|Title||Using E.164 numbers with the Session Initiation Protocol (SIP)
Peterson, H. Liu, J. Yu, B. Campbell
Network Working Group J. Peterson
Request for Comments: 3824 H. Liu
Category: Informational J. Yu
Using E.164 numbers with the Session Initiation Protocol (SIP)
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 (C) The Internet Society (2004).
There are a number of contexts in which telephone numbers are
employed by Session Initiation Protocol (SIP) applications, many of
which can be addressed by ENUM. Although SIP was one of the primary
applications for which ENUM was created, there is nevertheless a need
to define procedures for integrating ENUM with SIP implementations.
This document illustrates how the two protocols might work in
concert, and clarifies the authoring and processing of ENUM records
for SIP applications. It also provides guidelines for instances in
which ENUM, for whatever reason, cannot be used to resolve a
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Handling Telephone Numbers in SIP . . . . . . . . . . . . . . 3
4. Design Principles . . . . . . . . . . . . . . . . . . . . . . 5
5. Authoring NAPTR Records for SIP . . . . . . . . . . . . . . . 6
5.1. The Service Field . . . . . . . . . . . . . . . . . . . 6
5.2. Creating the Regular Expression: Matching . . . . . . . 6
5.3. Creating the Regular Expression: The URI . . . . . . . . 7
5.4. Setting Order and Preference amongst Records . . . . . . 8
5.5. Example of a Well-Formed ENUM NAPTR Record Set for SIP. 8
6. Processing ENUM Records . . . . . . . . . . . . . . . . . . . 8
6.1. Contending with Multiple SIP records . . . . . . . . . . 8
6.2. Processing the Selected NAPTR Record . . . . . . . . . . 9
7. Compatibility with RFC 3761. . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16
ENUM (E.164 Number Mapping, RFC 3761 ) is a system that uses DNS
(Domain Name Service, RFC 1034 ) in order to translate certain
telephone numbers, like '+12025332600', into URIs (Uniform Resource
Identifiers, RFC 2396 ), like 'sip:email@example.com'. ENUM
exists primarily to facilitate the interconnection of systems that
rely on telephone numbers with those that use URIs to route
transactions. E.164  is the ITU-T standard international
numbering plan, under which all globally-reachable telephone numbers
SIP (Session Initiation Protocol, RFC 3261 ) is a text-based
application protocol that allows two endpoints in the Internet to
discover one another in order to exchange context information about a
session they would like to share. Common applications for 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.
The most widespread application for SIP today is Voice-over-IP
(VoIP). As such, there are a number of cases in which SIP
applications are forced to contend with telephone numbers.
Unfortunately, telephone numbers cannot be routing in accordance with
the traditional DNS resolution procedures standardized for SIP (see
), which rely on SIP URIs. ENUM provides a method for
translating E.164 numbers into URIs, including potentially SIP URIs.
This document therefore provides an account of how SIP can handle
telephone numbers by making use of ENUM. Guidelines are proposed for
the authoring of the DNS records used by ENUM, and for client-side
processing once these DNS records have been received.
The guidelines in this document are oriented towards authoring and
processing ENUM records specifically for SIP applications. These
guidelines assume that the reader is familiar with Naming Authority
Pointer (NAPTR) records (RFC 3403 ) and ENUM (RFC 3761 ). Only
those aspects of NAPTR record authoring and processing that have
special bearing on SIP, or that require general clarification, are
covered in this document; these procedures do not update or override
the NAPTR or ENUM core documents.
Note that the ENUM specification has undergone a revision shortly
before the publication of this document, driven by the update of the
NAPTR system described in RFC 2915  to the Dynamic Delegation
Discovery System (DDDS) family of specifications (including RFC
3403). This document therefore provides some guidance for handling
records designed for the original RFC 2916 .
The remainder of this document is organized as follows: Section 3
suggests general behavior for SIP user agents that encounter
telephone numbers; Section 4 provides an overview of the intersection
of SIP and ENUM; proposed normative guidelines for ENUM record
authoring and processing in the context of SIP are described in
Section 5, and Section 6 respectively; some considerations relevant
to the revision of RFC 2916 are given in Section 7.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119  and indicate requirement levels for
compliant SIP implementations.
3. Handling Telephone Numbers in SIP
There are a number of reasons why a user might want to initiate a SIP
request that targets an E.164 number. One common reason is that the
user is calling from the PSTN through a PSTN-SIP gateway; such
gateways usually map routing information from the PSTN directly on to
SIP signaling. Or a native SIP user might intentionally initiate a
session addressed to an E.164 number - perhaps because the target
user is canonically known by that number, or the originator's SIP
user agent only supports a traditional numeric telephone keypad. A
request initially targeting a conventional SIP URI might also be
redirected to an E.164 number. In most cases, these are requests for
a telephony session (voice communication), though numerous other
services are also reached through telephone numbers (including
instant messaging services).
Unlike a URI, a telephone number does not contain a host name, or any
hints as to where one might deliver a request targeting a telephone
number on the Internet. While SIP user agents or proxy servers could
be statically provisioned with a mapping of destinations
corresponding to particular telephone numbers or telephone number
ranges, considering the size and complexity of a complete mapping, it
would be preferable for SIP user agents to be able to query as needed
for a destination appropriate for a particular telephone number.
In such cases a user agent might use ENUM to discover a URI
associated with the E.164 number - including a SIP URI. URIs
discovered through ENUM can then be used normally to route SIP
requests to their destination. Note that support for the NAPTR DNS
resource record format is specified for ordinary SIP URI processing
in , and thus support for ENUM is not a significant departure
from baseline SIP DNS routing.
Most of the remainder of this document provides procedures for the
use of ENUM, but a few guidelines are given in the remainder of this
section for cases in which ENUM is not used, for whatever reason.
If a user agent is unable to translate an E.164 number with ENUM, it
can create a type of SIP Request-URI that contains a telephone
number. Since one of the most common applications of SIP is
telephony, a great deal of attention has already been devoted to the
representation of telephone numbers in SIP. In particular, the tel
URL RFC 2806  has been identified as a way of carrying telephone
routing information within SIP. A tel URL usually consists of the
number in E.164 format preceded by a plus sign, e.g.,:
tel:+12025332600. This format is so useful that it has been
incorporated into the baseline SIP specification; the user portion of
a SIP URI can contain a tel URL (without the scheme string, like
sip:+firstname.lastname@example.org;user=phone). A SIP proxy server might
therefore receive a request from a user agent with a tel URL in the
Request-URI; one way in which the proxy server could handle this sort
of request is by launching an ENUM query itself, and proxying the SIP
request in accordance with the returned ENUM records.
In the absence of support for ENUM, or if ENUM requests return no
records corresponding to a telephone number, local policy can be used
to determine how to forward SIP requests with an E.164 number in the
Request-URI. Frequently, such calls are routed to gateways that
interconnect SIP networks with the PSTN. These proxy server policies
might be provisioned dynamically with routing information for
telephone numbers by TRIP . As a matter of precedence, SIP user
agents should attempt to translate telephone numbers to URIs with
ENUM, if implemented, before creating a tel URL, and deferring the
routing of this request to a SIP proxy server.
4. Design Principles
Although the applicability of ENUM to SIP has always been clear, the
exact way in which the two should cooperate has been a subject of
some controversy. How many SIP URIs should appear in ENUM, what kind
of URIs they are, whether or not the "service" field of NAPTR records
should contain capability information - numerous questions have
arisen around the authoring, and interpretation of ENUM records for
SIP consumers. The following, then, is a statement of the particular
philosophy that has motivated the recommendations in this document:
Address-of-record SIP URIs appear in ENUM, not contact address
URIs. Roughly speaking, an address-of-record is the canonical
identity of a SIP user - it usually appears in the From field of
SIP requests sent by that user; a contact address is the URI of a
device. The process of registration in SIP (using the REGISTER
method), for example, temporarily binds the contact address of a
device to the address-of-record of a user. A DNS record has a
long time-to-live when compared with the timeframe of SIP
registrations. The availability of an address-of-record also
transcends the availability of any single device. ENUM is more
suitable for representing an long-term identity than the URI of
any device with which a user is temporarily associated. If ENUM
were purposed to map to specific devices, it would be better to
translate telephone numbers to IPv4 addresses than to URIs (which
express something richer).
SIP URIs in ENUM do not convey capability information. SIP has
its own methods for negotiating capability information between
user agents (see SDP , the use of Require/Supported to
negotiate extensions in RFC 3261, and callee capabilities );
providing more limited capability information within ENUM is at
best redundant and at worst potentially misleading to SIP's
negotiation system. Also, addresses-of-record do not have
capabilities (only devices registered under an address-of-record
have actual capabilities), and putting contact addresses in ENUM
is not recommended.
Only one SIP URI, ideally, appears in an ENUM record set for a
telephone number. While it may initially seem attractive to
provide multiple SIP URIs that reach the same user within ENUM, if
there are multiple addresses at which a user can be contacted,
considerably greater flexibility is afforded if multiple URIs are
managed by a SIP location service that is identified by a single
record in ENUM. Behavior for parallel and sequential forking in
SIP, for example, is better managed in SIP than in a set of ENUM
User agents, rather than proxy servers, should process ENUM
records. The assumptions underlying the processing of NAPTR
records dictate that the ENUM client knows the set of enumservices
supported by the entity that is attempting to communicate. A SIP
proxy server is unlikely to know the enumservices supported by the
originator of a SIP request.
5. Authoring NAPTR Records for SIP
This document makes no assumptions about who authors NAPTR records
(service providers or end users), nor about any mechanisms by which a
record, once it is authored, may be uploaded to the appropriate DNS
servers. Authorship in the context of this document concerns only
the processes by which the NAPTR records themselves are constructed.
There are a few general guidelines which are applicable to the
authoring of DNS records that should be considered by the authors of
ENUM NAPTR record sets. The most important is that authors SHOULD
keep record sets relatively small - DNS is not optimized for the
transference of large files. Having five or six NAPTR records is
quite reasonable, but policies that encourage records sets of
hundreds of NAPTR records are not appropriate. Also, DNS records are
relatively permanent; authors SHOULD NOT use ENUM NAPTR records to
express relationships between E.164 numbers and URIs that potentially
exist for only a short time. DNS is most scalable when it can assume
records will be valid for a reasonable length of time (at least
5.1. The Service Field
The Service field of a NAPTR record (per RFC 3403) contains a string
token that designates the protocol or service associated with a
particular record (and which imparts some inkling of the sort of URI
that will result from the use of the record). ENUM  requires the
IANA registration of service fields known as "enumservices".
An enumservice for SIP has been developed in the ENUM working group
(see ) which uses the format 'E2U+sip' to designate that a SIP
address-of-record appears in the URI field of a NAPTR record. It is
strongly RECOMMENDED that authors of NAPTR records use the 'E2U+sip'
service field whenever the regexp contains a SIP address-of-record
5.2. Creating the Regular Expression: Matching
The authorship of the regular expression (henceforth regexp) in a
NAPTR record intended for use by ENUM is vastly simplified by the
absence of an antecedent in the substitution (i.e., the section
between the first two delimiters). It is RECOMMENDED that
implementations use an exclamation point as a delimiter, since this
is the only delimiter used throughout the ENUM core specification.
When a NAPTR record is processed, the expression in the antecedent is
matched against the starting string (for ENUM, the telephone number)
to assist in locating the proper record in a set; however, in ENUM
applications, since the desired record set is located through a
reverse resolution in the e164.arpa domain that is based on the
starting string, further analysis of the starting string on the
client side will usually be unnecessary. In such cases, the
antecedent of the regular expression is commonly 'greedy' - it uses
the regexp '^.*$', which matches any starting string. Some authors
of ENUM record sets may want to use the full power of regexps, and
create non-greedy antecedents; the DDDS standard requires that ENUM
resolvers support these regexps when they are present. For providing
a trivial mapping from a telephone number to a SIP URI, the use of a
greedy regexp usually suffices.
Note that when the antecedent of the regexp is greedy, this does not
mean that the replacement field in NAPTR records provides a viable
alternative to authoring with a regexp. Authors of NAPTR records for
ENUM MUST NOT use the replacement field in records with an 'E2U+sip'
5.3. Creating the Regular Expression: The URI
The consequent side of a regexp contains a URI; NAPTR records that
are intended to be used for session initiation (including SIP
telephony) SHOULD use a SIP URI. While this may not sound especially
controversial at first hearing, there are other sorts of URIs that
might be considered appropriate for SIP applications: 'tel' URIs,
'im' or 'pres' URIs, or others that describe specific services that
might be invoked through SIP are all potentially candidates. While
the use of these URIs might seem reasonable under some circumstances,
including these in NAPTR records rather than SIP URIs could weaken
the proper composition of services and negotiation of capabilities in
It is RECOMMENDED that authors of ENUM records should always use the
SIP or SIPS URI scheme when the service field is 'E2U+sip', and the
URIs in question MUST be addresses-of-record, not contact addresses.
Users of SIP can register one or more contact addresses with a SIP
registrar that will be consulted by the proxy infrastructure of an
administrative domain to contact the end user when requests are
received for their address-of-record. 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 needs
to target specific devices rather than URIs, then a hypothetical
'E2IPv4+sip' enumservice would be more appropriate.
5.4. Setting Order and Preference amongst Records
For maximal compatibility authors of ENUM records for SIP SHOULD
always use the same order value for all NAPTR records in an ENUM
record set. If relative preference among NAPTR records is desirable,
it should be expressed solely with the preference field.
5.5. Example of a Well-Formed ENUM NAPTR Record Set for SIP
IN NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:email@example.com!" .
IN NAPTR 100 20 "u" "E2U+mailto" "!^.*$!mailto:firstname.lastname@example.org!" .
6. Processing ENUM Records
These guidelines do not by any means exhaustively describe the NAPTR
algorithm or the processing of NAPTR records; implementers should
familiarize themselves with the DDDS algorithm and ENUM before
reviewing this section.
Although in some cases, ENUM record sets will consist only a single
'E2U+sip' record, this section assumes that integrators of ENUM and
SIP must be prepared for more complicated scenarios - however, just
because we recommend that clients should be generous in what they
receive, and try to make sense of potentially confusing NAPTR
records, that does not mean that we recommend any of the potentially
troublesome authoring practices that make this generosity necessary.
6.1. Contending with Multiple SIP records
If an ENUM query returns multiple NAPTR records that have a service
field of 'E2U+sip', or other service field that may be used by SIP
(such as 'E2U+pres', see ) the ENUM client must first determine
whether or not it should attempt to make use of multiple records or
select a single one. The pitfalls of intentionally authoring ENUM
record sets with multiple NAPTR records for SIP are detailed above in
If the ENUM client is a user agent, then at some point a single NAPTR
record must be selected to serve as the Request-URI of the desired
SIP request. If the given NAPTR records have different preferences,
the most preferred record SHOULD be used. If two or more records
share most preferred status, the ENUM client SHOULD randomly
determine which record will be used, though it MAY defer to a local
policy that employs some other means to select a record.
If the ENUM client is a SIP intermediary that can act a redirect
server, then it SHOULD return a 3xx response with more than one
Contact header field corresponding to the multiple selected NAPTR
records in an ENUM record set. If the NAPTR records have different
preferences, then 'q' values may be used in the Contact header fields
to correspond to these preferences. Alternatively, the redirect
server MAY select a single record in accordance with the NAPTR
preference fields (or randomly when no preference is specified) and
send this resulting URI in a Contact header field in a 3xx response.
Otherwise, if the ENUM client is a SIP intermediary that can act as a
proxy server, then it MAY fork the request when it receives multiple
appropriate NAPTR records in an ENUM record set. Depending on the
relative precedence values of the NAPTR records the proxy may wish to
fork sequentially or in parallel. However, the proxy MUST build a
route set from these NAPTR records that consists exclusively of SIP
or SIPS URIs, not other URI schemes. Alternatively, the proxy server
MAY select a single record in accordance with the NAPTR preference
fields (or randomly when no preference is specified, or in accordance
with local policy) and proxy the request with a Request-URI
corresponding to the URI field of this NAPTR record - though again,
it MUST select a record that contains a SIP or SIPS URI. Note that
there are significant limitations that arise if a proxy server
processes ENUM record sets instead of a user agent, and that
therefore it is RECOMMENDED that SIP network elements act as redirect
servers rather than proxy servers after performing an ENUM query.
6.2. Processing the Selected NAPTR Record
Obviously, when an appropriate NAPTR record has been selected, the
URI should be extracted from the regexp field. The URI is between
the second and third exclamation points in the string. Once a URI
has been extracted from the NAPTR record, it SHOULD be used as the
Request-URI of the SIP request for which the ENUM query was launched.
SIP clients should perform some sanity checks on the URI, primarily
to ensure that they support the scheme of the URI, but also to verify
that the URI is well-formed. Clients MUST at least verify that the
Request-URI does not target themselves.
Once an address-of-record has been extracted from the selected NAPTR
record, clients follow the standard SIP mechanisms (see ) for
determining how to forward the request. This may involve launching
subsequent NAPTR or SRV queries in order to determine how best to
route to the domain identified by an address-of-record; clients
however MUST NOT make the same ENUM query recursively (if the URI
returned by ENUM is or contains a tel URL, see ).
Note that SIP requests based on the use of NAPTR records may fail for
any number of reasons. If there are multiple NAPTR records relevant
to SIP present in an ENUM record set, then after a failure has
occurred on an initial attempt with one NAPTR record, SIP user agents
MAY try their request again with a different NAPTR record from the
ENUM record set.
7. Compatibility with RFC 2916
The ENUM specification is currently undergoing a revision in the ENUM
WG. The new specification, RFC 3761 , is based on the Dynamic
Delegation Discovery System  revision to the NAPTR resource record
specified in RFC 2915 . For the most part, DDDS is an
organizational revision that makes the algorithmic aspects of record
processing separable from any underlying database format (such as the
NAPTR DNS resource record).
The most important revision in RFC 3761 is the concept of
enumservices. The original ENUM specification, RFC 2916, specified a
number of "service" values that could be used for ENUM, including the
"sip+E2U" service field. RFC 3761 introduces an IANA registration
system with new guidelines for the registration of enumservices,
which are no longer necessarily divided into discreet "service" and
"protocol" fields, and which admit of more complex structures. In
order to differentiate enumservices in RFC 3761 from those in RFC
2916, the string "E2U" is the leading element in an enumservice
field, whereas by RFC 2916 it was the trailing element.
An enumservice for SIP addresses-of-record is described in . This
enumservice uses the enumservice field "E2U+sip". RFC 3761-compliant
authors of ENUM records for SIP MUST therefore use the "E2U+sip"
enumservice field instead of the "sip+E2U" field. For backwards
compatibility with existing legacy records, however, the 'sip+E2U'
field SHOULD be supported by an ENUM client that support SIP.
Also note that the terminology of DDDS differs in a number of
respects from the initial NAPTR terminology in RFC 2916. DDDS
introduces the concept of an Application, an Application Specific
String, a First Well Known Rule, and so on. The terminology used in
this document is a little looser (it refers to a 'starting string',
for example, where 'Application Specific String' would be used for
DDDS). The new terminology is reflected in RFC 3761.
8. Security Considerations
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.
Ordinarily, when you give someone your telephone number, you don't
expect that they will be able to trivially determine your full name
and place of employment. If, however, you create a NAPTR record for
use with ENUM that maps your telephone number to a SIP URI like
'email@example.com', expect to get a lot of calls from
Unlike a traditional telephone number, the target of 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.
Users of ENUM who are nevertheless uncomfortable with revealing their
names may, since identities on the Internet are not exactly at a
premium, publish a less revealing SIP URI, like
'sip:firstname.lastname@example.org' or even
'sip:email@example.com', which could in
turn point to their internal URI.
An analysis of threats specific to the dependence of ENUM on the DNS,
and the applicability of DNSSEC  to these, is provided in .
9.1. Normative References
 Faltstrom, P. and M. Mealling, "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.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Mockapetris, P., "Domain Names - Concepts and Facilities",
STD13, RFC 1034, November 1987.
 Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
 Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403,
 Peterson, J., "enumservice registration for SIP Addresses-of-
Record", RFC 3764, April 2004.
 Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, April
 Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
9.2. Informative References
 International Telecommunications Union, "Recommendation E.164:
The international public telecommunication numbering plan", May
 Rosenberg, J., Schulzrinne, H. and P. Kyzviat, "Indicating User
Agent Capabilities in the Session Initiation Protocol (SIP)",
Work in Progress, June 2003.
 Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, September 2000.
 Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
 Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol:
Locating SIP Servers", RFC 3263, June 2002.
 Rosenberg, J., Squire, M., and H. Salama, "Telephony Routing
over IP (TRIP)", RFC 3219, August 2001.
 Faltstrom, P., "E.164 number and DNS", RFC 2916, September
 Peterson, J., "Enumservice Registration for Presence Services",
Work in Progress, February 2003.
 Arends, R., et al., "Protocol Modifications for the DNS
Security Extensions", Work in Progress, May 2004.
Appendix A. Acknowledgments
The authors would like to thank Richard Shockey for his input on
privacy issues, and Tom McGarry and Rohan Mahy for overall comments
and analysis. Thanks are due as well to Juan Heinanen and Lawrence
E. Conroy for advice on updating this document to better reflect RFC
3761. Special thanks are given to Patrik Faltstrom and Michael
Mealling for significantly reducing the size of this document by
producing a tight and well-specified successor to RFC 2916. Richard
Stastny and Patrik Faltstrom also provided valuable notes on the
valid usage of non-greedy regexp antecedents.
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