tel URI Parameters: The Internet Assigned Number Authority (IANA) tel Uniform Resource Identifier (URI) Parameter Registry
A standard registry for the settings used in telephone-number links.
Jennings, C. and V. Gurbani, "The Internet Assigned Number Authority (IANA) tel Uniform Resource Identifier (URI) Parameter Registry", RFC 5341, DOI 10.17487/RFC5341, September 2008, <https://www.rfc-editor.org/info/rfc5341>.
@misc{rfc5341,
series = {Request for Comments},
number = 5341,
howpublished = {RFC 5341},
publisher = {RFC Editor},
doi = {10.17487/RFC5341},
url = {https://www.rfc-editor.org/info/rfc5341},
author = {C. Jennings and V. Gurbani},
title = {{The Internet Assigned Number Authority (IANA) tel Uniform Resource Identifier (URI) Parameter Registry}},
pagetotal = 7,
year = 2008,
month = sep,
} RFC text
Contents
Network Working Group C. Jennings
Request for Comments: 5341 Cisco Systems
Updates: 3966 V. Gurbani
Category: Standards Track Bell Laboratories, Alcatel-Lucent
September 2008
The Internet Assigned Number Authority (IANA)
tel Uniform Resource Identifier (URI) Parameter Registry
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.
Abstract
This document creates an Internet Assigned Number Authority (IANA)
registry for tel Uniform Resource Identifier (URI) parameters and
their values. It populates the registry with the parameters defined
in the tel URI specification, along with the parameters in tel URI
extensions defined for number portability and trunk groups.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Use of the Registry . . . . . . . . . . . . . . . . . . . . . . 2
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 3
4.1. tel URI Parameters Registry . . . . . . . . . . . . . . . . 3
4.2. Registration Policy for tel URI Parameters . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 5
1. Introduction
The tel URI (RFC 3966 [1]), 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.
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 [2].
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
features.
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
Parameters".
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 registry
4.2. Registration Policy for tel URI Parameters
As per the terminology in [3] 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
decision):
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 [5]) or H.323 [7], 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, [8])
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 [4], 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
them.
6. Acknowledgments
The structure of this document comes from [6], 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. References
7.1. Normative References
[1] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
7.2. Informative References
[4] 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.
[5] 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.
[6] 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.
[7] ITU-T H.323, "H.323: Packet-based multimedia communications
systems", June 2006.
[8] ITU-T Q.764, "Signaling System No. 7: ISDN User Part Signaling
Procedures", December 1999.
Authors' Addresses
Cullen Jennings
Cisco Systems
170 West Tasman Drive
Mailstop SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 902-3341
EMail: fluffy@cisco.com
Vijay K. Gurbani
Bell Laboratories, Alcatel-Lucent
2701 Lucent Lane
Room 9F-546
Lisle, IL 60532
USA
Phone: +1 630 224-0216
EMail: vkg@alcatel-lucent.com
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On this page
§1Introduction
The tel URI, defined in RFC 3966, provides a way to represent telephone numbers as web-style links. Like many other URIs, the tel scheme supports extensibility: you can add new parameters and new values to customize how a telephone number is used.
However, RFC 3966 did not specify where these parameters should be registered or managed. Without a central registry, different groups might invent the same parameter name for different purposes, or use conflicting syntax. This RFC creates an IANA registry to solve that problem and fills it with the parameters already defined in tel URI extensions.
A tel URI is like a phone number link on the web. But if lots of people want to add their own special settings to it, they need an official place to register those settings so they don't step on each other.
§2The tel URI Concept
A tel URI is a Uniform Resource Identifier that identifies a resource by a telephone number. It lets applications create clickable telephone numbers that can trigger calling, messaging, or other phone-based actions. The format is simple: tel:+1-201-555-0123 is a valid tel URI.
The tel scheme supports parameters, which are optional additions that provide extra information or modify the meaning of the number. For example, an extension might specify a phone context or a routing preference. These parameters use the query-string style syntax common in URIs.
A tel URI is a computer-friendly way to write a phone number as a link. You can add extra information to that link, like what company it's for or where it should be routed.
§3The Parameter Registry
The tel URI Parameter Registry is a table where each registered parameter has a name, a description of what values it accepts, and a reference to the RFC or specification that defines it.
Some parameters take predefined values (marked as "Constrained"), where the specification lists the valid options. Some parameters take no value at all and work as flags. Others accept any value. The registry indicates which type each parameter is, so implementors know what to expect.
The registry is a list that says what extra settings can be added to a tel URI and what kinds of values they accept.
§4Registration Requirements
To register a new parameter or parameter value with IANA, the specification must be documented in an RFC or other permanent and publicly available specification. The documentation must fully explain the syntax, intended usage, and semantics of the parameter.
This requirement ensures that independent implementations can interoperate correctly and prevents accidental namespace collisions. When registering a new value for an existing parameter, you must reference the specification that provides that value. The registration policy is "Specification Required, Designated Expert": a technical expert reviews all requests to ensure they meet the standards for interoperability and do not conflict with existing registrations.
Before you can add a new setting to the registry, you have to write down exactly what it does in a document that everyone can see. A technical expert then checks it to make sure it won't cause problems.
§5Initial Parameters
The RFC populates the registry with twelve initial parameters, sourced from the tel URI specification and from extensions for number portability and trunk groups.
Parameters from the base tel URI specification include isub (ISDN subaddress), isub-encoding (how the subaddress is encoded), ext (extension), and phone-context (context for the number). Extensions for number portability add npdi (Number Portability Dip Indicator, a flag), rn (routing number), rn-context, cic (Carrier Identification Code), and cic-context. Extensions for trunk groups add tgrp (trunk group) and trunk-context. Each entry in the registry references the RFC that defines it.
The registry starts with twelve settings that are already used by phone number links: things like extensions, context information, and special codes for handling number portability.
- isubISDN Subaddress — Additional routing information for ISDN calls.
- extExtension — Extension number at the destination.
- phone-contextPhone Context — Context to disambiguate the phone number.
- npdiNumber Portability Indicator — Flag indicating number portability lookups were done.
- rnRouting Number — Routing number for number-portability.
- tgrpTrunk Group — Identifies a trunk group for the call.
§6Security Considerations
The registry itself does not have direct security implications. However, any new parameter registered must be reviewed for security considerations by the RFC that proposes it. Each specification defining a new parameter MUST provide detailed security analysis for how that parameter could be misused or how its presence or absence could affect the security of the system processing the tel URI.
This ensures that the IETF reviews security impacts before parameters are standardized, preventing insecure parameters from being widely adopted.
The registry doesn't have its own security problems, but every new setting that gets registered has to explain if it could cause any security issues.
§7Status Today
RFC 3966 remains the current specification for tel URIs, and RFC 5341 updates it to add the formal registry. The registry is maintained by IANA and continues to accept new parameter registrations that meet the "Specification Required, Designated Expert" policy.
RFC 5341 created the official list for tel URI settings, and that list is still being used today whenever someone wants to add a new setting to a phone number link.