Network Working Group B. Rosen
Request for Comments: 4967 NeuStar
Category: Standards Track July 2007
Dial String Parameter for the
Session Initiation Protocol Uniform Resource Identifier
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
RFC 3966 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
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RFC 4967 Dial String Parameter July 2007
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 a user part consisting of a dial string from a user part
that happens to be composed of characters that would appear in a dial
string.
Use of DTMF (dual tone multi-frequency) 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, an 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 dial string 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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RFC 4967 Dial String Parameter July 2007
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 multi-frequency. 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 as the telephone dial pad implies.
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 (Public Switched Telephone Network). The
"digits" for the fourth column are usually expressed using the
letters A through D. Thus, "C" is 852/1633Hz. Some systems do 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 a user
part that happens to consist of the same characters.
4. Solution
A new alternative value for the "userinfo" 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
dial string is a sequence of the characters 0-9, A-F, P, X, '*' and
'#'. 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
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