Network Working Group H. Schulzrinne
Internet-Draft Columbia U.
Expires: August 15, 2004 February 15, 2004
The tel URI for Telephone Numbers
draft-ietf-iptel-tel-rfc2806bis-03
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies the URI (Uniform Resource Identifier) scheme
"tel". The ``tel'' URI describes resources identified by telephone
numbers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 6
3. URI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. URI Comparisons . . . . . . . . . . . . . . . . . . . . . . 9
5. Phone Numbers and Their Context . . . . . . . . . . . . . . 10
5.1 Phone Numbers . . . . . . . . . . . . . . . . . . . . . . . 10
5.1.1 Separators in Phone Numbers . . . . . . . . . . . . . . . . 10
5.1.2 Alphabetic Characters Corresponding to Digits . . . . . . . 11
5.1.3 Alphabetic, * and \\# Characters as Identifiers . . . . . . 11
5.1.4 Global Numbers . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.5 Local Numbers . . . . . . . . . . . . . . . . . . . . . . . 11
5.2 ISDN Subaddresses . . . . . . . . . . . . . . . . . . . . . 13
5.3 Extensions . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.4 Other Parameters . . . . . . . . . . . . . . . . . . . . . . 13
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1 Why Not Just Put Telephone Numbers in SIP URIs? . . . . . . 16
7.2 Why Not Distinguish Between Call Types? . . . . . . . . . . 16
7.3 Why tel? . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.4 Do Not Confuse Numbers with How They Are Dialed . . . . . . 16
8. Usage of Telephone URIs in HTML . . . . . . . . . . . . . . 17
9. Use of tel URIs with SIP (Informative) . . . . . . . . . . . 18
9.1 Local Translation . . . . . . . . . . . . . . . . . . . . . 18
9.2 Proxy Translation . . . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 22
12. Security Considerations . . . . . . . . . . . . . . . . . . 23
13. Change History . . . . . . . . . . . . . . . . . . . . . . . 24
13.1 Changes from ietf-02 to ietf-03 . . . . . . . . . . . . . . 24
13.2 Changes from ietf-00 to ietf-01 . . . . . . . . . . . . . . 24
13.3 Changes from -08 to draft-ietf-iptel-rfc2806bis-00 . . . . . 24
13.4 Changes Since -07 . . . . . . . . . . . . . . . . . . . . . 24
13.5 Changes Since -06 . . . . . . . . . . . . . . . . . . . . . 24
13.6 Changes Since -05 . . . . . . . . . . . . . . . . . . . . . 24
13.7 Changes Since -04 . . . . . . . . . . . . . . . . . . . . . 24
13.8 Changes Since -03 . . . . . . . . . . . . . . . . . . . . . 25
13.9 Changes Since -02 . . . . . . . . . . . . . . . . . . . . . 25
13.10 Changes Since -01 . . . . . . . . . . . . . . . . . . . . . 25
13.11 Changes Since RFC 2806 . . . . . . . . . . . . . . . . . . . 25
Normative References . . . . . . . . . . . . . . . . . . . . 26
Informative References . . . . . . . . . . . . . . . . . . . 27
Author's Address . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . 28
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1. Introduction
This document defines the URI scheme "tel". The "tel" URI describes
resources identified by telephone numbers. A telephone number is a
string of decimal digits that uniquely indicates the network
termination point. The number contains the information necessary to
route the call to this termination point. (This definition is
derived from [E.164], but encompasses both public and private
numbers.)
The "tel" URI telephone number is not restricted in the type of
termination point it refers to. The termination point can be in the
public telephone network, a private telephone network or the
Internet. The termination point can be fixed or wireless and address
a fixed wired, mobile or nomadic terminal. The terminal addressed
can support any electronic communication service (ECS) including
voice, data and fax. The URI can refer to resources identified by a
telephone number, including but not limited to originators or targets
of a telephone call.
The "tel" URI is a globally unique identifier ("name") only; it does
not describe the steps necessary to reach a particular number and
does not imply dialing semantics. Furthermore, it does not refer to a
specific physical device, only to a telephone number.
Telephone numbers as commonly understood actually comprise two
related, but distinct concepts: as a canonical address-of-record and
as a dial-string. We define the concepts below:
Address-of-record or identifier: The telephone number is understood
here as the canonical address-of-record or identifier for a
termination point within a specific network. For the public
network, these numbers follow the rules in E.164 [E.164], while
private numbers follow the rules of the owner of the private
numbering plan. Subscribers publish such identifiers phone number
as a universal means of being reached, independent of the location
of the caller. (Naturally, not all numbers are reachable from
everywhere, for a variety of technical and local policy reasons.
Also, a single termination point may be reachable from different
networks and may have multiple such identifiers.)
Dial string: "Dial strings" are the actual numbers, symbols and
pauses entered by a user to place a phone call. A dial-string is
consumed by one or more network entities, and understood in the
context of the configuration of these entities. It is used to
generate a telephone number so that a call can be routed.
Dial-strings may require pre-pended digits to handle local PBXs,
and they may include post-dial DTMF signaling that could control
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an IVR or reach an extension. Dial strings are beyond the scope
of this document.
To reach a telephone number from a phone on a PBX, for example, the
user of that phone has to know how to convert the telephone number
identifier into a dial string appropriate for that phone. The
telephone number itself does not convey what needs to be done for a
particular terminal. Instructions may include dialing "9" before
placing a call or prepending a "00" to reach a number in a foreign
country. The phone may also need to strip area and country codes.
The notation for phone numbers in this document is similar to that in
RFC 3191 [RFC3191] and RFC 3192 [RFC3192]. However, the syntax
differs since this document describes URIs whereas RFC 3191 and RFC
3192 specify electronic mail addresses. RFC 3191 and RFC 3192 use "/"
to indicate parameters (qualifiers). Since URI use the forward slash
to describe path hierarchy, the URI scheme described here uses the
semicolon, in keeping with Session Initiation Protocol (SIP) URI
conventions [RFC3261].
There are at least two ways one can envision making a telephone
connection. In the first approach, a URI contains the dial string,
which is then passed to an entity that can reproduce the actions
specified in the dial string. For example, in an analog phone
system, a dialer translates the dial string into a sequence of
actions such as waiting for dial tone, sending DTMF digits, pausing
and generating post-dial DTMF digits after the callee picks up. In
an ISDN or ISUP environment, the recipient of the dial string
performs the appropriate protocol actions.
Another approach has the URI specify the telephone number, which can
be either globally unique or only be valid within a local context.
The dialing application is aware of the local context, knowing, for
example, whether special digits need to be dialed to seize an outside
line, whether network, pulse or tone dialing is needed and what tones
indicate call progress. The dialing application then converts the
telephone number into a dial sequence and performs the necessary
signaling actions. The document below assumes the second model. The
dialer does not have to be a user application as found in traditional
desktop operating systems, but could well be part of an IP-to-PSTN
gateway.
The approach pursued here has the disadvantage that certain services,
such as electronic banking or voicemail, cannot be specified in a
URI.
The URI can be used as a request URI in SIP [RFC3261] requests. The
SIP specification also inherits the 'subscriber' part of the syntax
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as part of the 'user element' in the SIP URI. Other protocols may
use this URI for both query-based and prefix-based applications.
The "tel" URI does not specify the call type such as voice, fax, or
data call and does not provide the connection parameters for a data
call. The type and parameters are assumed to be negotiated either
in-band by the telephone device or through a signaling protocol such
as SIP.
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2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant
implementations.
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3. URI Syntax
The URI is defined using the ABNF (augmented Backus-Naur form)
described in RFC 2234 [RFC2234] and uses elements from the core
definitions (Appendix A of RFC 2234).
The syntax definition follows RFC 2396 [RFC2396], indicating the
actual characters contained in the URI. Note that the reserved
characters "+", ";", "=", and "?" MUST NOT be escaped if shown in the
grammar definitions below as they are delimiters for the "tel" URI
scheme. These reserved characters MUST be escaped if they appear in
parameter values.
Characters other than those in the "reserved" and "unsafe" sets (see
RFC 2396 [RFC2396]) are equivalent to their "% HEX HEX" encoding.
The "tel" URI has the following syntax:
telephone-uri = "tel:" telephone-subscriber
telephone-subscriber = global-number / local-number
global-number = global-number-digits *par
local-number = local-number-digits *par context *par
par = parameter / extension / isdn-subaddress
isdn-subaddress = ";isub=" 1*uric
extension = ";ext=" 1*phonedigit
context = ;phone-context=" descriptor
descriptor = domainname / global-number-digits
global-number-digits = "+" 1*phonedigit
local-number-digits = 1*phonedigit-hex
domainname = *( domainlabel "." ) toplabel [ "." ]
domainlabel = alphanum
/ alphanum *( alphanum / "-" ) alphanum
toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum
parameter = ";" pname ["=" pvalue ]
pname = 1*( alphanum / "-" )
pvalue = 1*paramchar
paramchar = param-unreserved / unreserved / escaped
unreserved = alphanum / mark
mark = "-" | "_" | "." | "!" | "~" | "*" |
"'" | "(" | ")"
escaped = "%" HEXDIG HEXDIG
param-unreserved = "[" / "]" / "/" / ":" / "&" / "+" / "$"
phonedigit = DIGIT / [ visual-separator ]
phonedigit-hex = HEXDIG / "*" / "#" / [ visual-separator ]
visual-separator = "-" / "." / "(" / ")"
alphanum = ALPHA / DIGIT
Each parameter name ("pname"), the ISDN subaddress, the extension and
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the context MUST NOT appear more than once. The order of the URL
parameters is immaterial. The ISDN subaddress or extension SHOULD
appear first, if present, followed by the context parameter, if
present, followed by any other parameters in lexicographical order.
This simplifies comparison when the "tel" URI is compared
character-by-character, such as in SIP URIs [RFC3261].
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4. URI Comparisons
Two "tel" URIs are equivalent according to the following rules:
o URI are not equal if one is a 'local-number and the other a
'global-number'.
o For mandatory additional parameters (Section 5.4) and the
'phone-context' and 'extension' parameters defined in this
document, 'phone-number' parameter values are compared
digit-by-digit after removing all 'visual-separator's from
consideration.
o Parameters are compared according to 'pname', regardless of the
order they appeared in the URI. If one URI has a parameter name
not found in the other, the two URIs are not equal.
o URI comparisons are case-insensitive.
All parameter names and values SHOULD use lower-case characters since
tel URIs may be used within contexts where comparisons are
case-sensitive.
Section 19.1.4 in the SIP specification [RFC3261] discusses one
such case.
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5. Phone Numbers and Their Context
5.1 Phone Numbers
The 'subscriber part of the URI indicates the number. The phone
number can be represented in either global (E.164) or local notation.
All phone numbers MUST use the global form unless they cannot be
represented as such. Numbers from private numbering plans, emergency
("911", "112") and some directory assistance numbers (e.g., "411")
and other "service codes" (numbers of the form N11 in the United
States) cannot be represented in global (E.164) form, and need to be
represented as a local number with a context. Local numbers MUST be
tagged with a 'phone-context' (Section 5.1.5).
Implementations MUST NOT assume that telephone numbers have a
maximum, minimum or fixed length, or that they always begin with a
certain number.
E.164 limits numbers to 15 digits. For geographic numbers, one to
three digits are the country code, with the remainder divided into
area or city code (national destination code) and subscriber
number. Alternatively, there is a global three-digit service
code, followed by a global subscriber number of up to 12 digits.
Finally, a "international public telecommunication number for
networks is composed of decimal digits arranged in three code
fields. The code fields are the 3-digit shared Country Code (CC)
field, the IC field, which varies in length between 1 to 4 digits,
and the Subscriber Number (SN) which can be up to 15 minus the
number of digits in the CC and IC fields." [E.164].
5.1.1 Separators in Phone Numbers
Phone numbers MAY contain visual separators. Visual separators
('visual-separator') merely aid readability and are not used for URI
comparison or placing a call.
Despite complicating comparisons, this specification retains the
visual separators to follow the spirit of RFC 2396 [RFC2396],
which remarks that "A URI often needs to be remembered by people,
and it is easier for people to remember a URI when it consists of
meaningful components." Also, ISBN URNs documented in RFC 3187
[RFC3187] use visual separators in a manner similar to this
specification.
Even though ITU-T E.123 [E.123] recommends the use of space
characters as visual separators in printed telephone numbers,
"tel" URIs cannot use spaces to avoid excessive escaping.
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5.1.2 Alphabetic Characters Corresponding to Digits
In some countries, it is popular to write phone numbers using
alphabetic characters which correspond to certain numbers on the
telephone keypad. The URI format does not support this notation
since the mapping from alphabetic characters to digits is not
completely uniform internationally, although there are standards
[E.161][T1.703] addressing this issue.
5.1.3 Alphabetic, * and \\# Characters as Identifiers
Since called and calling terminal numbers (TNs) are encoded in BCD in
ISUP, six additional values per digit can be encoded, sometimes
represented as the hexadecimal characters A through F. Similarly,
DTMF allows for the encoding of the symbols *, \# and A through D.
However, in accordance with E.164, they may not be included in global
numbers. Their use in local numbers is not defined, but is not
prohibited.
5.1.4 Global Numbers
Globally unique numbers are identified by the leading "+" character.
Global numbers MUST be composed with the country (CC) and national
(NSN) numbers as specified in E.123 [E.123] and E.164 [E.164].
Globally unique numbers have the property of being unambiguous
everywhere in the world and are RECOMMENDED.
5.1.5 Local Numbers
Local numbers are unique only within a certain geographical area or a
certain part of the telephone network, e.g., a private branch
exchange (PBX), a state or province, a particular local exchange
carrier or a particular country. URIs with local phone numbers
should only appear in environments where all local entities can
successfully set up the call by passing the number to the dialing
software. Digits needed for accessing an outside line, for example,
are not included in local numbers. Local numbers SHOULD NOT be used
unless there is no way to represent the number as a global number.
Local numbers require that the originator and recipient are
configured appropriately, so that they can insert and recognize
the correct descriptors. Since there is no algorithm to
independently pick the same descriptor, labeling numbers with
their context increases the chances of mis-configuration, so that
valid identifiers are rejected by mistake. The algorithm to
select descriptors was chosen that accidental collisions should be
rare, but they cannot be ruled out.
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Local numbers MUST have a 'phone-context' parameter that identifies
the scope of their validity. The parameter MUST be chosen to
unambiguously identify the local context within which the number is
unique. Thus, the combination of the descriptor in the
'phone-context' parameter and local number is again globally unique.
The parameter value is defined by the assignee of the local number.
It does NOT indicate a prefix that turns the local number into a
global (E.164) number.
There are two ways to label the context: via a global number or any
number of its leading digits (e.g., "+33") and via a domain name,
e.g., "houston.example.com". The choice between the two is left to
the "owner" of the local number and is governed by whether there is a
global number or domain name that is a valid identifier for a
particular local number.
The domain name does not have to resolve to any actual host, but
MUST"> be under the administrative control of the entity managing the
local phone context.
A global number context consists of the initial digits of a valid
global number. All global numbers matching these initial digits must
be assigned to the same organization that is describing the context
and no such matching number can be used by any other organization.
If such an initial string of digits does not exist, the organization
should use the lowest number of the global number range assigned to
it. (This can occur if two organizations share the same decimal
block of numbers. For example, assume an organization owns the
number range +1-212-939-7000 through +1-212-939-7199. +1-212-939-7
would not be a valid global number context, but +1-212-939-7000 would
work.) It is not required that local numbers within the context
actually begin with the chosen set of initial numbers.
For a local number defined within a PBX, the organization can choose
any number under its control to identify the context. For example, a
context consisting of any of the organization's global numbers may be
suitable, or a substring that is completely occupied by the
organization. For example, +49-6151-16 would be a suitable context
for the TU Darmstadt, as it uses all numbers starting with those
digits.
A context consisting of the initial digits of a global number does
not imply that adding these to the local number will generate a valid
E.164 number. It might do so by coincidence, but this cannot be
relied upon. (For example, "911" should be labeled with the context
"+1", but "+1-911" is not a valid E.164 number.)
National freephone numbers do not need a context, even though they
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are not necessarily reachable from outside a particular country code
or numbering plan. Recall that "tel" URIs are identifiers; it is
sufficient that a global number is unique, but it is not required
that it be reachable from everywhere.
Even non-freephone numbers may be out of date or not be reachable
from a particular location. For example, premium services such as
"900" numbers in the North American numbering plan are often not
dialable from outside the particular country code.
The two label types were chosen so that, in almost all cases, a
local administrator can pick an identifier that is reasonably
descriptive and does not require a new IANA-managed assigned
number. It is up to the administrator to assign an appropriate
identifier and to use it consistently. Often, an organization can
choose among several different identifiers.
If the recipient of a "tel" URI uses the URI simply for
identification, the receiver does not need to know anything about the
context descriptor. It simply treats it as one part of a globally
unique identifier, with the other being the local number. If a
recipient of the URI intends to place a call to the local number, it
MUST verify that it is within the same context as one of the
descriptors. If it is not within the same context, it MUST NOT
initiate the call and treat the URI like an invalid destination.
5.2 ISDN Subaddresses
A phone number MAY also contain an 'isdn-subaddress"> parameter which
indicates an ISDN subaddress.
ISDN subaddresses typically contain IA5 characters, but may contain
any octet value.
5.3 Extensions
Extensions identify stations behind a PBX and are roughly equivalent
to ISDN subaddresses. They are identified with the 'extension">
parameter. At most one of the 'isdn-subaddress and 'extension
parameters can appear in a tel URI, i.e., they cannot appear both at
the same time.
5.4 Other Parameters
Future extensions to this URI scheme may add other parameters
('parameter in the ABNF). Such parameters can be either mandatory or
optional. Mandatory parameters start with "m-". An implementation
MAY ignore optional parameters. An implementation MUST NOT use the
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URI if it contains unknown mandatory parameters. The "m-" prefix
cannot be added to parameters that were already registered (except to
create a new, logically distinct parameter). The "phone-context"
parameter in this document is mandatory.
For example, 'parameter' parameters can be used to store
application-specific additional data about the phone number, its
intended use, or any conversions that have been applied to the
number.
All new parameters MUST be registered with IANA.
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6. Examples
tel:+1-201-555-0123 This URI points to a phone number in the United
States. The hyphens are included to make the number more
human-readable; they separate country, area codes and subscriber
number.
tel:7042;phone-context=cs.columbia.edu: The URI describes a local
phone number valid within the context "cs.columbia.edu".
tel:863-1234;phone-context=+1-914-555: The URI describes a local
phone number that is valid within a particular phone prefix.
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7. Rationale
7.1 Why Not Just Put Telephone Numbers in SIP URIs?
The "tel" URI describes a service, reaching a telephone number, that
is independent of the means of doing so, be it via a SIP-to-PSTN
gateway, a direct SIP call via ENUM translation, some other signaling
protocols such as H.323 or a traditional circuit-switched call
initiated on the client side via, say, TAPI. It is thus, in spirit,
closer to the URN schemes that also leave the resolution to an
external mechanism. The same "tel" URI may get translated to any
number of other URIs in the process of setting up the call.
7.2 Why Not Distinguish Between Call Types?
Signaling protocols such as SIP allow to negotiate the call type and
parameters, making the very basic indication within the URL scheme
moot. Also, since the call type can change frequently, any such
indication in a URI is likely to be out of date. If such designation
is desired for a device that directly places calls without a
signaling protocol such as SIP, mechanisms such as the "type"
attribute for the "A" element in HTML may be more appropriate.
7.3 Why tel?
"Tel" was chosen since it is widely recognized none of the other
suggestions appeared appropriate. "Callto" was discarded since URI
schemes locate a resource and do not specify an action to be taken.
"Telephone" and "phone" were considered too long and not as
internationally recognized.
7.4 Do Not Confuse Numbers with How They Are Dialed
As an example, the E.164 number "+1-212-555-3141" will be dialed in
many countries as 00-1-212-555-3141, where the leading "00" is a
prefix for international calls. (In general, "+" in E.164 indicates
that an international prefix is required.) Tel URIs MUST NOT contain
the local dialing prefixes in numbers such as +1-212-555-3141, as the
transformation back to an international number is not guaranteed to
be correct or unique.
If a network entity receives a "tel" URI containing a local number,
it MUST make sure that it knows the context in which the local phone
number is to be processed, or else the number MUST NOT be used.
Equally, the originator of a "tel" URI must take into consideration
that the recipient may have insufficient information about the phone
number's context.
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8. Usage of Telephone URIs in HTML
Links using the "tel" URL SHOULD enclose the telephone number, so
that users can easily predict the action taken when following the
link.
Dial <a href="tel:+1-212-555-0101">+1-212-555-0101</a>
for assistance.
instead of
Dial <a href="tel:+1-212-555-0101">this number</a>
for assistance.
On a public HTML page, the telephone number in the URI SHOULD always
be in the global form, even if the text of the link uses some local
format.
Telephone (if dialing in the United States):
<a href="tel:+1-201-555-0111">(201) 555-0111</a>
or even
For having RFCs read aloud, call
<a href="tel:+1-555-438-3732">1-555-IETF-RFC</a>.
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9. Use of tel URIs with SIP (Informative)
SIP can use the "tel" URI as a Request-URI, along with "sip" and
"sips" URIs. For brevity, we will imply "sips" URIs when talking
about SIP URIs. Both "tel" and SIP URIs can contain telephone
numbers. In SIP URIs, they appear as the user part, i.e., before the
@ symbol (Section 19.1.6 in [RFC3261]).
Unless a SIP UA connects directly to a PSTN gateway, one of the SIP
proxy servers has to translate the tel URI to a SIP URI, with the
host part of that URI pointing to a gateway. Typically, the outbound
proxy server, as the first proxy server visited by a call request,
performs this translation. A proxy server can translate all tel URIs
to the same SIP host name or select a different gateway for different
tel prefixes, based, for example, on information learned from TRIP.
However, a proxy server could also delegate this translation task to
any other proxy server since proxy servers are free to apply whatever
routing logic they desire.
As noted earlier, all phone numbers MUST use the global form unless
they cannot be represented as such. If the local-number format is
used, it MUST be qualified by the 'phone-context' parameter.
Effectively, the combination of local number and phone context makes
the tel URI globally unique.
While web pages, vCard business cards, address books and directories
can easily contain global tel URIs, users on twelve-button (IP)
phones cannot dial such numbers directly and are typically accustomed
to dialing shorter strings, e.g., for PBX extensions or local
numbers. These so-called dial-strings (Section 1) are not directly
represented by tel URIs, as noted. We refer to the translation of
dial strings into SIP and tel URIs, global or local, as the dial
plan. There are at least two appropriate ways to deal with dial
strings in SIP terminals, local translation and proxy translation,
described in turn below.
9.1 Local Translation
A SIP UA can use a dial plan to translate dial strings into SIP or
"tel" URIs. The dial plan can be manually configured or, preferably,
be downloaded as part of a device configuration mechanism. (At this
time, there is no standardized mechanism for this.)
A mobile user can use at least two dial plans, namely the dial plan
for the network that he is currently visiting and the dial plan for
the user's home network. Generally, dialed numbers that are meant to
represent global numbers will look the same after the translation
regardless of the dial plan, even if, say, the visited network uses
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'0' for dialing an 'outside' number and the user's home network uses
'9', as long as the user is aware of the current dial plan. However,
local extensions that do not have a direct global equivalent may well
behave differently. To avoid any ambiguity, the dial plan MUST
insert a suitable 'phone-context' string when performing the
translation. If the 'phone-context' is a domain name, there are
three cases:
1. The outbound proxy recognizes the domain name in the SIP URI as
its local context and can route the request to a gateway that
understands the local number.
2. The outbound proxy does not use the same phone context, but can
route to a proxy that handles this phone context. This routing
can be done via a lookup table or the domain name of the phone
context might be set up to reflect the SIP domain name of a
suitable proxy. For example, a proxy may always route calls with
tel URIs like
tel:1234;phone-context=munich.example.com
to the SIP proxy located at munich.example.com. (Proxies that
receive a tel URI with a context they do not understand are
obligated to return a 404 (Not Found) status resonse, so that an
outbound proxy may decide to attempt such a heuristic.)
3. The outbound proxy does not recognize the phone context and
cannot find the appropriate proxy cognizant of that phone
context. In that case, the translation fails and the outbound
proxy returns a 404 (Not Found) error response.
9.2 Proxy Translation
In proxy translation mode, the SIP UA simply turns the dialed digits
into the user part of the SIP URI and appends a ';user=phone'
parameter and provides an appropriate phone context reflecting the
local dialing plan. The host name or IP address of the outbound
proxy is made the host part of the SIP request URI. The outbound
proxy can then apply the dial plan indicated by the phone context in
the URI to translate the SIP URI into a "tel" URI or other SIP URI.
Translation into a "tel" URI makes sense only if the proxy wants to
delegate finding the PSTN gateway to another proxy. For example,
after the user with a location-specified dial plan located in domain
"munich.example.com" dials the digits "1234", the device converts
this into a SIP URI:
sip:1234;phone-context=munich.example.com@example.com
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Alternatively, the SIP UA can issue a call with a "tel" URI. For this
example, it might be:
tel:1234;phone-context=munich.example.com
Using a SIP URI is more robust and is thus the preferred approach.
Since a single proxy may receive calls from many different
locations with different local dial plans, devices that rely on
the proxy for number translation SHOULD always be configured with
a context. Otherwise, for example, a provider or enterprise would
have to provision a separate proxy for each branch office or
geographic area with its own dial plan.
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10. IANA Considerations
IANA is requested to update the reference to RFC 2806 in the URI
scheme registry for the 'tel' scheme to this document.
"Tel" URI parameters ('parameter') MUST be registered with IANA.
Mandatory parameters must be described in a standards-track RFC,
while an informational RFC is sufficient for other parameters.
The registration must indicate:
Parameter name: The name used for the parameter, according to the BNF
in Section 3.
Applicability: A brief description of its applicability.
Mandatory? Whether the parameter is mandatory or not; only the names
of mandatory parameters must start with "m-" as described in
Section 5.4.;
Specification: A reference to the specification that defines the
parameter and its syntax.
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11. Acknowledgments
This document is derived from RFC 2806 [RFC2806], written by Antti
Vähä-Sipilä. Flemming Andreasen, Francois Audet, Lawrence Conroy,
Cullen Jennings, Andrew Main, Michael Hammer, Jon Peterson, Mike
Pierce, Jonathan Rosenberg and James Yu provided extensive comments.
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12. Security Considerations
The security considerations parallel those for the mailto URL
[RFC2368].
A recipient of a "tel" URI SHOULD NOT place calls without the consent
of its owner. Placing calls automatically without appropriate user
confirmation may incur a number of risks, such as those described
below.
o Calls may incur costs.
o The URI may be used to place malicious or annoying calls.
o A call will take the user's phone line off-hook, thus preventing
its use.
o A call may reveal the user's, possibly unlisted, phone number to
the remote host in the caller identification data, and may allow
the attacker to correlate the user's phone number with other
information such as the e-mail or IP address.
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13. Change History
13.1 Changes from ietf-02 to ietf-03
o Added BNF definition for 'mark'.
o Use fictitious phone numbers.
13.2 Changes from ietf-00 to ietf-01
o Editorial changes suggested by Francois Audet.
o Added * and \# as characters to local numbers.
13.3 Changes from -08 to draft-ietf-iptel-rfc2806bis-00
o Editorial clarifications.
o Remove multiple context descriptions.
13.4 Changes Since -07
o Added section on using tel URIs in SIP.
13.5 Changes Since -06
o Clarified semantics.
o Removed context from freephone numbers.
o Added phone extensions.
13.6 Changes Since -05
o URI comparisons are case-insensitive.
o Specified recommended order of parameters to simplify use within
SIP URIs.
13.7 Changes Since -04
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o ISDN subaddresses can contain any IA5 character or even binary
data; represented now as "uric".
13.8 Changes Since -03
o Clarified use of multiple contexts and how to express this, as a
comma-separated list.
13.9 Changes Since -02
o Clarifications and editorial fixes.
o Now, mandatory parameters are labeled, to avoid making
[I-D.yu-tel-url] obsolete.
13.10 Changes Since -01
The draft has been greatly simplified to reflect parts that have
actually been implemented.
o Removed references to carrier selection.
o Removed dial context.
o Removed fax and modem URIs.
o Removed post-dial strings.
o Removed pause characters.
13.11 Changes Since RFC 2806
The specification is backwards-compatible with RFC 2806.
o Editorial changes and clarifications. The document has been
shortened and reorganized. Most paragraphs have been rewritten to
be more concise.
o Syntax now conforms to RFC 2396 [RFC2396], in particular related
to escaping.
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Normative References
[E.123] , ITU., "Notation for national and international telephone
numbers, e-mail addresses and web addresses",
Recommendation E.123, February 2001.
[E.161] , ITU., "Arrangement of digits, letters and symbols on
telephones and other devices that can be used for gaining
access to a telephone network", Recommendation E.161, May
1995.
[E.164] , ITU., "The international public telecommunication
numbering plan", Recommendation E.164, May 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC3191] Allocchio, C., "Minimal GSTN address format in Internet
Mail", RFC 3191, October 2001.
[RFC3192] Allocchio, C., "Minimal FAX address format in Internet
Mail", RFC 3192, October 2001.
[RFC3261] 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.
[T1.703] , ANSI., "Allocation of Letters to the Keys of Numeric
Keypads for Telecommunications", Standard T1.703-1995
(R1999), 1999.
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Informative References
[I-D.yu-tel-url]
Yu, J., "New Parameters for the 'tel' URL to Support
Number Portability and Freephone Service",
draft-yu-tel-url-08 (work in progress), November 2003.
[RFC2368] Hoffman, P., Masinter, L. and J. Zawinski, "The mailto URL
scheme", RFC 2368, July 1998.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC2806] Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806,
April 2000.
[RFC3187] Hakala, J. and H. Walravens, "Using International Standard
Book Numbers as Uniform Resource Names", RFC 3187, October
2001.
Author's Address
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7042
EMail: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
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