Network Working Group H. Schulzrinne
Internet-Draft Columbia U.
Expires: October 4, 2004 April 5, 2004
The tel URI for Telephone Numbers
draft-ietf-iptel-tel-rfc2806bis-06
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. URI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. URI Comparisons . . . . . . . . . . . . . . . . . . . . . . . 6
5. Phone Numbers and Their Context . . . . . . . . . . . . . . . 7
5.1 Phone Numbers . . . . . . . . . . . . . . . . . . . . . . 7
5.1.1 Separators in Phone Numbers . . . . . . . . . . . . . 7
5.1.2 Alphabetic Characters Corresponding to Digits . . . . 8
5.1.3 Alphabetic, * and \\# Characters as Identifiers . . . 8
5.1.4 Global Numbers . . . . . . . . . . . . . . . . . . . . 8
5.1.5 Local Numbers . . . . . . . . . . . . . . . . . . . . 8
5.2 ISDN Subaddresses . . . . . . . . . . . . . . . . . . . . 10
5.3 Phone Extensions . . . . . . . . . . . . . . . . . . . . . 10
5.4 Other Parameters . . . . . . . . . . . . . . . . . . . . . 11
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1 Why Not Just Put Telephone Numbers in SIP URIs? . . . . . 11
7.2 Why Not Distinguish Between Call Types? . . . . . . . . . 12
7.3 Why tel? . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.4 Do Not Confuse Numbers with How They Are Dialed . . . . . 12
8. Usage of Telephone URIs in HTML . . . . . . . . . . . . . . . 12
9. Use of tel URIs with SIP (Informative) . . . . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 14
11. Security Considerations . . . . . . . . . . . . . . . . . . 14
12. Changes Since RFC 2806 . . . . . . . . . . . . . . . . . . . 15
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
13.1 Normative References . . . . . . . . . . . . . . . . . . . . 15
13.2 Informative References . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . 18
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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 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 an address-of-record or identifier in the sense of the
previous paragraph so that a call can be routed. Dial-strings may
require pre-pended digits to egress the private branch exchange
(PBX) the end system is connected to, and they may include
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post-dial dual-tone multi-frequency (DTMF) signaling that could
control an IVR or reach an extension. Dial strings are beyond the
scope of this document.
Both approaches can be encoded into a URI. For dial strings, this
URI is 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 integrated services
digital network (ISDN) or ISDN user part (ISUP) environment, the
signaling elements receiving protocol messages containing the dial
string perform the appropriate protocol actions. As noted, this
approach is beyond the scope of this specification.
The approach described here has the URI specify the telephone number
as an identifier, 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.
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 identifier approach described in this document has the
disadvantage that certain services, such as electronic banking or
voicemail, cannot be specified in a URI.
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].
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The 'tel' URI can be used as a request URI in SIP [RFC3261] 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.
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.
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.
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 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:
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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 = "[" / "]" / "/" / ":" / "&" / "+" / "$"
phonedigit = DIGIT / [ visual-separator ]
phonedigit-hex = HEXDIG / "*" / "#" / [ visual-separator ]
visual-separator = "-" / "." / "(" / ")"
alphanum = ALPHA / DIGIT
reserved = ";" | "/" | "?" | ":" | "@" | "&" |
"=" | "+" | "$" | ","
uric = reserved | unreserved | escaped
Each parameter name ("pname"), the ISDN subaddress, the 'extension'
and the 'context' MUST NOT appear more than once. The
'isdn-subaddress' or 'extension' MUST 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].
4. URI Comparisons
Two "tel" URIs are equivalent according to the following rules:
o Both must be either a 'local-number' or both must be a
'global-number', i.e., start with a '+'.
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o The 'global-number-digits' and the 'local-number-digits' must be
equal, after removing all visual separators.
o For mandatory additional parameters (Section 5.4) and the
'phone-context' and 'extension' parameters defined in this
document, The 'phone-context' parameter value is compared as a
host name if it is a 'domainname' or digit-by-digit if it is
'global-number-digits'. The latter is compared after removing all
'visual-separator' characters.
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.
5. Phone Numbers and Their Context
5.1 Phone Numbers
The 'telephone-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.
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
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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 in visual separators to avoid
excessive escaping.
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 meaning in local numbers is not defined here, but they
are 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 SHOULD be used.
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.
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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.
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.
For example, +49-6151-16 would be a suitable context for the
Technical University of Darmstadt, as it uses all numbers starting
with those digits. 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.
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
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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
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 International Alphabet 5 (IA5
[T.50]) characters, but may contain any octet value.
5.3 Phone Extensions
Phone extensions identify stations behind a non-ISDN PBX and are
roughly equivalent in functionality 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.
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5.4 Other Parameters
Future protocol 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 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, "isub"
and "ext" are optional.
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.
Entities that forward protocol requests containing tel URIs with
optional parameters MUST NOT delete or modify parameters they do not
understand.
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.
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 E.164 number ("ENUM") translation
[RFC2916], some other signaling protocols such as H.323 or a
traditional circuit-switched call initiated on the client side via,
say, the Telephony Application Programming Interface (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.
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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 URI 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, a "+" symbol in E.164
indicates that an international prefix is required.)
8. Usage of Telephone URIs in HTML
Links using the "tel" URI 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
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For having RFCs read aloud, call
<a href="tel:+1-555-438-3732">1-555-IETF-RFC</a>.
9. Use of tel URIs with SIP (Informative)
SIP can use the "tel" URI anywhere a URI is allowed, for example 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
[RFC3219]. 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. For local numbers,
the proxy MUST NOT translate "tel" URIs whose context it does not
understand.
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 rules that govern
the translation of dial strings into SIP and tel URIs, global or
local, as the dial plan. Currently, translations from dial strings
to tel URIs have to take place in end systems. Future efforts may
provide means to carry dial strings in a SIP URI, for example, but no
such mechanisms exist at the time of writing.
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.)
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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
his 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
'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 tel or 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.
10. Acknowledgments
This document is derived from RFC 2806 [RFC2806], written by Antti
Vähä-Sipilä. Flemming Andreasen, Francois Audet, Lawrence Conroy,
Cullen Jennings, Michael Hammer, Andrew Main, Xavier Marjou, Jon
Peterson, Mike Pierce, Jonathan Rosenberg and James Yu provided
extensive comments.
11. 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
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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.
12. Changes Since RFC 2806
The specification is syntactically backwards-compatible with the
"tel" URI defined in RFC 2806 [RFC2806], but has been completely
rewritten. This document more clearly distinguishes telephone
numbers as identifiers of network termination points from dial
strings and removes the latter from the purview of "tel" URIs.
Compared to RFC 2806, references to carrier selection, dial context,
fax and modem URIs, post-dial strings and pause characters have been
removed. The URI syntax now conforms to RFC 2396 [RFC2396].
A section on using tel URIs in SIP was added.
13. References
13.1 Normative References
[E.123] International Telecommunications Union, "Notation for
national and international telephone numbers, e-mail
addresses and web addresses", Recommendation E.123,
February 2001.
[E.161] International Telecommunications 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, May 1995.
[E.164] International Telecommunications Union, "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.
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[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.
13.2 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.
[RFC2916] Faltstrom, P., "E.164 number and DNS", RFC 2916, September
2000.
[RFC3187] Hakala, J. and H. Walravens, "Using International Standard
Book Numbers as Uniform Resource Names", RFC 3187, October
2001.
[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.
[RFC3219] Rosenberg, J., Salama, H. and M. Squire, "Telephony
Routing over IP (TRIP)", RFC 3219, January 2002.
[T.50] International Telecommunications Union, "International
Reference Alphabet (IRA) (Formerly International Alphabet
No. 5 or IA5) - Information technology - 7-bit coded
character set for information interchange", Recommendation
T.50, 1992.
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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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