iptel B. Rosen
Internet-Draft NeuStar
Expires: December 27, 2006 June 25, 2006
Dialstring parameter for the Session Initiation Protocol Uniform
Resource Identifier
draft-rosen-iptel-dialstring-04.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 dial string 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 dial string as a 'sip:' or
'sips:' URI.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. Normative 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). The user enters a series of digits that invoke
some kind of function. The entered sequence, called a "dial string",
may be translated to a telephone number, or it may invoke a special
service. In many newer designs, the mapping between a dial string
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 dial
string in a 'sip:' or 'sips:' URI [RFC3261] to an intermediary that
can transform the dial string to a phone number or a service
invocation. The intermediary is able to perform this transform
provided that it knows the context (i.e., dialing plan) within which
the number was dialed.
There is a problem here. The intermediary can apply its
transformation only if it recognizes that the user part of the SIP
URI is a dial string. However, there is currently no way to
distinguish an user part consisting of a dial string from an user
part that happens to be composed of characters that would appear in a
dial string.
Use of DTMF detectors after the initial number has been dialed is not
uncommon. A common function 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 a series of
DTMF digits 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.
This is another case where the ability to indicate that a dialstring
is being presented would be useful.
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].
It is assumed that the reader is familiar with the terminology and
acronyms defined in [RFC3261]
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3. Requirements
A mechanism to express a dial string in a 'sip:' or 'sips:' URI is
required. A dial string consists of a sequence of
* The digits 0-9
* The special characters # and *
* The DTMF digits A-D
* characters representing a short Pause, and a "Wait for call
completion" in a dial string
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 dial string.
A dial string always exists within a context. The context MUST be
specified when expressing a dial string.
It MUST be possible to distinguish between a dial string and an user
part that happens to consist of the same characters.
4. Solution
A new value for the "user" parameter of the 'sip:' or 'sips:' URI
schemes is defined, "dialstring". This value may be used in a 'sip:'
or 'sips:' URI when the user part is a dial string. The user part is
a sequence of the characters 0-9, A-F, P and X. E represents *, F
represents #, P is a pause (short wait, like a comma in a modem
string) and X represents "wait for call completion".
When the "user=dialstring" is used, a context parameter as defined in
[RFC3966] MUST be specified. The context parameter would normally be
a domain name. 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. The context parameter value
is normally configured in the user agent.
A proxy server or Back to Back User Agent (B2BUA) [RFC3261] which is
authoritative for the context may translate the dial string to a
telephone number or service invocation URI. If such a translation is
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performed, the proxy server MUST change the URI parameter value from
"user=dialstring" to "user=phone". This translation MUST occur prior
to the call leaving the domain of the context.
Examples of dial string use include:
; what a SIP Phone might emit when a user dials extension 123
sip:123@sippbx.example.com;user=dialstring;
phone-context=atlanta.example.com
;existing voicemail systems have a local access extension,
;then expect to see the extension number as DTMF for the mailbox
sips:4500X4123@sip.example.com;user=dialstring;
phone-context='biloxi.example.com'
5. IANA Considerations
[RFC3969] defines a 'sip:' or '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 must 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 [RFC3966]. This parameter does not have a set of
predefined values.
6. Security Considerations
dial strings 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 beyond the domain
of the UAC. If they are sent across the Internet, they SHOULD be
protected against eavesdropping with TLS per the procedures in
[RFC3261].
7. Normative 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,
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June 2002.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, December 2004.
[RFC3969] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Uniform Resource Identifier (URI) Parameter
Registry for the Session Initiation Protocol (SIP)",
BCP 99, RFC 3969, 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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