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Dial String Parameter for the Session Initiation Protocol Uniform Resource Identifier

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4967.
Author Brian Rosen
Last updated 2020-01-21 (Latest revision 2007-03-02)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 4967 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Jon Peterson
Send notices to (None)
iptel                                                           B. Rosen
Internet-Draft                                                   NeuStar
Intended status: Standards Track                       February 28, 2007
Expires: September 1, 2007

    Dialstring parameter for the Session Initiation Protocol Uniform
                          Resource Identifier

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 1, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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  . . . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7

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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",
   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 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
   dialstring 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.

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   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 RFC3966
       dialstring-digits = *dialstring-element dialstring-digit
       dialstring-digit = HEXDIG / "*" / "#"; HEXDIG from RFC3966
       dialstring-element =  dialstring-digit  / "P" / "X" /
                  visual-separator; visual-separator from RFC3966

   A dialstring SHOULD NOT be used for an AoR in a REGISTER.  Parameters
   are ignored in registration.  Thus, two registrations with different
   phone-contexts would be considered equivalent, which is probably not

   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 as a
   sip: or sips: URI with the URI parameter value changed from
   "user=dialstring" to "user=phone".

   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 must be added to the references listed for the "user"

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

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   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

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,
              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.

Author's Address

   Brian Rosen
   470 Conrad Dr
   Mars, PA  16046

   Phone: +1 724 382 1051

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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

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