OGC 10-124r1

Open Geospatial Consortium Inc.
Date: 2010-07-15
Reference number o f this document: 10-124r1
OGC Name of this document: http://www.opengis.net/doc/WhitePaper/Identifiers/1.0
Category : OpenGIS®

White Paper

Ed itor: Simon Cox

OGC Identifiers –the case for http URIs

Copyright © 2010 Open Geospatial Consortium

Warning
This document is an OGC Whitepaper. It is subject to change without notice.
This document is not an official position of the OGC membership on thisparticular topic.

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OpenGIS® Whitepapern/aApproved for Public Release

EnglishCopyright © 2010 Open Geospatial Consortium, Inc.

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Contents

Page

1

Introduction and context ........................................................................................ 1

2

URIs, URNs and URLs .......................................................................................... 2

3

Resolution of OGC URIs ....................................................................................... 3
3.1

Domain management and long term persistence ........................................... 3

3.2

OGC identifiers for resources served by other providers .............................. 3

3.3

Representation of resources ........................................................................... 3

3.4

Information vs. non-information resources.................................................... 4

4

Governance of OGC URIs ..................................................................................... 55

URI exploitation use-cases..................................................................................... 6

6

Resourcing implications......................................................................................... 7

7

Summary and recommendations ............................................................................ 88

References .............................................................................................................. 9
8.1

URI and http standards................................................................................... 9

8.2

IANA registries: ............................................................................................. 9

8.3

OGC policies.................................................................................................. 9

8.4

Best practices in identifier design and management ...................................... 9

Revision History .......................................................................................................... 10

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i.

PrefaceThis document discusses implications of recent changes to the OGC Identifier policy,
specifically as related to the use of http URIs for persistent OGC resources.

ii.

Document terms and definitions

No special terms.

iii.

Contributors

All questions regarding this whitepaper should be directed to the editor orcontributors:
CONTACT

iv.

COMPANY

Simon Cox

European Commission – Joint ResearchCentreSven SchadeEuropean Commission – Joint ResearchCentreLuis Bermudez

Southeastern Universities Research
Association

Ingo Simonis

IGSIIntellectual Property statement

Attention is drawn to the possibility that some of the elements of this document may be
the subject of patent rights. The Open Geospatial Consortium Inc. shall not be held
responsible for identifying any or all such patent rights.
Recipients of this document are requested to submit, with their comments, notificationof any relevant patent claims or other intellectual property rights of which they may
be aware that might be infringed by any implementation of the standard set forth in
this document, and to provide supporting documentation.

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OGC 10-124r1

OGC Identifiers – the case for http URIs
1

Introduction and context

The OGC provides a large number of resources to support the construction of spatial
data infrastructures, including documents, specifications, schemas and concept
definitions. When deployed, the infrastructures require persistent reference to theseresources, enabled by persistent identifiers. This may be at various level of
granularity.
Uniform Resource Names (URNs) have been used by OGC for the identification of
resources governed by OGC, and also for some resources governed externally forwhich Uniform Resource Identifiers (URIs) were not available (especially EPSG
definitions). This was formalized by RFC 5165 which establishes the OGC NID'urn:ogc:' [ URN-OGC ], along with several OGC Best Practice and Policy documentsthat define the 'ResourceSpecificString' that composes the remainder of a name [seehttp://www.opengeospatial.org/ogc/policies/directives] .
Revisions to the OGC naming policy allow for OGC names structured as http URIs,
as an alternative to URNs. The internal structure of the two forms is identical, withthe full name differing only in the prefix and field separators, so the information
content and governance is unchanged. According to the current policy either URNs or
http URIs may be used in specifications [OGC-NA].
However, the use of http URIs (a) resolves some deployment challenges and (b)provides an opportunity for easier engagement with broader communities. So OGC
should now consider taking the next step, and mandate the use of http URIs for
persistent identifiers in OGC standards.
This white paper discusses a number of issues related to this proposal.

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URIs, URNs and URLs

The Uniform Resource Identifier [ URI] uses short strings to refer to resources in theweb. Example of these resources can be documents, electronic mailboxes, images,
downloadable files, concepts (e.g. bay, buoy, specification) and real objects (e.g.
Monterey Bay, Mooring 1, OGC document). Various URI schemes have beendevised, including Uniform Resource Locators (URLs) and Uniform Resource Names(URNs) [ URN].
URLs use domain names to locate resources in the web. There is a well-defined
governance structure. The Domain Name System (DNS) runs under the auspices ofInternet Corporation for Assigned Names and Numbers (ICANN) to provide a ser vicefor mapping from names to machine addresses.
URNs are also URIs but do not necessarily imply web availability of the resource
being identified. The Internet Assigned Numbers Authorities (IANA) maintains aregistry of URN schemes, with each scheme described in an RFC document. The
latter may provide details of a resolver service.
At the time that OGC started using URNs, it appeared to be a good option for
persistent identification, avoiding some undesirable effects and expectations around
URLs. In particular, URN NID registrations (governed by IETF through IANA) areforever, while http domain registration is periodic, and http server maintenance is askilled job. There was also a principle at stake: identification and location are
different functions. However, deployment and maintenance of a resolver service for
URNs has been a challenge for OGC, particularly as there is no standard for a URN
resolution service, and no best practice or precedent to fall back on.
Meanwhile, however, the use and interpretation of URIs on the web has evolved
significantly. The contemporary version of the URL story is [ W3C/IE TF, URI-URN ]-a 'http URI' (often, though incorrectly, abbreviated to just URI) may ser ve as apersistent identifiero used in this sense, the URI is interpreted trivially as a text-string: thepresence of '/' and ':' characters is unimportant in that context-

a http URI has the highly desirable characteristic that it is also a URL and thus
provides the opportunity for automatic resolution, following standard internet
conventions
o use of the DNS system and http protocol mean that no special resolver
service is required

The latter overcomes a legitimate criticism of all non-http URI schemes.
For OGC this creates a significant opportunity. Generic tools (browsers, reasoners)
have built- in support for binding to http URIs, without any customization, so theadoption of http URIs would allow documents referring to resources with OGC
identifiers to be used more effectively in the generic web context. There is norequirement for the consumer to use a specific resolver service, so no requirement thateveryone who encounters an OGC document to know where the OGC resolver is,what the invocation method is, or to cache definitions corresponding to OGC names
in advance of encountering them.

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Resolution of OGC URIs

Http URIs provide the opportunity for resolution using standard tools, but also create
an expectation of immediate resolvability. To meet user expectations, a service should
be deployed at the http address that handles requests for resources and responds with
useful representations. In order to preserve coherence both conceptually andpractically this requires attention to a number of issues.
3.1

Domain management and long term persistence

For an identifier to be persistent it requires the governing body to arrange for theidentifier to be available for the long term. This will give confidence to application
developers to use those strings as identifiers, and will provide the necessary
persistence to knowledge bases (e.g. semantic web graphs) that use those concepts.
Use of http URIs as persistent identifiers requires a commitment on the part of theorganization issuing the identifier to maintain the http domain registration, and astrategy for managing the domain and the web servers. Organizational changes may
occur on a shorter timescale than the than that over which the identifiers are intended
to be useful, which can create instability in domain names change (e.g. opengis.org
changed to opengeospatial.org). Persistent identifiers may use domain names that are
not tied directly to the organization name to protect against this (e.g. geosciml.org vs.
auscope.org). This must be planned for when the scheme is established [URIlife].
3.2

OGC identifiers for resources se rved by other providers

With the consent of some external authorities, the OGC 'def' name scheme provides
identifiers for certain resources where that authority is not affiliated directly with
OGC, in particular EPSG (for coordinate reference systems) and UCUM (for units-ofmeasure) [OGC-def]. Requests to OGC for representations of these resources may be
handled by redirection to a service provided by the governing authority. For example,
the OGC namehttp://www.opengis.net/def/crs/EPSG/ 0/4326

redirects tohttp://www.epsgregistry.org/indicio/query?request=Get RepositoryItem&id=urn :ogc:def:crs:EPSG::4326

The http response code should be 302 (Found) or 307 (Temporary Redirect) [ HTTP].
The redirect is 'temporary' since while the OGC Name is persistent, the URL that t herequest is redirected to may change over time.
This illustrates an important principle: the OGC identifier should be persistent, anduseful in the long term. The redirect leads to only the current service that provides
representations of the resource. This requires that OGC maintains the current state of
mappings from the http URIs to the URLs.
3.3

Representation of resources

Under REST principles there may be alternative representations of the same resource[Webarch; LD]. The http protocol supports content negotiation between the client andserver to allow delivery of a representation that the client is capable of processing (inparticular using the Accept and Accept-Language headers) [HTTP].

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Within the 'def' branch of the OGC scheme, OGC provides alternative representations
for the key resources for which there may be alternative client applications. For
example, a http GET request for
http://www.opengis.net/def/cs/OGC/1.0/ GridCS2d

results in different responses depending on whether the http Accept value is text/html,
application/gml+xml or application/rdf+xml. The variant representations are also
resources in their own right, addressed directly as
http://www.opengis.net/def/cs/OGC/1.0/ GridCS2d.ht ml
http://www.opengis.net/def/cs/OGC/1.0/ GridCS2d.g ml
http://www.opengis.net/def/cs/OGC/1.0/ GridCS2d.rdf

If the client expresses no preference, the web-page (HTML) representation isdelivered, since this representation is supported by the most common web-client – thebrowser. Client applications that have more specific requirements (e.g. GML, RDF)
are expected to also know how to negotiate for these.
3.4

Information vs. non-information resources

In the 'semantic web' a distinction is made between 'things on the web' and 'things in
the world', also referred to as information resources and non- information resources.
Representations of information resources can be transmitted electronically, as web
pages, GML encoded feature representations, RDF documents, PDF documents, etc.
Non-information resources such as human beings, real world objects, or abstract
concepts can be named by an URI, but a representation on the wire can o nly be of a
description of the resource. Browsers can display representations of information
resources directly, or descriptions of non- information resources.
Http response codes should be used to indicate if a representation of an information or
non- information resource was returned when dereferencing a URI. If a client
dereferences an information resource, http code 200 ('OK') gets returned. In case of anon- information resources, http code 303 ('See other') gets returned [ RDFpub;
SemURIs ]A key topic area for which OGC provides standard identifiers is 'definitions' of key
concepts used in geographic information services. While there is some contention in
the semantics community about whether concepts are information resources or noninformation resources, OGC identifies definitions rather than concepts. Each of these
an information resource, corresponding to 'the definition of the concept according to
OGC', so there is no ambiguity in this context 1 .
Nevertheless, when deploying feature services, the GI community may want toconsider whether they need to make this distinction 2 .

1A corollary of this is that OGC does not provide URIs for the (abstract) concepts themselves.

2

OGC AS Topic 5 Features (also known as ISO 19101) is ambiguous on the issue, defining a feature
as "A feature is an abstraction of a real world phenomenon". Of course a particular feature-service
provides a representation of a real-world feature as it is modeled for a particular application-domain.
The same real-world feature may be modeled differently for different applications, and none of thesecan claim to be comp rehensive. It could be argued that the representation delivered by a specificservice is strictly an (incomp lete) feature description, and thus is by definition an information resource.

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Governance of OGC URIs

Every web site or service has a URL policy, typically embodied in their 'sitemap',though it is rarely documented explicitly. However, when the purpose of a 'http URI'
is to serve as a persistent identifier, such a policy is important. There have been many
discussions of URI design [e.g. CoolURIs ; CHIPS; SemURIs ; IV OAid; WebName; URIURN; UK URI ].
Three audiences for the URI policy or scheme must be considered:
1. the party that wishes to mint an identifier as part of an orderly publication process;-the URI policy will usually embody aspects of a publication process which
is a core part of the resource governance process

2. the party that wishes to use identifiers minted by the first group, within
expressions of their data-

for this group the stability of the URI scheme is most important. If thescheme uses a predictable pattern for new identifiers, then this is bonus.

3. the party that consumes identifiers, usually embedded in a dataset or otherdocument as links;-

it is common for URIs to be parsed by the user, who will construct variants
with a reasonable prospect of finding something useful.

The OGC-NA [http://www.opengeospatial.org/ogcna] oversees the assignment of
OGC Names to resources, primarily OGC resources including documents,
specifications, and definitions. OGC Names are composed of a sequence of fields,with specific governance arrangements for the values of each field. This means that acomplete OGC Name can only be assigned when certain pre-conditions are met.
Functionally this institutes and enforces a delegation policy. Since the nameproduction rules have been designed for use in either http URIs or URNs, OGC is in astrong position to transition to http URIs without compromising governance.
Note that persistent identifiers in the OGC URI scheme are based on 's lashnamespaces'. The register of OGC identifiers does not include 'hash namespaces',
which lead to URIs with a fragment after a # [ RDFpub]. The latter allow you toaddress a secondary resource in the context of the resource identified by the URI
before the #, but since the fragment is not normally communicated by an http client as
part of a request [ URI clause 3.5; Webarch clause 3.2.1] the http server is not required
to distinguish a secondary resource using the hash URI syntax 3 .
The other key aspect of URI governance is the stability of the http server. This was
discussed above in section 3.1.

3Since a resource may have more than one identifier, there may always be an exp licit non-hash URI to
provide primary identification for a resource which is also a secondary resource in a particu lar context.

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URI exploitation use-cases

There are two main use-cases using URI references. Both are supported by use of httpURIs either as well as or better than URNs.
1. "Is the value P in a document equal to a value R?" – two modes are anticipated:

The simplest operation is string comparison on P and R; this is a lexical
operation, so it can be done just as easily on a http URI as on a URN. This is
expected to account for the largest usage of references in a mature SDI.



However, a negative result for string comparison does not necessarily meanthat the underlying resources do not match. A single resource may have morethan one identifier, or there may be different representations that are logicallyequivalent. In order to determine equivalence it is necessary to look deeper.
The presence of the Semantic Web statements owl:sameAs or
skos:exactMatch associated with the RDF representation of one of theresources may be enough. More complex logic or reasoning might besupported by inspection of representations of resource P and resource R. Anyof these scenarios require that either or both URIs be resolved, which is easierif they are http URIs than URNs

2. "Tell me more about P" – there are two common applications
Within a dataset expressed in GML, xlink:href attributes 4 carry links toresources that may be logically embedded in the host document. In order to
execute the implied transclusion and also maintain XML document validity asuitable representation of the resource must be obtained. Http contentnegotiation provides a standard mechanism to support this.

More sophisticated processing of linked resources will typically be handled bya helper application. Dispatching the appropriate helper requires that theformat of the resource representation be known. The http protocol supports
this through the MIME-type reported in the response header.

4

The semantics of GM L's use of xlink:href is essentially the same as that of RDF's rdf:resource – amechanis m for using references in place of nesting. This is no coincidence: RDF provided the modelfor the orig inal GM L meta-model developed around 2000. It was only later re -interpreted in terms of
the UML conceptual schema language favoured by the g eographic informat ion community organizedthrough ISO/TC 211.

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Resourcing implications

Http domains are easy to register, but temporary. Stability of a http domain and thehosted service is the responsibility of its owner. Superficially this appears to impose agreater and more specific obligation on the owner of a http domain, which makes httpURIs less suitable for persistent identification compared with a URN NID which isperpetual.
Looking a bit deeper, however, while the 'resolver' for a URN is formally the
document (RFC) that specifies the URN namespace, current practice for registration
includes a requirement to identify a resolution mechanism, preferably a resolver
service at an http address. For example, the OGC URN NID [ URN-OGC] specifies thatan operational resolver shall be provided at http://urn.opengis.net/. So in practice evenwith URN identifiers there is a requirement on OGC to maintain a resolver at a
specific web location. Hence the resource implications for persistent http server
registration and management are not significantly different between the URN or httpURI route.

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Summary and recommendations

Given that
 In order to support SDI deployment, resources provided by OGC for ongoing use
should have well- governed, maintained, persistent identifiers A http URI or a URN is a text string, and may be used for persistent identification
 A http URI, unlike a URN, is implicitly bound to a resolver service known to all
web-clients. Documents that include references expressed as http URIs can be
consumed by generic web applications.
 Use of http URIs to identify OGC resources is likely to increase the impact of
OGC in the mass market
1. OGC s hould imme diately mandate the use of http URI for identification of
persistent OGC resources, in preference to URNs. (Approved June 17, 2010)
Note that this recommendation applies to identifiers for resources that are governedby OGC, which are intended to be persistent. There is no obligation on any otherorganization, including those using spatial data infrastructures based on OGC
technology. The use http URIs for transient resources may be inappropriate.
To support this:
2. OGC s hould carefully manage (maintain for the long term) the
http://www.opengis.net domain and identifiers in this domain
3. OGC s hould ensure that suitable representations of resources with identifie rs
in the domain http://www.opengis.net are provided through the http protocol.
This includes redirection and content negotiation, whe re appropriate.

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References

8.1

URI and http standards

HTTP

Hypertext Transfer P rotocol -- http/1.1 http://tools.ietf.org/html/rfc2616

URI

Uniform Res ource Identifier (URI): Generic Syntax
http://tools.ietf.org/html/rfc3986

URN

URN Syntax http://tools.ietf.org/html/rfc2141

W3C/IE TF

Report from the Joint W3C/IETF URI Planning Interest Group: Uniform
Resource Identifiers (URIs), URLs, and Uniform Res ourc e Names (URNs):
Clarifications and Recommendations http://tools.ietf.org/html/rfc3305

8.2

IANA registries:

MIME Registry MI ME Media Types http://www.iana.org/assignments/media-types/
URI Registry

Uniform Res ource Identifer (URI) Schemes http://www.iana.org/assignments/urischemes

URN Registry

Uniform Res ource Names (URN) Namespaces
http://www.iana.org/assignments/urn-namespaces

8.3

OGC policies

URN-OGC

A Uniform Resource Name (URN) Namespace for the Open Geospatial
Cons ortium (OGC) http://tools.ietf.org/html/rfc5165

OGC-NA

OGC Naming Authority (OGC-NA) Policies & Procedures
http://portal.opengeospatial.org/files/?artifact_id=37800

OGC-def

OGC-NA Name type specification - definitions: Part 1 - basic namehttp://portal.opengeospatial.org/files/?artifact_id=37801

8.4Best practices in identifier design and management

CoolURIs

Cool URIs don't change http://www.w3.org/Provider/Style/URI

CHIPS

Common http Implementation Problems http://www.w3.org/TR/chips

IVOAidInternational virtual observatory alliance identifiers
http://www.ivoa.net/Documents/latest/IDs.html

LD

Web Architecture (from How to Publish Link ed Data on the Web
http://www4.wiwiss.fu-berlin.de/bizer/pub/LinkedDataTutorial/#Terminology

RDFpubBest Practice Recipes for Publishing RDF Vocabularies
http://www.w3.org/TR/swbp -vocab-pub

SemURIs

Cool URIs for the Semantic Web http://www.w3.org/TR/cooluris

UKURI

Designing URI Sets for the UK Public Sector
http://www.cabinetoffice.gov.uk/media/308995/public_sector_uri.pdf

URIlife

The URI Lifecycle in Semantic Web Architecture http://dbooth.org/2009/lifecycle

URI-URN

URNs, Namespaces and Registries
http://www.w3.org/2001/tag/doc/URNsAndRegistries-50

WebarchArchitecture of the World Wide Web, Volume One http://www.w3.org/TR/webarch/

WebNameDirk and Nadia design a naming scheme or Web naming schemes goodpractices http://www.w3.org/2001/tag/doc/justSayhttp

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Revision History

Date

Internal
version

EditorSections
modified

Description

18 May 2010 1

Simon Cox

N/A

Initial Draft Document.

19 May2

Simon Cox,
Sven SchadeAll

Reorganized; nits; put into OGC
document template

20 May3

Simon Cox,
Luis Bermudez

3, 4Various nits throughout; clarify infovs non-info-resource argument; hash
URIs

27 May4

Simon Cox,
Ingo Simonis

3.4, 7

Reformatted references; simplified
explanation of info vs non-info
resources; clarified scope ofrecommendation

4 June5Simon Cox

3.4, 4, 5

Minor clarifications in response tocomments from Stuart Williams

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