Internet Engineering Task Force
Internet Draft Schulzrinne/Vaha-Sipila
Columbia U./Nokia
draft-antti-rfc2806bis-04.txt
May 24, 2002
Expires: November 2002
URIs for Telephone Calls
STATUS OF THIS MEMO
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Internet Draft RFC2806bis May 24, 2002
Abstract
This document is a revision of RFC 2806.
This document specifies the URI (Uniform Resource Identifier) schemes
"tel" for specifying the address of a terminal in the phone network.
The tel URI is service-independent and describes voice calls (phone
calls, answering machines and voice messaging systems), facsimile
(telefax) calls and data calls, for landline, ISDN and mobile
subscribers.
1 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 RFC 2119 [1] and
indicate requirement levels for compliant implementations.
2 Introduction
This document defines the URI scheme "tel" that contains telephone
numbers. The telephone number can refer to terminals in the telephone
network or the Internet, mobile and landline devices, voice, data and
fax devices. The URI can refer to originators or targets of a
telephone call.
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: The telephone number is understood here as
the canonical address-of-record, as they are handled by the
signaling network. Generally, this means E.164 numbers, or
numbers that can be trivially converted into them.
Subscribers publish a phone number as a universal means of
being reached.
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 an IVR. Dial strings are
beyond the scope of this document.
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To reach a telephone number from a particular terminal, the user of
the terminal or the terminal itself has to know how to convert that
telephone number into a dial string appropriate for that terminal.
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 prefixing a "00" to reach a number in a foreign
country. Telephone numbers are also the normalized way that addresses
are signaled in the PSTN.
The notation for phone numbers is the same as in RFC 2303 [2] and RFC
2304 [3]. However, the syntax differs since this document describes
URIs whereas RFC 2303 and RFC 2304 specify electronic mail addresses.
RFC 2303 and RFC 2304 use "/" to indicate parameters (qualifiers).
Since URI use the forward slash to describe path hierarchy, the URI
schemes described here use the semicolon, in keeping with SIP URI
conventions [4].
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, by sending DTMF digits, waiting for
dial tone, pausing and generating post-dial DTMF digits after the
callee picks up. Another approach has the URI specify the telephone
number, which can be either globally unique or only be valid within a
local context. A dialer application is aware of 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 dialer then
converts the telephone number into a dial string 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 extensions on a PBX (when direct inward dialing is not used)
or electronic banking, cannot be specified in a URI.
The URI is used as a request URI in SIP [4] requests. The SIP
specification also inherits the subscriber part of the syntax as part
of the user element in the SIP URI. Other protocols may use this URI
for both query-based and prefix-based applications.
A "client" is defined as software that can parse one or more of the
URIs defined in this document and possibly place a call to a remote
entity.
3 URI Comparisons
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URI comparisons are case-sensitive except that the scheme and
parameter names are case-insensitive.
4 URI Syntax
The URI is defined using the ABNF (augmented Backus-Naur form)
described in RFC 2234 [5] and uses elements from the core definitions
(Appendix A of RFC 2234).
The syntax definition follows RFC 2396, 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 schemes.
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) are equivalent to their "% HEX HEX" encoding.
The "tel" URI has the following syntax:
telephone-uri = "tel:" subscriber *param |
subscriber = global-number / local-number |
global-number = global-number-part [isdn-subaddress] |
global-number-part = "+" 1*phonedigit |
local-number = local-number-part [isdn-subaddress] |
[context] |
local-number-part = 1*phonedigit |
isdn-subaddress = ";isub=" 1*phonedigit |
context = ";phone-context=" descriptor *("," descriptor) |
descriptor = domainname / global-number-part |
domainname = *( domainlabel "." ) toplabel [ "." ] |
domainlabel = alphanum |
/ alphanum *( alphanum / "-" ) alphanum |
toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum |
param = ";" pname ["=" pvalue ] |
pname = 1*uric |
pvalue = *uric |
phonedigit = HEXDIG / visual-separator |
visual-separator = "-" / "." / "(" / ")" |
alphanum = ALPHA / DIGIT |
5 Phone Numbers and Their Scope
5.1 Phone Numbers
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The <subscriber> part of the URI indicates the number. The phone
number can be written in either global (E.164) or local notation.
All phone numbers SHOULD be written in the international form unless
they cannot be written 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 written in international form, and need to
be represented with a context.
Implementations MUST NOT assume that telephone numbers have a
maximum, minimum or fixed length, or that they always begin with a
certain number.
5.1.1 Separators in Phone Numbers
Phone numbers MAY contain visual separators. Visual separators
(<visual-separator>) merely aid readability and MUST be ignored by
the client.
Even though ITU-T E.123 [6] recommends the use of space characters as
visual separators in printed telephone numbers, tel URIs MUST NOT use
spaces.
5.1.2 Alphabetic Characters
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
[7,8] addressing this issue.
Since called and calling terminal numbers (TNs) are encoded in BCD in
ISUP, this allows for six additional values per digit, sometimes
represented as the hexadecimal characters A through F. However, in
accordance with E.164, they may not be included in global numbers.
Their use in local numbers is not defined, but it not prohibited.
5.1.3 Global and Local Numbers
Global (international) numbers are identified by the "+" character
prefix. International numbers MUST be written with the country (CC)
and national (NSN) numbers as specified in E.123 and E.164 [6,9].
International numbers have the property of being unambiguous
everywhere in the world and are RECOMMENDED.
Local numbers only work within a certain geographical area or a
certain part of the telephone network, e.g., a private branch
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Internet Draft RFC2806bis May 24, 2002
exchange (PBX). 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.
The phone-context parameter can be used to label the local context. |
The parameter value is implementation-defined; care must be taken |
that the URL does not "escape" the administrative domain that |
administers the namespace. The parameter can contain a list of |
contexts that enumerate all the contexts where this number is valid. |
There are two common ways to label the context: via a global number |
prefix (e.g., "+33") and via a domain name, e.g., |
"houston.example.com". 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. For example, |
";context=+1,+49" indicates that the number is valid in country codes |
1 and 49. |
The global number prefix does not imply that adding it to the number |
will generate a valid E.164 number. (For example, "911" should have |
be labeled with the context "+1", but "+1-911" is not a valid E.164 |
number.)
5.2 ISDN Subaddresses
A phone number MAY also contain an <isdn-subaddress> parameter which
indicates an ISDN subaddress. The client SHOULD support ISDN
subaddresses if it supports ISDN call setup. If ISDN subaddressing is
not supported by the caller, <isdn-subaddress> MUST be ignored and
the user SHOULD be notified. The user or the client MAY opt not to
place a call if this feature is not supported.
5.3 Other Parameters
Future extensions to these URI schemes may add other parameters
(<param> 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 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, <param> 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.
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All new parameters MUST be registered with IANA.
6 Examples
tel:+358-555-1234567
This URI points to a phone number in Finland capable of
receiving voice calls. The hyphens are included to make the
number more human-readable; they separate country, area
codes and subscriber number.
tel:7042;context=cs.columbia.edu
The URI describes a local phone number valid within the
context "cs.columbia.edu".
7 Rationale
7.1 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.
7.2 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.
7.3 Do Not Confuse Numbers with How They Are Dialed
As an example, the E.164 number "+123456789" will be dialed in many
countries as 00123456789, 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 00123456789, as the
transformation back to an international number is not guaranteed to
be correct or unique.
If a client 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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When overlap signaling is used, devices which collect dial strings
MUST construct a complete phone number before constructing the tel
URI and including the URI in a network request. In other words, such
devices must convert from overlap dialing to en bloc signaling.
8 Usage of Telephone URIs in HTML
The number SHOULD be visible to the end user if it is conceivable
that the user might not have a client which is able to use these
URIs.
Telephone: <a href="tel:+3585551234567">+358-555-1234567</a>
On a public HTML page, the telephone number in the URI SHOULD always
be in the international form, even if the text of the link uses some
local format.
Telephone: <a href="tel:+3585551234567">(0555) 1234567</a>
or even
For having RFCs read aloud, call
<a href="tel:+1-555-438-3732">1-555-IETF-RFC</a>.
9 IANA Considerations
<Other> parameters MUST be registered with IANA.
10 Security Considerations
The security considerations parallel those for the mailto URL [10].
A client 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.
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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 client's phone number with
other information such as the e-mail or IP address.
o A call may use the same local number in different contexts, in
which the number may have a different meaning.
The client SHOULD NOT rapidly redial busy numbers to avoid the
congestion of the (signaling) network. The client SHOULD detect if
the number is unavailable or if the call is terminated before the
dialing string has been completely processed (for example, the call
is terminated while waiting for user input) and not try to call
again, unless instructed by the user.
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, in particular related to
escaping.
12 Changes Since -03
o Clarified use of multiple contexts and how to express this, as
a comma-separated list.
13 Changes Since -02
o Clarifications and editorial fixes.
o Now, mandatory parameters are labeled, to avoid making [11]
obsolete.
14 Changes Since -01
The draft has been greatly simplified to reflect parts that have
actually been implemented.
o Removed references to carrier selection.
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o Removed dial context.
o Removed fax and modem URIs.
o Removed post-dial strings.
o Removed pause characters.
15 Acknowledgments
This document inherits a large amount of text from RFC 2806 [12].
Flemming Andreasen, Francois Audet, Lawrence Conroy, Jon Peterson,
Mike Pierce and James Yu provided extensive comments.
16 Authors' Addresses
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
Antti Vaha-Sipila
(ISO 8859-15 quoted-printable: Antti V=E4h=E4-Sipil=E4)
Nokia Mobile Phones
P. O. Box 321
FIN-00045 Nokia Group
Finland
electronic mail: avs@iki.fi, antti.vaha-sipila@nokia.com
17 Bibliography
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[2] C. Allocchio, "Minimal PSTN address format in internet mail," RFC
2303, Internet Engineering Task Force, Mar. 1998.
[3] C. Allocchio, "Minimal FAX address format in internet mail," RFC
2304, Internet Engineering Task Force, Mar. 1998.
[4] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," RFC 2543, Internet Engineering Task
Force, Mar. 1999.
[5] D. Crocker, Ed., and P. Overell, "Augmented BNF for syntax
Schulzrinne/Vaha-Sipila [Page 10]
Internet Draft RFC2806bis May 24, 2002
specifications: ABNF," RFC 2234, Internet Engineering Task Force,
Nov. 1997.
[6] International Telecommunication Union, "Notation for national and
international phone numbers," Recommendation E.123, Telecommunication
Standardization Sector of ITU, Geneva, Switzerland, 1998.
[7] International Telecommunication Union, "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,
Telecommunication Standardization Sector of ITU, Geneva, Switzerland,
May 1995.
[8] ANSI, "Allocation of letters to the keys of numeric keypads for
telecommunications," Standard T1.703-1995 (R1999), ANSI, 1999.
[9] International Telecommunication Union, "The international public
telecommunication numbering plan," Recommendation E.164,
Telecommunication Standardization Sector of ITU, Geneva, Switzerland,
May 1997.
[10] P. Hoffman, L. Masinter, and J. Zawinski, "The mailto URL
scheme," RFC 2368, Internet Engineering Task Force, July 1998.
[11] J. Yu, "Extensions to the 'tel' and 'fax' URLs to support number
portability and freephone service," Internet Draft, Internet
Engineering Task Force, Mar. 2002. Work in progress.
[12] A. Vaha-Sipila, "URLs for telephone calls," RFC 2806, Internet
Engineering Task Force, Apr. 2000.
Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved.
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or assist in its implementation may be prepared, copied, published
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developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
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Internet Draft RFC2806bis May 24, 2002
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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Schulzrinne/Vaha-Sipila [Page 12]
Table of Contents
1 Terminology ......................................... 2
2 Introduction ........................................ 2
3 URI Comparisons ..................................... 3
4 URI Syntax .......................................... 4
5 Phone Numbers and Their Scope ....................... 4
5.1 Phone Numbers ....................................... 4
5.1.1 Separators in Phone Numbers ......................... 5
5.1.2 Alphabetic Characters ............................... 5
5.1.3 Global and Local Numbers ............................ 5
5.2 ISDN Subaddresses ................................... 6
5.3 Other Parameters .................................... 6
6 Examples ............................................ 7
7 Rationale ........................................... 7
7.1 Why Not Distinguish Between Call Types? ............ 7
7.2 Why "tel"? ......................................... 7
7.3 Do Not Confuse Numbers with How They Are Dialed ..... 7
8 Usage of Telephone URIs in HTML ..................... 8
9 IANA Considerations ................................. 8
10 Security Considerations ............................. 8
11 Changes Since RFC 2806 .............................. 9
12 Changes Since -03 ................................... 9
13 Changes Since -02 ................................... 9
14 Changes Since -01 ................................... 9
15 Acknowledgments ..................................... 10
16 Authors' Addresses .................................. 10
17 Bibliography ........................................ 10
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