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rfc:rfc4967

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
 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

Rosen Standards Track [Page 3] RFC 4967 Dial String Parameter July 2007

 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-element
     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= "
 to "user=phone".
 Examples of dial string use include:
 ;what a SIP Phone might emit when a user dials extension 123

sip:123;phone-context=atlanta.example.com@example.com;user=dialstring

 ;existing voicemail systems have a local access extension,
 ;then expect to see the extension number as DTMF for the mailbox

sip:450X123;phone-context=biloxi.example.com@example.com;user=dialstring

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"
 parameter.

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

Rosen Standards Track [Page 4] RFC 4967 Dial String Parameter July 2007

 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,
            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
 NeuStar
 470 Conrad Dr
 Mars, PA  16046
 USA
 Phone: +1 724 382 1051
 EMail: br@brianrosen.net

Rosen Standards Track [Page 5] RFC 4967 Dial String Parameter July 2007

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
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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

Rosen Standards Track [Page 6]

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