ENUM (E.164 Number Mapping, RFC 2916 ) is a system that uses DNS
(Domain Name Service, STD 13, RFC 1034 ) to translate telephone
numbers, like '+12025332600', into URIs (Uniform Resource
Identifiers, RFC 2396 ), like 'sip:firstname.lastname@example.org'. ENUM exists
primarily to facilitate the interconnection of systems that rely on
telephone numbers with those that use URIs to route transactions.
This document applies to the revised version of ENUM described in RFC
SIP (Session Initiation Protocol, RFC 3261 ) is a text-based
application protocol that allows endpoints on the Internet to
discover one another in order to exchange context information about a
session they would like to share. Common forms of communication that
are set up by SIP include Internet telephony, instant messaging,
video, Internet gaming and other forms of real-time communications.
SIP is a multi-service protocol capable of initiating sessions
involving different forms of real-time communications simultaneously.
SIP is a protocol that finds the best way for parties to communicate.
2. ENUM Service Registration
As defined in , the following is a template covering information
needed for the registration of the enumservice specified in this
Enumservice Name: "E2U+SIP"
URI Scheme(s): "sip:", "sips:"
Functional Specification: see Section 4
Security considerations: see Section 6
Intended usage: COMMON
Author: Jon Peterson (email@example.com)
Any other information that the author deems interesting: See
3. Addresses-of-record in SIP
This document specifies an enumservice field that is appropriate for
SIP addresses-of-record URIs. Various other types of URIs can be
present in SIP requests. A URI that is associated with a particular
SIP user agent (for example, a SIP phone) is commonly known as a SIP
The difference between a contact address and an address-of-record is
like the difference between a device and its user. While there is no
formal distinction in the syntax of these two forms of addresses,
contact addresses are associated with a particular device, and may
have a very device-specific form (like sip:10.0.0.1, or
sip:firstname.lastname@example.org). An address-of-record, however,
represents an identity of the user, generally a long-term identity,
and it does not have a dependency on any device; users can move
between devices or even be associated with multiple devices at one
time while retaining the same address-of-record. A simple URI,
generally of the form 'sip:email@example.com', is used for an
When a SIP request is created by a user agent, it populates the
address-of-record of its target in its To header field and
(generally) Request-URI. The address-of-record of the user that is
sending the request populates the From header field of the message;
the contact address of the device from which the request is sent is
listed in the Contact header field.
By sending a registration to a registrar on behalf of its user, a SIP
device (i.e., a user agent) can temporarily associate its own contact
address with the user's address-of-record. In so doing, the device
becomes eligible to receive requests that are sent to the address-
of-record. Upon receiving the registration request, the registrar
modifies the provisioning data in a SIP location service to create a
mapping between the address-of-record for the user and the device
where the user can currently be reached. When future requests arrive
at the administrative domain of this location service for the user in
question, proxy servers ask the location service where to find the
user, and will in turn discover the registered contact address(es).
A SIP-based follow-me telephony service, for example, would rely on
this real-time availability data in order to find the best place to
reach the end user without having to cycle through numerous devices
from which the user is not currently registered. Note that
addresses-of-record can be registered with other addresses-of-record;
for example, while at home, a user might elect to register the
address-of-record they use as their personal identity under their
work address-of-record in order to direct requests for their work
identity to whatever devices they might have associated with their
When a SIP entity (be it a user agent or proxy server) needs to make
a forwarding decision for a Request-URI containing an address-of-
record, it uses the mechanisms described in the SIP specification
(RFC 3263) to locate the proper resource in the network. Ordinarily,
this entails resolving the domain portion of the URI (example.com in
the example above) in order to route the call to a proxy server that
is responsible for that domain.
SIP user agents have specific communications capabilities (such as
the ability to initiate voice communications with particular codecs,
or support for particular SIP protocol extensions). Because an
address-of-record does not represent any particular device or set of
devices, an address-of-record does not have capabilities as such.
When a SIP user agent sends a request to an address-of-record, it
begins a phase of capability negotiation that will eventually
discover the best way for the originator to communicate with the
target. The originating user agent first expresses capabilities of
its own in the request it sends (and preferences for the type of
session it would like to initiate). The expression of these
capabilities may entail the usage of SDP  to list acceptable types
of media supported and favored by the client, the inclusion of
Required/Supported headers to negotiate compatibility of extensions,
and possibly the usage of optional SIP extensions, for example using
callee capabilities  to communicate request handling dispositions.
Proxy servers or endpoints subsequently return responses that allow a
rich bidirectional capability negotiation process.
The process by which SIP endpoints negotiate capabilities can overlap
with the primary service provided by NAPTR records: permitting the
originating client to select a particular URI for communications
based on an ordered list of enumservices. However, ENUM's capability
management mechanism is decidedly one-way - the administrator of the
telephone number expresses capabilities (in the form of protocol
names) and preferences that the client must evaluate without
negotiation. Moreover, listing available protocols is not comparable
to agreement on session media (down to the codec/interval level) and
protocol extension support - it would be difficult to express, in the
level of detail necessary to arrange a desired session, the
capabilities of a SIP device within a NAPTR service field.
Provisioning contact addresses in ENUM rather than addresses-of-
record would compromise the SIP capability negotiation and discovery
process. Much of the benefit of using a URI comes from the fact that
it represents a logical service associated with a user, rather than a
device - indeed, if ENUM wished to target particular devices,
'E2IPv4' would be a more appropriate resolution service to define
SIP addresses-of-record may use the SIP URI scheme or the SIPS URI
scheme. The SIPS URI scheme, when used in an address-of-record,
indicates that the user it represents can only be reached over a
secure connection (using TLS).
4. The 'E2U+SIP' enumservice
Traditionally, the services field of a NAPTR record (as defined in
) contains a string that is composed of two subfields: a
'protocol' subfield and a 'resolution service' subfield. ENUM in
particular defines an 'E2U' (E.164 to URI) resolution service. This
document defines an 'E2U+SIP' enumservice for SIP.
The scheme of the URI that will appear in the regexp field of a NAPTR
record using the 'E2U+SIP' enumservice may either be 'SIP' or 'SIPS'.
This enumservice is best suited to SIP addresses-of-record.
When a SIP address-of-record appears in the regexp field of a NAPTR
record, there is no need to further qualify the enumservice field
with any capability data, since addresses-of-record do not have
There is also generally no need to have more than one NAPTR record
under a single telephone number that points to a SIP address-of-
Note that the user portion of a SIP URI may contain a telephone
number (e.g., 'sip:+firstname.lastname@example.org'). Clients should be
careful to avoid infinite loops when recursively performing ENUM
queries on URIs that result from an ENUM lookup.
5. Example of E2U+SIP enumservice
The following is an example of the use of the enumservice registered
by this document in a NAPTR resource record.
IN NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:email@example.com!" .
6. Security Considerations
A SIP address-of-record is a canonical address by which a user is
known - placing this address in ENUM is comparable to placing an
email address or a similar URI in the DNS.
DNS does not make policy decisions about the records that it shares
with an inquirer. 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 to be open to the public -
which is a cause for some privacy considerations.
Unlike a traditional telephone number, the resource identified by a
SIP URI may require that callers provide cryptographic credentials
for authentication and authorization before a user is alerted. In
this respect, ENUM in concert with SIP can actually provide far
greater protection from unwanted callers than the existing PSTN,
despite the public availability of ENUM records. An analysis of
threats specific to the dependence of ENUM on the DNS, and the
applicability of DNSSEC  to these, is provided in .
7. IANA Considerations
This document registers the 'E2U+SIP' enumservice under the
enumservice registry described in the IANA considerations in RFC
3761. Details of the registration are given in Section 2.
8.1. 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.
 Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, May 2002.
 Mockapetris, P., "Domain Names - Concepts and Facilities", STD
13, RFC 1034, November 1987.
 Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
 Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403, October
8.2. Informative References
 Faltstrom, P., "E.164 number and DNS", RFC 2916, September 2000.
 Rosenberg, J., Schulzrinne, H. and P. Kyzviat, "Indicating User
Agent Capabilities in the Session Initiation Protocol (SIP)",
Work in Progress.
 Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
 R. Arends, et al., "Protocol Modifications for the DNS Security
Extensions", Work in Progress.
Thanks to Richard Shockey for comments on the initial draft of this
document, and to Allison Mankin for valuable review comments.
10. Author's Address
1800 Sutter St
Concord, CA 94520
Phone: +1 925/363-8720
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