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IETF RFC 4759
The ENUM Dip Indicator Parameter for the 'tel' URI
Last modified on Thursday, December 7th, 2006
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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 © 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.
Stastny, et al. Standards Track PAGE 1 
RFC 4759 ENUM Dip Indicator "tel" URI parameter November 2006
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].
Stastny, et al. Standards Track PAGE 2
RFC 4759 ENUM Dip Indicator "tel" URI parameter November 2006
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).
Stastny, et al. Standards Track PAGE 3
RFC 4759 ENUM Dip Indicator "tel" URI parameter November 2006
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
Stastny, et al. Standards Track PAGE 4
RFC 4759 ENUM Dip Indicator "tel" URI parameter November 2006
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].
Stastny, et al. Standards Track PAGE 5
RFC 4759 ENUM Dip Indicator "tel" URI parameter November 2006
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.
Stastny, et al. Standards Track PAGE 6
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
Stastny, et al. Standards Track PAGE 7
RFC 4759 ENUM Dip Indicator "tel" URI parameter November 2006
Full Copyright Statement
Copyright © The IETF Trust (2006).
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.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Stastny, et al. Standards Track PAGE 8
The ENUM Dip Indicator Parameter for the 'tel' URI
RFC TOTAL SIZE: 15062 bytes
PUBLICATION DATE: Thursday, December 7th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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