Structural Metadata Dictionary for LC Digital Objects

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External Pointers to Digital Objects

External pointers to digital objects include MARC record 856 fields, HTML hypertext links (HREFs), and Encoded Archival Descriptions (EADs).

Such pointers can contain one of the following:

• Uniform Resource Name (URN)*
• HTTP Uniform Resource Locator (URL)*
• HTTP Uniform Resource Locator (URL) with Resolver and URN

*   In MARC record 856 fields the "g" subfield is used to hold URNs and the "u" subfield is used to hold URLs. For current LC plans for using 856 subfields g and u, click here.

Uniform Resource Name (URN)

• Components:
  1. URN tag
  2. A colon [:]
  3. Name Space Identifier (e.g. hdl, isbn)
  4. A colon [:]
  5. Local Object Identifier (e.g., a handle or ISBN)

• Layouts:

  URN tag:Name Space ID:Local Object ID

• Examples:

  URN:hdl:loc.ndlp/amrlp.3a16116.1

  URN:isbn:0-949999-03-2

• The URN is resolved by a specified server into one of the HTTP Uniform Resource Locator types listed below.

HTTP Uniform Resource Locator (URL)

• Components:
  1. Protocol Identifier
  2. A colon and two forward slashes [://]
  3. Host/domain name
  4. A colon [:]
  5. Port Identifier (Optional. If absent, defaults to Port 80)
  6. A forward slash [/]
  7. Path to software on the Host followed by a question mark (?) and a Search Argument or the Path to the object as it resides on the Host
• Layouts:

  Protocol ID://Host name:Port ID/Path?Search Argument

  Protocol ID://Host name/Path?Search Argument

  Protocol ID://Host name:Port ID/Path/Object Name

  Protocol ID://Host name/Path/Object Name

• Examples:

  http://rs10.loc.gov:91919/repository/getimage.cgi?URN=hdl:loc.ndlp/amrlp.3a16116.1

  http://rs10.loc.gov/repository/getimage.cgi?URN=hdl:loc.ndlp/amrlp.3a16116.1

  http://memory.loc.gov/ammem/techdocs/repository/attlist.html

  http://lcweb.loc.gov/ndlint/repository/attlist.html

HTTP Uniform Resource Locator (URL) where the Local Object ID contains a URN and the host/domain name is that of the server to be used in resolving the URN to a URL.

• Components:
  1. Protocol Identifier
  2. A colon and two forward slashes [://]
  3. Host/domain ID of resolver
  4. A colon [:]
  5. Port Identifier (If absent, defaults to Port 80)
  6. A forward slash [/]
  7. Local object ID (e.g., a handle or a PURL)
• Layouts:

  Protocol ID://Host Name of Resolver:Port ID/Local Object ID

  Protocol ID://Host Name of Resolver/Local Object ID

• Examples:

  http://hdl.loc.gov:9091/loc.ndlp/amrlp.3a16116.1

  http://hdl.loc.gov/loc.ndlp/amrlp.3a16116.1

  http://purl.oclc.org:8088/OCLC/PURL/FA

  http://purl.oclc.org/OCLC/PURL/FA

LC Plans for Using MARC 856 subfields g and u:

Ultimately, the Library of Congress will record the Uniform Resource Name (URN) in the 856 field of the MARC bibliographic record associated with an LC collection item that has been digitized. Such URNs will be constructed as "handles" and will be placed in subfield g (856 $g). Example:  $gurn:hdl:loc.ndlp/amrlp.3a16116

In the near term, however, both URLs and URNs will be recorded in the 856 field:

• 856 $u will contain the Uniform Resource Locator (URL) of a proxy server with the digital object's URN attached. The proxy server will be used to resolve the URN.   Example:  $uhttp://hdl.loc.gov/loc.ndlp/amrlp.3a16116
• 856 $g will contain the digital object's URN. Example:  $gurn:hdl:loc.ndlp/amrlp.3a16116