Network Working Group R. Mahy
Request for Comments: 5028 Plantronics
Category: Standards Track October 2007 A Telephone Number Mapping (ENUM) Service Registration for
Instant Messaging (IM) Services
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document registers a Telephone Number Mapping (ENUM) service for
Instant Messaging (IM). Specifically, this document focuses on
provisioning 'im:' URIs (Uniform Resource Identifiers) in ENUM.
ENUM (E.164 Number Mapping, RFC 3761 ) is a system that uses DNS
(Domain Name Service, RFC 1034 ) to translate telephone numbers,
such as '+12025550100', into URIs (Uniform Resource Identifiers, RFC
3986 ), such as 'im:email@example.com'. ENUM exists primarily to
facilitate the interconnection of systems that rely on telephone
numbers with those that use URIs to identify resources.
Instant Messaging (IM) is a service defined in RFC 2778  that
allows users to send and receive typically short, often textual
messages in near real-time. The IETF has defined a generic URI used
to identify an IM service for a particular resource: the 'im:' URI
scheme (defined in RFC 3861 ). RFC 3861  also defines rules
for discovering service running specific protocols, such as SIP (the
Session Initiation Protocol, RFC 3261 ) and XMPP (the eXtensible
Messaging and Presence Protocol, RFC 3921 ) from a specific 'im:'
RFC 3953  already defines an enumservice for presence services,
which returns 'pres:' URIs (also defined in RFC 3861 ). This
document registers an enumservice for advertising IM information
associated with an E.164 number.
2. ENUM Service Registration - im
As defined in RFC 3761 , the following is a template covering
information needed for the registration of the enumservice specified
in this document:
This Enumservice indicates that the resource identified is an
'im:' URI. The 'im:' URI scheme does not identify any particular
protocol that will be used to handle instant messaging receipt or
delivery, rather the mechanism in RFC 3861  is used to discover
whether an IM protocol supported by the party querying ENUM is
also supported by the target resource.
See section 3.
Rohan Mahy (firstname.lastname@example.org)
3. Security Considerations
The Domain Name System (DNS) does not make policy decisions about
which records it provides to a DNS resolver. All DNS records must be
assumed to be available to all inquirers at all times. The
information provided within an ENUM record set must therefore be
considered open to the public -- which is a cause for some privacy
Revealing an 'im:' URI by itself is unlikely to introduce many
privacy concerns, although, depending on the structure of the URI, it
might reveal the full name or employer of the target. The use of
anonymous URIs mitigates this risk.
As ENUM uses DNS, which in its current form is an insecure protocol,
there is no mechanism for ensuring that the answer returned to a
query is authentic. An analysis of threats specific to the
dependence of ENUM on the DNS is provided in RFC 3761, and a thorough
analysis of threats to the DNS itself is covered in RFC 3833 .
Many of these problems are prevented when the resolver verifies the
authenticity of answers to its ENUM queries via DNSSEC  in zones
where it is available.
More serious security concerns are associated with potential attacks
against an underlying Instant Messaging system (for example, message
forgery and tampering). For this reason, IM protocols have a number
of security requirements (detailed in RFC 2779 ) that call for
authentication, integrity and confidentiality properties, and similar
measures to prevent such attacks. Any instant messaging protocol
used in conjunction with the 'im:' URI scheme is required to meet
Unlike a traditional telephone number, the resource identified by an
'im:' URI may require that callers provide cryptographic credentials
for authentication and authorization before instant messages are
exchanged. In concert with instant messaging protocols, ENUM can
actually provide far greater protection from unwanted callers than
does the existing PSTN, despite the public availability of ENUM
4. IANA Considerations
This document requests registration of the "im" Enumservice according
to the definitions in this document and RFC 3761 .
5.1. Normative References
 Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
 Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
 Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
 Peterson, J., "Address Resolution for Instant Messaging and
Presence", RFC 3861, August 2004.
 Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions", RFC
4035, March 2005.
5.2. Informative References
 Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence
and Instant Messaging", RFC 2778, February 2000.
 Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
Messaging / Presence Protocol Requirements", RFC 2779, February
 Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
 Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence", RFC 3921,
 Peterson, J., "Telephone Number Mapping (ENUM) Service
Registration for Presence Services", RFC 3953, January 2005.
 Atkins, D. and R. Austein, "Threat Analysis of the Domain Name
System (DNS)", RFC 3833, August 2004.
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