iptel B. Rosen
Internet-Draft NeuStar
Expires: September 4, 2006 March 03, 2006
Dialstring parameter for the Session Initiation Protocol URI
draft-rosen-iptel-dialstring-03.txt
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Copyright (C) The Internet Society (2006).
Abstract
RFC3966 explicitly states that TEL URIs may not represent a dial
string. That leaves no way specify a dialstring in a standardized
way. Great confusion exists with the SIP URI parameter "user=phone",
and specifically, if it can represent a dial string. This memo
creates a new value for the user parameter "dialstring", so that one
may specify "user=dialstring" to encode a dialstring as a SIP or SIPS
URI.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
A user at a phone often has a limited User Interface, and in some
cases, is limited to a 10 key pad (and sometimes a "flash" function
with the switchhook). One enters a series of digits that invoke some
kind of function. The entered sequence, called a "dialstring" might
be translated to a telephone number, or it may invoke a special
service. In many newer designs, the mapping between a dialstring and
a phone number or service URI is contained within the phone
(digitmap). However, there are many phones and terminal adapters
that do not have internal translation mechanisms. Without a
translation mechanism in the phone, the phone must send the
dialstring in a SIP or SIPS URI [RFC3261]to an intermediary that can
transform the dialstring to a phone number or a service invocation.
At some point, a dialstring is translated to a phone number. After
that point, it is no longer a dialstring. However, there is no
current way for any entity to determine if translation has already
been accomplished. The "user=phone" parameter in a SIP or SIPS URI
could denote a telephone number, but would need another value to
denote a dialstring.
Use of DTMF detectors post dial is not uncommon. A common functions
some systems have is to express a string that incorporates fixed time
delays, or in some cases, actual "wait for call completion" after
which additional DTMF signals are emitted. For example, many
voicemail systems use a common phone number, after which the system
expects the desired mailbox number as DTMF to deposit a message for.
Many gateways have the ability to interpret such strings, but there
is no standardized way to express them, leading to interoperability
problems between endpoints.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Requirements
A mechanism to express a dialstring in a SIP or SIPS uri is required.
A dialstring consists of a sequence of
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* The digits 0-9
* The special characters # and *
* The MF digits A-D
Note: DTMF = Dual Tone MultiFrequency. Each "tone:" is actually two
frequencies superimposed. DTMF is a 4 x 4 matrix with four row
frequencies (697, 770, 852, 941 Hz) and four column frequencies
(1209, 1336, 1477, 1633). Most telephones only implement 3 of the 4
columns, which are used just like the telephone dial pad implies they
are. Thus, the digit 2 is the first row, second column, and consists
of 770Hz and 1209Hz frequencies mixed together. The fourth column is
not used in the PSTN. The "digits" for the fourth column are usually
expressed using the letters A through D. Thus, "C" is 852/1633Hz.
Some equipment does use these digits, so we include them in the
definition of the dialstring.
A dialstring always exists within a context. The context MUST be
specified when expressing a dialstring.
It MUST be possible to distinguish between a dialstring and a phone
number.
It MUST be possible to express a short Pause, and a "Wait for call
completion" in a dialstring.
4. Solution
A new value for the "user" parameter of the SIP/SIPS URI schemes is
defined, "dialstring". This value may be used in a SIP or SIPS URI
when the userpart is a dialstring. The userpart is a sequence of the
characters 0-9, A-F, P and X. E is represent *, F represents #, P is
a pause (short wait, like a comma in a modem string) and X represents
call completion.
When the "user=dialstring" is used, a context parameter as defined in
[RFC3966] MUST be specified.
A proxy server or B2BUA which is authoritative for the context may
translate the dialstring to a telephone number or service invocation
URI. If such a translation is performed, the proxy server MUST
change the URI to specify user=phone.
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(Examples of dialstring use include)
; what a SIP Phone might emit when a user dials extension 123
sip:123@sippbx.example.com;user=dialstring;
phone-context='AtlantaDialPlan'
;existing voicemail systems have a local access extension,
;then expect to see the extension number as DTMF for the mailbox
sips:4500P4123@sip.example.com;user=dialstring;
phone-context='BiloxiDialPlan'
()
5. IANA Considerations
[RFC3969] defines a SIP/SIPS URI Parameter sub registry. The "user"
parameter is specified as having predefined values.
This RFC defines a new value for the "user" parameter, "dialstring".
This RFC would be added to the references listed for the "user"
parameter.
This RFC defines a new parameter in the sub-registry, "phone-
context", whose meaning and syntax are derived from the same
parameter in [RFC3969]
6. Security Considerations
Dialstrings exposed to the Internet may reveal information about
internal network details or service invocations that could allow
attackers to use the PSTN or the Internet to attack such internal
systems. Dialstrings normally should not be sent over the open
Internet without some kind of protection against eavesdropping.
7. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, December 2004.
[RFC3969] Schulzrinne, H., "The tel URI for Telephone Numbers",
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RFC 3966, December 2004.
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Author's Address
Brian Rosen
NeuStar
470 Conrad Dr
Mars, PA 16046
US
Phone: +1 724 382 1051
Email: br@brianrosen.net
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