Dial String Parameter for the Session Initiation Protocol Uniform Resource Identifier
RFC 4967

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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

Rosen                       Standards Track                     [Page 1]
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].

Rosen                       Standards Track                     [Page 2]
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