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Network Working Group                                           B. RosenRequest for Comments: 4967                                       NeuStarCategory: Standards Track                                      July 2007Dial String Parameter for theSession Initiation Protocol Uniform Resource IdentifierStatus 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).AbstractRFC 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .23.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . . .34.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . . .35.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .46.  Security Considerations . . . . . . . . . . . . . . . . . . . .47.  Normative References  . . . . . . . . . . . . . . . . . . . . .5Rosen                       Standards Track                     [Page 1]

RFC 4967                 Dial String Parameter                 July 20071.  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 20073.  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 parameterRosen                       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 fromRFC 3966       dialstring-digits = *dialstring-element dialstring-digit                  *dialstring-element       dialstring-digit = HEXDIG / "*" / "#"; HEXDIG fromRFC 3966       dialstring-element =  dialstring-digit  / "P" / "X" /                  visual-separator; visual-separator fromRFC 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 123sip: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 mailboxsip:450X123;phone-context=biloxi.example.com@example.com;user=dialstring5.  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 internalRosen                       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.netRosen                       Standards Track                     [Page 5]

RFC 4967                 Dial String Parameter                 July 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Rosen                       Standards Track                     [Page 6]