The tel URI (RFC 3966 ), defines a URI that can be used to
represent resources identified by telephone numbers. The tel URI,
like many other URIs, provides extensibility through the definition
of new URI parameters and new values for existing parameters.
However, RFC 3966 did not specify an IANA registry where such
parameters and values can be listed and standardized. This
specification creates such a registry.
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 .
3. Use of the Registry
The tel URI parameters and values for these parameters MUST be
documented in a RFC or other permanent and readily available public
specification in order to be registered by IANA. This documentation
MUST fully explain the syntax, intended usage, and semantics of the
parameter. The intent of this requirement is to assure
interoperability between independent implementations, and to prevent
accidental namespace collisions between implementations of dissimilar
Documents defining tel URI parameters or parameter values MUST
register them with IANA, as described in Section 4. The IANA
registration policy for such parameters is "Specification Required,
Designated Expert," and is further discussed in Section 4.2.
Some tel URI parameters only accept a set of predefined parameter
values while others can take any value. There are also parameters
that do not have any value; they are used as flags.
Those URI parameters that take on predefined values typically take on
a large number of values. Registering each of those values, or
creating a sub-registry for each such parameter is not appropriate.
Instead, we have chosen to register URI parameter values by
reference. That is, the entry in the URI parameter registry for a
given URI parameter contains references to the RFCs defining new
values of that parameter.
Accordingly, the tel URI parameter registry contains a column that
indicates whether or not each parameter accepts a value. The column
may contain "No value" or "Constrained". A "Constrained" in the
column implies that certain predefined values exist for this
parameter and the accompanying RFC or other permanent and readily
available public specification should be consulted to find out the
accepted set of values. A "No Value" in the column implies that the
parameter is used either as a flag, or does not have a set of
predefined values. The accompanying RFC or other permanent and
readily available public specification should provide more
information on the semantics of the parameter.
4. IANA Considerations
The specification creates a new IANA registry named "tel URI
4.1. tel URI Parameters Registry
New tel URI parameters and new values for existing tel URI parameters
MUST be registered with IANA.
When registering a new tel URI parameter, the following information
MUST be provided:
o Name of the parameter.
o Whether the parameter only accepts a set of predefined values.
o Reference to the RFC or other permanent and readily available
public specification defining the parameter and new values.
When registering a new value for an existing tel URI parameter, the
following information MUST be provided:
o Name of the parameter.
o Reference to the RFC or other permanent and readily available
public specification providing the new value.
Table 1 contains the initial values for this registry.
Parameter Name Predefined Values Reference
-------------- ----------------- ---------
isub Constrained [RFC3966]
isub-encoding Constrained [RFC4715]
ext Constrained [RFC3966]
phone-context Constrained [RFC3966]
enumdi No value [RFC4759]
npdi No value [RFC4694]
rn Constrained [RFC4694]
rn-context Constrained [RFC4694]
cic Constrained [RFC4694]
cic-context Constrained [RFC4694]
tgrp Constrained [RFC4904]
trunk-context Constrained [RFC4904]
Table 1: IANA tel URI parameter registry4.2. Registration Policy for tel URI Parameters
As per the terminology in  and actions accorded to such a role,
the registration policy for tel URI parameters shall be
"Specification Required, Designated Expert" (the former implicitly
implies the latter).
The Designated Expert, when deliberating on whether to include a new
parameter in the tel URI registry, may use the criteria provided
below to reach a decision (this is not an exhaustive list but
representative of the issues to consider when rendering an equitable
o If the tel URI -- with the parameter under consideration -- will
be converted to a URI used by other signaling protocols such as
the Session Initiation Protocol (SIP ) or H.323 , then the
expert must consider whether this parameter merely encapsulates
signaling information that is not meaningful to the processing of
requests in the domain of the converted URI. For example, certain
Integrated Services Digital Network (ISDN) User Part (ISUP, )
parameters have no equivalent corollary in SIP; thus, their
presence or absence in a SIP URI will not hinder the normal rules
for processing that URI. Other parameters may affect the normal
processing rules associated with the URI; in such cases, the
expert must carefully consider the ramifications, if any, of the
presence of such parameters.
o Certain parameters of a tel URI can be optional. These parameters
act as metadata about the identifier in the tel URI. Optional
parameters should provide additional information to a service for
which they apply instead of acting as enablers of that service in
the first place. The service must continue to be invoked and
operate normally even in the absence of these parameters.
5. Security Considerations
The registry in this document does not in itself have security
considerations. However, as mentioned in , an important reason
for the IETF to manage the extensions of SIP is to ensure that all
extensions and parameters are able to provide secure usage. The
supporting RFC publications for parameter registrations described in
this specification MUST provide detailed security considerations for
The structure of this document comes from , which is the
equivalent work done in the SIP domain to establish a registry. Ted
Hardie, Alfred Hoenes, Jon Peterson, and Jonathan Rosenberg provided
substantive comments that have improved this document.
Brian Carpenter, Lars Eggert, Pasi Eronen, Chris Newman, and Glen
Zorn provided feedback during IESG review and Gen-ART review.
7.1. Normative References
 Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
7.2. Informative References
 Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B.
Rosen, "Change Process for the Session Initiation Protocol
(SIP)", BCP 67, RFC 3427, December 2002.
 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.
 Camarillo, G., "The Internet Assigned Number Authority (IANA)
Uniform Resource Identifier (URI) Parameter Registry for the
Session Initiation Protocol (SIP)", BCP 99, RFC 3969,
 ITU-T H.323, "H.323: Packet-based multimedia communications
systems", June 2006.
 ITU-T Q.764, "Signaling System No. 7: ISDN User Part Signaling
Procedures", December 1999.
170 West Tasman Drive
San Jose, CA 95134
Phone: +1 408 902-3341
Vijay K. Gurbani
Bell Laboratories, Alcatel-Lucent
2701 Lucent Lane
Lisle, IL 60532
Phone: +1 630 224-0216
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
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.
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 in BCP 78 and BCP 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 at
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