CENDI/2004-2

PERSISTENT IDENTIFICATION:

A KEY COMPONENT OF AN

E-GOVERNMENT INFRASTRUCTURE

CENDI Persistent Identification Task Group 

Final 

March 10, 2004 
CENDI/2004-1

Persistent Identification Whitepaper – Final 3/9/04 

Members of the CENDI Persistent Identification Task Group

George Barnum (US Government Printing Office) 

Ardie Bausenbach (Library of Congress) 

Charles Bradsher (Defense Technical Information Center) 

Robert Chadduck (National Archives and Records Administration) 

Sherry Davids (National Agricultural Library)

Walter Finch (National Technical Information Service) 

Evelyn Frangakis (National Agricultural Library)

Glenn Gardner (Library of Congress) 

Melanie Gardner (National Agricultural Library)

John Garrett (NASA Goddard Space Flight Center) 

Jeff Given (Dept. of Energy, Office of Scientific and Technical Information) 

Gail Hodge (CENDI Secretariat) 

Laurence Lannom (Corporation for National Research Initiatives) 

William LeFurgy (Library of Congress) 

Kurt Molholm (Defense Technical Information Center), Chair 

Barbara Nekoba (Defense Technical Information Center) 

David Pachter (Federal Library and Information Center Committee) 

Karen Spence (Dept. of Energy, Office of Scientific and Technical Information) 

CENDI is an interagency cooperative organization composed of the scientific and 

technical information (STI) managers from the Departments of Agriculture, Commerce, 

Energy, Education, Defense, the Environmental Protection Agency, Health and Human 

Services, Interior, the National Aeronautics and Space Administration (NASA), the U.S. 

Government Printing Office, and the National Archives and Records Administration.

CENDI's mission is to help improve the productivity of federal science- and technologybased programs through the development and management of effective scientific and 

technical information support systems. In fulfilling its mission, CENDI member agencies 

play an important role in helping to strengthen U.S. competitiveness and address scienceand technology-based national priorities. 

COPYRIGHT NOTICE:

This is a work of the U.S. Government and is not subject to copyright protection in the United 

States. Foreign copyrights may apply. 

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EXECUTIVE SUMMARY 

As more U.S. federal government information is generated in digital form, it is increasingly 

important to develop a digital information infrastructure to ensure effective management and 

access. A key component of this infrastructure is persistent identification of digital 

information resources. Currently, many government resources do not have any uniform type 

of identification; individual agencies instead devise their own methods for naming sources 

such as internal reports, presentations, and other documents. One exception is for 

information posted on agency World Wide Web sites. These sites make use of Uniform 

Resource Locators (URLs) to identify specific Web pages and objects. Unfortunately, this 

approach directly associates the name of the digital object with a physical location. When 

the object is removed from its original location, the association between the name and the 

location of the object is “broken” and accessing the original name yields and error message. 

Broken links are a major barrier to expanding electronic government, since citizens require 

consistent, reliable, and accurate access to government information on the Web. Current 

methods ensuring the association between the object and the name require maintenance, and 

if this is not performed consistently the association remains broken. Addressing this problem

requires incorporating methods for creating and maintaining persistent identification as a key 

component of the Federal Enterprise Architecture. 

Two primary persistent identifier applications have emerged: the Persistent URL (PURL) and 

the Handle System®. Both systems are in use in the government and private sectors to 

enable Web applications to redirect users from the “persistent URL” to the current location of 

the object. Handles and PURLs are globally unique and can support mechanisms such as 

OpenURL, which associates critical descriptive information (metadata) with identification to 

enable context-sensitive linking. Handles are also supportive of a federated implementation, 

are independent of any physical location, and can resolve to multiple locations or multiple 

versions of an object. The Handle System has been adopted by major publishers for 

persistent identification of commercially traded content through its implementation with the 

Digital Object Identifier (DOI) system. While most of today’s implementations of persistent 

identifiers use PURLs or Handles for document-like objects, there are a variety of other 

object types from events to agreements to data sets that could be managed using persistent 

identification schemes. 

Establishing methods for persistent identification of government resources requires extensive 

analysis of issues such as preferred identifier approaches, core metadata, identifier 

maintenance, and relationships with existing information management systems. 

Consideration must be given to all aspects of the government information life cycle from 

creation to long-term management and access to ultimate disposition, including permanent 

preservation. There is also potential impact on key federal information management 

requirements and directives such as A-130 and A-110. For these reasons, a logical next step 

is the formation of a group under the E-government Interagency Committee on Government 

Information representing a variety of stakeholder groups to study the implementation issues, 

analyze costs and present recommendations. 

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The Opportunity: Ensuring Persistent Location of Digital Objects in E-Government 

Services 

The President’s Management Agenda and the E-Government Act of 2002 emphasize the 

development of electronic services for citizens and industry and the efficient and effective 

sharing of information between and among federal government entities and other government 

levels. As more of these initiatives use the Internet, and in particular the Web, as their 

information dissemination platform, the ability to consistently locate digital objects over time 

becomes increasingly important. 

While there are no specific studies that address the percentage of broken links (“linkrot”) for 

government information objects, it can be assumed to be significant. While broken links on 

federal government Web sites may occur less frequently than on the Web at large, the 

realignment of responsibilities within agencies, fewer resources, changes in contractors, the 

ephemeral nature of some projects, and the closure of government programs and units all 

increase the possibility of resources being moved without warning. 

Persistent identification is a key component of a reliable digital information infrastructure 

and is essential for providing e-government services and information. Without such 

persistence, citizens who have bookmarked URLs or who try to access government services 

from an outdated reference will receive the ever-annoying 404 message rather than valuable 

government information and services. References to government Web sites will result in an 

increasing number of broken links over time; this will frustrate citizens, increase

maintenance costs, and potentially result in the withholding of services or information from

the public. 

As Is 

What is Persistent Identification? 

Bits and bytes not only need to be displayed, but to be labeled or referenced in such a way 

that they can be reliably found over time in a dynamic information environment. The current 

addressing structure for the Web is based on the Uniform Resource Locator (URL). This 

technology uses a physical location (IP address/server/path/file name) to identify and locate 

digital objects. While the URL provides direct, efficient access, URL-only naming fails 

whenever the resources are moved or reorganized. In addition, the URL may stay the same, 

but the object addressed by the URL may change significantly or be replaced with 

completely different content. Thus, while the URL permits interoperability in assigning an 

initial address for an object, it offers no assurance that this address will follow the object as it 

moves among locations. The lack of persistence or “linkrot” leads to 404 errors (file not 

found), inhibiting access to digital objects and causing problems when archiving material for 

long-term preservation and permanent access. A recent study by researchers at the 

University of Nebraska found a half-life of 55 months for what was considered to be stable 

information appropriate for inclusion in a Web-based curriculum in biochemistry and 

molecular biology (Markwell & Brooks, 2003). These statistics are of particular concern 

when URLs are used as links, citations or references, or bookmarks. 

Unlike the URL, a persistent identifier tracks a specific object regardless of its physical 

location or current ownership. It is similar in function to a Social Security number which is 

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assigned to an individual and does not change when that person’s address changes. 

Likewise, in the digital environment, digital object identifiers must be unique, persistent, 

independent of specific Web domain names, resolvable using standard Web protocols, and 

flexible enough to allow efficient management of digital information and accommodation of 

technological changes. 

 

Persistent Identifier Approaches 

Among the most commonly used persistent identifier applications are Persistent URLs 

(PURLs) and the Handle System. Both approaches provide registration and resolution 

services (similar to the resolver concept used in the Domain Name System (DNS) for URLs) 

to map the persistent identifier to the current physical location of the digital object. The 

PURL approach retains the URL construct, which can be used directly by today’s Web 

browsers, using the Web’s indirection techniques for resolving the old URL to a new one. 

PURL software, developed by OCLC, a third-party provider of library services, resolves 

Persistent URL identifiers using servers identified by their Web domain address. The PURL 

is structured as: 

http://purl.[resolver name, e.g. oclc.org]/[specific resource identifier] 

A PURL contains the URL for the PURL Resolver Service (in the example above, the 

resolver at OCLC is used) followed by an identifier for the resource. The PURL assigned to 

the document-like object points to the PURL resolver record, which contains information to 

redirect the PURL to the current URL of the object. Of course, the resolver table must be 

updated when the actual URL location changes, but the document’s PURL does not change. 

A PURL server can resolve only the PURLs it maintains (e.g., OCLC’s PURL server cannot 

resolve PURLs assigned by other PURL servers). The PURL Resolver software is available 

free from OCLC, or PURLs can be deposited on OCLC’s Resolver under an agreement with 

OCLC. 

The Handle System, developed by the Corporation for National Research Initiatives (CNRI) 

under contract to several U.S. government agencies, is an interoperable network of 

distributed resolver servers, linked through a Global Resolver currently maintained by CNRI. 

The Global Handle Server registers, maintains, and resolves the naming authorities of 

locally-maintained Handle Servers. Any local Handle Server can, therefore, resolve any 

Handle through the Global Resolver. Handles, as most commonly used, resolve to the 

current URL of a digital object. A Handle is structured as: 

[unique persistent naming authority for the assigning agency]/[unique, persistent identifier 

for the resource]

The Global Handle Server assures that each naming authority is unique; local Handle Servers 

assure that each resource identifier assigned by a naming authority is unique within that 

naming authority. The resource identifier portion of a Handle can be an intelligent string or 

an unintelligent “dumb string.” Many organizations use identifiers already developed for 

their internal systems in this portion of the Handle. 

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The Handle System also supports the resolution of one Handle to multiple targets, and 

priorities can be established for the order in which the multiple resolutions will be used. 

Handles can, therefore, resolve to different digital versions of the same content, to mirror 

sites, or to different business models (pay versus free). They can also resolve to different 

digital versions of differing content, such as a mix of resources required for a distancelearning course. For example, one Handle could provide the capability to access all of the 

digital materials for a course. In addition to URLs, Handles can resolve to email accounts or 

to other Handles (supporting various Web services applications). Each of these various 

target categories has a unique data type. Because current Web browsers cannot support the 

Handle resolution directly, it is necessary to use intervening software. The software can be 

downloaded as an add-on client or hosted on a proxy server. 

Therefore, both PURLS and Handles are redirection mechanisms that can be addressed 

through normal looking URLs, e.g., 

http://dx.doi.org/10.123/456 

http://purl.oclc.org/some-doc-name 

In addition there are other persistent identification schemes under development. For 

example, the XRI (eXtensible Resource Identifier) is a proposed scheme for distributed 

directory services to enable identification of resources and the sharing of data across 

disparate computer systems. The ARK (Archive Resource Key), developed by the California 

Digital Library, emphasizes archived digital objects. An ARK requires a link from the object 

to a promise statement regarding the degree of persistent maintenance, a link from the object 

to its metadata, and a link to the object itself. 

Who Uses Persistent Identifiers? 

The need for persistent identification for document-like objects has been recognized by a 

number of organizations and there are several implementations in the public and private 

sectors, in the United States and in other countries. Within the U.S. Government, for 

example, the Government Printing Office and the Department of Energy’s Office of 

Scientific and Technical Information use PURLs and their own installations of the PURL 

Resolver to manage their connections to the full text of documents. 

The Defense Technical Information Center uses the Handle System to control the 

identification and location of digital objects it receives from throughout the DoD. DTIC is a 

Handle Naming Authority. At the present time, Handles are assigned to DTIC's full text, 

publicly-releasable technical reports. Resolvable Handles are displayed on citations in 

DTIC’s Scientific and Technical Information Networks (Public STINET and Private 

STINET). In addition, a separate DTIC Central Handle Service Directory stored in an Oracle 

database, contains searchable key metadata for each Handle resource. A search of key 

metadata (i.e., Title, Corporate Author, Personal Author, Report No., DTIC AD No., 

Publication Year) returns a results list from which a Handle can be selected. The central 

directory serves two purposes: 1) To provide handle resolution for any known Handle when 

a Handle prefix and suffix are known; 2) To ‘discover’ a Handle when some information is 

known about a resource, but not its Handle. The Handle provides a direct link to the 

resource. This directory also provides the interface that will enable DTIC to manage 

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resources held by outside DoD agencies. DTIC is increasing the functionality of its Handle 

Service and will soon provide secure access to unclassified but protected digital assets,

support remote access and management of Handles and affiliated data (as part of its 

partnership building effort), and extend Handles to fit different digital models (e.g., distance 

learning objects). 

Additionally, DTIC is exploring new information technologies to make a variety of digital 

materials available to its user communities through a Defense Virtual Information 

Architecture (DVIA). These materials may include textual materials such as technical reports 

and electronic journals, plus maps, videos, photographs, sound, spatial data, architectural 

drawings, computer programs, instructional materials and possibly medical imagery. DTIC 

will store some of the materials in its own repository and also provide links to remote sites 

when linking is the best way to deliver the information. Seamless searching across diverse 

resources will be offered. The Handle System is an essential component of this application. 

The Library of Congress has naming authorities under CNRI’s Global Handle Resolver for 

its major units. Over 400,000 Handles have been assigned since 1995 to digital objects 

maintained by the Library. Handles are used as identifiers in the Library’s American 

Memory Collection (a large digital library collection), as identifiers for electronic finding 

aids for the Library’s archival collections, and as persistent links to digital content described 

by the Library’s distributed cataloging records and electronic finding aids. There is a current

project to investigate the assignment of persistent identifiers to the Library’s XML schemas. 

The largest single supplier of persistent identification is the International DOI Foundation 

(IDF). IDF developed the Handle-based Digital Object Identifier to support e-commerce. 

CrossRef is the largest user of the DOI system. CrossRef is a collaborative reference linking 

service that functions as a sort of digital switchboard. It holds no full text content itself, but 

rather links users to digital content through the DOIs, which are tagged to object-level 

metadata supplied by the participating publishers, including the URL for the digital content. 

The end result is an efficient, scalable linking system through which a researcher can click on 

a reference citation in a citation database, for example, and access the cited article. More than 

200 publishers deposit and maintain DOI persistent identifiers in the CrossRef System to link 

citations and references to the full text of journal articles and books. To date over 9 million 

identifiers have been registered in the CrossRef system and more are being added each day. 

CrossRef is a DOI Registration Agency. The primary role of Registration Agencies is to 

provide services to Registrants - allocating DOI prefixes, registering DOIs, and providing the 

necessary infrastructure to allow Registrants to declare and maintain metadata and state data. 

The Registration Agency concept allows the Handle System to be a distributed system. 

The TSO (The Stationery Office) in the UK is also a DOI Registration Agency. TSO 

publishes on behalf of UK’s Parliaments and assemblies and is the UK's definitive source of

official and regulatory information. The TSO recently joined the International DOI 

Foundation in order to help its government and private clients better manage digital 

resources. With a major emphasis in the UK on E-government and with support from the Egovernment Envoy, the TSO is seeking to provide a series of services that use the DOI to 

support management of government information over time. 

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What Could Be 

Persistent Identifiers and the Federal Enterprise Architecture 

How does this technology fit with the Federal Enterprise Architecture (represented by the 

figure below) being developed by OMB under the authority of the E-Government Act of 

2002?

Business Reference Model (BRM)

• Lines of Business

• Agencies, Customers, Partners

Service Component Reference Model (SRM)

• Capabilities and Functionality

• Services and Access Channels

Technical Reference Model (TRM)

• IT Services

• Standards

Data Reference Model (DRM)

• Business-focused data standardization 

• Cross-Agency Information exchanges

Business-Driven Approach

Performance Reference Model (PRM)

• Government-wide Performance Measures & Outcomes

• Line of Business-Specific Performance Measures & Outcomes

Federal Enterprise Architecture (FEA)

Component-Based Architecture

Page 9

Persistent identification is a core infrastructure component that touches every model of the 

FEA above. Persistent identifiers specifically support the Data Reference Model of the FEA 

by enabling information exchange and use within and even across FEA business lines. The 

technology also provides the Technical Reference Model with both an IT service (the 

resolution) and a standard identifier syntax (the structure of the identifier and the core 

metadata). 

The FEA’s emphasis on standards and best practices can be addressed by development of a 

Persistent Identifier framework. A federal-wide design, if not implementation, of a Persistent 

Identification Resolver would permit data types used in the resolver to be standardized. This 

would result in a common look and feel for users. In addition, a central approach can 

produce shared best practices, additional services and plug-ins, and non-redundant 

submission for services and products that are produced by multiple agencies. 

The government examples described above outline the use of persistent identifiers to link to 

the current location for document-like objects. However, persistent identifiers can also be 

used in “non standard” ways to integrate and link digital content. Identifiers, for example, 

can help track events, such as conferences (to manage conference proceedings as either a 

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single resource or separately as independent entities) or meetings, track the flow of 

correspondence, link versions and updates of departmental regulations, monitor the impact of 

training on its employees, bring together information about a terrorist from a variety of 

agencies, manage agreements, identify data sets, or manage electronic records (including 

appraisal, retention and disposition activities). 

Project Management and Maintenance Issues 

The successful implementation of persistent identifiers may well face more social and 

economic challenges than technical ones. The implementation of a Federal Persistent 

Identification Resolver requires ongoing maintenance and, therefore, ongoing resources. At 

the individual agency level, the resolver must be kept up-to-date with the current URLs for 

the locations of the government digital objects. The resolution provided by the system is 

only as up-to-date as the physical locations to which the persistent identifiers point. While 

some of this updating can be automated, responsibility for this updating and ensuring its 

reliability must be assigned within each agency, program or office or to a trusted third-party. 

It is not sufficient to create identifiers and leave them without maintenance; active 

management is needed in order to gain the benefits of such a system. 

The adoption of persistent identifiers will require the introduction of new technology, 

although the specifics of the identifier and the use of that identifier will determine exactly 

what new technology will be needed. In the case of both PURLs and Handles, an 

organization will run its own server, either a PURL server or a Handle server, or arrange to 

register its identifiers on another organization’s server. In the case of Handles the server 

containing the Handles and the proxy server mapping the URL requests to the id server can 

be one in the same or they can be separate while in the case of PURLs they are always the 

same. On the client side a normal Web browser can be used if the identifier is in the form of

a URL. However, use of the native Handle protocol, which allows added functionality, would 

require special plug-in software or a separate client application. 

Successful implementation of a Federal Persistent Identification Resolver may impact federal 

information policies, including A-130, A-10, the American Technology Preeminence Act, the 

data clauses of the DFARS, the FAR, and subsequent agency directives and regulations. 

These areas need further discussion and investigation. 

Specific Next Steps 

• The E-government Interagency Committee on Government Information should 

establish a group representing various stakeholders to address persistent identification 

as a component of the e-government infrastructure. This should include discussions 

of centralized versus distributed implementations, a framework for development of a 

government-wide persistent identification scheme or schemes, and discussions of the 

gateway and user interfaces. The group must analyze the costs of such a system

based on the specific implementation decisions. 

• Once criteria have been established, the FEA model should be modified to 

specifically reference the need for each agency to consider what information should 

be managed using persistent identifiers and the specifics of the scheme to be used. 

• Policies and guidelines should be established for the creation and maintenance of 

persistent identifiers and the management of related services.

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• A key area for discussion and consensus is the determination of a core set(s) of 

persistent identification metadata (and metadata formats) necessary to assist in 

discovery, digital rights management, and the provision of associated services for 

different user communities. 

• All the above discussions should consider the life cycle of information from creator to 

initial and secondary dissemination to preservation, long-term records management 

and archiving. 

• Since the Web crosses all sectors and national boundaries, it will be important to 

work with other organizations that are involved in the discussion of persistent 

identifiers on a national and international scale. This includes not only organizations 

such as the International DOI Foundation, CNRI and OCLC which are directly 

involved in these technologies, but also groups such as the World Wide Web 

Consortium (Web protocols and other technologies), the International Federation of 

Library and Museum Associations (libraries and museums), the International 

Publishers Association (publishers of all kinds), the International Standards 

Organization, the National Information Standards Organization, and the Dublin Core 

Metadata Initiative (which has created a special interest group on Persistent 

Identification within the Dublin Core Metadata Initiative). 

References 

Handles System. [http://www.handle.net/] 

International DOI Foundation. [http://www.doi.org] 

Kunze, J. and R. P. C. Rogers. “The ARK Persistent Identifier Scheme.” Internet Draft 

January 31, 2003. [http://www.cdlib.org/inside/diglib/ark/arkspec.pdf] 

Markwell, J. and D. Brooke. “Broken Links: Just How Rapidly Do Science Education 

Hyperlinks Go Extinct?” 2003. [http://www-class.unl.edu/biochem/url/broken_links.html] 

OASIS. “eXtensible Resource Identifier TC.” [http://www.oasisopen.org/committees/tc_home.php?wg_abbrev=xri] 

Persistent URLs. [http://www.purl.oclc.org] 

TSO. “Information Management & Interoperability Strategies: The Case of Digital 

Identifiers.” March 31, 2003. 

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