SIPPING H. Schulzrinne
Internet-Draft Columbia U.
Expires: August 19, 2006 February 15, 2006
A Uniform Resource Name (URN) for Services
draft-ietf-ecrit-service-urn-00
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Copyright (C) The Internet Society (2006).
Abstract
The content of many communication services depend on the context,
such as the user's location. We describe a 'service' URN that allows
to register such context-dependent services that can be resolved in a
distributed manner.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Registration Template . . . . . . . . . . . . . . . . . . . . 4
3. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1 Normative References . . . . . . . . . . . . . . . . . . . 6
5.2 Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8
A. Alternative Approaches Considered . . . . . . . . . . . . . . 8
B. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
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1. Introduction
In existing telecommunications systems, there are many well-known
communication and information services that are offered by loosely
coordinated entities across a large geographic region, with well-
known identifiers. Some of the services are operated by governments
or regulated monopolies, others by competing commercial enterprises.
Examples include emergency services (reached by 911 in North America,
112 in Europe), community services and volunteer opportunities (211
in some regions of the United States),telephone directory and repair
services (411 and 611 in the United States and Canada), government
information services (311 in some cities in the United States),
lawyer referral services (1-800-LAWYER), car roadside assistance
(automobile clubs) and pizza delivery services. Unfortunately,
almost all of them are limited in scope to a single country or
possibly a group of countries, such as those belonging to the North
American Numbering Plan or the European Union. The same identifiers
are often used for other purposes outside that region, making
accessing such services difficult when users travel or use devices
produced outside their home country.
These services are characterized by long-term stability of user-
visible identifiers, decentralized administration of the underlying
service and a well-defined resolution mechanism. (For example, there
is no national coordination or call center for 911; rather, various
local government organizations cooperate to provide this service,
based on jurisdictions.)
In this document, we propose a URN namespace that, together with
resolution protocols beyond the scope of this document, allows to
define such global, well-known services, while distributing the
actual implementation across a large number of service-providing
entities. While there are many ways to divide provision of such
services, we focus on geography as a common way to delineate service
regions. In addition, users can choose different directory providers
that in turn manage how geographic locations are mapped to service
providers.
Availability of such service identifiers simplifies end system
configuration. For example, an IP phone could have a special set of
short cuts or buttons that invoke emergency services, as it would not
be practical to manually re-configure the device with local emergency
contacts for each city or town a user visits with his or her mobile
device. Also, such identifiers allow to delegate routing decisions
to third parties and mark certain requests as having special
characteristics while preventing these characteristics to be
accidentally invoked on inappropriate requests.
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This URN allows to identify services independent of a particular
protocol to deliver the services. It may appear in protocols that
allow general URIs, such as SIP [5] request URIs, web pages or
mapping protocols.
Existing technologies address the mapping of service identifiers to a
service for a particular DNS domain (DNS SRV [9], DNS NAPTR [11]) or
a local area network (SLP [8]).
The tel URI [16] allows to express service codes such as 911 by
adding a context parameter, but does not address the problem of
global validity.
LUMP [20] is a prototype resolution system for mapping URNs to URLs
based on geographic location. However, it is anticipated that there
will be several such systems.
2. Registration Template
Below, we include the registration template for the URN scheme
according to RFC 3406 [15].
Namespace ID: service
Registration Information: Registration version: 1; registration date:
2005-07-10
Declared registrant of the namespace: TBD
Declaration of syntactic structure: The URN consists of a
hierarchical service identifier, with a sequence of labels
separated by periods. The left-most label is the most significant
one and is called 'top-level service', while names to the right
are called 'sub-services'. The set of allowable characters is the
same as that for IRIs, that used for domain names [1] except that
there is no restriction on the first character being a letter;
labels are case-insensitive and SHOULD be specified in all lower-
case. Any string of service labels can be used to request
services that are either more generic or more specific. In other
words, if a service 'x.y.z' exists, the URNs 'x' and 'x.y' are
also valid service URNs.
"URN:service:" top-level-service *("." service-identifier)
top-level-service = ALPHA / DIGIT / "-" /
service-identifier = ALPHA / DIGIT / "-" /
Relevant ancillary documentation: None
Community considerations: The service URN is believe to be relevant
to a large cross-section of Internet users, including both
technical and non-technical users, on a variety of devices, but
particularly for mobile and nomadic users. The service URN will
allow Internet users needing services to identify the service by
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kind, without having to determine manually who provides the
particular service in the user's current context, e.g., at his
current location. For example, a traveler will be able to use his
mobile device to request emergency services without having to know
the local emergency number. The assignment of identifiers is
described in the IANA Considerations (Section 4). The service URN
does not prescribe a particular resolution mechanism, but it is
assumed that a number of different entities could operate and
offer such mechanisms. The ECRIT working group is currently
discussing several approaches, including solutions based on DNS,
IRIS and a web-services protocol. Software prototypes for some of
these are currently already available and are believed to be
readily developed.
Namespace considerations: There do not appear to be other URN
namespaces that serve the same need of uniquely identifying
widely-available communication and information services. Unlike
most other currently registered URN namespaces, the service URN
does not identify documents and protocol objects (e.g., [13],
[14], [18], [19]), types of telecommunications equipment [17],
people or organizations [12]. tel URIs [16] identify telephone
numbers, but numbers commonly identifying services, such as 911 or
112, are specific to a particular region or country.
Identifier uniqueness considerations: A service URN identifies a
logical service, specified in the service registration (see IANA
considerations). Resolution of the URN, if successful, will
return a particular instance of the service, and this instance may
be different even for two users making the same request in the
same place at the same time; the logical service identified by the
URN, however, is persistent and unique.
Identifier persistence considerations: The 'service' URN for the same
service is expected to be persisent, although there naturally
cannot be a guarantee that a particular service will continue to
be available globally or at all times.
Process of identifier assignment: Details of the service assignment
depend on the service and national regulations. In general, it is
assumed that providers of services can register through a service
mapping mechanism for a particular service in a particular
geographic area. The provision of some services may be restricted
by local or national regulations. (As a hypothetical example,
providing emergency services may be restricted to government-
authorized entities, which may limit the region where each entity
can advertise its services.) The rules for each service are
described in a service-specific document.
Process for identifier resolution: 'service' identifiers are resolved
by the TBD mapping protocol, an instance of a Resolution Discovery
System (RDS) as described in RFC 2276 [3]. (In theory, there
could be several such mapping protocols in concurrent use, as long
as there are reasonable guarantees that all services are available
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in all mapping protocols.)
Rules for Lexical Equivalence: 'service' identifiers are compared
according to domain name comparison rules. The use of homographic
identifiers is NOT RECOMMENDED.
Conformance with URN Syntax: There are no special considerations.
Validation mechanism: The RDS mechanism is also used to validate the
existence of a resource. As noted, by its design, the
availability of a resource may depend on where service is desired
and there may not be service available in all or most locations.
(For example, roadside assistance service is unlikely to be
available on about 70% of the earth's surface.)
Scope: The scope for this URN is public and global.
3. Example
For discussion and illustration purposes only, we include an example
of a particular service. We choose emergency services as an example,
with the top-level service identifier 'sos'. A possible list of
identifiers might include:
urn:service:sos
urn:service:sos.fire
urn:service:sos.police
urn:service:sos.marine
urn:service:sos.mountain
urn:service:sos.rescue
urn:service:sos.poison
urn:service:sos.suicide
urn:service:sos.mental-health
urn:service:sos.animal-control
4. IANA Considerations
New service-identifying tokens and sub-registrations are to be
managed by IANA, according to the processes outlined in [4]. The
policy for top-level service names is TBD, but could be
'specification required', 'IETF Consensus' or 'Standards Action'.
The policy for assigning names to sub-services may differ for each
top-level service designation and MUST be defined by the document
describing the top-level service.
5. References
5.1 Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
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[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Sollins, K., "Architectural Principles of Uniform Resource Name
Resolution", RFC 2276, January 1998.
[4] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[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] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
5.2 Informative References
[7] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997.
[8] Guttman, E., Perkins, C., Veizades, J., and M. Day, "Service
Location Protocol, Version 2", RFC 2608, June 1999.
[9] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[10] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[11] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, September 2000.
[12] Mealling, M., "The Network Solutions Personal Internet Name
(PIN): A URN Namespace for People and Organizations", RFC 3043,
January 2001.
[13] Rozenfeld, S., "Using The ISSN (International Serial Standard
Number) as URN (Uniform Resource Names) within an ISSN-URN
Namespace", RFC 3044, January 2001.
[14] Hakala, J. and H. Walravens, "Using International Standard Book
Numbers as Uniform Resource Names", RFC 3187, October 2001.
[15] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"Uniform Resource Names (URN) Namespace Definition Mechanisms",
BCP 66, RFC 3406, October 2002.
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[16] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004.
[17] Tesink, K. and R. Fox, "A Uniform Resource Name (URN) Namespace
for the Common Language Equipment Identifier (CLEI) Code",
RFC 4152, August 2005.
[18] Kang, S., "Using Universal Content Identifier (UCI) as Uniform
Resource Names (URN)", RFC 4179, October 2005.
[19] Kameyama, W., "A Uniform Resource Name (URN) Namespace for the
TV-Anytime Forum", RFC 4195, October 2005.
[20] Schulzrinne, H., "Location-to-URL Mapping Protocol (LUMP)",
draft-schulzrinne-ecrit-lump-01 (work in progress),
October 2005.
Author's Address
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7004
Email: hgs+ecrit@cs.columbia.edu
URI: http://www.cs.columbia.edu
Appendix A. Alternative Approaches Considered
The "sos" SIP URI reserved user name proposed here follows the
convention of RFC 2142 [7] and the "postmaster" convention documented
in RFC 2822 [10]. The approach has the advantage that only the home
proxy for a user needs to understand the convention and that the
mechanism is likely backwards-compatible with most SIP user agents,
with the only requirement that they have to be able to generate
alphanumeric URLs. One drawback is that it may conflict with locally
assigned addresses of the form "sos@domain". Also, if proxies not
affiliated with the domain translate the URL, they violate the
current SIP protocol conventions.
There are a number of possible alternatives, each with their own set
of advantages and problems:
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tel:NNN;context=+C This approach uses tel URIs [16]. Here, NNN is
the national emergency number, where the country is identified by
the context C. This approach is easy for user agents to implement,
but hard for proxies and other SIP elements to recognize, as it
would have to know about all number-context combinations in the
world and track occasional changes. In addition, many of these
numbers are being used for other services. For example, the
emergency number in Paraguay (00) is also used to call the
international operator in the United States. A number of
countries, such as Italy, use 118 as an emergency number, but it
also connects to directory assistance in Finland.
tel:sos This solution avoids name conflicts, but is not a valid "tel"
[16] URI. It also only works if every outbound proxy knows how to
route requests to a proxy that can reach emergency services since
tel URIs. The SIP URI proposed here only requires a user's home
domain to be appropriately configured.
sip:sos@domain Earlier work had defined a special user identifier,
sos, within the caller's home domain in a SIP URI, for example,
sip:sos@example.com. This approach had the advantage that dial
plans in existing user agents could probably be converted to
generate such a URI and that only the home proxy for the domain
has to understand the user naming convention. However, it
overloads the user part of the URI with specific semantics rather
than being opaque, makes routing by the outbound proxy a special
case that does not conform to normal SIP request-URI handling
rules and is SIP-specific. The mechanism also does not extend
readily to other services.
SIP URI user parameter: One could create a special URI, such as "aor-
domain;user=sos". This avoids the name conflict problem, but
requires mechanism-aware user agents that are capable of emitting
this special URI. Also, the 'user' parameter is meant to describe
the format of the user part of the SIP URI, which this usage does
not do. Adding other parameters still leaves unclear what, if
any, conventions should be used for the user and domain part of
the URL. Neither solution is likely to be backward-compatible
with existing clients.
Special domain: A special domain, such as "sip:fire@sos.int" could be
used to identify emergency calls. This has similar properties as
the "tel:sos" URI, except that it is indeed a valid URI. To make
this usable, the special domain would have to be operational and
point to an appropriate emergency services proxy. Having a
single, if logical, emergency services proxy for the whole world
seems to have undesirable scaling and administrative properties.
Appendix B. Acknowledgments
This document is based on discussions with Jonathan Rosenberg and
benefitted from the comments of Benja Fallenstein and Leslie Daigle.
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