Network Working Group J. Kunze Request for Comments: 2731 Dublin Core Category: Informational Metadata Initiative December 1999 Encoding Dublin Core Metadata in HTML Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. 1. Abstract The Dublin Core [DC1] is a small set of metadata elements for describing information resources. This document explains how these elements are expressed using the META and LINK tags of HTML [HTML4.0]. A sequence of metadata elements embedded in an HTML file is taken to be a description of that file. Examples illustrate conventions allowing interoperation with current software that indexes, displays, and manipulates metadata, such as [SWISH-E], [freeWAIS-sf2.0], [GLIMPSE], [HARVEST], [ISEARCH], etc., and the Perl [PERL] scripts in the appendix. 2. HTML, Dublin Core, and Non-Dublin Core Metadata The Dublin Core (DC) metadata initiative [DCHOME] has produced a small set of resource description categories [DC1], or elements of metadata (literally, data about data). Metadata elements are typically small relative to the resource they describe and may, if the resource format permits, be embedded in it. Two such formats are the Hypertext Markup Language (HTML) and the Extensible Markup Language (XML); HTML is currently in wide use, but once standardized, XML [XML] in conjunction with the Resource Description Framework [RDF] promise a significantly more expressive means of encoding metadata. The [RDF] specification actually describes a way to use RDF within an HTML document by adhering to an abbreviated syntax. Kunze Informational [Page 1] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 This document explains how to encode metadata using HTML 4.0 [HTML4.0]. It is not concerned with element semantics, which are defined elsewhere. For illustrative purposes, some element semantics are alluded to, but in no way should semantics appearing here be considered definitive. The HTML encoding allows elements of DC metadata to be interspersed with non-DC elements (provided such mixing is consistent with rules governing use of those non-DC elements). A DC element is indicated by the prefix "DC", and a non-DC element by another prefix; for example, the prefix "AC" is used with elements from the A-Core [AC]. 3. The META Tag The META tag of HTML is designed to encode a named metadata element. Each element describes a given aspect of a document or other information resource. For example, this tagged metadata element, says that Homer Simpson is the Creator, where the element named Creator is defined in the DC element set. In the more general form, the capitalized words are meant to be replaced in actual descriptions; thus in the example, ELEMENT_NAME was: Creator ELEMENT_VALUE was: Simpson, Homer and PREFIX was: DC Within a META tag the first letter of a Dublin Core element name is capitalized. DC places no restriction on alphabetic case in an element value and any number of META tagged elements may appear together, in any order. More than one DC element with the same name may appear, and each DC element is optional. The next example is a book description with two authors, two titles, and no other metadata. Kunze Informational [Page 2] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 The prefix "DC" precedes each Dublin Core element encoded with META, and it is separated by a period (.) from the element name following it. Each non-DC element should be encoded with a prefix that can be used to trace its origin and definition; the linkage between prefix and element definition is made with the LINK tag, as explained in the next section. Non-DC elements, such as Email from the A-Core [AC], may appear together with DC elements, as in This example also shows how some special characters may be encoded. The author name in the first element contains a diacritic encoded as an HTML character entity reference -- in this case an accented letter E. Similarly, the last line contains two double-quote characters encoded so as to avoid being interpreted as element content delimiters. 4. The LINK Tag The LINK tag of HTML may be used to associate an element name prefix with the reference definition of the element set that it identifies. A sequence of META tags describing a resource is incomplete without one such LINK tag for each different prefix appearing in the sequence. The previous example could be considered complete with the addition of these two LINK tags: In general, the association takes the form Kunze Informational [Page 3] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 where, in actual descriptions, PREFIX is to be replaced by the prefix and LOCATION_OF_DEFINITION by the URL or URN of the defining document. When embedded in the HEAD part of an HTML file, a sequence of LINK and META tags describes the information in the surrounding HTML file itself. Here is a complete HTML file with its own embedded description. A Dirge
               Rough wind, that moanest loud
                 Grief too sad for song;
               Wild wind, when sullen cloud
                 Knells all the night long;
               Sad storm, whose tears are vain,
               Bare woods, whose branches strain,
               Deep caves and dreary main, -
                 Wail, for the world's wrong!
       
5. Encoding Recommendations HTML allows more flexibility in principle and in practice than is recommended here for encoding metadata. Limited flexibility encourages easy development of software for extracting and processing metadata. At this early evolutionary stage of internet metadata, easy prototyping and experimentation hastens the development of useful standards. Kunze Informational [Page 4] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Adherence is therefore recommended to the tagging style exemplified in this document as regards prefix and element name capitalization, double-quoting (") of attribute values, and not starting more than one META tag on a line. There is much room for flexibility, but choosing a style and sticking with it will likely make metadata manipulation and editing easier. The following META tags adhere to the recommendations and carry identical metadata in three different styles: Use of these recommendations is known to result in metadata that may be harvested, indexed, and manipulated by popular, freely available software packages such as [SWISH-E], [freeWAIS-sf2.0], [GLIMPSE], [HARVEST], and [ISEARCH], among others. These conventions also work with the metadata processing scripts appearing in the appendix, as well as with most of the [DCPROJECTS] applications referenced from the [DCHOME] site. Software support for the LINK tag and qualifier conventions (see the next section) is not currently widespread. Ordering of metadata elements is not preserved in general. Writers of software for metadata indexing and display should try to preserve relative ordering among META tagged elements having the same name (e.g., among multiple authors), however, metadata providers and searchers have no guarantee that ordering will be preserved in metadata that passes through unknown systems. 6. Dublin Core in Real Descriptions In actual resource description it is often necessary to qualify Dublin Core elements to add nuances of meaning. While neither the general principles nor the specific semantics of DC qualifiers are within scope of this document, everyday uses of the qualifier syntax are illustrated to lend realism to later examples. Without further explanation, the three ways in which the optional qualifier syntax is currently (subject to change) used to supplement the META tag may be summarized as follows: Kunze Informational [Page 5] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Accordingly, a posthumous work in Spanish might be described with Note that the qualifier syntax and label suffixes (which follow an element name and a period) used in examples in this document merely reflect current trends in the HTML encoding of qualifiers. Use of this syntax and these suffixes is neither a standard nor a recommendation. 7. Encoding Dublin Core Elements This section consists of very simple Dublin Core encoding examples, arranged by element. Title (name given to the resource) ----- Kunze Informational [Page 6] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Creator (entity that created the content) ------- Subject (topic or keyword) ------- Kunze Informational [Page 7] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Description (account, summary, or abstract of the content) ----------- Publisher (entity that made the resource available) --------- Contributor (other entity that made a contribution) ----------- Kunze Informational [Page 8] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Date (of an event in the life of the resource; [WTN8601] recommended) ---- Type (nature, genre, or category; [DCT1] recommended) ---- Kunze Informational [Page 9] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Format (physical or digital data format, plus optional dimensions) ------ Kunze Informational [Page 10] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Identifier (of the resource) ---------- Source (reference to the resource's origin) ------ Language (of the content of the resource; [RFC1766] recommended) -------- Kunze Informational [Page 11] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Relation (reference to a related resource) -------- Coverage (extent or scope of the content) -------- Kunze Informational [Page 12] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Rights (text or identifier of a rights management statement) ------ 8. Security Considerations The syntax rules for encoding Dublin Core metadata in HTML that are documented here pose no direct risk to computers and networks. People can use these rules to encode metadata that is inaccurate or even deliberately misleading (creating mischief in the form of "index spam"), however, this reflects a general pattern of HTML META tag abuse that is not limited to the encoding of metadata from the Dublin Core set. Even traditional metadata encoding schemes (e.g., [MARC]) are not immune to inaccuracy, although they are generally followed in environments where production quality greatly exceeds that of the average Web site. Systems that process metadata encoded with META tags need to consider issues related to its accuracy and validity as part of their design and implementation, and users of such systems need to consider the design and implementation assumptions. Various approaches may be relevant for certain applications, such as adding statements of metadata provenance, signing of metadata with digital signatures, and automating certain aspects of metadata creation; but these are far outside the scope of this document and the underlying META tag syntax that it describes. Kunze Informational [Page 13] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 9. Appendix -- Perl Scripts that Manipulate HTML Encoded Metadata This section contains two simple programs that work with versions 4 and 5 of the Perl [PERL] scripting language interpreter. They may be taken and freely adapted for local organizational needs, research proposals, venture capital bids, etc. A variety of applications are within easy reach of implementors that choose to build on these scripts. Script 1: Metadata Format Conversion ------------------------------------- Here is a simple Perl script that correctly recognizes every example of metadata encoding in this document. It shows how a modest scripting effort can produce a utility that converts metadata from one format to another. Minor changes are sufficient to support a number of output formats. #!/depot/bin/perl # # This simple perl script extracts metadata embedded in an HTML file # and outputs it in an alternate format. Issues warning about missing # element name or value. # # Handles mixed case tags and attribute values, one per line or spanning # several lines. Also handles a quoted string spanning multiple lines. # No error checking. Does not tolerate more than one ") { next if (! //i) { while (<>) { $meta .= $_; last if (/>/); } } $name = $meta =~ /name\s*=\s*"([^"]*)"/i ? $1 : "MISSING ELEMENT NAME"; $content = $meta =~ /content\s*=\s*"([^"]*)"/i ? $1 : "MISSING ELEMENT VALUE"; ($scheme) = $meta =~ /scheme\s*=\s*"([^"]*)"/i; ($lang) = $meta =~ /lang\s*=\s*"([^"]*)"/i; if ($lang || $scheme) { $mod = " ($lang"; if (! $scheme) Kunze Informational [Page 14] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 { $mod .= ")"; } elsif (! $lang) { $mod .= "$scheme)" } else { $mod .= ", $scheme)"; } } else { $mod = ""; } print " @|$name$mod; $content\n"; } print "@)urc;\n"; # ---- end of Perl script ---- When the conversion script is run on the metadata file example from the LINK tag section (section 4), it produces the following output. @(urc; @|DC.Title; A Dirge @|DC.Creator; Shelley, Percy Bysshe @|DC.Type; poem @|DC.Date; 1820 @|DC.Format; text/html @|DC.Language; en @)urc; Script 2: Automated Metadata Creation -------------------------------------- The creation and maintenance of high-quality metadata can be extremely expensive without automation to assist in processes such as supplying pre-set or computed defaults, validating syntax, verifying value ranges, spell checking, etc. Considerable relief could be had from a script that reduced an individual provider's metadata burden to just the title of each document. Below is such a script. It lets the provider of an HTML document abbreviate an entire embedded resource description using a single HTML comment statement that looks like Our script processes this statement specially as a kind of "metadata block" declaration with attached title. The general form is Kunze Informational [Page 15] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 This statement works much like a "Web server-side include" in that the script replaces it with a fully-specified block of metadata and triggers other replacements. Once installed, the script can output HTML files suitable for integration into one's production Web server procedures. The individual provider keeps a separate "template" file of infrequently changing pre-set values for metadata elements. If the provider's needs are simple enough, the only element values besides the title that differ from one document to the next may be generated automatically. Using the script, values may be referenced as variables from within the template or within the document. Our variable references have the form "(--mbVARNAME)", and here is what they look like inside a template: (--mbtitle) The above template represents the metadata block that will describe the document once the variable references are replaced with real values. By the conventions of our script, the following variables will be replaced in both the template and in the document: (--mbfilesize) size of the final output file (--mbtitle) title of the document (--mblanguage) language of the document (--mbbaseURL) beginning part of document identifier (--mbfilename) last part (minus .html) of identifier (--mbfilemodtime) last modification date of the document Kunze Informational [Page 16] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Here's an example HTML file to run the script on.

From: Acting Shift Supervisor To: Plant Control Personnel RE: (--mbtitle) Date: (--mbfilemodtime)