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 (C) The IETF Trust (2007).
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
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 22. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 23. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 34. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 35. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 46. Security Considerations . . . . . . . . . . . . . . . . . . . . 47. Normative References . . . . . . . . . . . . . . . . . . . . . 5
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
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.
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].
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
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.
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
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.
The syntax of the context parameter follows the same conventions as
the same parameter in [RFC3966], that is, it appears between the
digits and the "@" in the userinfo [RFC3261] of the URI:
dialstring = dialstring-digits context; context from RFC 3966
dialstring-digits = *dialstring-element dialstring-digit
dialstring-digit = HEXDIG / "*" / "#"; HEXDIG from RFC 3966
dialstring-element = dialstring-digit / "P" / "X" /
visual-separator; visual-separator from RFC 3966
A dial string SHOULD NOT be used for an AoR (Address of Record) in a
REGISTER. Parameters are ignored in registration. Thus, two
registrations with different phone-contexts would be considered
equivalent, which is probably not desirable.
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. The telephone number MAY
be expressed as a global or local tel: URI, or it MAY be left as a
sip: or sips: URI with the URI parameter value changed from "user= "
Examples of dial string use include:
;what a SIP Phone might emit when a user dials extension 123
;existing voicemail systems have a local access extension,
;then expect to see the extension number as DTMF for the mailbox
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 has been added to the references listed for the "user"
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. Dial strings normally SHOULD NOT be sent beyond the domain
of the UAC (User Agent Client). If they are sent across the
Internet, they SHOULD be protected against eavesdropping with TLS
(Transport Layer Security) 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,
[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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