





Network Working Group                                         R. Stastny
Request for Comments: 4759                                         Oefeg
Category: Standards Track                                     R. Shockey
                                                            Neustar Inc.
                                                               L. Conroy
                                                     Roke Manor Research
                                                           November 2006


           The ENUM Dip Indicator Parameter for the "tel" URI

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 (2006).

Abstract

   This document defines a new parameter "enumdi" for the "tel" Uniform
   Resource Identifier (URI) to support the handling of ENUM queries in
   Voice over Internet Protocol (VoIP) network elements.  A VoIP network
   element may receive a URI containing an E.164 number, where that URI
   contains an "enumdi" parameter.  The presence of the "enumdi"
   parameter indicates that an ENUM query has already been performed on
   the E.164 number by a previous VoIP network element.  Equally, if a
   VoIP network element sends such a URI, it asserts that an ENUM query
   has been carried out on this number.

















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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Formal Syntax ...................................................3
   4. Normative Rules .................................................3
      4.1. Options for ENUM Domain Providers ..........................3
      4.2. Client Behaviour for VoIP Network Elements .................3
           4.2.1. Handling a URI with the "enumdi" Parameter ..........3
           4.2.2. Adding the "enumdi" Parameter to URIs ...............4
           4.2.3. Handling a URI Retrieved from ENUM ..................4
   5. Examples ........................................................4
   6. Security Considerations .........................................5
   7. IANA Considerations .............................................5
   8. Acknowledgements ................................................6
   9. References ......................................................6
      9.1. Normative References .......................................6
      9.2. Informative References .....................................6

1.  Introduction

   VoIP network elements (including User Agent Servers and User Agent
   Clients) may be set up in different ways to handle E.164 [3] numbers
   during call setup, depending on the capabilities provided.  One
   common approach is to query ENUM as defined in RFC 3761 [4], and to
   use the set of NAPTR resource records that is returned.

   If the ENUM query leads to a result, the call is set up accordingly.
   If the ENUM query does not lead finally to a result, another database
   may be queried and/or the call may finally be routed to the Public
   Switched Telephone Network (PSTN).  In doing so, the call may be
   routed to another VoIP network element.  To indicate in signalling to
   this next VoIP element that an ENUM query has already been made for
   the "tel" URI (specified in RFC 3966 [5]), the "enumdi" parameter is
   used, to prevent the next VoIP network element from repeating
   redundant queries.

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 BCP 14, RFC 2119
   [1].








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3.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) as described in RFC 4234 [2] to extend the syntax of the
   "par" production defined in the ABNF of RFC 3966 [5].

   par =/ enum-dip-indicator

   enum-dip-indicator = ";enumdi"

   The enum-dip-indicator is an optional parameter for the "tel" URI.
   Note also that enum-dip-indicator can appear at most once in any
   "tel" URI.

4.  Normative Rules

4.1.  Options for ENUM Domain Providers

   A domain provider can, at its choosing, populate a NAPTR record with
   a "tel" URI that contains the enum dip indicator.  This would, as a
   consequence of the rules stated below, inform the client that it
   should not bother performing a query and pass the request on.

4.2.  Client Behaviour for VoIP Network Elements

   This section discusses how a VoIP network element handles a received
   "tel" URI that contains the "enumdi" parameter or has queried ENUM in
   e164.arpa. for a given E.164 number.

4.2.1.  Handling a URI with the "enumdi" Parameter

   If a VoIP network element receives a "tel" URI containing the
   "enumdi" parameter, the VoIP network element SHOULD NOT retrieve the
   related information for this number from ENUM in e164.arpa. even if
   it would normally do so.

   Note that the recipient network element may reasonably choose to
   query ENUM if it does not have a trust relationship with the
   immediate sender of the URI.

   If the "tel" URI (received from a trusted entity) is to be passed to
   the next network element, the VoIP network element MUST pass on the
   received URI containing the "enumdi" parameter unchanged.

   If, however, the URI has been received from an untrusted entity, then
   the recipient entity may either strip it before sending the URI
   onwards or instead carry out its own ENUM query and add the parameter
   accordingly to the URI (see next).



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4.2.2.  Adding the "enumdi" Parameter to URIs

   When a VoIP network element queries ENUM in e164.arpa. for a given
   E.164 number and the result of the query is DNS error code 3
   (commonly known as "NXDOMAIN"), then if that network element chooses
   to pass the call to another network element by using a "tel" URI, the
   "enumdi" parameter MUST be set.

4.2.3.  Handling a URI Retrieved from ENUM

   When a VoIP network element queries ENUM in e164.arpa. for a given
   E.164 number and either:

   o  the result of the query includes a NAPTR resource record
      containing a "tel" URI that has the same E.164 number, or

   o  the result of the query includes a NAPTR resource record
      containing a "tel" URI with the "enumdi" parameter set,

   then if that retrieved "tel" URI is chosen to be passed to another
   network element, the sending VoIP network element MUST pass on the
   retrieved URI with the "enumdi" parameter set.

   When a VoIP network element queries ENUM in e164.arpa. for a given
   E.164 number and the result is a "tel" URI with a different E.164
   number that lacks the enum dip indicator, the client can either
   perform another query against that number or pass the request on, as
   a matter of local policy.

5.  Examples


   a.  A VoIP network element called server.example.com receives a "tel"
       URI tel:+441632960038.  The VoIP network element queries the DNS
       for NAPTR resource records in 8.3.0.0.6.9.2.3.6.1.4.4.e164.arpa.,
       and gets an error response with code = 3 (commonly known as
       "NXDOMAIN").  The VoIP network element decides to route the call
       to the PSTN via another VoIP network element called
       gw.example.com.

          It therefore signals to the next VoIP network element with:
             tel:+441632960038;enumdi
          or (using the procedures of RFC 3261 [6] section 19.1.6):
             sip:+441632960038;enumdi@gw.example.com;user=phone







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   b.  A VoIP network element called server.example.com receives a "tel"
       URI tel:+441632960038.  The VoIP network element queries the DNS
       for NAPTR resource records in 8.3.0.0.6.9.2.3.6.1.4.4.e164.arpa.,
       and receives the same "tel" URI in reply (i.e.,
       tel:+441632960038).

       The VoIP network element decides to route the call to the PSTN
       via another VoIP network element called gw.example.com.

          It therefore signals to this next VoIP network element with:
             tel:+441632960038;enumdi
          or (using the procedures of RFC 3261 [6] section 19.1.6):
             sip:+441632960038;enumdi@gw.example.com;user=phone

6.  Security Considerations

   In addition to those security implications discussed in the "tel" URI
   [5] specification, there are new security implications associated
   with the defined parameter.

   If the "enumdi" is illegally inserted into the "tel" URI when the
   signalling message carrying the "tel" URI is en route to the
   destination entity, the call may be routed to the PSTN network,
   incurring unexpected charges or causing a downstream VoIP network
   element to reject the call setup.  Many network elements that will
   process URIs containing this parameter will maintain trust
   relationships with others.  If such a URI is received from an entity
   outside the trust boundary of the recipient, then that recipient
   entity may reasonably ignore it and make an ENUM query itself.  In so
   doing, it can avoid this potential attack.

   It is less a problem if the "enumdi" is illegally removed.  An
   additional ENUM query may be performed to retrieve the routing number
   information and have the "enumdi" included again.

   It is RECOMMENDED that protocols carrying the "tel" URI ensure
   message integrity during the message transfer between the two
   communicating network elements so as to detect any unauthorised
   changes to the content of the "tel" URI and other information.

7.  IANA Considerations

   This document does not itself require any IANA actions.

   It does define a parameter for the "tel" URI.  Further information on
   a registry for such parameters is covered in the IANA "tel" URI
   Parameter Registry [7].




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8.  Acknowledgements

   Many thanks for the thorough review provided by Alex Mayrhofer.

9.  References

9.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, BCP 14, March 1997.

   [2]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 4234, October 2005.

   [3]  ITU-T, "The International Public Telecommunication Number Plan",
        Recommendation E.164, February 2005.

   [4]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
        Identifiers (URI) Dynamic Delegation  Discovery System (DDDS)
        Application (ENUM)", RFC 3761, April 2004.

   [5]  Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
        December 2004.

   [6]  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.

9.2.  Informative References

   [7]  Jennings, C. and V. Gurbani, "The Internet Assigned Number
        Authority (IANA) tel Uniform Resource Identifier (URI) Parameter
        Registry", Work in Progress, May 2006.


















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RFC 4759         ENUM Dip Indicator "tel" URI parameter    November 2006


Authors' Addresses

   Richard Stastny
   Oefeg
   Postbox 147
   1103 Vienna
   Austria

   Phone: +43-664-420-4100
   EMail: Richard.stastny@oefeg.at


   Richard Shockey
   Neustar Inc.
   46000 Center Oak Plaza
   Sterling, VA  20166
   United States

   Phone: +1-571-434-5651
   EMail: richard.shockey@neustar.biz


   Lawrence Conroy
   Roke Manor Research
   Roke Manor
   Romsey
   United Kingdom

   Phone: +44-1794-833666
   EMail: lconroy@insensate.co.uk





















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