INTERNET-DRAFT A. Vaha-Sipila
Expires 29-Dec-1998 Nokia
25-Jun-1998
URLs for Telephone Calls
<draft-antti-telephony-url-05.txt>
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Abstract
This document specifies URL (Uniform Resource Locator) schemes
''tel'', ''fax'' and ''modem'' for specifying the location of a
terminal in the phone network and the connection types (modes of
operation) that can be used to connect to that entity. This
specification covers voice calls (normal phone calls, answering
machines and voice messaging systems), facsimile (telefax) calls
and data calls, both for POTS and digital/mobile subscribers.
Version History
| Changes to the previous versions are indicated by a bar in the
| left margin like in this section.
| In this file, the bars indicate changes since version 04.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 New URL Schemes . . . . . . . . . . . . . . . . . . . . . 2
1.2 Formal Definitions . . . . . . . . . . . . . . . . . . . 2
1.3 Requirements . . . . . . . . . . . . . . . . . . . . . . 3
2. URL Schemes for Telephone Calls . . . . . . . . . . . . . 3
2.1 Applicability . . . . . . . . . . . . . . . . . . . . . . 3
| 2.2 "tel" URL Scheme . . . . . . . . . . . . . . . . . . . . 3
2.3 "fax" URL Scheme . . . . . . . . . . . . . . . . . . . . 3
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2.4 "modem" URL Scheme . . . . . . . . . . . . . . . . . . . 4
2.5 Parsing telephone, fax and modem URLs . . . . . . . . . . 4
2.6 Examples of Use . . . . . . . . . . . . . . . . . . . . . 7
2.7 The Choice of the Scheme Specifier . . . . . . . . . . . 8
3. References . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . 9
5. Authors' Addresses . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
1.1 New URL Schemes
URLs that designate phone or fax numbers that can be dialed have
been brought forward in other Internet-Drafts. However, none of
these has reached the RFC status. This document tries to remedy
the situation. All interested parties are invited to submit
comments on this Internet-Draft. Contact information can be found
at the end of this document.
See also [CONV-URL] for more discussion on conversational URLs.
| This specification defines three new URL schemes: "tel",
"fax" and "modem". They are intended for describing a terminal
that can be contacted using the telephone network. The
description includes the subscriber (telephone) number of the
terminal and the necessary parameters for successfully
connecting to that terminal.
| The "tel" scheme describes a connection to a terminal that
handles normal voice telephone calls, a voice mailbox or another
voice messaging system or a service that can be operated using
DTMF codes.
The "fax" scheme describes a connection to a terminal that can
handle telefaxes (facsimiles). The name (scheme specifier) for
the URL is "fax" as recommended by [E.123].
The "modem" scheme describes a connection to a terminal that can
handle incoming data calls. The term "modem" refers to a device
that does digital-to-analog and analog-to-digital conversions;
in addition to these, a "modem" scheme can describe a fully
digital connection.
The notation for phone numbers is the same which is specified in
| [RFC2303] and [RFC2304]. However, the syntax definition is a
bit different due to the fact that this document specifies URLs
| whereas [RFC2303] and [RFC2304] specify electronic mail
addresses. For example, "/" (used in URLs to separate parts in a
hierarchical URL [RFC1738]) has been replaced by ";".
1.2 Formal Definitions
| Formal definitions follow [RFC2234]. This specification uses
elements from the 'core' definitions (Appendix A of [RFC2234]).
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1.3 Requirements
Compliant software MUST follow this specification. Requirements
are indicated by capitalized words as specified in [RFC2119].
2. URL Schemes for Telephone Calls
2.1 Applicability
In this document, "user agent" means software that can detect and
parse one or more of these URLs and place a call to the remote
| terminal using hardware and software at its disposal after it has
| been properly configured.
These URL schemes are used to direct the user agent to place a
call using the telephone network. The network in question may be a
landline or mobile phone network. If the phone network
differentiates between (for example) voice and data calls, or if
the user agent has several different telecommunications equipment
at its disposal, it is possible to specify which kind of call
(voice/fax/data) is requested. The URL can also contain
information about the capabilities of the remote entity, so that
the connection can be established successfully.
None of the URL schemes do have a 'path' in them - they are
always absolute. The URLs are always case-insensitive.
2.2 "tel" URL Scheme
The URL syntax is formally described as follows. For the basis
of this syntax, see [RFC2303].
telephone-url telephone-scheme ":"
telephone-subscriber
| telephone-scheme "tel"
telephone-subscriber global-phone-number / local-phone-number
global-phone-number "+" 1*phonedigit [isdn-subaddress]
[post-dial]
local-phone-number 1*(phonedigit / dtmf-digit /
pause-character) [isdn-subaddress]
[post-dial]
isdn-subaddress ";isub" 1*phonedigit
post-dial ";postd" 1*(phonedigit / dtmf-digit
/ pause-character)
phonedigit DIGIT / visual-separator
visual-separator "-" / "." / "(" / ")"
pause-character one-second-pause / wait-for-dial-tone
one-second-pause "p"
wait-for-dial-tone "w"
dtmf-digit "*" / "#" / "A" / "B" / "C" / "D"
2.3 "fax" URL Scheme
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The URL syntax is formally described as follows (the definition
reuses nonterminals from the definition above). For the basis of
this syntax, see [RFC2303] and [RFC2304].
fax-url fax-scheme ":" fax-subscriber
fax-scheme "fax"
fax-subscriber fax-global-phone / fax-local-phone
fax-global-phone "+" 1*phonedigit [isdn-subaddress]
[t33-subaddress] [post-dial]
fax-local-phone 1*(phonedigit / dtmf-digit /
pause-character) [isdn-subaddress]
[t33-subaddress] [post-dial]
t33-subaddress ";tsub" 1*phonedigit
2.4 "modem" URL Scheme
The URL syntax is formally described as follows. For the basis of
this syntax, see [RFC2303].
modem-url modem-scheme ":" remote-host
modem-scheme "modem"
remote-host telephone-subscriber *modem-params
modem-params ";type" data-capabilities
data-capabilities accepted-modem ["?" data-bits parity
stop-bits]
accepted-modem "V21" / "V22" / "V22b" /
"V23" / "V26t" / "V32" /
"V32b" / "V34" / "V110" /
"V120" / "B103" / "B212" /
"X75" / "vnd." vendor-name "."
modem-type
data-bits "7" / "8"
parity "n" / "e" / "o" / "m" / "s"
stop-bits "1" / "2"
vendor-name 1*(ALPHA / DIGIT / "-" / "+")
modem-type 1*(ALPHA / DIGIT / "-" / "+")
2.5 Parsing telephone, fax and modem URLs
A. The type of call is specified by the scheme specifier.
| "Tel" means that a voice call is opened. "Fax" indicates
that the call should be a facsimile (telefax) call. "Modem" means
that it should be a data call. Not all networks differentiate
between the types of call; in this case, the scheme specifier
indicates the telecommunications equipment type to use.
B. <telephone-subscriber> and <fax-subscriber> indicate the
phone number to be dialed. The phone number can be written in
either international or local notation. All phone numbers SHOULD
always be written in the international form if there is no good
reason to use the local form.
Any telephone number MUST contain at least one <DIGIT>, that is,
subscriber numbers consisting only of non-numbers are not allowed.
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International numbers MUST begin with the "+" character. Local
numbers MUST NOT contain that character. International numbers
MUST be written with the country (CC) and national (NSN) numbers
| as specified in [E.123] and [E.164]. International numbers have
| the property of being totally unambiguous everywhere in the world
| if the user agent is properly configured.
Local numbers MAY be used if the number only works from inside a
certain geographical area or a network. Note that some numbers may
work from several networks but not from the whole world - these
| SHOULD be written in international form. URLs containing local
| phone numbers should only appear in an environment where all
| user agents can get the call successfully set up by passing the
| number to the dialing entity "as is". An example could be a
| company intranet, where all user agents are located under a
| the same private telephone exchange. If local phone numbers
| are used, the document in which they are present SHOULD contain
| an indication of the context in which they are intended to be
| used.
| In some regions, it is popular to write phone numbers using
| alphabetic characters which correspond to certain numbers on the
| telephone keypad. Letters in <dtmf-digit> characters do not have
| anything to do with this, nor is this supported by this URL
| scheme.
C. All <visual-separator> characters MUST be removed from the
phone number by the user agent before using it do dial out.
These cracaters are present only to aid readability: they MUST
NOT have any other meaning. Note that although [E.123]
recommends the use of space (SP) characters as the separators,
spaces MUST NOT be used in these URLs.
D. After the telephone number has been extracted, it is
converted to the format that the user agent can use to place the
call. (For example, the "+" character might be replaced by the
international call prefix, or the international and trunk
prefixes might be removed to place a local call.) Numbers that
have been specified using <local-phone> or <fax-local-phone>
MUST be used by the user agent "as is", without any conversions.
E. The number may contain a <post-dial> sequence, which MUST be
dialled using Dual Tone Multifrequency (DTMF) in-band signalling
| or pulse dialing after the call setup is complete. If the user
| agent does not support DTMF or pulse dialing after the call has
| been set up, <post-dial> MUST be ignored. In that case, the user
SHOULD be notified.
F. A local phone number or a post-dial sequence may contain
<pause-character> characters which indicate a pause
while dialing ("p"), or a wait for dial tone ("w").
| User agents MAY support this method of dialing, and the final
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| interpretation of these characters is left to the user agent.
If it is not supported, user agents MUST ignore everything in the
dial string after the first <pause-character> and the user SHOULD
be notified. The user or the user agent MAY opt not to place a
call if this feature is not supported and these characters are
present in the URL.
Any <dtmf-digit> characters and all dial string characters after
the first <pause-character> or <dtmf-digit> SHOULD be sent to line
using DTMF (Dual Tone Multifrequency) in-band signaling, even if
dialing is done using direct network signaling (a digital
| subscriber loop or a mobile phone). If the local infrastructure
| does not support DTMF codes, the user agent MAY opt to use pulse
| dialing. However, it should be noted that certain services which
| are controlled using DTMF tones cannot be controlled with pulse
| dialing.
G. A phone number MAY also contain an <isdn-subaddress> which
indicates an ISDN subaddress. User agent SHOULD support ISDN
subaddresses. These addresses are sent to the network by using a
method available to the user agent (typically, ISDN subscribers
send the address with the call setup signalling). If ISDN
subaddressing is not supported by the caller, <isdn-subaddress>
MUST be ignored and the user SHOULD be notified. The user or the
user agent MAY opt not to place a call if this feature is not
supported.
H. A fax number MAY also contain a <t33-subaddress>, which
indicates the start of a T.33 subaddress [T.33]. User agents
SHOULD support this. Otherwise <t33-subaddress> MUST be ignored
and the user SHOULD be notified. The user or the user agent MAY
opt not to place a call if this feature is not supported.
I. <modem-params> indicate the minimum compliance required from
the user agent to be able to connect to the remote entity. The
minimum compliance is defined as being equal to or a superset of
the capabilities of the listed modem type.
The user agent MUST call out using compatible hardware, or request
that the network provides such a service.
For example, if the user agent only has access to a V.22bis modem
and the URL indicates that the minimum acceptable connection is
V.32bis, the user agent MUST NOT try to connect to the remote host
since V.22bis is a subset of V.32bis. However, if the URL lists
V.32 as the minimum acceptable connection, the user agent can use
V.32bis to create a connection since V.32bis is a superset of
V.32.
This feature is present because modem pools often have separate
numbers for slow modems and fast modems, or have different numbers
for analog and ISDN connections, or may use proprietary modems
that are incompatible with standards. It is somewhat analogous to
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the connection type specifier (typecode) in FTP URLs [RFC1738]: it
provides the user agent with information that can not be deduced
from the scheme specifier, but is helpful for successful
operation.
This also means that the number of data and stop bits and parity
MUST be set according to the information given in the URL, or to
default values, if the information is not present.
The capability tokens are listed below. If capabilities
suggest that it is impossible to create a connection, the
connection MUST NOT be created.
If new modem types are standardized by ITU-T, this list can be
extended with those capability tokens. Tokens are formed by taking
the number of the standard and joining together the first letter
| (for example, "V"), number (for example, 22) and the first letter
| of the postfix (for example "bis" would become "b").
| Proprietary modem types MUST be specified using the "vendor naming
| tree", which takes the form "vnd.x.y", in which "x" is the name of
| the entity from which the specifications for the modem type can be
| acquired and "y" is the type or model of the modem. Vendor names
| MUST share the same name space with vendor names used in MIME
| types [RFC2048]. Submitting the modem types to ietf-types list
| for review is strongly recommended.
| New capabilities MUST always be documented in an RFC.
Capability Explanation
V21 ITU-T V.21
V22 ITU-T V.22
V22b ITU-T V.22bis
V23 ITU-T V.23
V26t ITU-T V.26ter
V32 ITU-T V.32
V32b ITU-T V.32bis
V34 ITU-T V.34
V110 ITU-T V.110
V120 ITU-T V.120
X75 ITU-T X.75
B103 Bell 103
B212 Bell 212
Data bits: "8" or "7" The number of data bits. If not
specified, defaults to "8".
Parity: "n", "e", "o", Parity. None, even, odd, mark or
"m", "s" space parity, respectively. If
not specified, defaults to "n".
Stop bits: "1" or "2" The number of stop bits. If not
specified, defaults to "1".
2.6 Examples of Use
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| tel:+358-555-1234567
This URL instructs the user agent to place a voice call to the
specified number in Finland. The hyphens are included to make the
number more human-readable: country and area codes have been
separated from the subscriber number.
fax:+358.555.1234567
The above URL instructs the user agent to place a fax call to
the specified number. It uses dots instead of hyphens as
separators, but they have no effect on the functionality.
modem:+3585551234567;typev32b?7e1;typev110
This URL instructs the user agent to place a data call to the
specified number. The user agent may opt to use either a ITU-T
V.32bis modem (or a faster one, which is compatible with V.32bis),
using settings of 7 data bits, even parity and one stop bit, or an
ISDN connection using ITU-T V.110 protocol.
| tel:+358-555-1234567;postdpp22
The above URL instructs the user agent to place a voice call to
| +358-555-1234567, then wait for an implementation-dependent time
| (for example, two seconds) and emit two DTMF dialing
tones "2" on the line (for example, to choose a particular
extension number, or to invoke a particular service).
| tel:0w003585551234567
This URL places a voice call to the given number. The number
format is intended for local use: the first zero opens an
outside line, the "w" character waits for a second dial tone,
and the number already has the international access code
appended to it ("00"). This kind of phone number MUST NOT be
used in an environment where all users of this URL might not be
able to successfully dial out by using this number directly.
However, this might be appropriate for pages in a company
intranet.
2.7 The Choice of the Scheme Specifier
| There has been discussion on whether the scheme name "tel" is
| appropriate. To summarize, these are the points made against
| the other proposals.
| callto URL schemes locate a resource and do not specify
| an action to be taken.
| telephone Too long. Also, "tel" considered to be a more
| international form.
| phone Was countered on the basis that "tel" is more
| internationally acceptable.
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3. References
NOTE. References to Internet-Drafts will be removed from the final
document which will be submitted to the RFC-Editor.
[RFC2234] Augmented BNF for Syntax Specifications: ABNF.
November 1997. D. Crocker et al. RFC 2234.
<URL:ftp://ftp.ds.internic.net/rfc/rfc2234.txt>
[CONV-URL] Conversational Multimedia URLs. 1997. Pete Cordell. An
Internet-Draft (work in progress).
<URL:ftp://ftp.ds.internic.net/internet-drafts/
draft-cordell-sg16-conv-url-00.txt>
[RFC1738] Uniform Resource Locators (URL). December 1994. T.
Berners-Lee et al. RFC 1738.
<URL:ftp://ftp.ds.internic.net/rfc/rfc1738.txt>
[RFC2048] Multipurpose Internet Mail Extensions (MIME) Part Four:
Registration Procedures. November 1996. N. Freed et al. RFC 2048.
<URL:ftp://ftp.ds.internic.net/rfc/rfc2048.txt>
[RFC2119] Key Words for Use in RFCs to Indicate Requirement
Levels. March 1997. S. Bradner. RFC 2119.
<URL:ftp://ftp.ds.internic.net/rfc/rfc2119.txt>
[RFC2303] Minimal PSTN Address Format in Internet Mail. March 1998.
C. Allocchio. RFC 2303.
<URL:ftp://ftp.ds.internic.net/rfc/rfc2303.txt>
[RFC2304] Minimal FAX Address Format in Internet Mail. March 1998.
C. Allocchio. RFC 2304.
<URL:ftp://ftp.ds.internic.net/rfc/rfc2304.txt>
[E.123] ITU-T Recommendation E.123: Telephone Network and ISDN
Operation, Numbering, Routing and Mobile Service: Notation for
National and International Telephone Numbers. 1993.
[E.164] ITU-T Recommendation E.164: Telephone Network and ISDN
Operation, Numbering, Routing and Mobile Service: Numbering Plan
for the ISDN Era. 1991.
[T.33] ITU-T Recommendation T.33: Facsimile Routing Utilizing
the Subaddress. 1996.
4. Security Considerations
It should be noted that the user agent SHOULD NOT call out without
the knowledge of the user because of associated risks, which
include
- call costs (including long calls, long distance calls,
| international calls and premium rate calls)
- wrong numbers inserted on web pages by malicious users
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- making the user's phone line unavailable (off-hook) for a
malicious purpose
- opening a data call to a remote host, thus possibly opening a
back door to the user's computer
| - revealing the user's (possibly unlisted) phone number to the
| remote host in the caller identification data
All of these risks MUST be taken into consideration when
designing the user agent.
The user agent SHOULD have some mechanism that the user can use to
filter out unwanted numbers. The user agent SHOULD NOT use rapid
redialing of the number if it is busy to avoid the congestion of
the (signaling) network. Also, the user agent 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.
5. Authors' Addresses
Contact person and version control responsibility
for this specification:
Nokia Mobile Phones
Antti Vaha-Sipila
P. O. Box 68
FIN-33721 Tampere
Finland
Electronic mail: antti.vaha-sipila@nmp.nokia.com
Please include your name and electronic mail address in all
communications. If you want to receive the newest version of this
specification electronically, send mail to the address above.
This document expires on the 29th of December, 1998, or when a
new version is released.
A. Vaha-Sipila URLs for Telephone Calls June 1998