upon validation requests and provide validation results to
Registrars. Since Registrars could potentially communicate with
several Validation Entities, and Validation Entities could provide
services to several Registrars (worst case: full mesh), a
standardized protocol and data format should be used in this data
flow.

o If the local Registry policy mandates that validation-related
information is to be stored along with delegation records, a
validation-related data flow between Registry and Registrar is
required. Since the registration itself already requires
communication between those entities, validation-related
information in a standardized data format should be embedded into
the existing Registry-Registrar protocol data flow.

o Validation Entities may need to communicate with Assignees to
perform validation. A Validation Entity may choose to perform all
communication with the Assignee via the requesting Registrar
rather than contacting the Assignee by itself. Since the actual
communication form and process are expected to greatly vary, it
does not make sense to specify any data formats or processes for
this purpose.

5. Example Scenarios

5.1. E.164 Number Assignment along with ENUM Registration

In this simple scenario, we assume that the roles of the Registrar,
the VE, and the NAE are performed by the same entity, e.g., an
Internet Telephony Service Provider (ITSP). This ITSP is a CSP that
was assigned number ranges by the NNPA. Out of these ranges he
assigns numbers to his customers (Assignees) to provide those with
communication services. The ITSP chooses to assign an E.164 number
together with the corresponding ENUM domain name. Therefore, it can
perform the validation simply by reference to its subscriber
database.

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Figure 3 shows the external interactions needed for the ENUM domain
name provisioning process:

+----------+
| Registry |
+----------+
^
|
|(3)
|
+--------------------------------------+
| |
| ITSP |
| +-----------+ +----+ |
| | Registrar | | VE | |
| +-----------+ (2) +----+ |
| |
+--------------------------+ |
^ | |
| | |
|(1) | |
| | |
| | |
+------------+ (4) | +-----+ |
| Assignee = |<----------| | NAE | |
| Registrant | | +-----+ |
------------- | |
+-----------+

Legend:

ITSP: Internet Telephony Service Provider
NAE: Number Assignment Entity
VE: Validation Entity

Figure 3: E.164 Number Assignment along with ENUM Registration

(1) The ITSP receives an order for ENUM services.
(2) The ITSP assigns a free E.164 number and performs the validation
at the same time.
(3) The ITSP sends an ENUM registration request to the Registry,
which might contain additional information about the validation
applied.
(4) The ITSP sends a confirmation about the E.164 number assignment
and the ENUM registration to its customer, who is now Assignee
and Registrant.

This scenario is quite close to "ordinary" domain name registrations.

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5.2. Fully Disjoint Roles

In this more complex scenario, we assume that all roles of the ENUM
provisioning model are performed by different entities. In contrast
with the previous example (in Section 5.1), we assume that the ENUM
domain name to be registered is based on an already assigned E.164
number and the NAE in question provides the VE with access to the
subscriber database. We further assume that there is a requirement
for the VE to verify the intention of the Assignee. The validation
process therefore involves also contacting the Assignee.

Figure 4 shows the interactions needed for the ENUM domain name
provisioning process:

+----------+
| Registry |
+----------+
^
|
|(9)
|
|
| (3)
+-----------+ ---------->+----+
| Registrar |<---------- | VE |
+-----------+ (8) > +----+
^ / / ^ |
| / / | |
| (7)/ / | |
|(2) / / | |
| / / (5)| |
| / / | |
| / / | |
| / /(6) | |
| / / | |(4)
| / / | |
| / / | |
+------------+< | v
| Assignee = | +-----+
| Registrant |<---------- | NAE |
+------------+ (1) +-----+

Legend:

NAE: Number Assignment Entity
VE: Validation Entity

Figure 4: Fully Disjoint Roles

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RFC 4725 ENUM Validation Architecture November 2006

(1) The NAE assigns an E.164 number. This assignment could have
been done long before the ENUM domain name registration, e.g.,
at the time when the Assignee subscribed to a common telephony
service.
(2) The Assignee orders the corresponding ENUM domain name at a
Registrar of his choice.
(3) The Registrar requests validation at an independent VE.
(4) The VE contacts the subscriber database of the NAE, to verify
that the Assignee of the E.164 number corresponds to the
Registrant of the ENUM domain name.
(5) The result of the NAE subscriber database is positive.
(6) The VE performs a call-back to the E.164 number to be registered
as ENUM domain name, makes provisions for authentication, and
asks the Assignee to confirm his intention.
(7) The Assignee confirms and the VE documents this confirmation.
(8) The VE returns a positive answer to the Registrar. The answer
might contain some additional information about the validation
process, such as expiration date, validation method applied, and
so on.
(9) Finally, the Registrar sends an ENUM registration request to the
Registry. Additional information about the validation process
might be sent along with the registration request.

6. Security Considerations

6.1. Fraud Prevention

Situations where an entity has control over the ENUM domain of a
third party's E.164 number impose high fraud potential. Unauthorized
control over an ENUM domain of a bank could, for example, be used for
"man in the middle" attacks on telephone banking applications. Cases
of such attacks could discredit ENUM as a whole.

Implementing high-quality validation processes is therefore crucial
to any ENUM deployment and should receive high attention.

6.2. Assignee Data

When handling Assignee data, privacy and discretion issues must be
considered. Implementations transporting assignee data over the
Internet must use authenticated and encrypted transport protocols.
Local registration/validation policy and agreements should clearly
limit usage of Assignee data.

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7. Acknowledgements

The authors would like to thank the following persons for their
valuable suggestions and contributions: Lawrence Conroy, Michael
Haberler, Ted Hardie, Otmar Lendl, Hala Mowafy, Marcel Parodi, Jon
Peterson, Penn Pfautz, Patrik Schaefer, and Richard Stastny.

8. References

8.1. Normative References

[1] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.

[2] ITU-T, "The international public telecommunication numbering
plan", Recommendation E.164 (02/05), Feb 2005.

8.2. Informative References

[3] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.

[4] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[5] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
RFC 3730, March 2004.

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Authors' Addresses

Alexander Mayrhofer
enum.at GmbH
Karlsplatz 1/9
Wien A-1010
Austria

Phone: +43 1 5056416 34
EMail: [email protected]
URI: http://www.enum.at/

Bernie Hoeneisen
Switch
Neumuehlequai 6
CH-8001 Zuerich
Switzerland

Phone: +41 44 268 1515
EMail: [email protected], [email protected]
URI: http://www.switch.ch/

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RFC 4725 ENUM Validation Architecture November 2006

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