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Network Working Group J. Peterson
Request for Comments: 3824 H. Liu
Category: Informational J. Yu
NeuStar
B. Campbell
dynamicsoft
June 2004
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 Notice
Copyright (C) The Internet Society (2004).
Abstract
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
telephone number.
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
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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
1. Introduction
ENUM (E.164 Number Mapping, RFC 3761 [1]) is a system that uses DNS
(Domain Name Service, RFC 1034 [4]) in order to translate certain
telephone numbers, like '+12025332600', into URIs (Uniform Resource
Identifiers, RFC 2396 [9]), like 'sip:user@sipcarrier.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 [10] is the ITU-T standard international
numbering plan, under which all globally-reachable telephone numbers
are organized.
SIP (Session Initiation Protocol, RFC 3261 [2]) 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
[14]), 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 [6]) and ENUM (RFC 3761 [1]). Only
those aspects of NAPTR record authoring and processing that have
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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 [12] 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 [16].
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.
2. Terminology
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 [3] 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
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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 [14], 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 [8] 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:+12025332600@carrier.com;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 [15]. 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.
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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 [13], the use of Require/Supported to
negotiate extensions in RFC 3261, and callee capabilities [11]);
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
records.
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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
several hours).
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 [1] requires the
IANA registration of service fields known as "enumservices".
An enumservice for SIP has been developed in the ENUM working group
(see [7]) 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
URI.
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
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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 '^.*