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Versions: 00 01 02 03 04 05 RFC 4967

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

Status of this Memo

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Copyright Notice

   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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   Copyright (C) The Internet Society (2006).  This document is subject
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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